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<span>Terminology</span> </div> </a> <ul id="toc-Terminology-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Manual_service_exchanges" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Manual_service_exchanges"> <div class="vector-toc-text"> <span class="vector-toc-numb">3</span> <span>Manual service exchanges</span> </div> </a> <ul id="toc-Manual_service_exchanges-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Early_automatic_exchanges" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Early_automatic_exchanges"> <div class="vector-toc-text"> <span class="vector-toc-numb">4</span> <span>Early automatic exchanges</span> </div> </a> <button aria-controls="toc-Early_automatic_exchanges-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> <span class="vector-icon mw-ui-icon-wikimedia-expand"></span> <span>Toggle Early automatic exchanges subsection</span> </button> <ul id="toc-Early_automatic_exchanges-sublist" class="vector-toc-list"> <li id="toc-Electromechanical_signaling" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Electromechanical_signaling"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.1</span> <span>Electromechanical signaling</span> </div> </a> <ul id="toc-Electromechanical_signaling-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Noise" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Noise"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.2</span> <span>Noise</span> </div> </a> <ul id="toc-Noise-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Maintenance_tasks" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Maintenance_tasks"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.3</span> <span>Maintenance tasks</span> </div> </a> <ul id="toc-Maintenance_tasks-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Electronic_switches" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Electronic_switches"> <div class="vector-toc-text"> <span class="vector-toc-numb">5</span> <span>Electronic switches</span> </div> </a> <ul id="toc-Electronic_switches-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Digital_switches" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Digital_switches"> <div class="vector-toc-text"> <span class="vector-toc-numb">6</span> <span>Digital switches</span> </div> </a> <ul id="toc-Digital_switches-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-The_switch's_place_in_the_network" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#The_switch's_place_in_the_network"> <div class="vector-toc-text"> <span class="vector-toc-numb">7</span> <span>The switch's place in the network</span> </div> </a> <ul id="toc-The_switch's_place_in_the_network-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Switch_design" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Switch_design"> <div class="vector-toc-text"> <span class="vector-toc-numb">8</span> <span>Switch design</span> </div> </a> <button aria-controls="toc-Switch_design-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> <span class="vector-icon mw-ui-icon-wikimedia-expand"></span> <span>Toggle Switch design subsection</span> </button> <ul id="toc-Switch_design-sublist" class="vector-toc-list"> <li id="toc-Fault_tolerance" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Fault_tolerance"> <div class="vector-toc-text"> <span class="vector-toc-numb">8.1</span> <span>Fault tolerance</span> </div> </a> <ul id="toc-Fault_tolerance-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Fire_and_disaster_recovery" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Fire_and_disaster_recovery"> <div class="vector-toc-text"> <span class="vector-toc-numb">8.2</span> <span>Fire and disaster recovery</span> </div> </a> <ul id="toc-Fire_and_disaster_recovery-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-See_also" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#See_also"> <div class="vector-toc-text"> <span class="vector-toc-numb">9</span> <span>See also</span> </div> </a> <ul id="toc-See_also-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-References" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#References"> <div class="vector-toc-text"> <span class="vector-toc-numb">10</span> <span>References</span> </div> </a> <ul id="toc-References-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-External_links" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#External_links"> <div class="vector-toc-text"> <span class="vector-toc-numb">11</span> <span>External links</span> </div> </a> <ul id="toc-External_links-sublist" class="vector-toc-list"> </ul> </li> </ul> </div> </div> </nav> </div> </div> <div class="mw-content-container"> <main id="content" class="mw-body"> <header class="mw-body-header vector-page-titlebar"> <nav aria-label="Contents" class="vector-toc-landmark"> <div id="vector-page-titlebar-toc" class="vector-dropdown vector-page-titlebar-toc vector-button-flush-left" > <input type="checkbox" id="vector-page-titlebar-toc-checkbox" role="button" aria-haspopup="true" data-event-name="ui.dropdown-vector-page-titlebar-toc" class="vector-dropdown-checkbox " aria-label="Toggle the table of contents" > <label id="vector-page-titlebar-toc-label" for="vector-page-titlebar-toc-checkbox" class="vector-dropdown-label cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet cdx-button--icon-only " aria-hidden="true" ><span class="vector-icon mw-ui-icon-listBullet mw-ui-icon-wikimedia-listBullet"></span> <span class="vector-dropdown-label-text">Toggle the table of contents</span> </label> <div class="vector-dropdown-content"> <div id="vector-page-titlebar-toc-unpinned-container" class="vector-unpinned-container"> </div> </div> </div> </nav> <h1 id="firstHeading" class="firstHeading mw-first-heading"><span class="mw-page-title-main">Telephone exchange</span></h1> <div id="p-lang-btn" class="vector-dropdown mw-portlet mw-portlet-lang" > <input type="checkbox" id="p-lang-btn-checkbox" role="button" aria-haspopup="true" data-event-name="ui.dropdown-p-lang-btn" 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Available in 34 languages" > <label id="p-lang-btn-label" for="p-lang-btn-checkbox" class="vector-dropdown-label cdx-button cdx-button--fake-button cdx-button--fake-button--enabled cdx-button--weight-quiet cdx-button--action-progressive mw-portlet-lang-heading-34" aria-hidden="true" ><span class="vector-icon mw-ui-icon-language-progressive mw-ui-icon-wikimedia-language-progressive"></span> <span class="vector-dropdown-label-text">34 languages</span> </label> <div class="vector-dropdown-content"> <div class="vector-menu-content"> <ul class="vector-menu-content-list"> <li class="interlanguage-link interwiki-ar mw-list-item"><a href="https://ar.wikipedia.org/wiki/%D8%A8%D8%AF%D8%A7%D9%84%D8%A9_%D9%87%D8%A7%D8%AA%D9%81" title="بدالة هاتف – Arabic" lang="ar" hreflang="ar" data-title="بدالة هاتف" data-language-autonym="العربية" data-language-local-name="Arabic" class="interlanguage-link-target"><span>العربية</span></a></li><li class="interlanguage-link interwiki-bg mw-list-item"><a href="https://bg.wikipedia.org/wiki/%D0%A2%D0%B5%D0%BB%D0%B5%D1%84%D0%BE%D0%BD%D0%BD%D0%B0_%D1%86%D0%B5%D0%BD%D1%82%D1%80%D0%B0%D0%BB%D0%B0" title="Телефонна централа – Bulgarian" lang="bg" hreflang="bg" data-title="Телефонна централа" data-language-autonym="Български" data-language-local-name="Bulgarian" class="interlanguage-link-target"><span>Български</span></a></li><li class="interlanguage-link interwiki-ca mw-list-item"><a href="https://ca.wikipedia.org/wiki/Central_de_commutaci%C3%B3" title="Central de commutació – Catalan" lang="ca" hreflang="ca" data-title="Central de commutació" data-language-autonym="Català" data-language-local-name="Catalan" class="interlanguage-link-target"><span>Català</span></a></li><li class="interlanguage-link interwiki-cs mw-list-item"><a href="https://cs.wikipedia.org/wiki/Telefonn%C3%AD_%C3%BAst%C5%99edna" title="Telefonní ústředna – Czech" lang="cs" hreflang="cs" data-title="Telefonní ústředna" data-language-autonym="Čeština" data-language-local-name="Czech" class="interlanguage-link-target"><span>Čeština</span></a></li><li class="interlanguage-link interwiki-da mw-list-item"><a href="https://da.wikipedia.org/wiki/Telefoncentral" title="Telefoncentral – Danish" lang="da" hreflang="da" data-title="Telefoncentral" data-language-autonym="Dansk" data-language-local-name="Danish" class="interlanguage-link-target"><span>Dansk</span></a></li><li class="interlanguage-link interwiki-de mw-list-item"><a href="https://de.wikipedia.org/wiki/Vermittlungsstelle" title="Vermittlungsstelle – German" lang="de" hreflang="de" data-title="Vermittlungsstelle" data-language-autonym="Deutsch" data-language-local-name="German" class="interlanguage-link-target"><span>Deutsch</span></a></li><li class="interlanguage-link interwiki-es mw-list-item"><a href="https://es.wikipedia.org/wiki/Central_telef%C3%B3nica" title="Central telefónica – Spanish" lang="es" hreflang="es" data-title="Central telefónica" data-language-autonym="Español" data-language-local-name="Spanish" class="interlanguage-link-target"><span>Español</span></a></li><li class="interlanguage-link interwiki-eo mw-list-item"><a href="https://eo.wikipedia.org/wiki/Telefoncentralo" title="Telefoncentralo – Esperanto" lang="eo" hreflang="eo" data-title="Telefoncentralo" data-language-autonym="Esperanto" data-language-local-name="Esperanto" class="interlanguage-link-target"><span>Esperanto</span></a></li><li class="interlanguage-link interwiki-fa mw-list-item"><a href="https://fa.wikipedia.org/wiki/%D8%AA%D9%84%D9%81%D9%86%E2%80%8C%D8%AE%D8%A7%D9%86%D9%87" title="تلفن‌خانه – Persian" lang="fa" hreflang="fa" data-title="تلفن‌خانه" data-language-autonym="فارسی" data-language-local-name="Persian" class="interlanguage-link-target"><span>فارسی</span></a></li><li class="interlanguage-link interwiki-fr mw-list-item"><a href="https://fr.wikipedia.org/wiki/Commutateur_t%C3%A9l%C3%A9phonique" title="Commutateur téléphonique – French" lang="fr" hreflang="fr" data-title="Commutateur téléphonique" data-language-autonym="Français" data-language-local-name="French" class="interlanguage-link-target"><span>Français</span></a></li><li class="interlanguage-link interwiki-ko mw-list-item"><a href="https://ko.wikipedia.org/wiki/%EC%A0%84%ED%99%94_%EA%B5%90%ED%99%98%EA%B8%B0" title="전화 교환기 – Korean" lang="ko" hreflang="ko" data-title="전화 교환기" data-language-autonym="한국어" data-language-local-name="Korean" class="interlanguage-link-target"><span>한국어</span></a></li><li class="interlanguage-link interwiki-id mw-list-item"><a href="https://id.wikipedia.org/wiki/Sentral_telepon" title="Sentral telepon – Indonesian" lang="id" hreflang="id" data-title="Sentral telepon" data-language-autonym="Bahasa Indonesia" data-language-local-name="Indonesian" class="interlanguage-link-target"><span>Bahasa Indonesia</span></a></li><li class="interlanguage-link interwiki-it mw-list-item"><a href="https://it.wikipedia.org/wiki/Centrale_telefonica" title="Centrale telefonica – Italian" lang="it" hreflang="it" data-title="Centrale telefonica" data-language-autonym="Italiano" data-language-local-name="Italian" class="interlanguage-link-target"><span>Italiano</span></a></li><li class="interlanguage-link interwiki-he mw-list-item"><a href="https://he.wikipedia.org/wiki/%D7%9E%D7%A8%D7%9B%D7%96%D7%99%D7%99%D7%94" title="מרכזייה – Hebrew" lang="he" hreflang="he" data-title="מרכזייה" data-language-autonym="עברית" data-language-local-name="Hebrew" class="interlanguage-link-target"><span>עברית</span></a></li><li class="interlanguage-link interwiki-ka mw-list-item"><a href="https://ka.wikipedia.org/wiki/%E1%83%90%E1%83%A2%E1%83%A1" title="ატს – Georgian" lang="ka" hreflang="ka" data-title="ატს" data-language-autonym="ქართული" data-language-local-name="Georgian" class="interlanguage-link-target"><span>ქართული</span></a></li><li class="interlanguage-link interwiki-kk mw-list-item"><a href="https://kk.wikipedia.org/wiki/%D0%90%D0%B2%D1%82%D0%BE%D0%BC%D0%B0%D1%82%D1%82%D1%8B_%D1%82%D0%B5%D0%BB%D0%B5%D1%84%D0%BE%D0%BD_%D1%81%D1%82%D0%B0%D0%BD%D1%86%D0%B8%D1%8F%D1%81%D1%8B" title="Автоматты телефон станциясы – Kazakh" lang="kk" hreflang="kk" data-title="Автоматты телефон станциясы" data-language-autonym="Қазақша" data-language-local-name="Kazakh" class="interlanguage-link-target"><span>Қазақша</span></a></li><li class="interlanguage-link interwiki-hu mw-list-item"><a href="https://hu.wikipedia.org/wiki/Telefonk%C3%B6zpont" title="Telefonközpont – Hungarian" lang="hu" hreflang="hu" data-title="Telefonközpont" data-language-autonym="Magyar" data-language-local-name="Hungarian" class="interlanguage-link-target"><span>Magyar</span></a></li><li class="interlanguage-link interwiki-nl mw-list-item"><a href="https://nl.wikipedia.org/wiki/Telefooncentrale" title="Telefooncentrale – Dutch" lang="nl" hreflang="nl" data-title="Telefooncentrale" data-language-autonym="Nederlands" data-language-local-name="Dutch" class="interlanguage-link-target"><span>Nederlands</span></a></li><li class="interlanguage-link interwiki-ja mw-list-item"><a href="https://ja.wikipedia.org/wiki/%E9%9B%BB%E8%A9%B1%E4%BA%A4%E6%8F%9B%E6%A9%9F" title="電話交換機 – Japanese" lang="ja" hreflang="ja" data-title="電話交換機" data-language-autonym="日本語" data-language-local-name="Japanese" class="interlanguage-link-target"><span>日本語</span></a></li><li class="interlanguage-link interwiki-no mw-list-item"><a href="https://no.wikipedia.org/wiki/Telefonsentral" title="Telefonsentral – Norwegian Bokmål" lang="nb" hreflang="nb" data-title="Telefonsentral" data-language-autonym="Norsk bokmål" data-language-local-name="Norwegian Bokmål" class="interlanguage-link-target"><span>Norsk bokmål</span></a></li><li class="interlanguage-link interwiki-uz mw-list-item"><a href="https://uz.wikipedia.org/wiki/Avtomat_telefon_stansiya" title="Avtomat telefon stansiya – Uzbek" lang="uz" hreflang="uz" data-title="Avtomat telefon stansiya" data-language-autonym="Oʻzbekcha / ўзбекча" data-language-local-name="Uzbek" class="interlanguage-link-target"><span>Oʻzbekcha / ўзбекча</span></a></li><li class="interlanguage-link interwiki-pl mw-list-item"><a href="https://pl.wikipedia.org/wiki/Centrala_telefoniczna" title="Centrala telefoniczna – Polish" lang="pl" hreflang="pl" data-title="Centrala telefoniczna" data-language-autonym="Polski" data-language-local-name="Polish" class="interlanguage-link-target"><span>Polski</span></a></li><li class="interlanguage-link interwiki-pt mw-list-item"><a href="https://pt.wikipedia.org/wiki/Central_telef%C3%B4nica" title="Central telefônica – Portuguese" lang="pt" hreflang="pt" data-title="Central telefônica" data-language-autonym="Português" data-language-local-name="Portuguese" class="interlanguage-link-target"><span>Português</span></a></li><li class="interlanguage-link interwiki-ro mw-list-item"><a href="https://ro.wikipedia.org/wiki/Central%C4%83_telefonic%C4%83" title="Centrală telefonică – Romanian" lang="ro" hreflang="ro" data-title="Centrală telefonică" data-language-autonym="Română" data-language-local-name="Romanian" class="interlanguage-link-target"><span>Română</span></a></li><li class="interlanguage-link interwiki-rue mw-list-item"><a href="https://rue.wikipedia.org/wiki/%D0%90%D0%B2%D1%82%D0%BE%D0%BC%D0%B0%D1%82%D0%B8%D1%87%D0%BD%D0%B0_%D1%82%D0%B5%D0%BB%D0%B5%D1%84%D0%BE%D0%BD%D0%BD%D0%B0_%D1%81%D1%82%D0%B0%D1%86%D0%B8%D1%8F" title="Автоматична телефонна стация – Rusyn" lang="rue" hreflang="rue" data-title="Автоматична телефонна стация" data-language-autonym="Русиньскый" data-language-local-name="Rusyn" class="interlanguage-link-target"><span>Русиньскый</span></a></li><li class="interlanguage-link interwiki-ru mw-list-item"><a href="https://ru.wikipedia.org/wiki/%D0%90%D0%B2%D1%82%D0%BE%D0%BC%D0%B0%D1%82%D0%B8%D1%87%D0%B5%D1%81%D0%BA%D0%B0%D1%8F_%D1%82%D0%B5%D0%BB%D0%B5%D1%84%D0%BE%D0%BD%D0%BD%D0%B0%D1%8F_%D1%81%D1%82%D0%B0%D0%BD%D1%86%D0%B8%D1%8F" title="Автоматическая телефонная станция – Russian" lang="ru" hreflang="ru" data-title="Автоматическая телефонная станция" data-language-autonym="Русский" data-language-local-name="Russian" class="interlanguage-link-target"><span>Русский</span></a></li><li class="interlanguage-link interwiki-sk mw-list-item"><a href="https://sk.wikipedia.org/wiki/Telef%C3%B3nna_%C3%BAstred%C5%88a" title="Telefónna ústredňa – Slovak" lang="sk" hreflang="sk" data-title="Telefónna ústredňa" data-language-autonym="Slovenčina" data-language-local-name="Slovak" class="interlanguage-link-target"><span>Slovenčina</span></a></li><li class="interlanguage-link interwiki-sr mw-list-item"><a href="https://sr.wikipedia.org/wiki/%D0%A2%D0%B5%D0%BB%D0%B5%D1%84%D0%BE%D0%BD%D1%81%D0%BA%D0%B0_%D1%86%D0%B5%D0%BD%D1%82%D1%80%D0%B0%D0%BB%D0%B0" title="Телефонска централа – Serbian" lang="sr" hreflang="sr" data-title="Телефонска централа" data-language-autonym="Српски / srpski" data-language-local-name="Serbian" class="interlanguage-link-target"><span>Српски / srpski</span></a></li><li class="interlanguage-link interwiki-su mw-list-item"><a href="https://su.wikipedia.org/wiki/Sentral_telepon" title="Sentral telepon – Sundanese" lang="su" hreflang="su" data-title="Sentral telepon" data-language-autonym="Sunda" data-language-local-name="Sundanese" class="interlanguage-link-target"><span>Sunda</span></a></li><li class="interlanguage-link interwiki-fi mw-list-item"><a href="https://fi.wikipedia.org/wiki/Puhelinkeskus" title="Puhelinkeskus – Finnish" lang="fi" hreflang="fi" data-title="Puhelinkeskus" data-language-autonym="Suomi" data-language-local-name="Finnish" class="interlanguage-link-target"><span>Suomi</span></a></li><li class="interlanguage-link interwiki-sv mw-list-item"><a href="https://sv.wikipedia.org/wiki/Telefonv%C3%A4xel" title="Telefonväxel – Swedish" lang="sv" hreflang="sv" data-title="Telefonväxel" data-language-autonym="Svenska" data-language-local-name="Swedish" class="interlanguage-link-target"><span>Svenska</span></a></li><li class="interlanguage-link interwiki-tr mw-list-item"><a href="https://tr.wikipedia.org/wiki/Telefon_santrali" title="Telefon santrali – Turkish" lang="tr" hreflang="tr" data-title="Telefon santrali" data-language-autonym="Türkçe" data-language-local-name="Turkish" class="interlanguage-link-target"><span>Türkçe</span></a></li><li class="interlanguage-link interwiki-uk mw-list-item"><a href="https://uk.wikipedia.org/wiki/%D0%90%D0%B2%D1%82%D0%BE%D0%BC%D0%B0%D1%82%D0%B8%D1%87%D0%BD%D0%B0_%D1%82%D0%B5%D0%BB%D0%B5%D1%84%D0%BE%D0%BD%D0%BD%D0%B0_%D1%81%D1%82%D0%B0%D0%BD%D1%86%D1%96%D1%8F" title="Автоматична телефонна станція – Ukrainian" lang="uk" hreflang="uk" data-title="Автоматична телефонна станція" data-language-autonym="Українська" data-language-local-name="Ukrainian" class="interlanguage-link-target"><span>Українська</span></a></li><li class="interlanguage-link interwiki-zh mw-list-item"><a href="https://zh.wikipedia.org/wiki/%E7%94%B5%E4%BF%A1%E4%BA%A4%E6%8D%A2" title="电信交换 – Chinese" lang="zh" hreflang="zh" data-title="电信交换" data-language-autonym="中文" data-language-local-name="Chinese" class="interlanguage-link-target"><span>中文</span></a></li> </ul> <div class="after-portlet after-portlet-lang"><span class="wb-langlinks-edit wb-langlinks-link"><a href="https://www.wikidata.org/wiki/Special:EntityPage/Q256132#sitelinks-wikipedia" title="Edit interlanguage links" class="wbc-editpage">Edit links</a></span></div> </div> </div> </div> </header> <div class="vector-page-toolbar"> <div class="vector-page-toolbar-container"> <div id="left-navigation"> <nav aria-label="Namespaces"> <div 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style="display:none">Interconnects telephones for calls</div> <p class="mw-empty-elt"> </p> <figure typeof="mw:File/Thumb"><a href="/wiki/File:Jersey_Telecom_switchboard_and_operator.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/d/dc/Jersey_Telecom_switchboard_and_operator.jpg/250px-Jersey_Telecom_switchboard_and_operator.jpg" decoding="async" width="250" height="173" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/d/dc/Jersey_Telecom_switchboard_and_operator.jpg/375px-Jersey_Telecom_switchboard_and_operator.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/d/dc/Jersey_Telecom_switchboard_and_operator.jpg/500px-Jersey_Telecom_switchboard_and_operator.jpg 2x" data-file-width="766" data-file-height="531" /></a><figcaption>A <a href="/wiki/Switchboard_operator" title="Switchboard operator">telephone operator</a> manually connecting calls with cord pairs at a telephone switchboard</figcaption></figure> <figure typeof="mw:File/Thumb"><a href="/wiki/File:BDUK_Broadband_(14626513882).jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/8/80/BDUK_Broadband_%2814626513882%29.jpg/250px-BDUK_Broadband_%2814626513882%29.jpg" decoding="async" width="250" height="167" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/8/80/BDUK_Broadband_%2814626513882%29.jpg/375px-BDUK_Broadband_%2814626513882%29.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/8/80/BDUK_Broadband_%2814626513882%29.jpg/500px-BDUK_Broadband_%2814626513882%29.jpg 2x" data-file-width="6048" data-file-height="4032" /></a><figcaption>A modern central office, equipped with voice communication and <a href="/wiki/Broadband" title="Broadband">broadband</a> data capabilities</figcaption></figure> <p>A <b>telephone exchange</b>, also known as a <b>telephone switch</b> or <b>central office</b>, is a crucial component in the <a href="/wiki/Public_switched_telephone_network" title="Public switched telephone network">public switched telephone network</a> (PSTN) or large enterprise telecommunications systems. It facilitates the interconnection of telephone subscriber lines or digital system virtual circuits, enabling <a href="/wiki/Telephone_call" title="Telephone call">telephone calls</a> between subscribers. </p><p>The terminology used in telecommunications has evolved over time, with <i>telephone exchange</i> and <i>central office</i> often used interchangeably, the latter term originating from the <a href="/wiki/Bell_System" title="Bell System">Bell System</a>. A central office typically refers to a facility that houses the <a href="/wiki/Inside_plant" title="Inside plant">inside plant</a> equipment for one or several telephone exchanges, each catering to a specific geographical region. This region is sometimes known as the exchange area. In North America, the term <i>wire center</i> may be used to denote a central office location, indicating a facility that provides a telephone with a <a href="/wiki/Dial_tone" title="Dial tone">dial tone</a>.<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> Telecommunication carriers also define <i>rate centers</i> for business and billing purposes, which in large cities, might encompass clusters of central offices to specify geographic locations for distance measurement calculations. </p><p>In the 1940s, the Bell System in the United States and Canada introduced a <a href="/wiki/Original_North_American_area_codes" title="Original North American area codes">nationwide numbering system</a> that identified central offices with a unique three-digit code, along with a three-digit <a href="/wiki/Numbering_plan_area" class="mw-redirect" title="Numbering plan area">numbering plan area</a> code (NPA code or area code), making central office codes distinctive within each numbering plan area. These codes served as prefixes in subscriber telephone numbers. The mid-20th century saw similar organizational efforts in telephone networks globally, propelled by the advent of international and transoceanic telephone trunks and direct customer dialing. </p><p>For corporate or enterprise applications, a private telephone exchange is termed a <a href="/wiki/Private_branch_exchange" class="mw-redirect" title="Private branch exchange">private branch exchange</a> (PBX), which connects to the public switched telephone network. A PBX serves an organization's telephones and any private leased line circuits, typically situated in large office spaces or organizational campuses. Smaller setups might use a PBX or <a href="/wiki/Key_telephone_system" class="mw-redirect" title="Key telephone system">key telephone system</a> managed by a receptionist, catering to the telecommunication needs of the enterprise. </p> <meta property="mw:PageProp/toc" /> <div class="mw-heading mw-heading2"><h2 id="History">History</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Telephone_exchange&action=edit&section=1" title="Edit section: History"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Tivadar_Puskas.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/e/ea/Tivadar_Puskas.jpg/220px-Tivadar_Puskas.jpg" decoding="async" width="220" height="272" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/e/ea/Tivadar_Puskas.jpg/330px-Tivadar_Puskas.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/e/ea/Tivadar_Puskas.jpg 2x" data-file-width="370" data-file-height="457" /></a><figcaption><a href="/wiki/Tivadar_Pusk%C3%A1s" title="Tivadar Puskás">Tivadar Puskás</a></figcaption></figure> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Bell_telephone_magazine_(1922)_(14569847558).jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/f/f8/Bell_telephone_magazine_%281922%29_%2814569847558%29.jpg/220px-Bell_telephone_magazine_%281922%29_%2814569847558%29.jpg" decoding="async" width="220" height="149" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/f/f8/Bell_telephone_magazine_%281922%29_%2814569847558%29.jpg/330px-Bell_telephone_magazine_%281922%29_%2814569847558%29.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/f/f8/Bell_telephone_magazine_%281922%29_%2814569847558%29.jpg/440px-Bell_telephone_magazine_%281922%29_%2814569847558%29.jpg 2x" data-file-width="2816" data-file-height="1910" /></a><figcaption>1922 diagram of 1877 Boston exchange</figcaption></figure> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Telephone_switchboard_cross-switching_(Rankin_Kennedy,_Electrical_Installations,_Vol_V,_1903).jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/c/c3/Telephone_switchboard_cross-switching_%28Rankin_Kennedy%2C_Electrical_Installations%2C_Vol_V%2C_1903%29.jpg/220px-Telephone_switchboard_cross-switching_%28Rankin_Kennedy%2C_Electrical_Installations%2C_Vol_V%2C_1903%29.jpg" decoding="async" width="220" height="283" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/c/c3/Telephone_switchboard_cross-switching_%28Rankin_Kennedy%2C_Electrical_Installations%2C_Vol_V%2C_1903%29.jpg/330px-Telephone_switchboard_cross-switching_%28Rankin_Kennedy%2C_Electrical_Installations%2C_Vol_V%2C_1903%29.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/c/c3/Telephone_switchboard_cross-switching_%28Rankin_Kennedy%2C_Electrical_Installations%2C_Vol_V%2C_1903%29.jpg/440px-Telephone_switchboard_cross-switching_%28Rankin_Kennedy%2C_Electrical_Installations%2C_Vol_V%2C_1903%29.jpg 2x" data-file-width="553" data-file-height="712" /></a><figcaption>1903 manual switch for four subscriber lines (top) with four cross-bar talking circuits (horizontal), and one bar to connect the operator (T). The lowest cross-bar (E) connects idle stations to ground (earth) to enable the signaling indicators (F).</figcaption></figure> <p>In the era of the electrical telegraph, its principal users were post offices, railway stations, the more important governmental centers (ministries), stock exchanges, very few nationally distributed newspapers, the largest internationally important corporations, and wealthy individuals.<sup id="cite_ref-SidneyMorningHerald-1878.04.19_2-0" class="reference"><a href="#cite_note-SidneyMorningHerald-1878.04.19-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> Despite the fact that telephone devices existed before the invention of the telephone exchange, their success and economical operation would have been impossible on the same <a href="/wiki/Model_(abstract)" class="mw-redirect" title="Model (abstract)">schema</a> and structure of the contemporary telegraph, as prior to the invention of the telephone exchange switchboard, early telephones were hardwired to and communicated with only a single other telephone (such as from an individual's home to the person's business<sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup>). </p><p>A telephone exchange is a telephone system for a small geographic area that provides the switching (interconnection) of subscriber lines for calls made between them. Telephone exchanges replaced small telephone systems that connected its users with direct lines between each and every subscriber station. Exchanges made telephony an available and comfortable technology for everyday use and it gave the impetus for the creation of a new industrial sector. </p><p>As with the <a href="/wiki/Invention_of_the_telephone" title="Invention of the telephone">invention of the telephone</a> itself, the honour of "first telephone exchange" has several claimants. One of the first to propose a telephone exchange was <a href="/wiki/Hungary" title="Hungary">Hungarian</a> <a href="/wiki/Tivadar_Pusk%C3%A1s" title="Tivadar Puskás">Tivadar Puskás</a> in 1877 while he was working for <a href="/wiki/Thomas_Edison" title="Thomas Edison">Thomas Edison</a>.<sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-6" class="reference"><a href="#cite_note-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-7" class="reference"><a href="#cite_note-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-8" class="reference"><a href="#cite_note-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup> The first experimental telephone exchange was based on the ideas of Puskás, and it was built by the <a href="/wiki/Bell_Telephone_Company" title="Bell Telephone Company">Bell Telephone Company</a> in <a href="/wiki/Boston" title="Boston">Boston</a> in 1877.<sup id="cite_ref-9" class="reference"><a href="#cite_note-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup> The world's first state-administered telephone exchange opened on November 12, 1877 in Friedrichsberg close to <a href="/wiki/Berlin" title="Berlin">Berlin</a> under the direction of <a href="/wiki/Heinrich_von_Stephan" title="Heinrich von Stephan">Heinrich von Stephan</a>.<sup id="cite_ref-10" class="reference"><a href="#cite_note-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup> <a href="/wiki/George_Willard_Coy" title="George Willard Coy">George W. Coy</a> designed and built the first commercial US telephone exchange which opened in <a href="/wiki/New_Haven,_Connecticut" title="New Haven, Connecticut">New Haven, Connecticut</a> in January, 1878, and the first telephone booth was built in nearby <a href="/wiki/Bridgeport,_Connecticut" title="Bridgeport, Connecticut">Bridgeport</a>.<sup id="cite_ref-11" class="reference"><a href="#cite_note-11"><span class="cite-bracket">[</span>11<span class="cite-bracket">]</span></a></sup> The switchboard was built from "carriage bolts, handles from teapot lids and bustle wire" and could handle two simultaneous conversations.<sup id="cite_ref-12" class="reference"><a href="#cite_note-12"><span class="cite-bracket">[</span>12<span class="cite-bracket">]</span></a></sup> <a href="/wiki/Charles_Glidden" class="mw-redirect" title="Charles Glidden">Charles Glidden</a> is also credited with establishing an exchange in Lowell, MA. with 50 subscribers in 1878. </p><p>In Europe other early telephone exchanges were based in <a href="/wiki/London" title="London">London</a> and <a href="/wiki/Manchester" title="Manchester">Manchester</a>, both of which opened under Bell patents in 1879.<sup id="cite_ref-13" class="reference"><a href="#cite_note-13"><span class="cite-bracket">[</span>13<span class="cite-bracket">]</span></a></sup> Belgium had its first <a href="/wiki/International_Bell_Telephone_Company" title="International Bell Telephone Company">International Bell</a> exchange (in <a href="/wiki/Antwerp" title="Antwerp">Antwerp</a>) a year later. </p><p>In 1887 Puskás introduced the <a href="/wiki/Multiplexer" title="Multiplexer">multiplex</a> <a href="/wiki/Telephone_switchboard" title="Telephone switchboard">switchboard</a>. <sup class="noprint Inline-Template" style="margin-left:0.1em; white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Vagueness" title="Wikipedia:Vagueness"><span title="This information is too vague. (August 2016)">vague</span></a></i>]</sup>.<sup id="cite_ref-14" class="reference"><a href="#cite_note-14"><span class="cite-bracket">[</span>14<span class="cite-bracket">]</span></a></sup> </p><p>Later exchanges consisted of one to several hundred <a href="/wiki/Telephone_switchboard" title="Telephone switchboard">plug boards</a> staffed by <a href="/wiki/Switchboard_operator" title="Switchboard operator">switchboard operators</a>. Each operator sat in front of a vertical panel containing banks of ¼-inch <a href="/wiki/TRS_connector" class="mw-redirect" title="TRS connector">tip-ring-sleeve</a> (3-conductor) jacks, each of which was the local termination of a <a href="/wiki/Subscriber" class="mw-redirect" title="Subscriber">subscriber</a>'s <a href="/wiki/Telephone_line" title="Telephone line">telephone line</a>. In front of the jack panel lay a horizontal panel containing two rows of patch cords, each pair connected to a <a href="/wiki/Cord_circuit" title="Cord circuit">cord circuit</a>. </p><p>When a <a href="/wiki/Calling_party" title="Calling party">calling party</a> lifted the receiver, the local loop current lit a signal lamp near the jack.<sup id="cite_ref-calvert2003_15-0" class="reference"><a href="#cite_note-calvert2003-15"><span class="cite-bracket">[</span>15<span class="cite-bracket">]</span></a></sup> The operator responded by inserting the rear cord (<i>answering cord</i>) into the subscriber's jack and switched their headset into the circuit to ask, "Number, please?" For a local call, the operator inserted the front cord of the pair (<i>ringing cord</i>) into the called party's local jack and started the ringing cycle. For a long-distance call, the operator plugged into a <a href="/wiki/Trunking" title="Trunking">trunk</a> circuit to connect to another operator in another bank of boards or at a remote central office. In 1918, the average time to complete the connection for a long-distance call was 15 minutes.<sup id="cite_ref-calvert2003_15-1" class="reference"><a href="#cite_note-calvert2003-15"><span class="cite-bracket">[</span>15<span class="cite-bracket">]</span></a></sup> </p><p>Early manual switchboards required the operator to operate listening keys and ringing keys, but by the late 1910s and 1920s, advances in switchboard technology led to features which allowed the call to be automatically answered immediately as the operator inserted the answering cord, and ringing would automatically begin as soon as the operator inserted the ringing cord into the called party's jack. The operator would be disconnected from the circuit, allowing them to handle another call, while the caller heard an audible ringback signal, so that that operator would not have to periodically report that they were continuing to ring the line.<sup id="cite_ref-16" class="reference"><a href="#cite_note-16"><span class="cite-bracket">[</span>16<span class="cite-bracket">]</span></a></sup> </p><p>In the <a href="/wiki/Ringdown" title="Ringdown">ringdown</a> method, the originating operator called another intermediate operator who would call the called subscriber, or passed it on to another intermediate operator.<sup id="cite_ref-17" class="reference"><a href="#cite_note-17"><span class="cite-bracket">[</span>17<span class="cite-bracket">]</span></a></sup> This chain of intermediate operators could complete the call only if intermediate trunk lines were available between all the centers at the same time. In 1943 when military calls had priority, a cross-country US call might take as long as 2 hours to request and schedule in cities that used manual switchboards for toll calls. </p><p>On March 10, 1891, <a href="/wiki/Almon_Brown_Strowger" title="Almon Brown Strowger">Almon Brown Strowger</a>, an undertaker in <a href="/wiki/Kansas_City,_Missouri" title="Kansas City, Missouri">Kansas City, Missouri</a>, patented the <a href="/wiki/Stepping_switch" title="Stepping switch">stepping switch</a>, a device which led to the automation of telephone circuit switching. While there were many extensions and adaptations of this initial patent, the one best known consists of 10 levels or banks, each having 10 contacts arranged in a semicircle. When used with a rotary <a href="/wiki/Telephone_dial" class="mw-redirect" title="Telephone dial">telephone dial</a>, each pair of digits caused the shaft of the central contact "hand" of the stepping switch to first step (ratchet) up one level for each pulse in the first digit and then to swing horizontally in a contact row with one small rotation for each pulse in the next digit. </p><p>Later stepping switches were arranged in banks, the first stage of which was a <i><a href="/w/index.php?title=Linefinder&action=edit&redlink=1" class="new" title="Linefinder (page does not exist)">linefinder</a></i>. If one of up to a hundred subscriber lines (two hundred lines in later linefinders) had the receiver lifted "off hook", a linefinder connected the subscriber's line to a free first selector, which returned the subscriber a <a href="/wiki/Dial_tone" title="Dial tone">dial tone</a> to show that it was ready to receive dialled digits. The subscriber's dial pulsed at about 10 pulses per second, although the speed depended on the standard of the particular telephone administration. </p><p>Exchanges based on the Strowger switch were eventually challenged by <a href="/wiki/Panel_switch" title="Panel switch">other exchange types</a> and later by <a href="/wiki/Crossbar_switch" title="Crossbar switch">crossbar</a> technology. These exchange designs promised faster switching and would accept inter-switch pulses faster than the Strowger's typical 10 pps—typically about 20 pps. At a later date many also accepted <a href="/wiki/DTMF" title="DTMF">DTMF</a> "touch tones" or other tone signaling systems. </p><p>A transitional technology (from pulse to DTMF) had converters to convert DTMF to pulse, to feed to older Strowger, panel, or crossbar switches. This technology was used as late as mid-2002. </p> <div class="mw-heading mw-heading2"><h2 id="Terminology">Terminology</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Telephone_exchange&action=edit&section=2" title="Edit section: Terminology"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Many terms used in telecommunication technology differ in meaning and usage among the various English speaking regions. For the purpose of this article the following definitions are made: </p> <ul><li><i>Manual service</i> is telephone service in which a human <a href="/wiki/Switchboard_operator" title="Switchboard operator">telephone operator</a> routes calls as instructed by a subscriber with a telephone set that does not have a dial.</li> <li><i>Dial service</i> is when an exchange routes calls by interpreting subscriber-dialed digits.</li> <li>A <i>telephone switch</i> is the switching equipment of an exchange.</li> <li>A <i>wire center</i> is the area served by a particular switch or central office.</li> <li>A <i><a href="/wiki/Concentrator" title="Concentrator">concentrator</a></i> is a device that concentrates traffic, be it remote or co-located with the switch.</li> <li>An <i><a href="/wiki/Off-hook" class="mw-redirect" title="Off-hook">off-hook</a></i> condition represents a circuit that is in use, e.g., when a telephone call is in progress.</li> <li>An <i><a href="/wiki/On-hook" class="mw-redirect" title="On-hook">on-hook</a></i> condition represents an idle circuit, i.e. no telephone call is in progress.</li></ul> <p>A <i>central office</i> originally was a primary exchange in a city with other exchanges service parts of the area. The term became to mean any switching system including its facilities and operators. It is also used generally for the building that houses switching and related <a href="/wiki/Inside_plant" title="Inside plant">inside plant</a> equipment. In United States <a href="/wiki/Telecommunication" class="mw-redirect" title="Telecommunication">telecommunication</a> jargon, a central office (C.O.) is a <a href="/wiki/Common_carrier" title="Common carrier">common carrier</a> <a href="/wiki/Switching_center" class="mw-redirect" title="Switching center">switching center</a> <a href="/wiki/Class_5_telephone_switch" class="mw-redirect" title="Class 5 telephone switch">Class 5 telephone switch</a> in which trunks and <a href="/wiki/Local_loop" title="Local loop">local loops</a> are terminated and switched.<sup id="cite_ref-FS1037C_18-0" class="reference"><a href="#cite_note-FS1037C-18"><span class="cite-bracket">[</span>18<span class="cite-bracket">]</span></a></sup> In the UK, a <i>telephone exchange</i> means an exchange building, and is also the name for a telephone switch. </p> <div class="mw-heading mw-heading2"><h2 id="Manual_service_exchanges">Manual service exchanges</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Telephone_exchange&action=edit&section=3" title="Edit section: Manual service exchanges"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1236090951">.mw-parser-output .hatnote{font-style:italic}.mw-parser-output div.hatnote{padding-left:1.6em;margin-bottom:0.5em}.mw-parser-output .hatnote i{font-style:normal}.mw-parser-output .hatnote+link+.hatnote{margin-top:-0.5em}@media print{body.ns-0 .mw-parser-output .hatnote{display:none!important}}</style><div role="note" class="hatnote navigation-not-searchable">Main article: <a href="/wiki/Telephone_switchboard" title="Telephone switchboard">Telephone switchboard</a></div> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Switchboardof1924.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/2/22/Switchboardof1924.jpg/220px-Switchboardof1924.jpg" decoding="async" width="220" height="330" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/2/22/Switchboardof1924.jpg/330px-Switchboardof1924.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/2/22/Switchboardof1924.jpg/440px-Switchboardof1924.jpg 2x" data-file-width="1011" data-file-height="1515" /></a><figcaption>1924 PBX switchboard</figcaption></figure> <p>With <b>manual service</b>, the customer lifts the receiver <a href="/wiki/Off-hook" class="mw-redirect" title="Off-hook">off-hook</a> and asks the <a href="/wiki/Switchboard_operator" title="Switchboard operator">operator</a> to connect the call to a requested number. Provided that the number is in the same central office, and located on the operator's switchboard, the operator connects the call by plugging the ringing cord into the jack corresponding to the called customer's line. If the called party's line is on a different switchboard in the same office, or in a different central office, the operator plugs into the trunk for the destination switchboard or office and asks the operator answering (known as the "B" operator) to connect the call. </p><p>Most urban exchanges provided <a href="/wiki/Common_battery" title="Common battery">common-battery</a> service, meaning that the central office provided power to the subscriber telephone circuits for operation of the transmitter, as well as for automatic signaling with <a href="/wiki/Rotary_dial" title="Rotary dial">rotary dials</a>. In common-battery systems, the pair of wires from a subscriber's telephone to the exchange carry 48V (nominal) DC potential from the telephone company end across the conductors. The telephone presents an open circuit when it is <a href="/wiki/On-hook" class="mw-redirect" title="On-hook">on-hook</a> or idle.<sup id="cite_ref-autogenerated1_19-0" class="reference"><a href="#cite_note-autogenerated1-19"><span class="cite-bracket">[</span>19<span class="cite-bracket">]</span></a></sup> </p><p>When a subscriber's phone is off-hook, it presents an electrical resistance across the line which causes current to flow through the telephone and wires to the central office. In a manually operated switchboard, this current flowed through a relay coil, and actuated a buzzer or a lamp on the operator's switchboard, signaling the operator to perform service.<sup id="cite_ref-autogenerated1_19-1" class="reference"><a href="#cite_note-autogenerated1-19"><span class="cite-bracket">[</span>19<span class="cite-bracket">]</span></a></sup> </p><p>In the largest cities, it took many years to convert every office to automatic equipment, such as a <a href="/wiki/Panel_switch" title="Panel switch">panel switch</a>. During this transition period, once numbers were standardized to the <a href="/wiki/Telephone_exchange_names" title="Telephone exchange names"> 2L-4N or 2L-5N</a> format (two-letter exchange name and either four or five digits), it was possible to dial a number located in a manual exchange and be connected without requesting operator assistance. The policy of the <a href="/wiki/Bell_System" title="Bell System">Bell System</a> stated that customers in large cities should not need to be concerned with the type of office, whether they were calling a manual or an automatic office. </p><p>When a subscriber dialed the number of a manual station, an operator at the destination office answered the call after seeing the number on an <a href="/wiki/Panel_Call_Indicator" class="mw-redirect" title="Panel Call Indicator">indicator</a>, and connected the call by plugging a cord into the outgoing circuit and ringing the destination station. For example, if a dial customer calling from TAylor 4725 dialed a number served by a manual exchange, e.g., ADams 1383-W, the call was completed, from the subscriber's perspective, exactly as a call to LEnnox 5813, in an automated exchange. The party line letters W, R, J, and M were only used in manual exchanges with jack-per-line party lines. </p> <figure class="mw-default-size mw-halign-left" typeof="mw:File/Thumb"><a href="/wiki/File:Telephone_exchange_Montreal_QE3_33.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/1/1a/Telephone_exchange_Montreal_QE3_33.jpg/220px-Telephone_exchange_Montreal_QE3_33.jpg" decoding="async" width="220" height="161" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/1/1a/Telephone_exchange_Montreal_QE3_33.jpg/330px-Telephone_exchange_Montreal_QE3_33.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/1/1a/Telephone_exchange_Montreal_QE3_33.jpg/440px-Telephone_exchange_Montreal_QE3_33.jpg 2x" data-file-width="3115" data-file-height="2273" /></a><figcaption>Montreal telephone exchange (<abbr title="circa">c.</abbr><span style="white-space:nowrap;"> 1895</span>)</figcaption></figure> <p>In contrast to the listing format <i>MAin 1234</i> for an automated office with two capital letters, a manual office, having listings such as Hillside 834 or East 23, was recognizable by the format in which the second letter was not capitalized. </p><p>Rural areas, as well as the smallest towns, had manual service and signaling was accomplished with <a href="/wiki/Telephone_magneto" title="Telephone magneto">magneto</a> telephones, which had a crank for the signaling generator. To alert the operator, or another subscriber on the same line, the subscriber turned the crank to generate ringing current. The switchboard responded by interrupting the circuit, which dropped a metal tab above the subscriber's line jack and sounded a buzzer. <a href="/wiki/Dry_cell" title="Dry cell">Dry cell</a> batteries, normally two large N°. 6 cells in the subscriber's telephone, provided the direct current for the transmitter. Such magneto systems were in use in the US as late as 1983, as in the small town, Bryant Pond, <a href="/wiki/Woodstock,_Maine" title="Woodstock, Maine">Woodstock, Maine</a>. </p><p>Many small town magneto systems featured <a href="/wiki/Party_line_(telephony)" title="Party line (telephony)">party lines</a>, anywhere from two to ten or more subscribers sharing a single line. When calling a party, the operator used code ringing, a distinctive <a href="/wiki/Ringing_signal" class="mw-redirect" title="Ringing signal">ringing signal</a> sequence, such as two long rings followed by one short ring. Everyone on the line could hear the signals, and could pick up and monitor other people's conversations. </p> <div class="mw-heading mw-heading2"><h2 id="Early_automatic_exchanges">Early automatic exchanges</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Telephone_exchange&action=edit&section=4" title="Edit section: Early automatic exchanges"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <figure class="mw-default-size mw-halign-right" typeof="mw:File/Thumb"><a href="/wiki/File:KarawinnaTelstraExchange.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/7/73/KarawinnaTelstraExchange.jpg/220px-KarawinnaTelstraExchange.jpg" decoding="async" width="220" height="165" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/7/73/KarawinnaTelstraExchange.jpg/330px-KarawinnaTelstraExchange.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/7/73/KarawinnaTelstraExchange.jpg/440px-KarawinnaTelstraExchange.jpg 2x" data-file-width="2304" data-file-height="1728" /></a><figcaption>A rural telephone exchange building in Australia</figcaption></figure> <p><b>Automatic exchanges</b>, which provided <b>dial service</b>, were invented by <a href="/wiki/Almon_Strowger" class="mw-redirect" title="Almon Strowger">Almon Strowger</a> in 1888. First used commercially in 1892, they did not gain widespread use until the first decade of the 20th century. They eliminated the need for human <a href="/wiki/Switchboard_operator" title="Switchboard operator">switchboard operators</a> who completed the connections required for a <a href="/wiki/Telephone_call" title="Telephone call">telephone call</a>. Automation replaced human operators with electromechanical systems, and telephones were equipped with a dial by which a caller transmitted the destination telephone number to the automatic switching system. </p><p>A telephone exchange automatically senses an off-hook condition of the <a href="/wiki/Telephone" title="Telephone">telephone</a> when the user removes the handset from the switchhook or cradle. The exchange provides <a href="/wiki/Dial_tone" title="Dial tone">dial tone</a> at that time to indicate to the user that the exchange is ready to receive dialed digits. The pulses or <a href="/wiki/DTMF" title="DTMF">DTMF</a> tones generated by the telephone are processed and a connection is established to the destination telephone within the same exchange or to another distant exchange. </p><p>The exchange maintains the connection until one of the parties hangs up. This monitoring of connection status is called <i>supervision.</i> Additional features, such as billing equipment, may also be incorporated into the exchange. </p><p>The Bell System dial service implemented a feature called <a href="/wiki/Automatic_number_identification" title="Automatic number identification">automatic number identification</a> (ANI) which facilitated services like automated billing, <a href="/wiki/Toll-free_telephone_number" title="Toll-free telephone number">toll-free 800-numbers</a>, and <a href="/wiki/9-1-1" class="mw-redirect" title="9-1-1">9-1-1</a> service. In manual service, the operator knows where a call is originating by the light on the switchboard jack field. Before ANI, long-distance calls were placed into an operator queue and the operator asked the calling party's number and recorded it on a paper toll ticket. </p><p>Early exchanges were electromechanical systems using motors, shaft drives, rotating switches and <a href="/wiki/Relays" class="mw-redirect" title="Relays">relays</a>. Some types of automatic exchanges were the <a href="/wiki/Strowger_switch" title="Strowger switch">Strowger switch</a> or step-by-step switch, All Relay, <a href="/wiki/Panel_switch" title="Panel switch">panel switch</a>, <a href="/wiki/Rotary_system" title="Rotary system">Rotary system</a> and the <a href="/wiki/Crossbar_switch" title="Crossbar switch">crossbar switch</a>. </p> <div class="mw-heading mw-heading3"><h3 id="Electromechanical_signaling">Electromechanical signaling</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Telephone_exchange&action=edit&section=5" title="Edit section: Electromechanical signaling"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1236090951"><div role="note" class="hatnote navigation-not-searchable">Main article: <a href="/wiki/Signaling_(telecommunications)" title="Signaling (telecommunications)">Signaling (telecommunications)</a></div> <p>Circuits interconnecting switches are called <i>trunks</i>. Before <a href="/wiki/Signalling_System_7" class="mw-redirect" title="Signalling System 7">Signalling System 7</a>, <a href="/wiki/Bell_System" title="Bell System">Bell System</a> electromechanical switches in the United States originally communicated with one another over trunks using a variety of DC voltages and signaling tones. Today, those simple digital signals have been replaced by more modern coded digital signals (typically using binary code). </p><p>Some signaling communicated dialed digits. An early form called <a href="/wiki/Panel_call_indicator" title="Panel call indicator">Panel Call Indicator</a> Pulsing used <a href="/wiki/Quaternary_numeral_system" title="Quaternary numeral system">quaternary</a> pulses to set up calls between a <a href="/wiki/Panel_switch" title="Panel switch">panel switch</a> and a manual switchboard. Probably the most common form of communicating dialed digits between electromechanical switches was sending <a href="/wiki/Dial_pulse" class="mw-redirect" title="Dial pulse">dial pulses</a>, equivalent to a <a href="/wiki/Rotary_dial" title="Rotary dial">rotary dial</a>'s pulsing, but sent over trunk circuits between switches. </p><p>In Bell System trunks, it was common to use 20 pulse-per-second between crossbar switches and crossbar tandems. This was twice the rate of Western Electric/Bell System telephone dials. Using the faster pulsing rate made trunk utilization more efficient because the switch spent half as long listening to digits. DTMF was not used for trunk signaling. </p><p><a href="/wiki/Multi-frequency" class="mw-redirect" title="Multi-frequency">Multi-frequency (MF)</a> was the last of the pre-computerized methods. It used a different set of tones sent in pairs like DTMF. Dialing was preceded by a special <i>keypulse</i> (KP) signal and followed by a <i>start</i> (ST). Variations of the Bell System MF tone scheme became a <a href="/wiki/ITU-T" title="ITU-T">CCITT</a> standard. Similar schemes were used in the Americas and in some European countries including Spain. Digit strings between switches were often abbreviated to further improve utilization. </p><p>For example, one switch might send only the last four or five digits of a <a href="/wiki/Telephone_number" title="Telephone number">telephone number</a>. In one case, seven digit numbers were preceded by a digit 1 or 2 to differentiate between two area codes or office codes, (a two-digit-per-call savings). This improved revenue per trunk and reduced the number of digit receivers needed in a switch. Every task in electromechanical switches was done in big metallic pieces of hardware. Every fractional second cut off of call set up time meant fewer racks of equipment to handle call traffic. </p><p>Examples of signals communicating supervision or call progress include <a href="/wiki/E_and_M_signaling" title="E and M signaling">E and M signaling</a>, SF signaling, and robbed-bit signaling. In physical (not carrier) E and M trunk circuits, trunks were four wire. Fifty trunks would require a hundred pair cable between switches, for example. Conductors in one common circuit configuration were named tip, ring, ear (E) and mouth (M). Tip and ring were the voice-carrying pair, and named after the tip and ring on the three conductor cords on the manual operator's console. </p><p>In two-way trunks with <b>E and M signaling</b>, a handshake took place to prevent both switches from colliding by dialing calls on the same trunk at the same time. By changing the state of these leads from ground to −48 volts, the switches stepped through a handshake protocol. Using DC voltage changes, the local switch would send a signal to get ready for a call and the remote switch would reply with an acknowledgment (a wink) to go ahead with dial pulsing. This was done with relay logic and discrete electronics. </p><p>These voltage changes on the trunk circuit would cause pops or clicks that were audible to the subscriber as the electrical handshaking stepped through its protocol. Another handshake, to start timing for billing purposes, caused a second set of clunks when the called party answered. </p><p>A second common form of signaling for supervision was called <b>single-frequency</b> or <i>SF signaling</i>. The most common form of this used a steady 2,600 Hz tone to identify a trunk as idle. Trunk circuitry hearing a 2,600 Hz tone for a certain duration would go idle. (The duration requirement reduced <a href="/wiki/Falsing" title="Falsing">falsing</a>.) Some systems used tone frequencies over 3,000 Hz, particularly on SSB <a href="/wiki/Frequency-division_multiplex" class="mw-redirect" title="Frequency-division multiplex">frequency-division multiplex</a> <a href="/wiki/Microwave_radio_relay" class="mw-redirect" title="Microwave radio relay">microwave radio relays</a>. </p><p>On <a href="/wiki/T-carrier" title="T-carrier">T-carrier</a> binary digital transmission systems, bits within the T-1 data stream were used to transmit supervision. By careful design, the appropriated bits did not change voice quality appreciably. <b>Robbed bits</b> were translated to changes in contact states (opens and closures) by electronics in the channel bank hardware. This allowed direct current E and M signaling, or dial pulses, to be sent between electromechanical switches over a pure digital carrier which did not have DC continuity. </p> <div class="mw-heading mw-heading3"><h3 id="Noise">Noise</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Telephone_exchange&action=edit&section=6" title="Edit section: Noise"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1235681985">.mw-parser-output .side-box{margin:4px 0;box-sizing:border-box;border:1px solid #aaa;font-size:88%;line-height:1.25em;background-color:var(--background-color-interactive-subtle,#f8f9fa);display:flow-root}.mw-parser-output .side-box-abovebelow,.mw-parser-output .side-box-text{padding:0.25em 0.9em}.mw-parser-output .side-box-image{padding:2px 0 2px 0.9em;text-align:center}.mw-parser-output .side-box-imageright{padding:2px 0.9em 2px 0;text-align:center}@media(min-width:500px){.mw-parser-output .side-box-flex{display:flex;align-items:center}.mw-parser-output .side-box-text{flex:1;min-width:0}}@media(min-width:720px){.mw-parser-output .side-box{width:238px}.mw-parser-output .side-box-right{clear:right;float:right;margin-left:1em}.mw-parser-output .side-box-left{margin-right:1em}}</style><style data-mw-deduplicate="TemplateStyles:r1096940132">.mw-parser-output .listen .side-box-text{line-height:1.1em}.mw-parser-output .listen-plain{border:none;background:transparent}.mw-parser-output .listen-embedded{width:100%;margin:0;border-width:1px 0 0 0;background:transparent}.mw-parser-output .listen-header{padding:2px}.mw-parser-output .listen-embedded .listen-header{padding:2px 0}.mw-parser-output .listen-file-header{padding:4px 0}.mw-parser-output .listen .description{padding-top:2px}.mw-parser-output .listen .mw-tmh-player{max-width:100%}@media(max-width:719px){.mw-parser-output .listen{clear:both}}@media(min-width:720px){.mw-parser-output .listen:not(.listen-noimage){width:320px}.mw-parser-output .listen-left{overflow:visible;float:left}.mw-parser-output .listen-center{float:none;margin-left:auto;margin-right:auto}}</style><div class="side-box side-box-right listen noprint"><style data-mw-deduplicate="TemplateStyles:r1126788409">.mw-parser-output .plainlist ol,.mw-parser-output .plainlist ul{line-height:inherit;list-style:none;margin:0;padding:0}.mw-parser-output .plainlist ol li,.mw-parser-output .plainlist ul li{margin-bottom:0}</style> <div class="side-box-flex"> <div class="side-box-image"><figure class="mw-halign-center" typeof="mw:File"><span><img alt="" src="//upload.wikimedia.org/wikipedia/commons/thumb/8/87/Gnome-mime-sound-openclipart.svg/50px-Gnome-mime-sound-openclipart.svg.png" decoding="async" width="50" height="50" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/8/87/Gnome-mime-sound-openclipart.svg/75px-Gnome-mime-sound-openclipart.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/8/87/Gnome-mime-sound-openclipart.svg/100px-Gnome-mime-sound-openclipart.svg.png 2x" data-file-width="160" data-file-height="160" /></span><figcaption></figcaption></figure></div> <div class="side-box-text plainlist"><div class="haudio"> <div class="listen-file-header"><a href="/wiki/File:Sxs_intra_to_busy.ogg" title="File:Sxs intra to busy.ogg">Step-by-step call</a></div> <div><span typeof="mw:File"><span><audio id="mwe_player_0" controls="" preload="none" data-mw-tmh="" class="mw-file-element" width="232" style="width:232px;" data-durationhint="18" data-mwtitle="Sxs_intra_to_busy.ogg" data-mwprovider="wikimediacommons"><source src="//upload.wikimedia.org/wikipedia/commons/f/f5/Sxs_intra_to_busy.ogg" type="audio/ogg; codecs="vorbis"" data-width="0" data-height="0" /><source src="//upload.wikimedia.org/wikipedia/commons/transcoded/f/f5/Sxs_intra_to_busy.ogg/Sxs_intra_to_busy.ogg.mp3" type="audio/mpeg" data-transcodekey="mp3" data-width="0" data-height="0" /></audio></span></span></div> <div class="description">Subscribers hear a different-sounding dialtone in a step-by-step call.</div></div></div></div> <div class="side-box-abovebelow"><hr /><i class="selfreference">Problems playing this file? See <a href="/wiki/Help:Media" title="Help:Media">media help</a>.</i></div> </div> <p>Bell System installations typically had alarm bells, gongs, or chimes to announce alarms calling attention to a failed switch element. A trouble reporting card system was connected to switch common control elements. These trouble reporting systems punctured cardboard <a href="/wiki/Punched_card" title="Punched card">cards</a> with a code that logged the nature of a failure. </p> <div class="mw-heading mw-heading3"><h3 id="Maintenance_tasks">Maintenance tasks</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Telephone_exchange&action=edit&section=7" title="Edit section: Maintenance tasks"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Panel_Office_Typical_OGT-colourbalance-2.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/a/a9/Panel_Office_Typical_OGT-colourbalance-2.jpg/220px-Panel_Office_Typical_OGT-colourbalance-2.jpg" decoding="async" width="220" height="148" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/a/a9/Panel_Office_Typical_OGT-colourbalance-2.jpg/330px-Panel_Office_Typical_OGT-colourbalance-2.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/a/a9/Panel_Office_Typical_OGT-colourbalance-2.jpg/440px-Panel_Office_Typical_OGT-colourbalance-2.jpg 2x" data-file-width="2972" data-file-height="2000" /></a><figcaption>Manual test board in an electromechanical switching office staffed by a technician</figcaption></figure> <p>Electromechanical switching systems required sources of electricity in form of direct current (DC), as well as alternating ring current (AC), which were generated on-site with mechanical generators. In addition, telephone switches required adjustment of many mechanical parts. Unlike modern switches, a circuit connecting a dialed call through an electromechanical switch had DC continuity within the local exchange area via metallic conductors. </p><p>The design and maintenance procedures of all systems involved methods to avoid that subscribers experienced undue changes in the quality of the service or that they noticed failures. A variety of tools referred to as <i>make-busy</i>s were plugged into electromechanical switch elements upon failure and during repairs. A make-busy identified the part being worked on as in-use, causing the switching logic to route around it. A similar tool was called a <i>TD tool.</i> Delinquent subscribers had their service temporarily denied (TDed). This was effected by plugging a tool into the subscriber's office equipment on Crossbar systems or line group in step-by-step switches. The subscriber could receive calls but could not dial out. </p><p>Strowger-based, step-by-step offices in the Bell System required continuous maintenance, such as cleaning. Indicator lights on equipment bays alerted staff to conditions such as blown fuses (usually white lamps) or a <a href="/wiki/Permanent_signal" title="Permanent signal">permanent signal</a> (stuck off-hook condition, usually green indicators). Step offices were more susceptible to single-point failures than newer technologies. </p><p>Crossbar offices used more shared, common control circuits. For example, a digit receiver (part of an element called an <i>Originating Register</i>) would be connected to a call just long enough to collect the subscriber's dialed digits. Crossbar architecture was more flexible than step offices. Later crossbar systems had punch-card-based trouble reporting systems. By the 1970s, <i><a href="/wiki/Automatic_number_identification" title="Automatic number identification">automatic number identification</a></i> had been retrofitted to nearly all step-by-step and crossbar switches in the Bell System. </p> <div class="mw-heading mw-heading2"><h2 id="Electronic_switches">Electronic switches</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Telephone_exchange&action=edit&section=8" title="Edit section: Electronic switches"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p><a href="/wiki/Electronic_switching_system" title="Electronic switching system">Electronic switching systems</a> gradually evolved in stages from electromechanical hybrids with <a href="/wiki/Stored_program_control" title="Stored program control">stored program control</a> to the fully digital systems. Early systems used <a href="/wiki/Reed_relay" title="Reed relay">reed relay</a>-switched metallic paths under digital control. Equipment testing, phone numbers reassignments, circuit lockouts and similar tasks were accomplished by data entry on a terminal. </p><p>Examples of these systems included the <a href="/wiki/Western_Electric" title="Western Electric">Western Electric</a> <a href="/wiki/1ESS_switch" class="mw-redirect" title="1ESS switch">1ESS switch</a>, Northern Telecom <a href="/wiki/SP1_switch" class="mw-redirect" title="SP1 switch">SP1</a>, Ericsson AXE, Automatic Electric EAX-1 & EAX-2, Philips <a href="/wiki/PRX_(telephony)" title="PRX (telephony)">PRX</a>/A, ITT Metaconta, <a href="/wiki/General_Post_Office" title="General Post Office">British GPO/BT</a> <a href="/wiki/TXE" title="TXE">TXE</a> series and several other designs were similar. Ericsson also developed a fully computerized version of their ARF crossbar exchange called ARE. These used a crossbar switching matrix with a fully computerized control system and provided a wide range of advanced services. Local versions were called ARE11 while tandem versions were known as ARE13. They were used in Scandinavia, Australia, Ireland and many other countries in the late 1970s and into the 1980s when they were replaced with digital technology. </p><p>These systems could use the old signaling methods inherited from electromechanical crossbar and step-by-step switches. They also introduced a new form of data communications: two 1ESS exchanges could communicate with one another using a data link called <a href="/wiki/Common-channel_signaling" title="Common-channel signaling">Common Channel Interoffice Signaling, (CCIS)</a>. This data link was based on CCITT 6, a predecessor to <a href="/wiki/Signaling_System_7" class="mw-redirect" title="Signaling System 7">SS7</a>. In European systems R2 signalling was normally used. </p> <div class="mw-heading mw-heading2"><h2 id="Digital_switches">Digital switches</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Telephone_exchange&action=edit&section=9" title="Edit section: Digital switches"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Pbx-satellite-telephone-exchange-0a.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/b/bb/Pbx-satellite-telephone-exchange-0a.jpg/220px-Pbx-satellite-telephone-exchange-0a.jpg" decoding="async" width="220" height="165" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/b/bb/Pbx-satellite-telephone-exchange-0a.jpg/330px-Pbx-satellite-telephone-exchange-0a.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/b/bb/Pbx-satellite-telephone-exchange-0a.jpg/440px-Pbx-satellite-telephone-exchange-0a.jpg 2x" data-file-width="2816" data-file-height="2112" /></a><figcaption>A typical satellite PABX with front cover removed</figcaption></figure> <style data-mw-deduplicate="TemplateStyles:r1251242444">.mw-parser-output .ambox{border:1px solid #a2a9b1;border-left:10px solid #36c;background-color:#fbfbfb;box-sizing:border-box}.mw-parser-output .ambox+link+.ambox,.mw-parser-output .ambox+link+style+.ambox,.mw-parser-output .ambox+link+link+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+style+.ambox,.mw-parser-output .ambox+.mw-empty-elt+link+link+.ambox{margin-top:-1px}html body.mediawiki .mw-parser-output .ambox.mbox-small-left{margin:4px 1em 4px 0;overflow:hidden;width:238px;border-collapse:collapse;font-size:88%;line-height:1.25em}.mw-parser-output .ambox-speedy{border-left:10px solid #b32424;background-color:#fee7e6}.mw-parser-output .ambox-delete{border-left:10px solid #b32424}.mw-parser-output .ambox-content{border-left:10px solid #f28500}.mw-parser-output .ambox-style{border-left:10px solid #fc3}.mw-parser-output .ambox-move{border-left:10px solid #9932cc}.mw-parser-output .ambox-protection{border-left:10px solid #a2a9b1}.mw-parser-output .ambox .mbox-text{border:none;padding:0.25em 0.5em;width:100%}.mw-parser-output .ambox .mbox-image{border:none;padding:2px 0 2px 0.5em;text-align:center}.mw-parser-output .ambox .mbox-imageright{border:none;padding:2px 0.5em 2px 0;text-align:center}.mw-parser-output .ambox .mbox-empty-cell{border:none;padding:0;width:1px}.mw-parser-output .ambox .mbox-image-div{width:52px}@media(min-width:720px){.mw-parser-output .ambox{margin:0 10%}}@media print{body.ns-0 .mw-parser-output .ambox{display:none!important}}</style><table class="box-More_citations_needed_section plainlinks metadata ambox ambox-content ambox-Refimprove" role="presentation"><tbody><tr><td class="mbox-image"><div class="mbox-image-div"><span typeof="mw:File"><a href="/wiki/File:Question_book-new.svg" class="mw-file-description"><img alt="" src="//upload.wikimedia.org/wikipedia/en/thumb/9/99/Question_book-new.svg/50px-Question_book-new.svg.png" decoding="async" width="50" height="39" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/en/thumb/9/99/Question_book-new.svg/75px-Question_book-new.svg.png 1.5x, //upload.wikimedia.org/wikipedia/en/thumb/9/99/Question_book-new.svg/100px-Question_book-new.svg.png 2x" data-file-width="512" data-file-height="399" /></a></span></div></td><td class="mbox-text"><div class="mbox-text-span">This section <b>needs additional citations for <a href="/wiki/Wikipedia:Verifiability" title="Wikipedia:Verifiability">verification</a></b>.<span class="hide-when-compact"> Please help <a href="/wiki/Special:EditPage/Telephone_exchange" title="Special:EditPage/Telephone exchange">improve this article</a> by <a href="/wiki/Help:Referencing_for_beginners" title="Help:Referencing for beginners">adding citations to reliable sources</a> in this section. Unsourced material may be challenged and removed.</span> <span class="date-container"><i>(<span class="date">November 2023</span>)</i></span><span class="hide-when-compact"><i> (<small><a href="/wiki/Help:Maintenance_template_removal" title="Help:Maintenance template removal">Learn how and when to remove this message</a></small>)</i></span></div></td></tr></tbody></table> <p>First concepts of digital switching and transmission were developed by various labs in the United States and in Europe starting in the 1930s.<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (November 2020)">citation needed</span></a></i>]</sup> The first prototype digital switch was developed by <a href="/wiki/Bell_Labs" title="Bell Labs">Bell Labs</a> as part of the ESSEX project while the first true digital exchange to be combined with digital transmission systems was designed by LCT (Laboratoire Central de Telecommunications) in Paris.<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (September 2014)">citation needed</span></a></i>]</sup> The first digital switch to be placed into a public network in England was the Empress Exchange in <a href="/wiki/London" title="London">London</a> which was designed by the <a href="/wiki/General_Post_Office" title="General Post Office">General Post Office</a> research labs.<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (September 2014)">citation needed</span></a></i>]</sup> It was a tandem switch that connected three <a href="/wiki/Strowger_switch" title="Strowger switch">Strowger</a> exchanges. The first commercial roll-out of a fully digital local switching system was <a href="/wiki/Alcatel-Lucent" title="Alcatel-Lucent">Alcatel</a>'s E10 system which began serving customers in <a href="/wiki/Brittany" title="Brittany">Brittany</a> in Northwestern France in 1972.<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (September 2014)">citation needed</span></a></i>]</sup> </p><p>Prominent examples of digital switches include: </p> <ul><li><a href="/wiki/Ericsson" title="Ericsson">Ericsson</a>'s <a href="/wiki/AXE_telephone_exchange" title="AXE telephone exchange">AXE telephone exchange</a> is the most widely used digital switching platform in the world and can be found throughout Europe and in most countries around the world. It is also very popular in mobile applications. This highly modular system was developed in Sweden in the 1970s as a replacement for the very popular range of Ericsson <a href="/wiki/Crossbar_switch" title="Crossbar switch">crossbar switches</a> ARF, ARM, ARK and ARE used by many European networks from the 1950s onwards.</li> <li><a href="/wiki/Alcatel-Lucent" title="Alcatel-Lucent">Alcatel-Lucent</a> inherited three of the world's most iconic digital switching systems : <a href="/wiki/Alcatel-Lucent" title="Alcatel-Lucent">Alcatel</a> E10, <a href="/wiki/ITT_System_12" title="ITT System 12">1000-S12</a>, and the <a href="/wiki/Western_Electric" title="Western Electric">Western Electric</a> <a href="/wiki/5ESS_switch" class="mw-redirect" title="5ESS switch">5ESS</a>.</li></ul> <dl><dd>Alcatel developed the E10 system in France during the late 1960s and 1970s. This widely used family of digital switches was one of the earliest TDM switches to be widely used in public networks. Subscribers were first connected to E10A switches in France in 1972. This system is used in France, Ireland, China, and many other countries. It has been through many revisions and current versions are even integrated into <a href="/wiki/All-IP_network" class="mw-redirect" title="All-IP network">all-IP networks</a>.</dd> <dd>Alcatel also acquired <a href="/wiki/ITT_System_12" title="ITT System 12">ITT System 12</a> which when it bought ITT's European operations. The S12 system and E10 systems were merged into a single platform in the 1990s. The S12 system is used in Germany, Italy, Australia, Belgium, China, India, and many other countries around the world.</dd> <dd>Finally, when Alcatel and Lucent merged, the company acquired Lucent's <a href="/wiki/5ESS_switch" class="mw-redirect" title="5ESS switch">5ESS</a> and <a href="/wiki/4ESS_switch" class="mw-redirect" title="4ESS switch">4ESS</a> systems used throughout the United States of America and in many other countries.</dd></dl> <ul><li><a href="/wiki/Nokia_Siemens_Networks" class="mw-redirect" title="Nokia Siemens Networks">Nokia Siemens Networks</a> <a href="/wiki/EWSD" title="EWSD">EWSD</a> originally developed by <a href="/wiki/Siemens" title="Siemens">Siemens</a>, <a href="/wiki/Robert_Bosch_GmbH" class="mw-redirect" title="Robert Bosch GmbH">Bosch</a> and <a href="/w/index.php?title=DeTeWe&action=edit&redlink=1" class="new" title="DeTeWe (page does not exist)">DeTeWe</a><sup class="noprint" style="font-style: normal;"> [<a href="https://de.wikipedia.org/wiki/DeTeWe" class="extiw" title="de:DeTeWe">de</a>]</sup> for the German market is used throughout the world.</li> <li><a href="/wiki/Nortel" title="Nortel">Nortel</a> then <a href="/wiki/Genband" class="mw-redirect" title="Genband">Genband</a>, and now Ribbon Communications <a href="/wiki/DMS100" class="mw-redirect" title="DMS100">DMS100</a> and other versions are very popular with operators all over the world.</li> <li>GTD-5 EAX developed by GTE Automatic Electric, the GTD-5 was acquired by Lucent which became Alcatel-Lucent, which then became Nokia</li> <li><a href="/wiki/NEC" title="NEC">NEC</a> NEAX used in Japan, New Zealand and many other countries.</li> <li><a href="/wiki/Marconi_Company" title="Marconi Company">Marconi</a> <a href="/wiki/System_X_(telephony)" title="System X (telephony)">System X</a> originally developed by the British Post Office (later BT), GEC, Plessey and STC, is a type of digital exchange used by <a href="/wiki/BT_Group" title="BT Group">BT Group</a> in the UK public telephone network.</li></ul> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:Nortel_DMS100_public_voice_exchange.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/a/ab/Nortel_DMS100_public_voice_exchange.jpg/220px-Nortel_DMS100_public_voice_exchange.jpg" decoding="async" width="220" height="127" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/a/ab/Nortel_DMS100_public_voice_exchange.jpg/330px-Nortel_DMS100_public_voice_exchange.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/a/ab/Nortel_DMS100_public_voice_exchange.jpg 2x" data-file-width="413" data-file-height="238" /></a><figcaption>A digital exchange (<a href="/wiki/Nortel" title="Nortel">Nortel</a> <a href="/wiki/DMS-100" title="DMS-100">DMS-100</a>) used by an operator to offer local and long-distance services in <a href="/wiki/France" title="France">France</a>. Each switch typically serves 10,000–100,000+ subscribers depending on the geographic area.</figcaption></figure> <p>Digital switches encode the speech going on, in 8,000 time slices per second. (A <a href="/wiki/Sampling_rate" class="mw-redirect" title="Sampling rate">sampling rate</a> of 8khz). At each time slice, a digital <a href="/wiki/Pulse-code_modulation" title="Pulse-code modulation">PCM</a> representation of the sound is made. The digital PCM signals are then sent to the receiving end of the line, where the reverse process occurs using a DAC (<a href="/wiki/Digital-to-analog_converter" title="Digital-to-analog converter">digital-to-analog converter</a>), to produce the sound for the receiving phone. In other words, when someone uses a telephone, the speaker's voice is "encoded" using PCM for switching then reconstructed for the person on the other end. The speaker's voice is delayed in the process by a small fraction of one second — it is not "live", it is reconstructed — delayed only minutely. </p><p>Individual <a href="/wiki/Local_loop" title="Local loop">local loop</a> telephone lines are connected to a <a href="/wiki/Remote_concentrator" title="Remote concentrator">remote concentrator</a>. In many cases, the concentrator is co-located in the same building as the switch. The interface between remote concentrators and telephone switches has been standardised by <a href="/wiki/ETSI" class="mw-redirect" title="ETSI">ETSI</a> as the <a href="/wiki/V5_interface" title="V5 interface">V5</a> protocol. Concentrators are used because most telephones are idle most of the day, hence the traffic from hundreds or thousands of them may be concentrated into only tens or hundreds of shared connections. </p><p>Some telephone switches do not have concentrators directly connected to them, but rather are used to connect calls between other telephone switches. These complex machines are referred to as "carrier-level" switches or <a href="/wiki/Tandem_switch" class="mw-redirect" title="Tandem switch">tandem switches</a>. </p><p>Some telephone exchange buildings in small towns house only <i>remote</i> or <i>satellite</i> switches, and are homed upon a "parent" switch, usually several kilometres away. The remote switch is dependent on the parent switch for routing. Unlike a <a href="/wiki/Digital_loop_carrier" title="Digital loop carrier">digital loop carrier</a>, a remote switch can route calls between local phones itself, without using trunks to the parent switch. </p> <figure typeof="mw:File/Thumb"><a href="/wiki/File:Wire_Center_Locations.png" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/0/09/Wire_Center_Locations.png/300px-Wire_Center_Locations.png" decoding="async" width="300" height="232" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/0/09/Wire_Center_Locations.png/450px-Wire_Center_Locations.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/0/09/Wire_Center_Locations.png/600px-Wire_Center_Locations.png 2x" data-file-width="1056" data-file-height="816" /></a><figcaption>Map of wire center locations in the US</figcaption></figure> <figure typeof="mw:File/Thumb"><a href="/wiki/File:Central_Office_Locations.png" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/0/00/Central_Office_Locations.png/300px-Central_Office_Locations.png" decoding="async" width="300" height="232" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/0/00/Central_Office_Locations.png/450px-Central_Office_Locations.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/0/00/Central_Office_Locations.png/600px-Central_Office_Locations.png 2x" data-file-width="1056" data-file-height="816" /></a><figcaption>Map of central office locations in the US</figcaption></figure> <div class="mw-heading mw-heading2"><h2 id="The_switch's_place_in_the_network"><span id="The_switch.27s_place_in_the_network"></span>The switch's place in the network</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Telephone_exchange&action=edit&section=10" title="Edit section: The switch's place in the network"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Telephone switches are a small component of a large network. A major part, in terms of expense, maintenance, and logistics of the telephone system is <a href="/wiki/Outside_plant" title="Outside plant">outside plant</a>, which is the wiring outside the central office. While many subscribers were served with party-lines in the middle of the 20th century, it was the goal that each subscriber telephone station were connected to an individual pair of wires from the switching system. </p><p>A typical central office may have tens of thousands of pairs of wires that appear on terminal blocks called the <a href="/wiki/Main_distribution_frame" title="Main distribution frame">main distribution frame</a> (MDF). A component of the MDF is protection: fuses or other devices that protect the switch from lightning, shorts with electric power lines, or other foreign voltages. In a typical telephone company, a large database tracks information about each subscriber pair and the status of each jumper. Before computerization of Bell System records in the 1980s, this information was handwritten in pencil in accounting ledger books. </p><p>To reduce the expense of outside plant, some companies use "<a href="/wiki/Pair_gain" title="Pair gain">pair gain</a>" devices to provide telephone service to subscribers. These devices are used to provide service where existing copper facilities have been exhausted or by siting in a neighborhood, can reduce the length of copper pairs, enabling digital services such as <a href="/wiki/Integrated_Services_Digital_Network" class="mw-redirect" title="Integrated Services Digital Network">Integrated Services Digital Network</a> (ISDN) or <a href="/wiki/Digital_subscriber_line" title="Digital subscriber line">digital subscriber line</a> (DSL). </p><p>Pair gain or <a href="/wiki/Digital_loop_carrier" title="Digital loop carrier">digital loop carriers</a> (DLCs) are located outside the central office, usually in a large neighborhood distant from the CO. DLCs are often referred to as <a href="/wiki/Subscriber_Loop_Carrier" class="mw-redirect" title="Subscriber Loop Carrier">Subscriber Loop Carriers</a> (SLCs), after a <a href="/wiki/Lucent" class="mw-redirect" title="Lucent">Lucent</a> proprietary product. </p><p>DLCs can be configured as universal (UDLCs) or integrated (IDLCs). <i>Universal DLC</i>s have two terminals, a central office terminal (COT) and a remote terminal (RT), that function similarly. Both terminals interface with analog signals, convert to digital signals, and transport to the other side where the reverse is performed. </p><p>Sometimes, the transport is handled by separate equipment. In an <i>Integrated DLC</i>, the COT is eliminated. Instead, the RT is connected digitally to equipment in the telephone switch. This reduces the total amount of equipment required. </p><p>Switches are used in both local central offices and in <a href="/wiki/Long-distance_calling" title="Long-distance calling">long distance</a> centers. There are two major types in the <a href="/wiki/Public_switched_telephone_network" title="Public switched telephone network">Public switched telephone network</a> (PSTN), the <a href="/wiki/Class_4_telephone_switch" class="mw-redirect" title="Class 4 telephone switch">Class 4 telephone switches</a> designed for toll or switch-to-switch connections, and the <a href="/wiki/Class_5_telephone_switches" class="mw-redirect" title="Class 5 telephone switches">Class 5 telephone switches</a> or subscriber switches, which manage connections from subscriber telephones. Since the 1990s, hybrid Class 4/5 switching systems that serve both functions have become common. </p><p>Another element of the telephone network is time and timing. Switching, transmission and billing equipment may be slaved to very high accuracy <a href="/wiki/Network_Time_Protocol#Clock_strata" title="Network Time Protocol">10 MHz standards</a> which synchronize time events to very close intervals. Time-standards equipment may include Rubidium- or Caesium-based standards and a <a href="/wiki/Global_Positioning_System" title="Global Positioning System">Global Positioning System</a> receiver. </p> <div class="mw-heading mw-heading2"><h2 id="Switch_design">Switch design</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Telephone_exchange&action=edit&section=11" title="Edit section: Switch design"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Long-distance switches may use a slower, more efficient switch-allocation algorithm than <a href="/wiki/Class_5_telephone_switches" class="mw-redirect" title="Class 5 telephone switches">local central offices</a>, because they have near 100% utilization of their input and output channels. Central offices have more than 90% of their channel capacity unused. </p><p>Traditional telephone switches connected physical circuits (e.g., wire pairs) while modern telephone switches use a combination of <a href="/wiki/Multiplexers" class="mw-redirect" title="Multiplexers">space-</a> and time-division switching. In other words, each voice channel is represented by a <a href="/wiki/Time-division_multiplexing" title="Time-division multiplexing">time slot</a> (say 1 or 2) on a physical wire pair (A or B). In order to connect two voice channels (say A1 and B2) together, the telephone switch interchanges the information between A1 and B2. It switches both the time slot and physical connection. To do this, it exchanges data between the time slots and connections 8,000 times per second, under control of digital logic that cycles through electronic lists of the current connections. Using both types of switching makes a modern switch far smaller than either a space or time switch could be by itself. </p><p>The <a href="/wiki/Nonblocking_minimal_spanning_switch" title="Nonblocking minimal spanning switch">structure of a switch</a> is an odd number of layers of smaller, simpler subswitches. Each layer is interconnected by a web of wires that goes from each subswitch, to a set of the next layer of subswitches. In some designs, a physical (space) switching layer alternates with a time switching layer. The layers are symmetric, because in a telephone system callers can also <a href="/wiki/Called_party" title="Called party">be called</a>. Other designs use time-switching only, throughout the switch. </p><p>A time-division subswitch reads a complete cycle of time slots into a memory, and then writes it out in a different order, also under control of a cyclic computer memory. This causes some delay in the signal. </p><p>A space-division subswitch switches electrical paths, often using some variant of a <a href="/wiki/Nonblocking_minimal_spanning_switch" title="Nonblocking minimal spanning switch">nonblocking minimal spanning switch</a>, or a <a href="/wiki/Crossover_switch" title="Crossover switch">crossover switch</a>. </p> <div class="mw-heading mw-heading3"><h3 id="Fault_tolerance">Fault tolerance</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Telephone_exchange&action=edit&section=12" title="Edit section: Fault tolerance"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Composite switches are inherently fault-tolerant. If a subswitch fails, the controlling computer can sense the failure during a periodic test. The computer marks all the connections to the subswitch as "in use". This prevents new calls, and does not interrupt established calls. As established calls end, the subswitch becomes unused, and can be repaired. When the next test succeeds, the switch returns to full operation. </p><p>To prevent frustration with unsensed failures, all the connections between layers in the switch are allocated using <a href="/wiki/FIFO_(computing_and_electronics)" title="FIFO (computing and electronics)">first-in-first-out lists</a> (queues). As a result, if a connection is faulty or noisy and the customer hangs up and redials, they will get a different set of connections and subswitches. A <a href="/wiki/LIFO_(computing)" class="mw-redirect" title="LIFO (computing)">last-in-first-out</a> (stack) allocation of connections might cause a continuing string of very frustrating failures. </p> <div class="mw-heading mw-heading3"><h3 id="Fire_and_disaster_recovery">Fire and disaster recovery</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Telephone_exchange&action=edit&section=13" title="Edit section: Fire and disaster recovery"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <figure class="mw-default-size" typeof="mw:File/Thumb"><a href="/wiki/File:East_Village_20191129_-_06_-_Second_btwn_12_%26_13.jpg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/commons/thumb/0/0e/East_Village_20191129_-_06_-_Second_btwn_12_%26_13.jpg/170px-East_Village_20191129_-_06_-_Second_btwn_12_%26_13.jpg" decoding="async" width="170" height="227" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/0/0e/East_Village_20191129_-_06_-_Second_btwn_12_%26_13.jpg/255px-East_Village_20191129_-_06_-_Second_btwn_12_%26_13.jpg 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/0/0e/East_Village_20191129_-_06_-_Second_btwn_12_%26_13.jpg/340px-East_Village_20191129_-_06_-_Second_btwn_12_%26_13.jpg 2x" data-file-width="3024" data-file-height="4032" /></a><figcaption>Second Avenue exchange, NYC, site of the <a href="/wiki/1975_New_York_Telephone_Exchange_fire" class="mw-redirect" title="1975 New York Telephone Exchange fire">1975 New York Telephone Exchange fire</a></figcaption></figure> <p>A central exchange is almost always a <a href="/wiki/Single_point_of_failure" title="Single point of failure">single point of failure</a> for local calls. As the capacity of individual switches and the <a href="/wiki/Optical_fibre" class="mw-redirect" title="Optical fibre">optical fibre</a> which interconnects them increases, potential disruption caused by destruction of one local office will only be magnified. Multiple fibre connections can be used to provide redundancy to voice and data connections between switching centres, but careful network design is required to avoid situations where a main fibre and its backup both go through the same damaged central office as a potential <a href="/wiki/Common_mode_failure" class="mw-redirect" title="Common mode failure">common mode failure</a>.<sup id="cite_ref-nyt-hinsdale_20-0" class="reference"><a href="#cite_note-nyt-hinsdale-20"><span class="cite-bracket">[</span>20<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Telephone_exchange&action=edit&section=14" title="Edit section: See also"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1259569809">.mw-parser-output .portalbox{padding:0;margin:0.5em 0;display:table;box-sizing:border-box;max-width:175px;list-style:none}.mw-parser-output .portalborder{border:1px solid var(--border-color-base,#a2a9b1);padding:0.1em;background:var(--background-color-neutral-subtle,#f8f9fa)}.mw-parser-output .portalbox-entry{display:table-row;font-size:85%;line-height:110%;height:1.9em;font-style:italic;font-weight:bold}.mw-parser-output .portalbox-image{display:table-cell;padding:0.2em;vertical-align:middle;text-align:center}.mw-parser-output .portalbox-link{display:table-cell;padding:0.2em 0.2em 0.2em 0.3em;vertical-align:middle}@media(min-width:720px){.mw-parser-output .portalleft{clear:left;float:left;margin:0.5em 1em 0.5em 0}.mw-parser-output .portalright{clear:right;float:right;margin:0.5em 0 0.5em 1em}}</style><ul role="navigation" aria-label="Portals" class="noprint portalbox portalborder portalright"> <li class="portalbox-entry"><span class="portalbox-image"><span class="noviewer" typeof="mw:File"><span><img alt="telephone icon" src="//upload.wikimedia.org/wikipedia/commons/thumb/b/b8/Telephone_icon_blue_gradient.svg/28px-Telephone_icon_blue_gradient.svg.png" decoding="async" width="28" height="28" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/b/b8/Telephone_icon_blue_gradient.svg/42px-Telephone_icon_blue_gradient.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/b/b8/Telephone_icon_blue_gradient.svg/56px-Telephone_icon_blue_gradient.svg.png 2x" data-file-width="722" data-file-height="722" /></span></span></span><span class="portalbox-link"><a href="/wiki/Portal:Telephones" title="Portal:Telephones">Telephones portal</a></span></li></ul> <ul><li><a href="/wiki/History_of_telecommunication" title="History of telecommunication">History of telecommunication</a></li> <li><a href="/wiki/List_of_telephone_switches" title="List of telephone switches">List of telephone switches</a></li> <li><a href="/wiki/Pair_gain_system" class="mw-redirect" title="Pair gain system">Pair gain system</a></li> <li><a href="/wiki/Limited_availability" title="Limited availability">Limited availability</a></li> <li><a href="/wiki/Softswitch" title="Softswitch">Softswitch</a></li> <li><a href="/wiki/Plesiochronous_digital_hierarchy" title="Plesiochronous digital hierarchy">Plesiochronous digital hierarchy</a></li> <li><a href="/wiki/Telephone_exchange_names" title="Telephone exchange names">Telephone exchange names</a></li> <li><a href="/wiki/Automatic_call_distributor" title="Automatic call distributor">Automatic call distributor</a></li> <li><a href="/wiki/Public_utility_building" title="Public utility building">Public utility building</a></li></ul> <div class="mw-heading mw-heading2"><h2 id="References">References</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Telephone_exchange&action=edit&section=15" title="Edit section: References"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1239543626">.mw-parser-output .reflist{margin-bottom:0.5em;list-style-type:decimal}@media screen{.mw-parser-output .reflist{font-size:90%}}.mw-parser-output .reflist .references{font-size:100%;margin-bottom:0;list-style-type:inherit}.mw-parser-output .reflist-columns-2{column-width:30em}.mw-parser-output .reflist-columns-3{column-width:25em}.mw-parser-output .reflist-columns{margin-top:0.3em}.mw-parser-output .reflist-columns ol{margin-top:0}.mw-parser-output .reflist-columns li{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .reflist-upper-alpha{list-style-type:upper-alpha}.mw-parser-output .reflist-upper-roman{list-style-type:upper-roman}.mw-parser-output .reflist-lower-alpha{list-style-type:lower-alpha}.mw-parser-output .reflist-lower-greek{list-style-type:lower-greek}.mw-parser-output .reflist-lower-roman{list-style-type:lower-roman}</style><div class="reflist"> <div class="mw-references-wrap mw-references-columns"><ol class="references"> <li id="cite_note-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-1">^</a></b></span> <span class="reference-text"><style data-mw-deduplicate="TemplateStyles:r1238218222">.mw-parser-output cite.citation{font-style:inherit;word-wrap:break-word}.mw-parser-output .citation q{quotes:"\"""\"""'""'"}.mw-parser-output .citation:target{background-color:rgba(0,127,255,0.133)}.mw-parser-output .id-lock-free.id-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/6/65/Lock-green.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-limited.id-lock-limited a,.mw-parser-output .id-lock-registration.id-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/d/d6/Lock-gray-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-subscription.id-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/a/aa/Lock-red-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .cs1-ws-icon a{background:url("//upload.wikimedia.org/wikipedia/commons/4/4c/Wikisource-logo.svg")right 0.1em center/12px no-repeat}body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-free a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-limited a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-registration a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-subscription a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .cs1-ws-icon a{background-size:contain;padding:0 1em 0 0}.mw-parser-output .cs1-code{color:inherit;background:inherit;border:none;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;color:var(--color-error,#d33)}.mw-parser-output .cs1-visible-error{color:var(--color-error,#d33)}.mw-parser-output .cs1-maint{display:none;color:#085;margin-left:0.3em}.mw-parser-output .cs1-kern-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right{padding-right:0.2em}.mw-parser-output .citation .mw-selflink{font-weight:inherit}@media screen{.mw-parser-output .cs1-format{font-size:95%}html.skin-theme-clientpref-night .mw-parser-output .cs1-maint{color:#18911f}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .cs1-maint{color:#18911f}}</style><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://www.verizonenterprise.com/external/service_guide/reg/g_general_definitions.htm">"General Definitions"</a>. <i>Verizon service</i>. Verizon Enterprise Solutions.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=unknown&rft.jtitle=Verizon+service&rft.atitle=General+Definitions&rft_id=http%3A%2F%2Fwww.verizonenterprise.com%2Fexternal%2Fservice_guide%2Freg%2Fg_general_definitions.htm&rfr_id=info%3Asid%2Fen.wikipedia.org%3ATelephone+exchange" class="Z3988"></span></span> </li> <li id="cite_note-SidneyMorningHerald-1878.04.19-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-SidneyMorningHerald-1878.04.19_2-0">^</a></b></span> <span class="reference-text"><a rel="nofollow" class="external text" href="https://news.google.com/newspapers?nid=1301&dat=18780419&id=ceVhAAAAIBAJ&sjid=dpEDAAAAIBAJ&pg=6902,636781">Private Telegraphs</a>, <i><a href="/wiki/The_Sydney_Morning_Herald" title="The Sydney Morning Herald">The Sydney Morning Herald</a></i>, credited to <i><a href="/wiki/The_Times" title="The Times">The Times</a></i>, April 19, 1878, p. 6.</span> </li> <li id="cite_note-3"><span class="mw-cite-backlink"><b><a href="#cite_ref-3">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFBo_Leuf2002" class="citation book cs1"><a href="/wiki/Bo_Leuf" title="Bo Leuf">Bo Leuf</a> (2002). <a rel="nofollow" class="external text" href="https://books.google.com/books?id=YCLxL_aKvMMC&q=%22The+analog+telephone+was+an+early+extension%22"><i>Peer to Peer: Collaboration and Sharing Over the Internet</i></a>. Addison-Wesley. p. 15. <a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/9780201767322" title="Special:BookSources/9780201767322"><bdi>9780201767322</bdi></a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=Peer+to+Peer%3A+Collaboration+and+Sharing+Over+the+Internet&rft.pages=15&rft.pub=Addison-Wesley&rft.date=2002&rft.isbn=9780201767322&rft.au=Bo+Leuf&rft_id=https%3A%2F%2Fbooks.google.com%2Fbooks%3Fid%3DYCLxL_aKvMMC%26q%3D%2522The%2Banalog%2Btelephone%2Bwas%2Ban%2Bearly%2Bextension%2522&rfr_id=info%3Asid%2Fen.wikipedia.org%3ATelephone+exchange" class="Z3988"></span></span> </li> <li id="cite_note-4"><span class="mw-cite-backlink"><b><a href="#cite_ref-4">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFAlvin_K._Benson2010" class="citation book cs1">Alvin K. Benson (2010). <a rel="nofollow" class="external text" href="https://books.google.com/books?id=JqVZAAAAYAAJ&q=puskas+%22switchboard%22"><i>Inventors and inventions Great lives from history Volume 4 of Great Lives from History: Inventors & Inventions</i></a>. Salem Press. p. 1298. <a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/9781587655227" title="Special:BookSources/9781587655227"><bdi>9781587655227</bdi></a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=Inventors+and+inventions+Great+lives+from+history+Volume+4+of+Great+Lives+from+History%3A+Inventors+%26+Inventions&rft.pages=1298&rft.pub=Salem+Press&rft.date=2010&rft.isbn=9781587655227&rft.au=Alvin+K.+Benson&rft_id=https%3A%2F%2Fbooks.google.com%2Fbooks%3Fid%3DJqVZAAAAYAAJ%26q%3Dpuskas%2B%2522switchboard%2522&rfr_id=info%3Asid%2Fen.wikipedia.org%3ATelephone+exchange" class="Z3988"></span></span> </li> <li id="cite_note-5"><span class="mw-cite-backlink"><b><a href="#cite_ref-5">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://web.archive.org/web/20110204071321/http://hungarian-history.hu/mszh/epuskas.htm">"TIVADAR PUSKÁS (1844–1893)"</a>. 4 February 2011. Archived from <a rel="nofollow" class="external text" href="http://hungarian-history.hu/mszh/epuskas.htm">the original</a> on 4 February 2011.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=TIVADAR+PUSK%C3%81S+%281844%E2%80%931893%29&rft.date=2011-02-04&rft_id=http%3A%2F%2Fhungarian-history.hu%2Fmszh%2Fepuskas.htm&rfr_id=info%3Asid%2Fen.wikipedia.org%3ATelephone+exchange" class="Z3988"></span></span> </li> <li id="cite_note-6"><span class="mw-cite-backlink"><b><a href="#cite_ref-6">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://www.mszh.hu/English/feltalalok/puskas.html">"SZTNH"</a>. Mszh.hu. <a rel="nofollow" class="external text" href="https://web.archive.org/web/20050224203936/http://www.mszh.hu/English/feltalalok/puskas.html">Archived</a> from the original on 2005-02-24<span class="reference-accessdate">. Retrieved <span class="nowrap">2012-07-01</span></span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=SZTNH&rft.pub=Mszh.hu&rft_id=http%3A%2F%2Fwww.mszh.hu%2FEnglish%2Ffeltalalok%2Fpuskas.html&rfr_id=info%3Asid%2Fen.wikipedia.org%3ATelephone+exchange" class="Z3988"></span></span> </li> <li id="cite_note-7"><span class="mw-cite-backlink"><b><a href="#cite_ref-7">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://www.omikk.bme.hu/archivum/angol/htm/puskas_t.htm">"Puskás, Tivadar"</a>. Omikk.bme.hu. <a rel="nofollow" class="external text" href="https://web.archive.org/web/20210421152416/https://dokutar.omikk.bme.hu/archivum/angol/htm/puskas_t.htm">Archived</a> from the original on 2021-04-21<span class="reference-accessdate">. Retrieved <span class="nowrap">2012-07-01</span></span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=Pusk%C3%A1s%2C+Tivadar&rft.pub=Omikk.bme.hu&rft_id=http%3A%2F%2Fwww.omikk.bme.hu%2Farchivum%2Fangol%2Fhtm%2Fpuskas_t.htm&rfr_id=info%3Asid%2Fen.wikipedia.org%3ATelephone+exchange" class="Z3988"></span></span> </li> <li id="cite_note-8"><span class="mw-cite-backlink"><b><a href="#cite_ref-8">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://web.archive.org/web/20120316155715/http://www.hunreal.com/known-hungarians/tivadar-puskas/">"Welcome hunreal.com - BlueHost.com"</a>. Hunreal.com. Archived from the original on 2012-03-16<span class="reference-accessdate">. Retrieved <span class="nowrap">2012-07-01</span></span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=Welcome+hunreal.com+-+BlueHost.com&rft.pub=Hunreal.com&rft_id=http%3A%2F%2Fwww.hunreal.com%2Fknown-hungarians%2Ftivadar-puskas%2F&rfr_id=info%3Asid%2Fen.wikipedia.org%3ATelephone+exchange" class="Z3988"></span><span class="cs1-maint citation-comment"><code class="cs1-code">{{<a href="/wiki/Template:Cite_web" title="Template:Cite web">cite web</a>}}</code>: CS1 maint: unfit URL (<a href="/wiki/Category:CS1_maint:_unfit_URL" title="Category:CS1 maint: unfit URL">link</a>)</span></span> </li> <li id="cite_note-9"><span class="mw-cite-backlink"><b><a href="#cite_ref-9">^</a></b></span> <span class="reference-text">Frank Lewis Dyer: Edison His Life And Inventions. (page: 71)</span> </li> <li id="cite_note-10"><span class="mw-cite-backlink"><b><a href="#cite_ref-10">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://web.archive.org/web/20141106151341/http://cdrecord.org/private/tel.html">"120 Year Telephone anniversary"</a>. <i>cdrecord.org</i>. Archived from <a rel="nofollow" class="external text" href="http://cdrecord.org/private/tel.html">the original</a> on 6 November 2014.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=unknown&rft.jtitle=cdrecord.org&rft.atitle=120+Year+Telephone+anniversary&rft_id=http%3A%2F%2Fcdrecord.org%2Fprivate%2Ftel.html&rfr_id=info%3Asid%2Fen.wikipedia.org%3ATelephone+exchange" class="Z3988"></span></span> </li> <li id="cite_note-11"><span class="mw-cite-backlink"><b><a href="#cite_ref-11">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFSundqvist2018" class="citation news cs1">Sundqvist, By Viktoria (22 April 2018). <a rel="nofollow" class="external text" href="https://www.nhregister.com/news/article/It-s-for-you-New-Haven-was-the-birth-place-of-12854778.php">"Top 50: New Haven was birth place of the modern telephone"</a>. <i>New Haven Register</i>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.jtitle=New+Haven+Register&rft.atitle=Top+50%3A+New+Haven+was+birth+place+of+the+modern+telephone&rft.date=2018-04-22&rft.aulast=Sundqvist&rft.aufirst=By+Viktoria&rft_id=https%3A%2F%2Fwww.nhregister.com%2Fnews%2Farticle%2FIt-s-for-you-New-Haven-was-the-birth-place-of-12854778.php&rfr_id=info%3Asid%2Fen.wikipedia.org%3ATelephone+exchange" class="Z3988"></span></span> </li> <li id="cite_note-12"><span class="mw-cite-backlink"><b><a href="#cite_ref-12">^</a></b></span> <span class="reference-text"><a rel="nofollow" class="external text" href="https://web.archive.org/web/20130302120900/http://www.cr.nps.gov/nhl/DOE_dedesignations/Telephone.htm">National Park Service "first switchboard" page</a>.</span> </li> <li id="cite_note-13"><span class="mw-cite-backlink"><b><a href="#cite_ref-13">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://web.archive.org/web/20130605155116/http://www.mosi.org.uk/media/33871608/early%20manchester%20telephone%20exchanges.pdf">"Early Manchester telephone exchanges"</a> <span class="cs1-format">(PDF)</span>. mosi.org.uk. Archived from <a rel="nofollow" class="external text" href="http://www.mosi.org.uk/media/33871608/early%20manchester%20telephone%20exchanges.pdf">the original</a> <span class="cs1-format">(PDF)</span> on 2013-06-05<span class="reference-accessdate">. Retrieved <span class="nowrap">2013-07-30</span></span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=Early+Manchester+telephone+exchanges&rft.pub=mosi.org.uk&rft_id=http%3A%2F%2Fwww.mosi.org.uk%2Fmedia%2F33871608%2Fearly%2520manchester%2520telephone%2520exchanges.pdf&rfr_id=info%3Asid%2Fen.wikipedia.org%3ATelephone+exchange" class="Z3988"></span></span> </li> <li id="cite_note-14"><span class="mw-cite-backlink"><b><a href="#cite_ref-14">^</a></b></span> <span class="reference-text">Francis S. Wagner: Hungarian Contributions to World Civilization – Page 68</span> </li> <li id="cite_note-calvert2003-15"><span class="mw-cite-backlink">^ <a href="#cite_ref-calvert2003_15-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-calvert2003_15-1"><sup><i><b>b</b></i></sup></a></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFCalvert2003" class="citation web cs1">Calvert, J. B. (2003-09-07). <a rel="nofollow" class="external text" href="https://web.archive.org/web/20031227031717/http://www.du.edu/~jcalvert/tech/phones.htm">"Basic Telephones"</a>. Archived from <a rel="nofollow" class="external text" href="http://www.du.edu/~jcalvert/tech/phones.htm">the original</a> on 2003-12-27<span class="reference-accessdate">. Retrieved <span class="nowrap">2007-09-13</span></span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=Basic+Telephones&rft.date=2003-09-07&rft.aulast=Calvert&rft.aufirst=J.+B.&rft_id=http%3A%2F%2Fwww.du.edu%2F~jcalvert%2Ftech%2Fphones.htm&rfr_id=info%3Asid%2Fen.wikipedia.org%3ATelephone+exchange" class="Z3988"></span></span> </li> <li id="cite_note-16"><span class="mw-cite-backlink"><b><a href="#cite_ref-16">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://www.strombergcarlsontelephone.com/kellogg/PDF/1921%20SW%20BD%20INSERT.pdf">"Stromberg-Carlson Telephone Mfg. Co. Resource Page"</a> <span class="cs1-format">(PDF)</span>. <i>www.strombergcarlsontelephone.com</i><span class="reference-accessdate">. Retrieved <span class="nowrap">2020-09-09</span></span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=unknown&rft.jtitle=www.strombergcarlsontelephone.com&rft.atitle=Stromberg-Carlson+Telephone+Mfg.+Co.+Resource+Page&rft_id=http%3A%2F%2Fwww.strombergcarlsontelephone.com%2Fkellogg%2FPDF%2F1921%2520SW%2520BD%2520INSERT.pdf&rfr_id=info%3Asid%2Fen.wikipedia.org%3ATelephone+exchange" class="Z3988"></span></span> </li> <li id="cite_note-17"><span class="mw-cite-backlink"><b><a href="#cite_ref-17">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFCalvert2003" class="citation web cs1">Calvert, J. B. (2003-09-07). <a rel="nofollow" class="external text" href="https://web.archive.org/web/20031227031717/http://www.du.edu/~jcalvert/tech/phones.htm">"Basic Telephones, The Switchboard (ringdown is near bottom)"</a>. Archived from <a rel="nofollow" class="external text" href="http://www.du.edu/~jcalvert/tech/phones.htm">the original</a> on 2003-12-27<span class="reference-accessdate">. Retrieved <span class="nowrap">2006-09-13</span></span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=Basic+Telephones%2C+The+Switchboard+%28ringdown+is+near+bottom%29&rft.date=2003-09-07&rft.aulast=Calvert&rft.aufirst=J.+B.&rft_id=http%3A%2F%2Fwww.du.edu%2F~jcalvert%2Ftech%2Fphones.htm&rfr_id=info%3Asid%2Fen.wikipedia.org%3ATelephone+exchange" class="Z3988"></span></span> </li> <li id="cite_note-FS1037C-18"><span class="mw-cite-backlink"><b><a href="#cite_ref-FS1037C_18-0">^</a></b></span> <span class="reference-text">Source: from <a href="/wiki/Federal_Standard_1037C" title="Federal Standard 1037C">Federal Standard 1037C</a>.</span> </li> <li id="cite_note-autogenerated1-19"><span class="mw-cite-backlink">^ <a href="#cite_ref-autogenerated1_19-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-autogenerated1_19-1"><sup><i><b>b</b></i></sup></a></span> <span class="reference-text">Connected to a switch, an off-hook condition operates a relay to connect the line to a dial tone generator and a device to collect dialed digits.</span> </li> <li id="cite_note-nyt-hinsdale-20"><span class="mw-cite-backlink"><b><a href="#cite_ref-nyt-hinsdale_20-0">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFAndrew_Pollack1988" class="citation news cs1">Andrew Pollack (1988-05-26). <a rel="nofollow" class="external text" href="https://web.archive.org/web/20140709010940/http://www.billhamel.net/uverse/Phone_System_Vulnerable_CO_fire_1988_Hinsdale.pdf">"Phone System Feared Vulnerable To Wider Disruptions of Service"</a> <span class="cs1-format">(PDF)</span>. <i><a href="/wiki/The_New_York_Times" title="The New York Times">The New York Times</a></i>. Archived from <a rel="nofollow" class="external text" href="http://www.billhamel.net/uverse/Phone_System_Vulnerable_CO_fire_1988_Hinsdale.pdf">the original</a> <span class="cs1-format">(PDF)</span> on 2014-07-09<span class="reference-accessdate">. Retrieved <span class="nowrap">2013-07-30</span></span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.jtitle=The+New+York+Times&rft.atitle=Phone+System+Feared+Vulnerable+To+Wider+Disruptions+of+Service&rft.date=1988-05-26&rft.au=Andrew+Pollack&rft_id=http%3A%2F%2Fwww.billhamel.net%2Fuverse%2FPhone_System_Vulnerable_CO_fire_1988_Hinsdale.pdf&rfr_id=info%3Asid%2Fen.wikipedia.org%3ATelephone+exchange" class="Z3988"></span></span> </li> </ol></div></div> <div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Telephone_exchange&action=edit&section=16" title="Edit section: External links"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1235681985"><style data-mw-deduplicate="TemplateStyles:r1237033735">@media print{body.ns-0 .mw-parser-output .sistersitebox{display:none!important}}@media screen{html.skin-theme-clientpref-night .mw-parser-output .sistersitebox img[src*="Wiktionary-logo-en-v2.svg"]{background-color:white}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .sistersitebox img[src*="Wiktionary-logo-en-v2.svg"]{background-color:white}}</style><div class="side-box side-box-right plainlinks sistersitebox"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1126788409"> <div class="side-box-flex"> <div class="side-box-image"><span class="noviewer" typeof="mw:File"><span><img alt="" src="//upload.wikimedia.org/wikipedia/en/thumb/4/4a/Commons-logo.svg/30px-Commons-logo.svg.png" decoding="async" width="30" height="40" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/en/thumb/4/4a/Commons-logo.svg/45px-Commons-logo.svg.png 1.5x, //upload.wikimedia.org/wikipedia/en/thumb/4/4a/Commons-logo.svg/59px-Commons-logo.svg.png 2x" data-file-width="1024" data-file-height="1376" /></span></span></div> <div class="side-box-text plainlist">Wikimedia Commons has media related to <span style="font-weight: bold; font-style: italic;"><a href="https://commons.wikimedia.org/wiki/Category:Telephone_exchanges" class="extiw" title="commons:Category:Telephone exchanges">Telephone exchanges</a></span>.</div></div> </div> <ul><li><a rel="nofollow" class="external text" href="https://patents.google.com/patent/US252576">patent 252,576 for the first telephone switchboard in 1881</a></li> <li><a rel="nofollow" class="external text" href="http://www.adslgeek.com/dslforum/index.php?topic=207.0">A Telecom Exchange Tour in NZ</a></li></ul> <div class="navbox-styles"><style data-mw-deduplicate="TemplateStyles:r1129693374">.mw-parser-output .hlist dl,.mw-parser-output .hlist ol,.mw-parser-output .hlist ul{margin:0;padding:0}.mw-parser-output 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td.hlist ul{padding:0.125em 0}.mw-parser-output .navbox .navbar{display:block;font-size:100%}.mw-parser-output .navbox-title .navbar{float:left;text-align:left;margin-right:0.5em}body.skin--responsive .mw-parser-output .navbox-image img{max-width:none!important}@media print{body.ns-0 .mw-parser-output .navbox{display:none!important}}</style></div><div role="navigation" class="navbox" aria-labelledby="Telecommunications" style="padding:3px"><table class="nowraplinks hlist mw-collapsible mw-collapsed navbox-inner" style="border-spacing:0;background:transparent;color:inherit"><tbody><tr><th scope="col" class="navbox-title" colspan="2"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1129693374"><style data-mw-deduplicate="TemplateStyles:r1239400231">.mw-parser-output .navbar{display:inline;font-size:88%;font-weight:normal}.mw-parser-output .navbar-collapse{float:left;text-align:left}.mw-parser-output .navbar-boxtext{word-spacing:0}.mw-parser-output .navbar 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href="/wiki/Template:Telecommunications" title="Template:Telecommunications"><abbr title="View this template">v</abbr></a></li><li class="nv-talk"><a href="/wiki/Template_talk:Telecommunications" title="Template talk:Telecommunications"><abbr title="Discuss this template">t</abbr></a></li><li class="nv-edit"><a href="/wiki/Special:EditPage/Template:Telecommunications" title="Special:EditPage/Template:Telecommunications"><abbr title="Edit this template">e</abbr></a></li></ul></div><div id="Telecommunications" style="font-size:114%;margin:0 4em"><a href="/wiki/Telecommunications" title="Telecommunications">Telecommunications</a></div></th></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/History_of_telecommunication" title="History of telecommunication">History</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Beacon#For_defensive_communications" title="Beacon">Beacon</a></li> <li><a href="/wiki/History_of_broadcasting" title="History of broadcasting">Broadcasting</a></li> <li><a href="/wiki/Cable_protection_system" title="Cable protection system">Cable protection system</a></li> <li><a href="/wiki/Cable_television" title="Cable television">Cable TV</a></li> <li><a href="/wiki/Communications_satellite#History" title="Communications satellite">Communications satellite</a></li> <li><a href="/wiki/Computer_network#History" title="Computer network">Computer network</a></li> <li><a href="/wiki/Data_compression" title="Data compression">Data compression</a> <ul><li><a href="/wiki/Audio_coding_format" title="Audio coding format">audio</a></li> <li><a href="/wiki/Discrete_cosine_transform" title="Discrete cosine transform">DCT</a></li> <li><a href="/wiki/Image_compression" title="Image compression">image</a></li> <li><a href="/wiki/Video_coding_format" title="Video coding format">video</a></li></ul></li> <li><a href="/wiki/Digital_media" title="Digital media">Digital media</a> <ul><li><a href="/wiki/Internet_video" title="Internet video">Internet video</a></li> <li><a href="/wiki/Online_video_platform" title="Online video platform">online video platform</a></li> <li><a href="/wiki/Social_media" title="Social media">social media</a></li> <li><a href="/wiki/Streaming_media" title="Streaming media">streaming</a></li></ul></li> <li><a href="/wiki/Drums_in_communication" title="Drums in communication">Drums</a></li> <li><a href="/wiki/Edholm%27s_law" title="Edholm's law">Edholm's law</a></li> <li><a href="/wiki/Electrical_telegraph#History" title="Electrical telegraph">Electrical telegraph</a></li> <li><a href="/wiki/Fax#History" title="Fax">Fax</a></li> <li><a href="/wiki/Heliograph#History" title="Heliograph">Heliographs</a></li> <li><a href="/wiki/Hydraulic_telegraph#Greek_hydraulic_semaphore_system" title="Hydraulic telegraph">Hydraulic telegraph</a></li> <li><a href="/wiki/Information_Age" title="Information Age">Information Age</a></li> <li><a href="/wiki/Information_revolution" class="mw-redirect" title="Information revolution">Information revolution</a></li> <li><a href="/wiki/History_of_the_Internet" title="History of the Internet">Internet</a></li> <li><a href="/wiki/Mass_media#History" title="Mass media">Mass media</a></li> <li><a href="/wiki/History_of_mobile_phones" title="History of mobile phones">Mobile phone</a> <ul><li><a href="/wiki/Smartphone" title="Smartphone">Smartphone</a></li></ul></li> <li><a href="/wiki/Optical_communication" title="Optical communication">Optical telecommunication</a></li> <li><a href="/wiki/Optical_telegraph" title="Optical telegraph">Optical telegraphy</a></li> <li><a href="/wiki/Pager" title="Pager">Pager</a></li> <li><a href="/wiki/Photophone" title="Photophone">Photophone</a></li> <li><a href="/wiki/History_of_prepaid_mobile_phones" title="History of prepaid mobile phones">Prepaid mobile phone</a></li> <li><a href="/wiki/History_of_radio" title="History of radio">Radio</a></li> <li><a href="/wiki/Radiotelephone" title="Radiotelephone">Radiotelephone</a></li> <li><a href="/wiki/Communications_satellite" title="Communications satellite">Satellite communications</a></li> <li><a href="/wiki/Semaphore" title="Semaphore">Semaphore</a> <ul><li><a href="/wiki/Phryctoria" title="Phryctoria">Phryctoria</a></li></ul></li> <li><a href="/wiki/Semiconductor" title="Semiconductor">Semiconductor</a> <ul><li><a href="/wiki/Semiconductor_device" title="Semiconductor device">device</a></li> <li><a href="/wiki/MOSFET" title="MOSFET">MOSFET</a></li> <li><a href="/wiki/History_of_the_transistor" title="History of the transistor">transistor</a></li></ul></li> <li><a href="/wiki/Smoke_signal" title="Smoke signal">Smoke signals</a></li> <li><a href="/wiki/History_of_telecommunication" title="History of telecommunication">Telecommunications history</a></li> <li><a href="/wiki/Telautograph" title="Telautograph">Telautograph</a></li> <li><a href="/wiki/Telegraphy" title="Telegraphy">Telegraphy</a></li> <li><a href="/wiki/Teleprinter" title="Teleprinter">Teleprinter</a> (teletype)</li> <li><a href="/wiki/History_of_the_telephone" title="History of the telephone">Telephone</a></li> <li><i><a href="/wiki/The_Telephone_Cases" title="The Telephone Cases">The Telephone Cases</a></i></li> <li><a href="/wiki/History_of_television" title="History of television">Television</a> <ul><li><a href="/wiki/Digital_television" title="Digital television">digital</a></li> <li><a href="/wiki/Streaming_television" title="Streaming television">streaming</a></li></ul></li> <li><a href="/wiki/Submarine_communications_cable#Early_history:_telegraph_and_coaxial_cables" title="Submarine communications cable">Undersea telegraph line</a></li> <li><a href="/wiki/History_of_videotelephony" title="History of videotelephony">Videotelephony</a></li> <li><a href="/wiki/Whistled_language" title="Whistled language">Whistled language</a></li> <li><a href="/wiki/Wireless_revolution" class="mw-redirect" title="Wireless revolution">Wireless revolution</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Pioneers</th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Nasir_Ahmed_(engineer)" title="Nasir Ahmed (engineer)">Nasir Ahmed</a></li> <li><a href="/wiki/Edwin_Howard_Armstrong" title="Edwin Howard Armstrong">Edwin Howard Armstrong</a></li> <li><a href="/wiki/Mohamed_M._Atalla" title="Mohamed M. Atalla">Mohamed M. Atalla</a></li> <li><a href="/wiki/John_Logie_Baird" title="John Logie Baird">John Logie Baird</a></li> <li><a href="/wiki/Paul_Baran" title="Paul Baran">Paul Baran</a></li> <li><a href="/wiki/John_Bardeen" title="John Bardeen">John Bardeen</a></li> <li><a href="/wiki/Alexander_Graham_Bell" title="Alexander Graham Bell">Alexander Graham Bell</a></li> <li><a href="/wiki/Emile_Berliner" title="Emile Berliner">Emile Berliner</a></li> <li><a href="/wiki/Tim_Berners-Lee" title="Tim Berners-Lee">Tim Berners-Lee</a></li> <li><a href="/wiki/Francis_Blake_(inventor)" title="Francis Blake (inventor)">Francis Blake</a></li> <li><a href="/wiki/Jagadish_Chandra_Bose" title="Jagadish Chandra Bose">Jagadish Chandra Bose</a></li> <li><a href="/wiki/Charles_Bourseul" title="Charles Bourseul">Charles Bourseul</a></li> <li><a href="/wiki/Walter_Houser_Brattain" title="Walter Houser Brattain">Walter Houser Brattain</a></li> <li><a href="/wiki/Vint_Cerf" title="Vint Cerf">Vint Cerf</a></li> <li><a href="/wiki/Claude_Chappe" title="Claude Chappe">Claude Chappe</a></li> <li><a href="/wiki/Yogen_Dalal" class="mw-redirect" title="Yogen Dalal">Yogen Dalal</a></li> <li><a href="/wiki/Daniel_Davis_Jr." title="Daniel Davis Jr.">Daniel Davis Jr.</a></li> <li><a href="/wiki/Donald_Davies" title="Donald Davies">Donald Davies</a></li> <li><a href="/wiki/Amos_Dolbear" title="Amos Dolbear">Amos Dolbear</a></li> <li><a href="/wiki/Thomas_Edison" title="Thomas Edison">Thomas Edison</a></li> <li><a href="/wiki/Lee_de_Forest" title="Lee de Forest">Lee de Forest</a></li> <li><a href="/wiki/Philo_Farnsworth" title="Philo Farnsworth">Philo Farnsworth</a></li> <li><a href="/wiki/Reginald_Fessenden" title="Reginald Fessenden">Reginald Fessenden</a></li> <li><a href="/wiki/Elisha_Gray" title="Elisha Gray">Elisha Gray</a></li> <li><a href="/wiki/Oliver_Heaviside" title="Oliver Heaviside">Oliver Heaviside</a></li> <li><a href="/wiki/Robert_Hooke" title="Robert Hooke">Robert Hooke</a></li> <li><a href="/wiki/Erna_Schneider_Hoover" title="Erna Schneider Hoover">Erna Schneider Hoover</a></li> <li><a href="/wiki/Harold_Hopkins_(physicist)" title="Harold Hopkins (physicist)">Harold Hopkins</a></li> <li><a href="/wiki/Gardiner_Greene_Hubbard" title="Gardiner Greene Hubbard">Gardiner Greene Hubbard</a></li> <li><a href="/wiki/List_of_Internet_pioneers" title="List of Internet pioneers">Internet pioneers</a></li> <li><a href="/wiki/Bob_Kahn" class="mw-redirect" title="Bob Kahn">Bob Kahn</a></li> <li><a href="/wiki/Dawon_Kahng" title="Dawon Kahng">Dawon Kahng</a></li> <li><a href="/wiki/Charles_K._Kao" title="Charles K. Kao">Charles K. Kao</a></li> <li><a href="/wiki/Narinder_Singh_Kapany" title="Narinder Singh Kapany">Narinder Singh Kapany</a></li> <li><a href="/wiki/Hedy_Lamarr" title="Hedy Lamarr">Hedy Lamarr</a></li> <li><a href="/wiki/Roberto_Landell_de_Moura" title="Roberto Landell de Moura">Roberto Landell de Moura</a></li> <li><a href="/wiki/Innocenzo_Manzetti" title="Innocenzo Manzetti">Innocenzo Manzetti</a></li> <li><a href="/wiki/Guglielmo_Marconi" title="Guglielmo Marconi">Guglielmo Marconi</a></li> <li><a href="/wiki/Robert_Metcalfe" title="Robert Metcalfe">Robert Metcalfe</a></li> <li><a href="/wiki/Antonio_Meucci" title="Antonio Meucci">Antonio Meucci</a></li> <li><a href="/wiki/Samuel_Morse" title="Samuel Morse">Samuel Morse</a></li> <li><a href="/wiki/Jun-ichi_Nishizawa" title="Jun-ichi Nishizawa">Jun-ichi Nishizawa</a></li> <li><a href="/wiki/Charles_Grafton_Page" title="Charles Grafton Page">Charles Grafton Page</a></li> <li><a href="/wiki/Radia_Perlman" title="Radia Perlman">Radia Perlman</a></li> <li><a href="/wiki/Alexander_Stepanovich_Popov" class="mw-redirect" title="Alexander Stepanovich Popov">Alexander Stepanovich Popov</a></li> <li><a href="/wiki/Tivadar_Pusk%C3%A1s" title="Tivadar Puskás">Tivadar Puskás</a></li> <li><a href="/wiki/Johann_Philipp_Reis" title="Johann Philipp Reis">Johann Philipp Reis</a></li> <li><a href="/wiki/Claude_Shannon" title="Claude Shannon">Claude Shannon</a></li> <li><a href="/wiki/Almon_Brown_Strowger" title="Almon Brown Strowger">Almon Brown Strowger</a></li> <li><a href="/wiki/Henry_Sutton_(inventor)" title="Henry Sutton (inventor)">Henry Sutton</a></li> <li><a href="/wiki/Charles_Sumner_Tainter" title="Charles Sumner Tainter">Charles Sumner Tainter</a></li> <li><a href="/wiki/Nikola_Tesla" title="Nikola Tesla">Nikola Tesla</a></li> <li><a href="/wiki/Camille_Tissot" title="Camille Tissot">Camille Tissot</a></li> <li><a href="/wiki/Alfred_Vail" title="Alfred Vail">Alfred Vail</a></li> <li><a href="/wiki/Thomas_A._Watson" title="Thomas A. Watson">Thomas A. Watson</a></li> <li><a href="/wiki/Charles_Wheatstone" title="Charles Wheatstone">Charles Wheatstone</a></li> <li><a href="/wiki/Vladimir_K._Zworykin" title="Vladimir K. Zworykin">Vladimir K. Zworykin</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Transmission_medium" title="Transmission medium">Transmission<br />media</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Coaxial_cable" title="Coaxial cable">Coaxial cable</a></li> <li><a href="/wiki/Fiber-optic_communication" title="Fiber-optic communication">Fiber-optic communication</a> <ul><li><a href="/wiki/Optical_fiber" title="Optical fiber">optical fiber</a></li></ul></li> <li><a href="/wiki/Free-space_optical_communication" title="Free-space optical communication">Free-space optical communication</a></li> <li><a href="/wiki/Molecular_communication" title="Molecular communication">Molecular communication</a></li> <li><a href="/wiki/Radio_wave" title="Radio wave">Radio waves</a> <ul><li><a href="/wiki/Wireless" title="Wireless">wireless</a></li></ul></li> <li><a href="/wiki/Transmission_line" title="Transmission line">Transmission line</a> <ul><li><a href="/wiki/Telecommunication_circuit" title="Telecommunication circuit">telecommunication circuit</a></li></ul></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Network_topology" title="Network topology">Network topology</a><br />and switching</th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Bandwidth_(computing)" title="Bandwidth (computing)">Bandwidth</a></li> <li><a href="/wiki/Telecommunications_link" title="Telecommunications link">Links</a></li> <li><a href="/wiki/Node_(networking)" title="Node (networking)">Nodes</a> <ul><li><a href="/wiki/Terminal_(telecommunication)" title="Terminal (telecommunication)">terminal</a></li></ul></li> <li><a href="/wiki/Network_switch" title="Network switch">Network switching</a> <ul><li><a href="/wiki/Circuit_switching" title="Circuit switching">circuit</a></li> <li><a href="/wiki/Packet_switching" title="Packet switching">packet</a></li></ul></li> <li><a class="mw-selflink selflink">Telephone exchange</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Multiplexing" title="Multiplexing">Multiplexing</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Space-division_multiple_access" title="Space-division multiple access">Space-division</a></li> <li><a href="/wiki/Frequency-division_multiplexing" title="Frequency-division multiplexing">Frequency-division</a></li> <li><a href="/wiki/Time-division_multiplexing" title="Time-division multiplexing">Time-division</a></li> <li><a href="/wiki/Polarization-division_multiplexing" title="Polarization-division multiplexing">Polarization-division</a></li> <li><a href="/wiki/Orbital_angular_momentum_multiplexing" title="Orbital angular momentum multiplexing">Orbital angular-momentum</a></li> <li><a href="/wiki/Code-division_multiple_access" title="Code-division multiple access">Code-division</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Concepts</th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Communication_protocol" title="Communication protocol">Communication protocol</a></li> <li><a href="/wiki/Computer_network" title="Computer network">Computer network</a></li> <li><a href="/wiki/Data_communication" title="Data communication">Data transmission</a></li> <li><a href="/wiki/Store_and_forward" title="Store and forward">Store and forward</a></li> <li><a href="/wiki/Telecommunications_equipment" title="Telecommunications equipment">Telecommunications equipment</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Telecommunications_network" title="Telecommunications network">Types of network</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Cellular_network" title="Cellular network">Cellular network</a></li> <li><a href="/wiki/Ethernet" title="Ethernet">Ethernet</a></li> <li><a href="/wiki/Integrated_Services_Digital_Network" class="mw-redirect" title="Integrated Services Digital Network">ISDN</a></li> <li><a href="/wiki/Local_area_network" title="Local area network">LAN</a></li> <li><a href="/wiki/Mobile_telephony" title="Mobile telephony">Mobile</a></li> <li><a href="/wiki/Next-generation_network" title="Next-generation network">NGN</a></li> <li><a href="/wiki/Public_switched_telephone_network" title="Public switched telephone network">Public Switched Telephone</a></li> <li><a href="/wiki/Radio_network" title="Radio network">Radio</a></li> <li><a href="/wiki/Television_broadcasting" class="mw-redirect" title="Television broadcasting">Television</a></li> <li><a href="/wiki/Telex" title="Telex">Telex</a></li> <li><a href="/wiki/UUCP" title="UUCP">UUCP</a></li> <li><a href="/wiki/Wide_area_network" title="Wide area network">WAN</a></li> <li><a href="/wiki/Wireless_network" title="Wireless network">Wireless network</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Telecommunications_network" title="Telecommunications network">Notable networks</a></th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/ARPANET" title="ARPANET">ARPANET</a></li> <li><a href="/wiki/BITNET" title="BITNET">BITNET</a></li> <li><a href="/wiki/CYCLADES" title="CYCLADES">CYCLADES</a></li> <li><a href="/wiki/FidoNet" title="FidoNet">FidoNet</a></li> <li><a href="/wiki/Internet" title="Internet">Internet</a></li> <li><a href="/wiki/Internet2" title="Internet2">Internet2</a></li> <li><a href="/wiki/JANET" title="JANET">JANET</a></li> <li><a href="/wiki/NPL_network" title="NPL network">NPL network</a></li> <li><a href="/wiki/Toasternet" title="Toasternet">Toasternet</a></li> <li><a href="/wiki/Usenet" title="Usenet">Usenet</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Locations</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Category:Telecommunications_in_Africa" title="Category:Telecommunications in Africa">Africa</a></li> <li>Americas <ul><li><a href="/wiki/Category:Telecommunications_in_North_America" title="Category:Telecommunications in North America">North</a></li> <li><a href="/wiki/Category:Telecommunications_in_South_America" title="Category:Telecommunications in South America">South</a></li></ul></li> <li><a href="/wiki/Category:Communications_in_Antarctica" title="Category:Communications in Antarctica">Antarctica</a></li> <li><a href="/wiki/Category:Telecommunications_in_Asia" title="Category:Telecommunications in Asia">Asia</a></li> <li><a href="/wiki/Category:Telecommunications_in_Europe" title="Category:Telecommunications in Europe">Europe</a></li> <li><a href="/wiki/Category:Telecommunications_in_Oceania" title="Category:Telecommunications in Oceania">Oceania</a></li> <li>(<a href="/wiki/List_of_telecommunications_regulatory_bodies" title="List of telecommunications regulatory bodies">Global telecommunications regulation bodies</a>)</li></ul> </div></td></tr><tr><td class="navbox-abovebelow" colspan="2"><div> <ul><li><span class="nowrap"><span class="noviewer" typeof="mw:File"><span><img alt="" src="//upload.wikimedia.org/wikipedia/commons/thumb/4/4e/Telecom-icon.svg/16px-Telecom-icon.svg.png" decoding="async" width="16" height="16" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/4/4e/Telecom-icon.svg/24px-Telecom-icon.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/4/4e/Telecom-icon.svg/32px-Telecom-icon.svg.png 2x" data-file-width="500" data-file-height="500" /></span></span> </span><a href="/wiki/Portal:Telecommunication" title="Portal:Telecommunication">Telecommunication portal</a></li> <li><span class="noviewer" typeof="mw:File"><span title="Category"><img alt="" src="//upload.wikimedia.org/wikipedia/en/thumb/9/96/Symbol_category_class.svg/16px-Symbol_category_class.svg.png" decoding="async" width="16" height="16" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/en/thumb/9/96/Symbol_category_class.svg/23px-Symbol_category_class.svg.png 1.5x, //upload.wikimedia.org/wikipedia/en/thumb/9/96/Symbol_category_class.svg/31px-Symbol_category_class.svg.png 2x" data-file-width="180" data-file-height="185" /></span></span> <a href="/wiki/Category:Telecommunications" 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