Telecommunication - Multiple Access, Radio, Networks

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Three schemes have been devised for efficient sharing of a single channel under these conditions; they are called frequency-division multiple access (FDMA), time-division multiple access (TDMA), and code-division multiple access (CDMA). 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border-right-sm border-left-sm open"> <div class="drawer d-flex flex-column open"> <div class="left-rail-section-content"> <div class="topic-left-rail-header text-truncate bg-gray-50 position-relative text-right d-flex align-items-center"> <div class="tlr-title px-20 py-15 text-left"> <em class="material-icons text-gray-400 d-lg-none" data-icon="toc"></em> <a class="font-serif font-weight-bold text-black link-blue" href="https://www.britannica.com/technology/telecommunication">telecommunication</a> </div> <button aria-label="Close" class="js-sections-close-button btn-link btn-sm btn d-lg-none position-absolute top-0 p-10 right-0" > <em class="material-icons font-26" data-icon="close"></em> </button> </div> <div class="section-content pl-10 pr-20 pl-sm-50 pr-sm-60 pl-lg-5 pr-lg-10 pt-10 pt-lg-0 bg-gray-50 clear-catfish-ad"> <div class="toc mb-20"> <div class="font-serif font-14 font-weight-bold mx-15 mb-15 mt-20"> Table of Contents </div> <ul class="list-unstyled my-0" data-level="h1"><li data-target="#ref1"><div class="pl-25"><a class="link-gray-900 w-100" href="/technology/telecommunication">Introduction</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref76266"><div class="d-flex align-items-center"><button class="h1-link-drawer-button btn btn-xs btn-circle d-flex rounded" type="button" aria-label="Toggle Heading"><em class="material-icons font-18" data-icon="keyboard_arrow_right"></em></button><a class="w-100 link-gray-900" href="/technology/telecommunication#ref76266">Analog-to-digital conversion</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref76267"><a class="w-100 link-gray-900" href="/technology/telecommunication#ref76267">Sampling</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref76268"><a class="w-100 link-gray-900" href="/technology/telecommunication#ref76268">Quantization</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref76269"><a class="w-100 link-gray-900" href="/technology/telecommunication/Bit-mapping">Bit mapping</a></li></ul></div></li><li data-target="#ref76270"><div class="d-flex align-items-center"><button class="h1-link-drawer-button btn btn-xs btn-circle d-flex rounded" type="button" aria-label="Toggle Heading"><em class="material-icons font-18" data-icon="keyboard_arrow_right"></em></button><a class="w-100 link-gray-900" href="/technology/telecommunication/Bit-mapping#ref76270">Source encoding</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref76271"><a class="w-100 link-gray-900" href="/technology/telecommunication/Bit-mapping#ref76271">Huffman codes</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref76272"><a class="w-100 link-gray-900" href="/technology/telecommunication/Bit-mapping#ref76272">The Lempel-Ziv algorithm</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref76273"><a class="w-100 link-gray-900" href="/technology/telecommunication/Bit-mapping#ref76273">Run-length codes</a></li></ul></div></li><li data-target="#ref76274"><div class="d-flex align-items-center"><button class="h1-link-drawer-button btn btn-xs btn-circle d-flex rounded" type="button" aria-label="Toggle Heading"><em class="material-icons font-18" data-icon="keyboard_arrow_right"></em></button><a class="w-100 link-gray-900" href="/technology/telecommunication/Channel-encoding">Channel encoding</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref76275"><a class="w-100 link-gray-900" href="/technology/telecommunication/Channel-encoding#ref76275">Repetition codes</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref76276"><a class="w-100 link-gray-900" href="/technology/telecommunication/Channel-encoding#ref76276">The Hamming code</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref76277"><a class="w-100 link-gray-900" href="/technology/telecommunication/Channel-encoding#ref76277">Convolutional encoding</a></li></ul></div></li><li data-target="#ref76278"><div class="d-flex align-items-center"><button class="h1-link-drawer-button btn btn-xs btn-circle d-flex rounded" type="button" aria-label="Toggle Heading"><em class="material-icons font-18" data-icon="keyboard_arrow_right"></em></button><a class="w-100 link-gray-900" href="/technology/telecommunication/Modulation">Modulation</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref76279"><a class="w-100 link-gray-900" href="/technology/telecommunication/Modulation#ref76279">Analog modulation</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref76280"><a class="w-100 link-gray-900" href="/technology/telecommunication/Modulation#ref76280">Digital modulation</a><ul class="list-unstyled" data-level="h3"><li data-target="#ref76281"><a class="w-100 link-gray-900" href="/technology/telecommunication/Modulation#ref76281">Amplitude-shift keying</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref76282"><a class="w-100 link-gray-900" href="/technology/telecommunication/Modulation#ref76282">Frequency-shift keying</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref76283"><a class="w-100 link-gray-900" href="/technology/telecommunication/Modulation#ref76283">Phase-shift keying</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref76284"><a class="w-100 link-gray-900" href="/technology/telecommunication/Modulation#ref76284">Advanced methods</a></li></ul></li></ul></div></li><li data-target="#ref76285"><div class="d-flex align-items-center"><button class="h1-link-drawer-button btn btn-xs btn-circle d-flex rounded" type="button" aria-label="Toggle Heading"><em class="material-icons font-18" data-icon="keyboard_arrow_right"></em></button><a class="w-100 link-gray-900" href="/technology/telecommunication/Multiplexing">Multiplexing</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref76286"><a class="w-100 link-gray-900" href="/technology/telecommunication/Multiplexing#ref76286">Frequency-division multiplexing</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref76287"><a class="w-100 link-gray-900" href="/technology/telecommunication/Multiplexing#ref76287">Time-division multiplexing</a></li></ul></div></li><li data-target="#ref76288"><div class="d-flex align-items-center"><button class="h1-link-drawer-button btn btn-xs btn-circle d-flex rounded" type="button" aria-label="Toggle Heading"><em class="material-icons font-18" data-icon="keyboard_arrow_right"></em></button><a class="w-100 link-gray-900" href="/technology/telecommunication/Multiple-access">Multiple access</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref76289"><a class="w-100 link-gray-900" href="/technology/telecommunication/Multiple-access#ref76289">Frequency-division multiple access</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref76290"><a class="w-100 link-gray-900" href="/technology/telecommunication/Multiple-access#ref76290">Time-division multiple access</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref76291"><a class="w-100 link-gray-900" href="/technology/telecommunication/Multiple-access#ref76291">Code-division multiple access</a></li></ul></div></li></ul> <a class="toc-extra-link link-gray-900" href="https://www.britannica.com/technology/telecommunication/additional-info">References & Edit History</a> <a class="toc-extra-link link-gray-900" href="/facts/telecommunication">Quick Facts & Related Topics</a> </div> <div class="tlr-media-slider pb-10 mb-30"> <a 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src="https://cdn.britannica.com/16/4616-004-39B3985A/steps-conversion-analog-signal-intervals-receiver-value.jpg" alt="Basic steps in analog-to-digital conversionAn analog signal is sampled at regular intervals. The amplitude at each interval is quantized, or assigned a value, and the values are mapped into a series of binary digits, or bits. The information is transmitted as a digital signal to the receiver, where it is decoded and the analog signal reconstituted." height="50" /> </a> <a href="https://cdn.britannica.com/18/4618-050-5E36758C/signal-modulation-methods-binary-digits-amplitudes-series.jpg" data-href="/media/1/585799/3693" class="media-overlay-link d-inline-block mr-5"> <img loading="lazy" src="https://cdn.britannica.com/18/4618-004-79CDE7BF/signal-modulation-methods-binary-digits-amplitudes-series.jpg" alt="digital signal modulation" height="50" /> </a> <a href="https://cdn.britannica.com/19/4619-004-41851F3A/multiplexing-North-American-frequency-division-telephone-system-voice.jpg" data-href="/media/1/585799/3694" class="media-overlay-link d-inline-block mr-5"> <img loading="lazy" src="https://cdn.britannica.com/19/4619-004-41851F3A/multiplexing-North-American-frequency-division-telephone-system-voice.jpg" alt="Analog multiplexing, as employed in the North American telephone systemIn frequency-division multiplexing (FDM), 12 separate voice signals, each of 4-kilohertz bandwidth, are modulated onto carrier waves in the 60–108-kilohertz range. 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Let us know if you have suggestions to improve this article (requires login). </div> <div class="type-menu"> <label for="feedback-type" class="label mb-10">Feedback Type</label> <select id="feedback-type" class="form-select mb-30" name="feedbackTypeId" required> <option value="" selected="selected">Select a type (Required)</option> <option value="1">Factual Correction</option> <option value="2">Spelling/Grammar Correction</option> <option value="3">Link Correction</option> <option value="4">Additional Information</option> <option value="5">Other</option> </select> </div> <label for="feedback" class="label mb-10">Your Feedback</label> <textarea id="feedback" class="form-control mb-30" name="feedback" maxlength="3000" rows="7" required></textarea> <button class="btn btn-blue" type="submit">Submit Feedback</button> </form> <div class="success-messaging d-none mt-30"> <div class="title">Thank you for your feedback</div> <p>Our editors will review what you’ve submitted and determine whether to revise the article.</p> </div> </div> </div> <div class="md-websites-modal size-lg d-none"> <div class="md-modal-body"> <div class="h2 font-serif pb-15 border-bottom font-weight-bold"> External Websites </div> <div class="pb-20"> <ul class="list-unstyled mt-20 lh-lg"> <li><a class="external" href="https://www.umsl.edu/~joshik/msis480/chapt07.htm" target="_blank" rel="noopener ">University of Missouri-St. Louis - Telecommunications, the Internet, and Information System Architecture</a></li> <li><a class="external" href="https://www.academia.edu/4768572/Telecommunication_Introduction" target="_blank" rel="noopener ">Academia - Telecommunication Introduction</a></li> <li><a class="external" href="https://www.eolss.net/sample-chapters/c05/e6-108-02-00.pdf" target="_blank" rel="noopener ">UNESCO-EOLSS - Fundamentals of Telecommunications</a></li> <li><a class="external" href="https://ops.fhwa.dot.gov/publications/telecomm_handbook/chapter2_01.htm" target="_blank" rel="noopener ">Federal Highway Administration - Office of Operations - Fundamentals of Telecommunications</a></li> <li><a class="external" href="https://pmc.ncbi.nlm.nih.gov/articles/PMC4296789/" target="_blank" rel="noopener ">National Center for Biotechnology Information - PubMed Central - Telecommunications and Internet Broadband Policy: Sorting Out the Pieces for Telerehabilitation</a></li> <li><a class="external" href="https://www.theiet.org/media/11266/selected-landmarks-in-telecommunications.pdf" target="_blank" rel="noopener ">The Institute of Engineering and Technology - Selected landmarks in The History of Telecommunications</a></li> <li><a class="external" href="https://workforce.libretexts.org/Bookshelves/Information_Technology/Computer_Applications/Introduction_to_Computer_Applications_and_Concepts_(Lumen)/07%3A_Networks_and_Security/7.02%3A_Reading-_Networks_and_Telecommunications" target="_blank" rel="noopener ">Workforce LibreTexts - Reading- Networks and Telecommunications</a></li> <li><a class="external" href="https://utsa.pressbooks.pub/networking/chapter/history-of-telecommunications/" target="_blank" rel="noopener ">UTSA Pressbooks - History of Telecommunications</a></li> <li><a class="external" href="https://www.thecanadianencyclopedia.ca/en/article/telecommunications" target="_blank" rel="noopener ">The Canadian Encyclopedia - Telecommunication</a></li> </ul> </div> <div class="md-websites-ebk-title">Britannica Websites</div> <div class="md-websites-ebk-subtitle">Articles from Britannica Encyclopedias for elementary and high school students.</div> <ul class="list-unstyled bps-topic-web-sites lh-lg"> <li><a class="external" href="https://kids.britannica.com/kids/article/telecommunication/390860" target="_blank" rel="noopener">telecommunication - Children's Encyclopedia (Ages 8-11)</a></li> <li><a class="external" href="https://kids.britannica.com/students/article/telecommunication/277287" target="_blank" rel="noopener">telecommunication - Student Encyclopedia (Ages 11 and up)</a></li> </ul> </div> </div> </div> </div> <div class="toc-header-marker"></div> <button class="ai-ask-button btn btn-sm border-2 btn-outline-red-400 border-red-400 d-none mr-0 mr-lg-10 ml-5 ml-sm-10 ml-lg-0 p-10 px-sm-5 px-md-10 js-header-ai-ask-button"> Ask the Chatbot a Question </button> <div class="caption alternate-titles">Also known as: electronic communication</div> <div class="md-byline module-spacing "> <div class="font-serif font-12"> <span class="written-by text-gray-700"> Written by </span> <div class="editor-popover popover p-0"> <a class="d-block p-20 qa-editor-popup gtm-byline font-12 byline-contributor" href="/contributor/James-S-Lehnert/4193" > <div class="editor-title font-16 font-weight-bold">James S. Lehnert</div> <div class="editor-description font-12 font-serif mt-5 clamp-description text-black">Associate Professor of Electrical and Computer Engineering, Purdue University, West Lafayette, Indiana. Author of <i>Direct-Sequence Spread-Spectrum Signaling with Applications to Packet Radio Systems.</i></div> </a> <div data-popper-arrow></div> </div> <span class="btn btn-link editor-link p-0 qa-byline-link gtm-byline font-12 byline-contributor text-decoration-underline"> James S. Lehnert</span>, <div class="editor-popover popover p-0"> <a class="d-block p-20 qa-editor-popup gtm-byline font-12 byline-contributor" href="/contributor/David-E-Borth/4163" > <div class="editor-title font-16 font-weight-bold">David E. Borth</div> <div class="editor-description font-12 font-serif mt-5 clamp-description text-black">Corporate Vice President and Director, Communications Research Laboratories, Motorola Inc., Schaumburg, Illinois. Coauthor of <i>Introduction to Spread Spectrum Communications.</i></div> </a> <div data-popper-arrow></div> </div> <span class="btn btn-link editor-link p-0 qa-byline-link gtm-byline font-12 byline-contributor text-decoration-underline"> David E. Borth</span><span class="text-gray-700 mx-5">•</span><a class="see-all border-gray-700 gtm-byline" rel="nofollow" href="https://www.britannica.com/technology/telecommunication/additional-info#contributors">All</a> </div> <div class="font-serif font-12 text-gray-700"> <span class="qa-fact-checked-by">Fact-checked by</span> <div class="editor-popover popover p-0"> <a class="d-block p-20 qa-editor-popup font-12" href="/editor/The-Editors-of-Encyclopaedia-Britannica/4419" > <div class="editor-title font-16 font-weight-bold">The Editors of Encyclopaedia Britannica</div> <div class="editor-description font-12 font-serif mt-5 text-black">Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree. They write new content and verify and edit content received from contributors.</div> </a> <div data-popper-arrow></div> </div> <span class="btn btn-link editor-link p-0 qa-byline-link font-12 "> The Editors of Encyclopaedia Britannica</span></div> <div class="last-updated font-12 font-serif"> <span class="text-gray-700"> Last Updated: <time datetime="2025-01-31T00:00:00CST" >Jan 31, 2025</time> •</span> <a class="byline-edit-history" href="https://www.britannica.com/technology/telecommunication/additional-info#history" rel="nofollow">Article History</a> </div></div> </div> <button class="d-flex d-lg-none btn btn-outline-blue border rounded-sm shadow-sm mobile-toc-button gtm-mobile-toc-inline-button d-none d-sm-block js-sections-inline-button module-spacing btn d-lg-none"> <em class="material-icons mr-5 ml-n10 my-n5 md-icon" data-icon="toc"></em> Table of Contents </button> <div class="d-flex d-sm-none flex-row"> <button class="d-flex d-lg-none btn btn-outline-blue border rounded-sm shadow-sm mobile-toc-button gtm-mobile-toc-inline-button js-sections-inline-button module-spacing"> <em class="material-icons mr-5 ml-n10 my-n5 md-icon" data-icon="toc"></em> Table of Contents </button> <button class="ai-ask-button btn btn-sm border-2 btn-outline-red-400 border-red-400 module-spacing js-inline-ai-ask-button p-10 ml-5"> Ask the Chatbot </button> </div> <div class="js-qf-module qf-module px-40 px-sm-20 py-15 mx-auto module-spacing font-14 bg-gray-50 rounded"> <div class="facts-list mt-10"> <div class=""> <div class="js-fact mb-10 line-clamp clamp-3"> <dl> <dt>Key People: </dt> <dd><a href="/biography/Guglielmo-Marconi" topicid="364287">Guglielmo Marconi</a></dd> <dd><a href="/money/Carlos-Slim-Helu" topicid="915128">Carlos Slim Helú</a></dd> <dd><a href="/biography/Saad-al-Hariri" topicid="1383553">Saad al-Hariri</a></dd> <dd><a href="/money/Morita-Akio" topicid="392429">Morita Akio</a></dd> <dd><a href="/money/Anousheh-Ansari" topicid="1267667">Anousheh Ansari</a></dd> </dl> <button class="js-more-btn d-none btn btn-unstyled font-12 bg-gray-50" aria-label="Toggle more/less fact data"> <em class="js-content link-blue">(Show more)</em> </button> </div> </div> <div class=""> <div class="js-fact mb-10 line-clamp clamp-3"> <dl> <dt>Related Topics: </dt> <dd><a href="/technology/television-technology" topicid="1262241">television</a></dd> <dd><a href="/technology/broadcasting" topicid="80543">broadcasting</a></dd> <dd><a href="/technology/telephone" topicid="585993">telephone</a></dd> <dd><a href="/technology/Internet" topicid="291494">Internet</a></dd> <dd><a href="/technology/radio-technology" topicid="1262240">radio technology</a></dd> </dl> <button class="js-more-btn d-none btn btn-unstyled font-12 bg-gray-50" aria-label="Toggle more/less fact data"> <em class="js-content link-blue">(Show more)</em> </button> </div> <div class="text-center"> <a class="btn btn-sm btn-link p-0" href="/facts/telecommunication"> See all related content </a> </div> </div> </div> </div><span class="marker before-article"></span><span class="marker h7"></span><section data-level="1" id="ref76288"> <span class="marker PREMOD1 mod-inline"></span><p class="topic-paragraph">Multiplexing is defined as the sharing of a communications channel through local combining at a common point. In many cases, however, the communications channel must be efficiently shared among many users that are geographically distributed and that sporadically attempt to communicate at random points in time. Three schemes have been <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="devised" href="https://www.britannica.com/dictionary/devised" data-type="EB">devised</a> for efficient sharing of a single channel under these conditions; they are called frequency-division multiple access (FDMA), time-division multiple access (TDMA), and code-division multiple access (CDMA). These techniques can be used alone or together in <a href="https://www.britannica.com/technology/telephone" class="md-crosslink autoxref " data-show-preview="true">telephone</a> systems, and they are well illustrated by the most advanced mobile cellular systems.</p><span class="marker MOD1 mod-inline"></span> <section data-level="2" id="ref76289"><h2 class="h2"><span id="ref608235"></span>Frequency-division multiple access</h2> <span class="marker PREMOD2 mod-inline"></span><p class="topic-paragraph">In FDMA the goal is to divide the <a href="https://www.britannica.com/science/frequency-physics" class="md-crosslink autoxref " data-show-preview="true">frequency</a> spectrum into slots and then to separate the signals of different users by placing them in separate frequency slots. The difficulty is that the frequency spectrum is limited and that there are typically many more potential communicators than there are available frequency slots. In order to make efficient use of the communications channel, a system must be devised for managing the available slots. In the <span id="ref608236"></span><a href="https://www.britannica.com/technology/advanced-mobile-phone-system" class="md-crosslink ">advanced mobile phone system</a> (AMPS), the <span id="ref608237"></span><a href="https://www.britannica.com/technology/cell-phone" class="md-crosslink " data-show-preview="true">cellular</a> system employed in the <a href="https://www.britannica.com/place/United-States" class="md-crosslink autoxref " data-show-preview="true">United States</a>, different callers use separate frequency slots via FDMA. When one telephone call is completed, a network-managing computer at the cellular base station reassigns the released frequency slot to a new caller. A key goal of the AMPS system is to reuse frequency slots whenever possible in order to accommodate as many callers as possible. Locally within a cell, frequency slots can be reused when corresponding calls are terminated. In addition, frequency slots can be used simultaneously by multiple callers located in separate cells. The cells must be far enough apart geographically that the <a href="https://www.britannica.com/topic/radio" class="md-crosslink autoxref " data-show-preview="true">radio</a> signals from one cell are sufficiently <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="attenuated" href="https://www.merriam-webster.com/dictionary/attenuated" data-type="MW">attenuated</a> at the location of the other cell using the same frequency slot.</p><span class="marker MOD2 mod-inline"></span></section> <section data-level="2" id="ref76290"><h2 class="h2"><span id="ref608238"></span><a href="https://www.britannica.com/technology/time-division-multiple-access" class="md-crosslink ">Time-division multiple access</a></h2> <span class="marker PREMOD3 mod-inline"></span><p class="topic-paragraph">In TDMA the goal is to divide time into slots and separate the signals of different users by placing the signals in separate time slots. The difficulty is that requests to use a single communications channel occur randomly, so that on occasion the number of requests for time slots is greater than the number of available slots. In this case information must be buffered, or stored in memory, until time slots become available for transmitting the data. The buffering introduces delay into the system. In the <span id="ref608239"></span>IS54 cellular system, three digital signals are interleaved using TDMA and then transmitted in a 30-kilohertz frequency slot that would be occupied by one <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="analog" href="https://www.merriam-webster.com/dictionary/analog" data-type="MW">analog</a> signal in AMPS. Buffering digital signals and interleaving them in time causes some extra delay, but the delay is so brief that it is not ordinarily noticed during a call. The IS54 system uses aspects of both TDMA and FDMA.</p><span class="marker MOD3 mod-inline"></span></section> <section data-level="2" id="ref76291"><h2 class="h2"><span id="ref608240"></span><a href="https://www.britannica.com/technology/code-division-multiple-access" class="md-crosslink ">Code-division multiple access</a></h2> <span class="marker PREMOD4 mod-inline"></span><p class="topic-paragraph">In CDMA, signals are sent at the same time in the same frequency band. Signals are either selected or rejected at the <a href="https://www.britannica.com/technology/receiver" class="md-crosslink autoxref " data-show-preview="true">receiver</a> by recognition of a user-specific signature waveform, which is constructed from an assigned spreading code. The <span id="ref608241"></span>IS95 cellular system employs the CDMA technique. In IS95 an analog <a href="https://www.britannica.com/topic/speech-language" class="md-crosslink autoxref " data-show-preview="true">speech</a> signal that is to be sent to a cell site is first quantized and then organized into one of a number of digital frame structures. In one frame structure, a frame of 20 milliseconds’ duration consists of 192 bits. Of these 192 bits, 172 represent the speech signal itself, 12 form a cyclic <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="redundancy" href="https://www.merriam-webster.com/dictionary/redundancy" data-type="MW">redundancy</a> check that can be used for error <a href="https://www.britannica.com/technology/detection" class="md-crosslink autoxref " data-show-preview="true">detection</a>, and 8 form an encoder “tail” that allows the decoder to work properly. These bits are formed into an encoded data stream. After interleaving of the encoded data stream, bits are organized into groups of six. Each group of six bits indicates which of 64 possible waveforms to transmit. Each of the waveforms to be transmitted has a particular pattern of alternating polarities and occupies a certain portion of the radio-frequency spectrum. Before one of the waveforms is transmitted, however, it is multiplied by a code sequence of polarities that <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="alternate" href="https://www.britannica.com/dictionary/alternate" data-type="EB">alternate</a> at a rate of 1.2288 megahertz, spreading the <a href="https://www.britannica.com/technology/bandwidth" class="md-crosslink autoxref " data-show-preview="true">bandwidth</a> occupied by the signal and causing it to occupy (after filtering at the transmitter) about 1.23 megahertz of the radio-frequency spectrum. At the cell site one user can be selected from multiple users of the same 1.23-megahertz bandwidth by its assigned code sequence.</p><span class="marker MOD4 mod-inline"></span> <span class="marker PREMOD5 mod-inline"></span><p class="topic-paragraph">CDMA is sometimes referred to as <span id="ref608242"></span><a href="https://www.britannica.com/technology/spread-spectrum-multiple-access" class="md-crosslink ">spread-spectrum multiple access</a> (SSMA), because the process of multiplying the signal by the code sequence causes the power of the transmitted signal to be spread over a larger bandwidth. Frequency management, a necessary feature of FDMA, is eliminated in CDMA. When another user wishes to use the communications channel, it is assigned a code and immediately <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="transmits" href="https://www.britannica.com/dictionary/transmits" data-type="EB">transmits</a> instead of being stored until a frequency slot opens.</p><div class="module-spacing"> </div><span class="marker MOD5 mod-inline"></span> <span class="md-signature font-12"><a href="/contributor/James-S-Lehnert/4193">James S. Lehnert</a></span> <span class="md-signature font-12"><a href="/contributor/Wayne-Eric-Stark/4216">Wayne Eric Stark</a></span> <span class="md-signature font-12"><a href="/contributor/David-E-Borth/4163">David E. 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Telecommunication - Multiple Access, Radio, Networks | Britannica