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Statistical time-division multiplexing
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<h2 id="comparison-with-static-tdm">Comparison with static TDM</h2> <p>Time domain statistical multiplexing (packet mode communication) is similar to <a href="/facts/Time-division_multiplexing/tv7bMh1v">time-division multiplexing</a> (TDM), except that, rather than assigning a data stream to the same recurrent time slot in every <a href="/facts/Time-division_multiplexing/tv7bMh1v">TDM</a>, each data stream is assigned time slots (of fixed length) or <a href="/facts/Data_frame/ltGCbJIq">data frames</a> (of variable lengths) that often appear to be scheduled in a randomized order, and experience varying delay (while the delay is fixed in TDM). </p><p>Statistical multiplexing allows the bandwidth to be divided arbitrarily among a variable number of channels (while the number of channels and the channel data rate are fixed in TDM). </p><p>Statistical multiplexing ensures that slots will not be wasted (whereas TDM can waste slots). The transmission capacity of the link will be shared by only those users who have packets. </p><p>Static TDM and other circuit switching is carried out at the <a href="/facts/Physical_layer/WuATCrVw">physical layer</a> in the <a href="/facts/OSI_model/oxN9Xhjz">OSI model</a> and <a href="/facts/TCP%2fIP_model/KNkxtSpw">TCP/IP model</a>, while statistical multiplexing is carried out at the <a href="/facts/Data_link_layer/K9yPufbt">data link layer</a> and above. </p> <h2 id="channel-identification">Channel identification</h2> <p>In statistical multiplexing, each packet or frame contains a channel/data stream identification number, or (in the case of <a href="/facts/Datagram/TMHVctMz">datagram</a> communication) complete destination address information. </p> <h2 id="usage">Usage</h2> <p>Examples of statistical multiplexing are: </p> <ul><li>The <a href="/facts/MPEG_transport_stream/2wBLnyeD">MPEG transport stream</a> for <a href="/facts/Digital_TV/KQCyILQa">digital TV</a> transmission. Statistical multiplexing is used to allow several video, audio and data streams of different data rates to be transmitted over a bandwidth-limited channel (see <a href="/facts/Statistical_multiplexer/rLGcaGHV">Statistical multiplexer</a>). The packets have constant lengths. The channel number is denoted Program ID (PID).</li> <li>The <a href="/facts/User_Datagram_Protocol/dxLU0D6q">UDP</a> and <a href="/facts/Transmission_Control_Protocol/WW4sPrt9">TCP</a> protocols, where data streams from several application processes are multiplexed together. The packets may have varying lengths. The <a href="/facts/Port_number/ygoHVKNC">port numbers</a> constitute channel identification numbers (and also address information).</li> <li>The <a href="/facts/X.25/yaGPqw2Y">X.25</a> and <a href="/facts/Frame_Relay/bFdv0qIP">Frame Relay</a> packet-switching protocols, where the packets have varying lengths, and the channel number is denoted virtual connection identifier (VCI). The international collection of <a href="/facts/X.25/yaGPqw2Y">X.25</a> providers, using the <a href="/facts/X.25/yaGPqw2Y">X.25</a> <a href="/facts/Protocol_suite/Btj9FtwJ">protocol suite</a> was colloquially known as "the Packet switched network" in the 1980s and into the beginning of the 1990s.</li> <li>The <a href="/facts/Asynchronous_Transfer_Mode/SzA8ewFL">Asynchronous Transfer Mode</a> packet-switched protocol, where the packets have fixed length. The channel identification number consists of a <a href="/facts/Virtual_circuit_identifier/SzA8ewFL">virtual connection identifier</a> (VCI) and a <a href="/facts/Asynchronous_Transfer_Mode/SzA8ewFL">Virtual Path Identifier</a> (VPI).</li></ul> <h2 id="statistical-multiplexer">Statistical multiplexer</h2> <p>In digital audio and video broadcasting, for example, a statistical multiplexer is a content aggregating device that allows broadcasters to provide the greatest number of audio or video services for a given bandwidth by sharing a pool of fixed bandwidth among multiple services or streams of varying bitrates. The multiplexer allocates to each service the bandwidth required for its real-time needs so that services with complex scenes receive more bandwidth than services with less complex ones. This bandwidth sharing technique produces the best video quality at the lowest possible aggregate bandwidth. </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Fragmentation_(computing)/SflHry2p">Data fragmentation</a></li> <li><a href="/facts/Dynamic_bandwidth_allocation/a9t97IU9">Dynamic bandwidth allocation</a></li> <li><a href="/facts/Dynamic_TDMA/ue5UzN7B">Dynamic TDMA</a></li> <li><a href="/facts/Packet_(information_technology)/xCYf8SJu">Packet</a></li> <li><a href="/facts/Packet_switching/olDlDP1R">Packet switching</a></li></ul> <h2 id="external-links">External links</h2> <ul><li><a href="http://igorfuna.com/dvb-t/slovenia/multiplex-a-usage-chart">Example of Statistical Multiplexing</a> (Chart from a real DVB-T multiplex)</li></ul>