| Multiplexing |
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| Analog modulation |
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Statistical multiplexing is a type of digital communication link sharing, sometimes abbreviated as STDM.[1][2] It is very similar todynamic bandwidth allocation (DBA). In statistical multiplexing, acommunication channel is divided into an arbitrary number of variable bitrate digital channels or data streams. The link sharing is adapted to the instantaneous traffic demands of the data streams that are transferred over each channel. This is an alternative to creating a fixed sharing of a link, such as in generaltime division multiplexing (TDM) andfrequency division multiplexing (FDM). When performed correctly, statistical multiplexing can provide a link utilization improvement, called thestatistical multiplexing gain.
Statistical multiplexing is facilitated throughpacket mode orpacket-oriented communication, which among others is utilized inpacket switched computer networks. Eachstream is divided intopackets that normally are delivered asynchronously in a first-come first-served fashion. In alternative fashion, the packets may be delivered according to some scheduling discipline for fair queuing or differentiated and/or guaranteed quality of service. It is also found in fibre optic circuits where communications are made on a statistical basis.[3]
Statistical multiplexing of an analog channel, for example a wireless channel, is also facilitated through the following schemes:
Statistical multiplexing normally implies "on-demand" service rather than one that preallocates resources for each data stream. Statistical multiplexing schemes do not control user data transmissions.
Time domain statistical multiplexing (packet mode communication) is similar totime-division multiplexing (TDM), except that, rather than assigning a data stream to the same recurrent time slot in everyTDM, each data stream is assigned time slots (of fixed length) ordata frames (of variable lengths) that often appear to be scheduled in a randomized order, and experience varying delay (while the delay is fixed in TDM).[1]
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).
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.
Static TDM and other circuit switching is carried out at thephysical layer in theOSI model andTCP/IP model, while statistical multiplexing is carried out at thedata link layer and above.
In statistical multiplexing, each packet or frame contains a channel/data stream identification number, or (in the case ofdatagram communication) complete destination address information.
Examples of statistical multiplexing are:
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.