STATISTICAL MULTIPLEXING

'Statistical multiplexing' is a type of communication link sharing. In statistical multiplexing, a communication channel is divided into an arbitrary number of variable bit-rate 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 general time division multiplexing and frequency division multiplexing. When performed correctly, statistical multiplexing can provide a link utilization improvement, denoting the ''statistical multiplexing gain''.
Statistical multiplexing is facilitated through 'packet mode' or 'packet oriented' communication, which amongst others is utilized in packet switched computer networks. Each stream is divided into packets that normally are delivered asynchronously in a first-come first-serve fashion. Alternatively, the packets may be delivered according to some scheduling discipline for fair queuing or differentiated and/or guaranteed Quality of service.
Statistical multiplexing of an analog channel, for example a wireless channel, is also facilitated through the following schemes:

★ Random frequency-hopping orthogonal frequency division multiple access (RFH-OFDMA)

★ Code-division multiple access (CDMA), where different amount of spreading codes or spreading factors can be assigned to different users.
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.

Contents

Comparison with static TDM

Channel identification

Usage

Statistical multiplexer

See also

Comparison with static TDM

Time domain statistical multiplexing (packet mode communication) is similar to time-division multiplexing (TDM), except that, rather than assigning a data stream to the same recurrent time slot in every TDM frame, each data stream is assigned time slots (of fixed length) or data 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).
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 the physical layer in the OSI model and TCP/IP model, while statistical multiplexing is carried out at the data link layer and above.

Channel identification

In statistical multiplexing, each packet or frame contains a channel/data stream identification number, or (in the case of datagram communication) complete destination address information.

Usage

Examples of statistical multiplexing are:

★ The MPEG transport stream for digital TV 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 #Statistical multiplexer). The packets have constant lengths. The channel number is denoted Program ID (PID).

★ The UDP and TCP protocols, where data streams from several application processes are multiplexed together. The packets may have varying lengths. The port numbers constitute channel identification numbers (and also address information).

★ The X.25 and Frame relay packet-switching protocols, where the packets have varying lengths, and the channel number is denoted Virtual Connection Identifier (VCI). The international collection of X.25 providers, using the X.25 protocol suite was colloquially known as "the Packet switched network" in the 1980s and into the beginning of the 1990s.

★ The Asynchronous Transfer Mode packet-switched protocol, where the packets have fixed length. The channel identification number consists of a Virtual Connection Identifier (VCI) and a Virtual Path Identifier (VPI).

Statistical multiplexer

A ''Statistical multiplexer'' (or ''stat-mux'') is a device which allows broadcasters to change the bitrate of channels according to needs of different channels at different times.
Different types of images require different amounts of bandwidth to produce a good quality image. Programs with very little movement, such as a news broadcast, require much less bandwidth than programs that contain rapidly-moving images, such as hockey game.
If a broadcaster is showing news on one channel and ice hockey on another channel, with fixed multiplexing each channel has, say, 2 Mbit/s. The channel with the news show would have good image quality at this bitrate, because news shows have little movement and require little bandwidth. However, the channel showing the hockey game would suffer poorer image quality, because transmitting clear moving images of fast movement and crowd shots require large bandwidth.
A statistical multiplexer allows the broadcaster to allocate bandwidth to wherever it is needed. Thus 0.5 Mbit/s are allocated to the channel with the news program and 3.5 Mbit/s to the channel with the hockey game. This results in a good quality picture for both channels.

See also

★ Dynamic bandwidth allocation

★ Packet switching

★ Packet

★ Data frame

★ Dynamic TDMA

★ Reverse Statistical multiplexing