Statistical multiplexing
Encyclopedia
Statistical multiplexing is a type of communication link sharing, very similar to dynamic bandwidth allocation
(DBA). 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 (TDM) and frequency division multiplexing (FDM). When performed correctly, statistical multiplexing can provide a link utilization improvement, called the statistical multiplexing gain.
Statistical multiplexing is facilitated through packet mode or packet-oriented communication, which among 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. In alternative fashion, 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:
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.
(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 frame
s (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.
communication) complete destination address information.
Dynamic bandwidth allocation
Dynamic bandwidth allocation is a technique by which traffic bandwidth in a shared telecommunications medium can be allocated on demand and fairly between different users of that bandwidth. This is a form of bandwidth management, and is essentially the same thing as statistical multiplexing...
(DBA). In statistical multiplexing
Multiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
, 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 (TDM) and frequency division multiplexing (FDM). When performed correctly, statistical multiplexing can provide a link utilization improvement, called the statistical multiplexing gain.
Statistical multiplexing is facilitated through packet mode or packet-oriented communication, which among 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. In alternative fashion, the packets may be delivered according to some scheduling discipline for fair queuing or differentiated and/or guaranteed quality of service
Quality of service
The quality of service refers to several related aspects of telephony and computer networks that allow the transport of traffic with special requirements...
.
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.
Comparison with static TDM
Time domain statistical multiplexing (packet mode communication) is similar to time-division multiplexingTime-division multiplexing
Time-division multiplexing is a type of digital multiplexing in which two or more bit streams or signals are transferred apparently simultaneously as sub-channels in one communication channel, but are physically taking turns on the channel. The time domain is divided into several recurrent...
(TDM), except that, rather than assigning a data stream to the same recurrent time slot in every TDM frame
Data frame
In computer networking and telecommunication, a frame is a digital data transmission unit or data packet that includes frame synchronization, i.e. a sequence of bits or symbols making it possible for the receiver to detect the beginning and end of the packet in the stream of symbols or bits...
, each data stream is assigned time slots (of fixed length) or data frame
Data frame
In computer networking and telecommunication, a frame is a digital data transmission unit or data packet that includes frame synchronization, i.e. a sequence of bits or symbols making it possible for the receiver to detect the beginning and end of the packet in the stream of symbols or bits...
s (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
Physical layer
The physical layer or layer 1 is the first and lowest layer in the seven-layer OSI model of computer networking. The implementation of this layer is often termed PHY....
in the OSI model
OSI model
The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
and TCP/IP model, while statistical multiplexing is carried out at the data link layer
Data link layer
The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
and above.
Channel identification
In statistical multiplexing, each packet or frame contains a channel/data stream identification number, or (in the case of datagramDatagram
A datagram is a basic transfer unit associated with a packet-switched network in which the delivery, arrival time, and order are not guaranteed....
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 UDPUser Datagram ProtocolThe User Datagram Protocol is one of the core members of the Internet Protocol Suite, the set of network protocols used for the Internet. With UDP, computer applications can send messages, in this case referred to as datagrams, to other hosts on an Internet Protocol network without requiring...
and TCPTransmission Control ProtocolThe Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
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.25X.25X.25 is an ITU-T standard protocol suite for packet switched wide area network communication. An X.25 WAN consists of packet-switching exchange nodes as the networking hardware, and leased lines, Plain old telephone service connections or ISDN connections as physical links...
and Frame relayFrame relayFrame Relay is a standardized wide area network technology that specifies the physical and logical link layers of digital telecommunications channels using a packet switching methodology...
packet-switching protocols, where the packets have varying lengths, and the channel number is denoted Virtual Connection Identifier (VCI). The international collection of X.25X.25X.25 is an ITU-T standard protocol suite for packet switched wide area network communication. An X.25 WAN consists of packet-switching exchange nodes as the networking hardware, and leased lines, Plain old telephone service connections or ISDN connections as physical links...
providers, using the X.25X.25X.25 is an ITU-T standard protocol suite for packet switched wide area network communication. An X.25 WAN consists of packet-switching exchange nodes as the networking hardware, and leased lines, Plain old telephone service connections or ISDN connections as physical links...
protocol suite was colloquially known as "the Packet switched network" in the 1980s and into the beginning of the 1990s.
- The Asynchronous Transfer ModeAsynchronous Transfer ModeAsynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...
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
In for example digital audio and video broadcasting, 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.See also
- Dynamic bandwidth allocationDynamic bandwidth allocationDynamic bandwidth allocation is a technique by which traffic bandwidth in a shared telecommunications medium can be allocated on demand and fairly between different users of that bandwidth. This is a form of bandwidth management, and is essentially the same thing as statistical multiplexing...
- Packet switchingPacket switchingPacket switching is a digital networking communications method that groups all transmitted data – regardless of content, type, or structure – into suitably sized blocks, called packets. Packet switching features delivery of variable-bit-rate data streams over a shared network...
- Packet
- Data frameData frameIn computer networking and telecommunication, a frame is a digital data transmission unit or data packet that includes frame synchronization, i.e. a sequence of bits or symbols making it possible for the receiver to detect the beginning and end of the packet in the stream of symbols or bits...
- Dynamic TDMA
External links
- Example of Statistical Multiplexing (Chart from a real DVB-T multiplex)