Intelecommunications andcomputer networks, achannel access method ormultiple access method allows more than twoterminals connected to the sametransmission medium to transmit over it and to share its capacity.[1] Examples of shared physical media arewireless networks,bus networks,ring networks andpoint-to-point links operating inhalf-duplex mode.
A channel access method is based onmultiplexing, which allows severaldata streams or signals to share the samecommunication channel or transmission medium. In this context, multiplexing is provided by thephysical layer.
A channel access method may also be a part of the multiple access protocol and control mechanism, also known asmedium access control (MAC). Medium access control deals with issues such as addressing, assigning multiplex channels to different users and avoiding collisions. Media access control is a sub-layer in thedata link layer of theOSI model and a component of thelink layer of theTCP/IP model.
Several ways of categorizing multiple-access schemes and protocols have been used in the literature. For example, Daniel Minoli (2009)[2] identifies five principal types of multiple-access schemes:FDMA,TDMA,CDMA,SDMA, andrandom access.R. Rom andM. Sidi (1990)[3] categorize the protocols intoConflict-free access protocols,Aloha protocols, andCarrier Sensing protocols.
The Telecommunications Handbook (Terplan and Morreale, 2000)[4] identifies the following MAC categories:
Channel access schemes generally fall into the following categories.[1][5][6]
Thefrequency-division multiple access (FDMA) channel-access scheme is the most standard analog system, based on thefrequency-division multiplexing (FDM) scheme, which provides different frequency bands to different data streams. In the FDMA case, the frequency bands are allocated to different nodes or devices. An example of FDMA systems was the first-generation1G cell-phone systems, where each phone call was assigned to a specific uplink frequency channel and another downlink frequency channel. Each message signal (each phone call) ismodulated on a specificcarrier frequency.
A related technique is wavelength division multiple access (WDMA), based onwavelength-division multiplexing (WDM), where different data streams get different colors in fiber-optical communications. In the WDMA case, different network nodes in a bus or hub network get a different color.[7]
An advanced form of FDMA is theorthogonal frequency-division multiple access (OFDMA) scheme, for example, used in4G cellular communication systems. In OFDMA, each node may use several sub-carriers, making it possible to provide different quality of service (different data rates) to different users. The assignment of sub-carriers to users may be changed dynamically, based on the current radio channel conditions and traffic load.Single-carrier FDMA (SC-FDMA), a.k.a. linearly-precoded OFDMA (LP-OFDMA), is based on single-carrier frequency-domain-equalization (SC-FDE).
Thetime-division multiple access (TDMA) channel access scheme is based on thetime-division multiplexing (TDM) scheme. TDMA provides different time slots to different transmitters in a cyclically repetitive frame structure. For example, node 1 may use time slot 1, node 2 time slot 2, etc., until the last transmitter when it starts over. An advanced form is dynamic TDMA (DTDMA), where an assignment of transmitters to time slots varies on each frame.
Multi-frequency time-division multiple access (MF-TDMA) combines time and frequency multiple access. As an example,2G cellular systems are based on a combination of TDMA and FDMA. Each frequency channel is divided into eight time slots, of which seven are used for seven phone calls and one forsignaling data.
Statistical time-division multiplexing multiple access is typically also based on time-domain multiplexing, but not in a cyclically repetitive frame structure. Due to its random character, it can be categorized asstatistical multiplexing methods and capable ofdynamic bandwidth allocation. This requires amedia access control (MAC) protocol, i.e., a principle for the nodes to take turns on the channel and to avoid collisions. Common examples areCSMA/CD, used inEthernet bus networks and hub networks, andCSMA/CA, used in wireless networks such asIEEE 802.11.
Thecode-division multiple access (CDMA) scheme is based onspread spectrum, meaning that a wider radio channel bandwidth is used than the data rate of individual bit streams requires, and several message signals are transferred simultaneously over the same carrier frequency, utilizing different spreading codes. Per theShannon–Hartley theorem, the wide bandwidth makes it possible to send with asignal-to-noise ratio of much less than 1 (less than 0 dB), meaning that the transmission power can be reduced to a level below the level of the noise andco-channel interference from other message signals sharing the same frequency range.
One form isdirect-sequence CDMA (DS-CDMA), based ondirect-sequence spread spectrum (DSSS), used for example in3G cell phone systems. Each information bit (or each symbol) is represented by a long code sequence of several pulses, called chips. The sequence is the spreading code, and each message signal (for example each phone call) uses a different spreading code.
Another form isfrequency-hopping CDMA (FH-CDMA), based onfrequency-hopping spread spectrum (FHSS), where the channel frequency is changed rapidly according to a sequence that constitutes the spreading code. As an example, theBluetooth communication system is based on a combination of frequency-hopping and either CSMA/CA statistical time-division multiplexing communication (fordata communication applications) or TDMA (for audio transmission). All nodes belonging to the same user (to the samepiconet) use the same frequency hopping sequence synchronously, meaning that they send on the same frequency channel, but CDMA/CA or TDMA is used to avoid collisions within the virtual personal area network (VPAN). Frequency-hopping is used by Bluetooth to reduce the cross-talk and collision probability between nodes in different VPANs.
Other techniques include OFDMA andmulti-carrier code-division multiple access (MC-CDMA).
Space-division multiple access (SDMA) transmits different information in different physical areas. Examples include simplecellular radio systems and more advanced cellular systems that use directional antennas and power modulation to refine spatial transmission patterns.
Power-division multiple access (PDMA) scheme is based on using variable transmission power between users in order to share the available power on the channel. Examples include multipleSCPC modems on a satellite transponder, where users get on demand a larger share of the power budget to transmit at higher data rates.[8]
Packet mode channel access methods select a single network transmitter for the duration of a packet transmission. Some methods are more suited to wired communication, while others are more suited to wireless.[1]
Common statistical time-division multiplexing multiple access protocols for wired multi-drop networks include:
Common multiple access protocols that may be used in packet radio wireless networks include:
Where these methods are used for dividing forward and reverse communication channels, they are known asduplexing methods. A duplexing communication system can be eitherhalf-duplex orfull duplex. In a half-duplex system, communication only works in one direction at a time. A walkie-talkie is an example of a half-duplex system because both users can communicate with one another, but not at the same time, someone has to finish transmitting before the next person can begin. In a full-duplex system, both users can communicate at the same time. A telephone is the most common example of a full-duplex system because both users can speak and be heard at the same time on each end. Some types of full-duplexing methods are:
Note that hybrids of these techniques are frequently used. Some examples:
Different channel access constraints and schemes apply to different applications.
Inlocal area networks (LANs) andmetropolitan area networks (MANs), multiple access methods enable bus networks, ring networks, star networks, wireless networks and half-duplex point-to-point communication, but are not required in full-duplex point-to-point serial lines between network switches and routers. The most common multiple access method is CSMA/CD, which is used inEthernet. Although today's Ethernet installations use full-duplex connections directly toswitches. CSMA/CD is still implemented to achieve compatibility with olderrepeater hubs.
Insatellite communications, multiple access is the capability of acommunications satellite to function as a portion of a communications link between more than one pair of ground-based terminals concurrently. Three types of multiple access presently used with communications satellites arecode-division,frequency-division, andtime-division multiple access.
Incellular networks the two most widely adopted technologies are CDMA and TDMA. TDMA technology works by identifying natural breaks in speech and utilizing one radio wave to support multiple transmissions in turn. In CDMA technology, each individual packet receives a unique code that is broken up over a wide frequency spectrum and is then reassembled on the other end. CDMA allows multiple people to speak at the same time over the same frequency, allowing more conversations to be transmitted over the same amount of spectrum; this is one reason why CDMA eventually became the most widely adopted channel access method in the wireless industry.[9]
The origins of CDMA can be traced back to the 1940s where it was patented by the United States government and used throughout World War II to transmit messages. However, following the war the patent expired and the use of CDMA diminished and was widely replaced by TDMA.[9] That was untilIrwin M. Jacobs an MIT engineer, and fellow employees from the companyLinkabit founded the telecommunications companyQualcomm.[10] At the time Qualcomm was founded, Jacobs had already been working on addressing telecommunications problems for the military using digital technology to increase the capacity of spectrum.[11] Qualcomm knew that CDMA would greatly increase the efficiency and availability of wireless, but the wireless industry having already invested millions of dollars into TDMA was skeptical.[11] Jacobs and Qualcomm spent several years improving infrastructure and performing tests and demonstrations of CDMA. In 1993, CDMA became accepted as the wireless industry standard. By 1995, CDMA was being used commercially in the wireless industry as the foundation of2G.[9]
Cellular network standards | |||||||||
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| 0Gradio telephones (1946) | |||||||||
| 1G (1979) |
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| 2G (1991) |
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| 2G transitional (2.5G, 2.75G, 2.9G) |
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| 3G (1998) IMT-2000 (2001) |
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| 3G transitional (3.5G, 3.75G, 3.9G) |
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| 4G (2009) IMT Advanced (2013) |
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| 5G (2018) IMT-2020 (2021) |
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| Related articles | |||||||||
This article incorporatespublic domain material fromFederal Standard 1037C.General Services Administration. Archived fromthe original on 2022-01-22. (in support ofMIL-STD-188).
