Editing Channel access method

{{Short description|Means for multiple terminals to communicate over one medium}}
{{Use American English|date = March 2019}}

In [[telecommunications]] and [[computer networks]], a '''channel access method''' or '''multiple access method''' allows more than two [[terminal (telecommunication)|terminal]]s connected to the same [[transmission medium]] to transmit over it and to share its capacity.<ref name="Miao">{{cite book |author1=Guowang Miao |author2=Jens Zander |author3=Ki Won Sung |author4=Ben Slimane |title=Fundamentals of Mobile Data Networks |publisher=Cambridge University Press |isbn=978-1107143210 |date=2016}}</ref> Examples of shared physical media are [[wireless network]]s, [[bus network]]s, [[ring network]]s and [[point-to-point link]]s operating in [[half-duplex]] mode.

A channel access method is based on [[multiplexing]], which allows several [[data stream]]s or signals to share the same [[communication channel]] or transmission medium. In this context, multiplexing is provided by the [[physical layer]].

A channel access method may also be a part of the multiple access protocol and control mechanism, also known as [[medium 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 the [[data link layer]] of the [[OSI model]] and a component of the [[link layer]] of the [[TCP/IP model]].

==Fundamental schemes==
Several ways of categorizing multiple-access schemes and protocols have been used in the literature. For example, Daniel Minoli (2009)<ref>{{cite book|author=Daniel Minoli|title=Satellite Systems Engineering in an IPv6 Environment|url=https://books.google.com/books?id=4yJi1UQDPp8C&pg=PA136|access-date=1 June 2012|date=3 February 2009|publisher=CRC Press|isbn=978-1-4200-7868-8|pages=136–}}</ref> identifies five principal types of multiple-access schemes: [[FDMA]], [[Time-division multiple access|TDMA]], [[CDMA]], [[Space-division multiple access|SDMA]], and [[random access]]. [[Raphael Rom|R. Rom]] and [[Moshe Sidi|M. Sidi]] (1990)<ref>{{cite book |last1=Rom |first1=Raphael |last2=Sidi |first2=Moshe |title=Multiple Access Protocols |series=Telecommunication Networks and Computer Systems |date=1990 |publisher=Springer-Verlag/University of Michigan |doi=10.1007/978-1-4612-3402-9|isbn=978-1-4612-7997-6 }}</ref> categorize the protocols into ''Conflict-free access protocols'', ''Aloha protocols'', and ''Carrier Sensing protocols''.

The Telecommunications Handbook (Terplan and Morreale, 2000)<ref>{{cite book|author=Kornel Terplan|title=The Telecommunications Handbook|url=https://books.google.com/books?id=_lLZLE6-SRsC&pg=PT266|access-date=1 June 2012|year=2000|publisher=CRC Press|isbn=978-0-8493-3137-4|pages=266–}}</ref> identifies the following MAC categories:

* Fixed assigned: TDMA, FDMA+WDMA, CDMA, SDMA
* Demand assigned (DA)
** Reservation: DA/TDMA, DA/FDMA+DA/WDMA, DA/CDMA, DA/SDMA
** Polling: Generalized polling, Distributed polling, Token Passing, Implicit polling, Slotted access
* Random access (RA): Pure RA (ALOHA, GRA), Adaptive RA (TRA), CSMA, CSMA/CD, CSMA/CA

Channel access schemes generally fall into the following categories.<ref name=Miao/><ref>{{cite web |title=Fundamentals of Communications Access Technologies: FDMA, TDMA, CDMA, OFDMA, AND SDMA |url=http://electronicdesign.com/communications/fundamentals-communications-access-technologies-fdma-tdma-cdma-ofdma-and-sdma |publisher=Electronic Design |date=2013-01-22 |access-date=2014-08-28}}</ref><ref>{{cite book |title=Wireless Sensors and Instruments: Networks, Design, and Applications |publisher=CRC Press |author=Halit Eren |date=Nov 16, 2005 |isbn=9781420037401 |page=112}}</ref>

===Frequency-division multiple access===
The [[frequency-division multiple access]] (FDMA) channel-access scheme is the most standard analog system, based on the [[frequency-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-generation [[1G]] 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) is [[modulated]] on a specific [[carrier frequency]].

A related technique is wavelength division multiple access (WDMA), based on [[wavelength-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.<ref>{{Cite web|url=https://www.abcofnetworks.com/2019/02/Multiple-Access-Techniques-in-communication-FDMA-TDMA-CDMA.html|title=Multiple Access Techniques in communication: FDMA, TDMA, CDMA|last=Sadique|first=Abubaker|archive-url=https://web.archive.org/web/20191009145423/https://www.abcofnetworks.com/2019/02/Multiple-Access-Techniques-in-communication-FDMA-TDMA-CDMA.html|archive-date=2019-10-09}}</ref>

An advanced form of FDMA is the [[orthogonal frequency-division multiple access]] (OFDMA) scheme, for example, used in [[4G]] 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).

===Time-division multiple access===
The [[time-division multiple access]] (TDMA) channel access scheme is based on the [[time-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 for [[signaling (telecommunications)|signaling]] 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 as [[statistical multiplexing]] methods and capable of [[dynamic bandwidth allocation]]. This requires a [[media access control]] (MAC) protocol, i.e., a principle for the nodes to take turns on the channel and to avoid collisions. Common examples are [[CSMA/CD]], used in [[Ethernet]] bus networks and hub networks, and [[CSMA/CA]], used in wireless networks such as [[IEEE 802.11]].

===Code-division multiple access and spread spectrum multiple access===
The [[code-division multiple access]] (CDMA) scheme is based on [[spread 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 the [[Shannon–Hartley theorem]], the wide bandwidth makes it possible to send with a [[signal-to-noise ratio]] of much less than 1 (less than 0&nbsp;dB), meaning that the transmission power can be reduced to a level below the level of the noise and [[co-channel interference]] from other message signals sharing the same frequency range.

One form is [[direct-sequence CDMA]] (DS-CDMA), based on [[direct-sequence spread spectrum]] (DSSS), used for example in [[3G]] 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 is [[frequency-hopping CDMA]] (FH-CDMA), based on [[frequency-hopping spread spectrum]] (FHSS), where the channel frequency is changed rapidly according to a sequence that constitutes the spreading code. As an example, the [[Bluetooth]] communication system is based on a combination of frequency-hopping and either CSMA/CA statistical time-division multiplexing communication (for [[data communication]] applications) or TDMA (for audio transmission). All nodes belonging to the same user (to the same [[piconet]]) 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 and [[multi-carrier code-division multiple access]] (MC-CDMA).

===Space-division multiple access===
[[Space-division multiple access]] (SDMA) transmits different information in different physical areas. Examples include simple [[cellular radio]] systems and more advanced cellular systems that use directional antennas and power modulation to refine spatial transmission patterns.

===Power-division multiple access===
'''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 multiple [[SCPC]] modems on a satellite transponder, where users get on demand a larger share of the power budget to transmit at higher data rates.<ref>{{Citation|last1=Elinav|first1=Doron|title=Power Division Multiple Access|date=Mar 6, 2014|url=https://patents.google.com/patent/US20140064125|last2=Rubin|last3=Brener|first2=Mati E.|first3=Snir|access-date=2016-06-29}}</ref>

===Packet mode methods===
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.<ref name=Miao/>

Common statistical time-division multiplexing multiple access protocols for wired multi-drop networks include:

* [[Carrier-sense multiple access with collision detection]] (CSMA/CD), used in [[Ethernet]] and [[IEEE 802.3]]
* [[Multiple Access with Collision Avoidance]] (MACA)
* [[Multiple Access with Collision Avoidance for Wireless]] (MACAW)
* [[Carrier-sense multiple access]] (CSMA)
* [[Carrier-sense multiple access with collision avoidance and resolution using priorities]] (CSMA/CARP)
* Bitwise Arbitration based on constructive interference as used on [[CAN bus]]
* [[Token bus]] (IEEE 802.4)
* [[Token Ring]] (IEEE 802.5)
* [[Token passing]], used in [[FDDI]]
* [[Dynamic time-division multiple access]] (Dynamic TDMA)

Common multiple access protocols that may be used in packet radio wireless networks include:
* [[Carrier-sense multiple access with collision avoidance]] (CSMA/CA), used in [[IEEE 802.11]]/[[WiFi]], potentially using a [[distributed coordination function]]
* ALOHA and slotted ALOHA, used in [[ALOHAnet]]
* [[Reservation ALOHA]] (R-ALOHA)
* [[Mobile Slotted Aloha]] (MS-ALOHA)
* [[Code-division multiple access]] (CDMA)
* [[Orthogonal frequency-division multiple access]] (OFDMA)
* [[Orthogonal frequency-division multiplexing]] (OFDM)

===Duplexing methods===
Where these methods are used for dividing forward and reverse communication channels, they are known as [[duplexing]] methods. A duplexing communication system can be either [[half-duplex]] or [[full 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:

* [[Time-division duplex]] (TDD)
* [[Frequency-division duplex]] (FDD)
* [[Echo cancellation]]

==Hybrid application examples==
Note that hybrids of these techniques are frequently used. Some examples:
 
* The [[GSM]] cellular system combines the use of frequency-division duplex (FDD) to prevent interference between outward and return signals, with FDMA and TDMA to allow multiple handsets to work in a single cell.
* [[GSM]] with the [[GPRS]] packet-switched service combines FDD and FDMA with [[slotted Aloha]] for reservation inquiries and a [[dynamic TDMA]] scheme for transferring the actual data.
* [[Bluetooth]] packet mode communication combines [[frequency hopping]] for shared channel access among several private area networks in the same room with [[CSMA/CA]] for shared channel access within a network.
* [[IEEE 802.11b]] [[wireless local area network]]s (WLANs) are based on FDMA and [[DS-CDMA]] for avoiding interference among adjacent WLAN cells or access points. This is combined with [[CSMA/CA]] for multiple access within the cell.
* [[HIPERLAN/2]] wireless networks combine [[FDMA]] with dynamic TDMA, meaning that resource reservation is achieved by [[packet scheduling]].
* [[G.hn]], an [[ITU-T]] standard for high-speed networking over home wiring (power lines, phone lines and coaxial cables) employs a combination of TDMA, token passing and [[CSMA/CARP]] to allow multiple devices to share the medium.

==Application-specific definitions==
Different channel access constraints and schemes apply to different applications.

===Local and metropolitan area networks===
In [[local area network]]s (LANs) and [[metropolitan area network]]s (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 in [[Ethernet]]. Although today's Ethernet installations use full-duplex connections directly to [[Ethernet switch|switches]]. CSMA/CD is still implemented to achieve compatibility with older [[repeater hub]]s.

===Satellite communications===
In [[satellite communications]], multiple access is the capability of a [[communications 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 are [[code-division]], [[frequency-division multiple access|frequency-division]], and [[time-division multiple access|time-division]] multiple access.

=== Cellular networks ===
In [[cellular 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.<ref name=":0">{{Cite web|url=https://www.qualcomm.com/invention/stories/world-changing-technology|title=The world-changing technology that almost wasn't|last=Qualcomm|first=Qualcomm|website=Qualcomm}}</ref>

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.<ref name=":0" /> That was until [[Irwin M. Jacobs]] an MIT engineer, and fellow employees from the company [[Linkabit]] founded the telecommunications company [[Qualcomm]].<ref>{{Cite news|url=https://www.wsj.com/articles/SB10001424052970204879004577111313063790248|title=Qualcomm Founder Set to Retire|last=Tibken|first=Shara|date=2011-12-21|work=Wall Street Journal|access-date=2019-12-03|language=en-US|issn=0099-9660}}</ref> 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.<ref name=":1">{{Cite book|url=https://books.google.com/books?id=JcH4C2eAsJEC&q=%25E2%2580%259CDisrupting%2520the%2520Cellular%2520Status%2520Quo%2520Qualcomm%2520Goes%2520to%2520Bat%252C%25201989%25E2%2580&pg=PR5|title=The Qualcomm Equation: How a Fledgling Telecom Company Forged a New Path to Big Profits and Market Dominance|last=Mock|first=Dave|date=2005|publisher=Amacom|isbn=978-0-8144-2858-0|language=en}}</ref> 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.<ref name=":1" /> 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 of [[2G]].<ref name=":0" />

==See also==
* [[Diversity scheme]]
* [[Dynamic bandwidth allocation]]
* [[Radio resource management]] for inter-base station interference control

==References==
{{Reflist}}

{{Channel access methods}}
{{Mobile telecommunications standards}}
{{telecommunications}}
{{FS1037C MS188}}
{{Authority control}}

[[Category:Channel access methods| ]]