In telecommunication and computer networks a channel access method or multiple access method allows several terminal (telecommunication) connected to the same multi-point transmission medium to transmit over it and to share its capacity. Examples of shared physical media are wireless networks bus network , ring network , hub network and half-duplex point-to-point links. A channel-access scheme is based on a multiplexing method, that allows several data streams or signals to share the same communication channel or physical medium Multiplexing is in this context provided by the physical layer Note that multiplexing also may be used in full-duplex Point-to-point (telecommunications) communication between nodes in a switched network, which should not be considered as multiple access. A channel-access scheme is also based on a multiple access protocol and control mechanism, also known as media access control (MAC). This protocol deals with issues such as addressing, assigning multiplex channels to different users, and avoiding collisions. The MAC-layer is a sub-layer in Layer 2 (Data Link Layer of the OSI model and a component of the Link Layer of the TCP/IP model

Fundamental types of channel access schemes

These are the four fundamental types of channel access schemes:

Frequency division multiple access (FDMA)

The frequency division multiple access (FDMA) channel-access scheme is based on the frequency-division multiplex (FDM) scheme, which provides different frequency bands to different data-streams. In the FDMA case, the data streams are allocated to different users or nodes. An example of FDMA systems were the first-generation (1G) cell-phone systems. A related technique is wave-length division multiple access (WDMA), based on wavelength division multiplex (WDM), where different users get different colors in fiber-optical communication.

Time division multiple access (TDMA)

The time division multiple access (TDMA) channel access scheme is based on the time division multiplex (TDM) scheme, which provides different time-slots to different data-streams (in the TDMA case to different transmitters) in a cyclically repetitive frame structure. For example, user 1 may use time slot 1, user 2 time slot 2, etc. until the last user. Then it starts all over again. Packet mode Packet mode multiple-access is typically also based on time-domain multiplexing, but not in a cyclically repetitive frame structure, and therefore it is not considered as TDM or TDMA. Due to its random character it can be categorised as statistical multiplexing methods, making it possible to provide dynamic bandwidth allocation

Code division multiple access (CDMA)

The code division multiple access (CDMA) scheme is based on spread spectrum An example is the 3G cell phone system.

Space division multiple access (SDMA)

Space-division_multiple_access (SDMA).

List of channel access methods

Circuit mode and channelization methods

The following are common circuit mode and channelization channel access methods: *[[Frequency division multiple access]] (FDMA) , based on [[frequency-division multiplex]] (FDM) **[[Wavelength division multiplexing|Wavelength division multiple access]] (WDMA) **[[Orthogonal frequency division multiple access]] (OFDMA), based on [[Orthogonal frequency-division multiplexing]] (OFDM) **[[Single-carrier FDMA]] (SC-FDMA), a.k.a. linearly-precoded OFDMA (LP-OFDMA), based on single-carrier frequency-domain-equalization (SC-FDE). * Time-division multiple access (TDMA), based on [[time-division multiplex]] (TDM) **[[Multi-Frequency Time Division Multiple Access]] (MF-TDMA) * [[Code division multiple access]] (CDMA) , a.k.a. Spread spectrum (SSMA) **Direct-sequence CDMA (DS-CDMA), based on Direct-sequence spread spectrum (DSSS) **Frequency-hopping CDMA (FH-CDMA), based on Frequency-hopping spread spectrum (FHSS) **Orthogonal frequency-hopping multiple access (OFHMA) **Multi-carrier code division multiple access (MC-CDMA) *[[Space division multiple access]] (SDMA) ===Packet mode methods=== The following are examples of [[packet mode]] channel access methods: * Contention (telecommunications) based random multiple access methods **[[Aloha protocol|Aloha]] **[[Slotted Aloha]] **[[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 detection]] (CSMA/CD) - suitable for wired networks **[[Carrier sense multiple access with collision avoidance]] (CSMA/CA) - suitable for wireless networks ***[[Distributed Coordination Function]] (DCF) ***[[Point Coordination Function]] (PCF) **[[Carrier sense multiple access with collision avoidance and Resolution using Priorities]] ([[CSMA/CARP]]) **[[Carrier Sense Multiple Access/Bitwise Arbitration]] ([[CSMA/BA]]) Based on constructive interference ([[CAN-bus]]) * [[Token passing]] : **IBM token ring **Token bus * [[Polling (computer science)|Polling]] * Resource reservation (scheduled) packet-mode protocols ** [[Dynamic Time Division Multiple Access]] (Dynamic TDMA) ** [[Packet reservation multiple access]] (PRMA) ** [[Reservation ALOHA]] (R-ALOHA) ===Duplexing methods=== Where these methods are used for dividing forward and reverse communication channels, they are known as [[duplexing]] methods, such as: *[[Duplex (telecommunications)|Time division duplex]] (TDD) *[[Frequency division duplex]] (FDD) ==Hybrid channel access scheme application examples== Note that hybrids of these techniques can be - and frequently are - 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 inside a medium). * [[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 [[Time division multiple access|TDMA]], [[Token passing]] and [[CSMA/CARP]] to allow multiple devices to share the medium. ==Definition within certain application areas== ===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, hubbed 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, or in switched networks (logical star topology). The most common multiple access method is [[CSMA/CD]], which is used in [[Ethernet]]. Although today Ethernet installations typically are switched, CSMA/CD is utilized anyway to achieve compatibility with hubs.

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 satellite terminals concurrently. Three types of multiple access presently used with communications satellites are code-division multiple access frequency-division multiple access and time-division multiple access multiple access.

Switching centers

In telecommunication switching center , multiple access is the telecommunication connection of a User (telecommunications) to two or more switching centers by separate access network lines using a single message routing indicator or telephone number

Classifications in the literature

Several ways of categorizing multiple-access schemes and protocols have been used in the literature. For example, Daniel Minoli (2009)http://books.google.com/books?id4yJi1UQDPp8C&pgPA136&dq%22access+schemes%22&hlsv Daniel Minoli, Satellite Systems Engineering in an IPv6 Environment, CRC Press, 2009, ISBN 1420078682, 9781420078688] identifies five principal types of multiple-access schemes: FDMA Time division multiple access CDMA SDMA and Random access R. Rom and M. Sidi (1990)Raphael Rom, Moshe Sidi, Multiple Access Protocols: Performance and Analysis Springer-Verlag, 1990, Originally from University of Michigan categorize the protocols into Conflict-free access protocols Aloha protocols and Carrier Sensing protocols The Telecommunications Handbook (Terplan and Morreale, 2000)http://books.google.com/books?id_lLZLE6-SRsC&pgPT266&dq%22access+schemes%22&hlsv#PPT264,M1 Kornel Terplan, Patricia Morreale, The Telecommunications Handbook CRC Press, 2000, ISBN 0849331374, 9780849331374] 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

See also

*Radio resource management for inter-base station interference control *Statistical multiplexing *Dynamic bandwidth allocation *Diversity scheme

References

Category:Media Access Control ar:طريقة الوصول المتعدد ca:Accés múltiple de:Vielfachzugriff eu:Sarerako sarbide-protokolo ja:多元接続