In telecommunications and computer networks, a channel access method or multiple access method allows several terminals 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 networks, ring networks, hub networks 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 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.
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These are the four fundamental types of channel access schemes:
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.
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,but some times user 1 may use time slot 1 in first frame and use another time slot in next frame.
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.
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).
The following are common circuit mode and channelization channel access methods:
The following are examples of packet mode channel access methods:
Where these methods are used for dividing forward and reverse communication channels, they are known as duplexing methods, such as:
Note that hybrids of these techniques can be - and frequently are - used. Some examples:
In local area networks (LANs) and metropolitan area networks (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's Ethernet installations typically are switched, CSMA/CD is utilized anyway to achieve compatibility with hubs.
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, frequency-division, and time-division multiple access.
In telecommunication switching centers, multiple access is the connection of a user to two or more switching centers by separate access lines using a single message routing indicator or telephone number.
Several ways of categorizing multiple-access schemes and protocols have been used in the literature. For example, Daniel Minoli (2009)[1] identifies five principal types of multiple-access schemes: FDMA, TDMA, CDMA, SDMA, and Random access. R. Rom and M. Sidi (1990)[2] categorize the protocols into Conflict-free access protocols, Aloha protocols, and Carrier Sensing protocols.
The Telecommunications Handbook (Terplan and Morreale, 2000)[3] identifies the following MAC categories:
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This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).