Channel access method

In telecommunications and computer networks, a channel access method or multiple access method allows several terminals connected to the same physical medium to transmit over it and to share its capacity. Examples of shared physical media are bus networks, ring networks, hub networks, wireless networks and half-duplex point-to-point links.

Multiple access protocols and control mechanisms are called media access control (MAC), which is provided by the data link protocol layer in the OSI seven layer model as well as in the TCP/IP five layer model.

A multiple access method is based on a multiplex method, that allows several data streams or signals to share the same communication channel or physical media. Multiplexing is 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.

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)
  • Orthogonal frequency division multiple access (OFDMA)
  • Wavelength division multiple access (WDMA)
• Time-division multiple access (TDMA)
  • Multi-Frequency Time Division Multiple Access (MF-TDMA)
• Spread spectrum multiple access (SSMA)
  • Direct-sequence spread spectrum (DSSS)
  • Frequency-hopping spread spectrum (FHSS)
  • Orthogonal Frequency-Hopping Multiple Access (OFHMA)
  • Code division multiple access (CDMA) - the overarching form of DS-SS and FH-SS
  • 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 based random multiple access methods:
  • 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)
  • Carrier sense multiple access with collision avoidance (CSMA/CA)
    • Distributed Coordination Function (DCF)
    • Point Coordination Function (PCF)
  • Carrier sense multiple access with collision avoidance and Resolution using Priorities (CSMA/CARP)
• Token passing:
  • Token ring
  • Token bus
• 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:

• 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 networks (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.

Definition within certain application areas

Local and metropolitan area networks

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.

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, frequency-division, and time-division multiple access.

Switching centers

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.

See also

• Media access control
• Multiplex
• Statistical multiplexing
• Dynamic bandwidth allocation
• Diversity schemeca:Accés múltiple

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