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
point-to-point links. Respective wording is recommended with IETF on Mobile Ad Hoc Networking Terminology
Multiple access protocols and control mechanisms are called media access control (MAC) for Data links, which is provided by the Data Link Layer in the OSI model and the Link Layer of the TCP/IP 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
channel access methods:
Packet mode 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:
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.
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
, 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