- Media Access Control
The Media Access Control (MAC)
data communication protocolsub-layer, also known as the Medium Access Control, is a sublayer of the Data Link Layerspecified in the seven-layer OSI model(layer 2). It provides addressing and channel accesscontrol mechanisms that make it possible for several terminals or network nodes to communicate within a multipoint network, typically a local area network(LAN) or metropolitan area network(MAN).
The MAC sub-layer acts as an interface between the
Logical Link Control(LLC) sublayer and the network's physical layer.The MAC layer emulates a full-duplex logical communication channel in a multipoint network. This channel may provide unicast, multicastor broadcastcommunication service.
The MAC layer addressing mechanism is called physical address or
MAC address. This is a unique serial number assigned to each network adapter, making it possible to deliver data packets to a destination within a subnetwork, i.e. a physical network consisting of one or several network segments interconnected by repeaters, hubs, bridges and switches, but not by IP routers. An IP router may interconnect several subnets.
An example of a physical network is an Ethernet network, perhaps extended by Wireless Local Area Networks (WLAN) accesspoints and WLAN network adapters, since these share the same 48-bit MAC address hierarchy as Ethernet.
A MAC layer is not required in
full-duplex point-to-pointcommunication, but address fields are included in some point-to-point protocols for compatibility reasons.
Channel access control mechanism
The channel access control mechanisms provided by the MAC layer are also known as a
multiple access protocol. This makes it possible for several stations connected to the same physical mediumto share it. Examples of shared physical media are bus networks, ring networks, hub networks, wireless networksand half-duplexpoint-to-point links. The multiple access protocol may detect or avoid data packet collisions if a packet mode contention based channel access methodis used, or reserve resources to establish a logical channel if a circuit switchedor channelization based channel access method is used. The channel access control mechanism relies on a physical layer multiplexscheme.
The most widespread multiple access protocol is the contention based
CSMA/CDprotocol used in Ethernet networks. This mechanism is only utilized within a network collision domain, for example an Ethernet bus network or a hub network. An Ethernet network may be divided into several collision domains, interconnected by bridges and switches. A multiple access protocol is not required in a switched full-duplexnetwork, such as today's switched Ethernet networks, but is often available in the equipment for compatibility reasons.
Common multiple access protocols
Examples of common
packet modemultiple access protocols for wired multi-drop networks are:
CSMA/CD(used in Ethernetand IEEE 802.3)
Token bus( IEEE 802.4)
Token ring(IEEE 802.5)
Token passing(used in FDDI)
Examples of common multiple access protocols that may be used in packet radio wireless networks are:
CSMA/CA(used in IEEE 802.11/ WiFiWLANs)
* Slotted ALOHA
For a more extensive list, see List of channel access methods.
Channel access method
*Token Ring and
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