- Media Access Control
The Media Access Control (MAC)
data communication protocol sub-layer, also known as the Medium Access Control, is a sublayer of theData Link Layer specified in theseven-layer OSI model (layer 2). It provides addressing andchannel access control mechanisms that make it possible for several terminals or network nodes to communicate within a multipoint network, typically alocal area network (LAN) ormetropolitan area network (MAN).The MAC sub-layer acts as an interface between the
Logical Link Control (LLC) sublayer and the network'sphysical layer .The MAC layer emulates a full-duplex logical communication channel in a multipoint network. This channel may provideunicast ,multicast orbroadcast communication service.Addressing mechanism
The MAC layer addressing mechanism is called physical address or
MAC address . This is a unique serial number assigned to eachnetwork adapter , making it possible to deliver data packets to a destination within asubnetwork , i.e. a physical network consisting of one or several network segments interconnected byrepeater s,hub s,bridge s andswitch es, but not by IProuter s. 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-point communication, 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 samephysical medium to share it. Examples of shared physical media arebus network s,ring network s,hub network s,wireless networks andhalf-duplex point-to-point links. The multiple access protocol may detect or avoid data packet collisions if a packet mode contention basedchannel access method is used, or reserve resources to establish a logical channel if acircuit switched or channelization based channel access method is used. The channel access control mechanism relies on a physical layermultiplex scheme.The most widespread multiple access protocol is the contention based
CSMA/CD protocol 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 switchedfull-duplex network, 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 mode multiple access protocols for wired multi-drop networks are:
*CSMA/CD (used inEthernet andIEEE 802.3 )
*Token bus (IEEE 802.4 )
*Token ring (IEEE 802.5)
*Token passing (used inFDDI )Examples of common multiple access protocols that may be used in packet radio wireless networks are:
*CSMA/CA (used inIEEE 802.11 /WiFi WLANs)
* Slotted ALOHA
*Dynamic TDMA
*Reservation ALOHA (R-ALOHA)
*CDMA
*OFDMA For a more extensive list, see List of channel access methods.
ee also
*
Channel access method
*MAC address
*Ethernet and CSMA/CD
*Token Ring andtoken passing
* CSMA/CA
*MAC-Forced Forwarding
*MLME
*SoftMAC
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