Media access control

The OSI model
7	Application layer
6	Presentation layer
5	Session layer
4	Transport layer
3	Network layer
2	Data link layer LLC sublayer MAC sublayer
1	Physical layer
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The media access control (MAC) data communication protocol sub-layer, also known as the medium access control, is a sublayer of the data link layer specified in the seven-layer OSI model (layer 2), and in the four-layer TCP/IP model (layer 1). It provides addressing and channel access control mechanisms that make it possible for several terminals or network nodes to communicate within a multiple access network that incorporates a shared medium, e.g. Ethernet. The hardware that implements the MAC is referred to as a medium access controller.

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 multi-point network. This channel may provide unicast, multicast or broadcast communication service.

Functions performed in the MAC layer

According to 802.3-2002 section 4.1.4, the functions required of a MAC are:^[1]

• receive/transmit normal frames
• half-duplex retransmission and backoff functions
• append/check FCS (frame check sequence)
• interframe gap enforcement
• discard malformed frames
• append(tx)/remove(rx) preamble, SFD, and padding
• half-duplex compatibility: append(tx)/remove(rx) MAC address

In 100Mbps and faster MACs, the MAC address is not actually handled in the MAC layer. Doing so would make it impossible to implement IP because the ARP layer of IP-Ethernet needs access to the MAC address.

Addressing mechanism

In 100Mbps and faster Ethernet MACs, there is no required addressing mechanism. However, the MAC address inherited from the original MAC layer specification is used in many higher level protocols such as Internet Protocol (IP) over Ethernet.

The local network address used in IP-Ethernet is called MAC address because it historically was part of the MAC layer in early Ethernets. The MAC layer's addressing mechanism is called physical address or MAC address. A MAC address is a unique serial number. Once a MAC address has been assigned to a particular network interface (typically at time of manufacture), that device should be uniquely identifiable amongst all other network devices in the world. This guarantees that each device in a network will have a different MAC address (analogous to a street address). This makes it possible for data packets to be delivered to a destination within a subnetwork, i.e. hosts interconnected by some combination of repeaters, hubs, bridges and switches, but not by IP routers. Thus, when an IP packet reaches its destination (sub)network, the destination IP address (a layer-3, network layer, construct) is resolved into the MAC address (a layer-2 construct) of the destination host.

An example of a physical network is an Ethernet network, perhaps extended by wireless local area network (WLAN) access points 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 same physical medium to share it. Examples of shared physical media are bus networks, ring networks, hub networks, wireless networks and half-duplex point-to-point links. The multiple access protocol may detect or avoid data packet collisions if a packet mode contention based channel access method is used, or reserve resources to establish a logical channel if a circuit switched or channelization based channel access method is used. The channel access control mechanism relies on a physical layer multiplex 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 switched full-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 in Ethernet and 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/WiFi WLANs)
• Slotted ALOHA
• Dynamic TDMA
• Reservation ALOHA (R-ALOHA)
• Mobile Slotted Aloha (MS-ALOHA)
• CDMA
• OFDMA

For a more extensive list, see List of channel access methods.

See also

• MAC-Forced Forwarding
• MLME
• SoftMAC

References

1. ^ IEEE 802.3

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.