Editing Medium access control (section)

{{short description|Service layer in IEEE 802 network standards}}
{{Distinguish|Mandatory access control}}

{{OSI model|data}}
{{IPstack}}

In [[IEEE 802|IEEE 802 LAN/MAN standards]], the '''medium access control''' ('''MAC'''), also called '''media access control''', is the layer that controls the hardware responsible for interaction with the [[wired communication|wired]] (electrical or optical) or wireless [[transmission medium]]. The MAC sublayer and the [[logical link control]] (LLC) sublayer together make up the [[data link layer]]. The LLC provides [[flow control (data)|flow control]] and [[multiplexing]] for the logical link (i.e. [[EtherType]], [[802.1Q VLAN tag]] etc), while the MAC provides flow control and multiplexing for the transmission medium.

These two sublayers together correspond to layer 2 of the [[OSI model]]. For compatibility reasons,<!--backwards compatibility with Ethernet II/DIX (which pre-dates both 802.2/LLC and 802.3/"Ethernet")--> LLC is optional for implementations of [[IEEE 802.3]] (the frames are then "raw"), but compulsory for implementations of other IEEE 802 physical layer standards. Within the hierarchy of the OSI model and IEEE 802 standards, the MAC sublayer provides a [[control abstraction]] of the physical layer such that the complexities of physical link control are invisible to the LLC and upper layers of the network stack. Thus any LLC sublayer (and higher layers) may be used with any MAC. In turn, the medium access control block is formally connected to the [[PHY]] via a [[media-independent interface]]. Although the MAC block is today typically integrated with the PHY within [[system in package|the same device package]], historically any MAC could be used with any PHY, independent of the transmission medium.

When sending data to another device on the network, the MAC sublayer encapsulates higher-level frames into frames appropriate for the transmission medium (i.e. the MAC adds a [[syncword]] preamble and also padding if necessary), adds a [[frame check sequence]] to identify transmission errors, and then forwards the data to the physical layer as soon as the appropriate [[channel access method]] permits it. For topologies with a [[collision domain]] (bus, ring, mesh, point-to-multipoint topologies), controlling when data is sent and when to wait is necessary to avoid [[Collision (telecommunications)|collisions]]. Additionally, the MAC is also responsible for compensating for collisions by initiating retransmission if a [[jam signal]] is detected. When receiving data from the physical layer, the MAC block ensures data integrity by verifying the sender's frame check sequences, and strips off the sender's preamble and padding before passing the data up to the higher layers.

{{Multiplex techniques}}