(Media Access Control layer) The part of the data link protocol that controls access to the physical transmission medium in IEEE 802 networks (LANs). The common MAC layer standards are CSMA/CD and CSMA/CA, which are used in Ethernet and Wi-Fi. Earlier MAC layers were the token passing methods in Token Ring, FDDI and MAP. MAC layer functionality is built into the network adapters, which includes a unique serial number that identifies the vendor and adapter (see OUI). See data link protocol, OSI model, CSMA/CD, CSMA/CA and Ethernet.