  in the seven-layer osi model of computer networking, the logical link control (llc) data communication protocol layer is the upper sublayer of the data link layer, which is itself layer 2. the llc sublayer provides multiplexing mechanisms that make it possible for several network protocols (ip, ipx, decnet and appletalk) to coexist within a multipoint network and to be transported over the same network medium. it can also provide flow control and automatic repeat request (arq) error management mechanisms. the llc sublayer acts as an interface between the media access control (mac) sublayer and the network layer. ==operation== the llc sublayer is primarily concerned with: * multiplexing protocols transmitted over the mac layer (when transmitting) and decoding them (when receiving). * providing node-to-node flow and error control in today's networks, flow control and error management is typically taken care of by a transport layer protocol such as tcp, or by some application layer protocol, in an end-to-end fashion, i.e. retransmission is done from source to end destination. this implies that the need for llc sublayer flow control and error management has reduced. llc is consequently only a multiplexing feature in today's link layer protocols. an llc header tells the data link layer what to do with a packet once a frame is received. it works like this: a host will receive a frame and look in the llc header to find out to what protocol stack the packet is destined - for example, the ip protocol at the network layer or ipx. however, today most non-ip network protocols are abandoned. ==application examples== ===x.25 and lapb=== an llc sublayer was a key component in early packet switching networks such as x.25 networks with the lapb data link layer protocol, where flow control and error management were carried out in a node-to-node fashion, meaning that if an error was detected in a frame, the frame was retransmitted from one switch to next instead. this extensive handshaking between the nodes made the networks slow. ===local area network (lan) and metropolitan area network (man) protocols=== the ieee 802.2 standard specifies the llc sublayer for all ieee 802 local area networks, such as ieee 802.3/ethernet (if the ethertype field is not used), ieee 802.5, and ieee 802.11. ieee 802.2 is also used in some non-ieee 802 networks such as fddi. ====ethernet ==== since bit errors are very rare in wired networks, ethernet does not provide flow control or automatic repeat request (arq), meaning that incorrect packets are detected but only cancelled, not retransmitted (except in case of collisions detected by the csma/cd mac layer protocol). in
Logical_link_control