The LAN Layer

The LAN layer, at its upper end, supplies to the network layer a mechanism for addressing a packet and sending it from one station to another. At its lower end, it handles interactions with the physical layer. The LAN layer covers packet addressing, delivery and receipt, forwarding, error detection, collision detection and collision-related retransmission attempts.

In IEEE protocols, the LAN layer is divided into the media access control, or MAC, sublayer and a higher logical link control, or LLC, sublayer for higher-level flow-control functions that today have moved largely to the transport layer. For example, the HDLC protocol supports sliding windows (6.2 Sliding Windows) as an option, as did the early X.25 protocol. ATM, 3.5 Asynchronous Transfer Mode: ATM, also supports some higher-level functions, though not sliding windows.

Because the LLC layer is so often insignificant, and because the most well-known LAN-layer functions are in fact part of the MAC sublayer, it is common to identify the LAN layer with its MAC sublayer, especially for IEEE protocols where the MAC layer has official standing. In particular, LAN-layer addresses are perhaps most often called MAC addresses. Generally speaking, much of the operation of the LAN/MAC layer takes place in the network card. Host systems (including drivers) are, for example, generally oblivious to collisions (although they may query the card for collision statistics). In some cases, eg with Wi-Fi rate scaling, the host-system driver may get involved.