Layer 2. The Data Link Layer

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This layer concerns itself with the procedures and protocols for operating the communications channels (transmission protocol knowledge and management), and provides error detection / correction and synchronization for the Physical Level. Devices at this layer include switches, bridges, and intelligent hubs. The Data Link layer consists of two sublayers: The Logical Link Control (LLC) and the Media Access Control (MAC). The LLC sublayer is in charge of establishing and maintaining links between communicating devices. Ethernet and Token Ring protocols operate in the Data Link Layer, as do protocols such as ATM, CSMA/CD, FDDI, PPP and SLIP. The MAC sublayer is responsible for framing data. Computers or other communicating devices identify themselves via MAC addresses and Network Interface Cards (NICs), while the Data Link Layer at the same time organizes information from the higher layers into "blocks of bits," called frames (such as the "Synchronous Data Link Control" frame), for orderly transfer and error control. Sometimes this involves bit-stuffing to pad out strings of 1's.

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