The MAC sublayer

The MAC sublayer defines the processes that enable the hardware components such as the NIC to access media (wire) on the network. MAC is responsible for handling data encapsulation and media access control.

The data encapsulation phase adds a layer 2 header, which contains the source and destination MAC addresses, inserting a preamble which is used for bit synchronization and helps any receiving devices identify the start of a bit. Finally, a trailer is applied to the end that is used for error handling during transmission of traffic across the network, as shown:

The second function of the MAC sublayer is to handle how frames access physical media, and how they are removed and pass upward to the LLC to the rest of the host system.