• The MAC sublayer acts as an interface between the logical link control (LLC) sublayer and the network's physical layer. The MAC layer emulates a full-duplex logical communication channel in a multi-point network. This channel may provide unicast, multicast or broadcast communication service.
• One of the key operations performed by the MAC sub-layer of the Data Link layer is known as "framing," which is the process of encapsulating the higher-layer protocol packet (which may or may not be a Network layer protocol packet) in a set of MAC sub-layer headers, and appending a MAC sub-layer trailer, which most often is simply an error-detecting sequence.
• Framing also includes defining an interface to the Physical layer that can support the successful exchange of long sequences of bits across the Physical medium, which in the case of WLANs is especially unreliable.

The Data Link layer header will include at least the following three items:

• MAC sub-layer Destination Address (MAC-DA)
• MAC sub-layer Source Address (MAC-SA)
• Information to identify the higher-layer protocol payload (carried in the LLC sub-layer, by either the LLC sub-layer protocol or the LLC sub-layer protocol in conjunction with the Sub-Network Access Protocol (SNAP)

The first two items are MAC sub-layer items, and the third item in the list is frequently found in the form of a "type" or "protocol" field that indicates which kind of higher layer packet is embedded in the frame. Strictly speaking, this is an LLC sub-layer function. Note that I did not say "LLC sub-layer protocol," since it is possible to have Data Link protocols that are not divided into LLC and MAC sub-layers. The original Ethernet is an example of such a Data Link layer protocol.