Chapter 5. Data Link Layer

The Data Link Layer is usually depicted as having two sublayers: Media Access Control (MAC) and Logical Link Control (LLC), though additional sublayers are becoming more common (see Figure 5-1). The Media Access Control sublayer was described in the preceding chapter on Ethernet, and is the lowest sublayer. Above the MAC Layer is an increasingly more common (optional) MAC Control sublayer. If the MAC Layer for the interface is capable, then another optional sublayer called Operations, Administration, and Maintenance (OAM) may be used. OAM was added to Ethernet in conjunction with Ethernet in the First Mile (EFM) implementations in 802.3 Clauses 56–67. The MAC, MAC Control, and OAM sublayers apply to a single interface. Also optionally, a link aggregation may be created that spans multiple interfaces and appears as a single logical interface to higher layers. Above that, as the highest sublayer in the Data Link Layer, is where the MAC Control Client resides.