Field Guide to Wireless LANs for Administrators and Power Users, A

Higher-Layer Protocol Encapsulation

The frame payload of the IEEE 802.11 MPDU must always start with an LLC sub-layer protocol header, since the IEEE 802.11 MAC sub-layer protocol header has no ability to describe the encapsulated higher layer protocol. The LLC sub-layer protocol identifies its higher layer protocol payloads using assigned Service Access Point (SAP) values. Certain higher layer protocols can be layered directly over the LLC sub-layer protocol header (e.g., Novell's Internetwork Packet Exchange (IPX), NetBEUI, etc.).

Due to the small size of the LLC SAP fields (eight bits each, of which only six bits are really usable^[19] to enumerate higher layer protocols), the LLC sub-layer protocol provides an “escape valve” by defining the ...