Encapsulation of Higher-Layer Protocols Within 802.11

Like all other 802 link layers, 802.11 can transport any network-layer protocol. Unlike Ethernet, 802.11 relies on 802.2 logical-link control (LLC) encapsulation to carry higher-level protocols. Figure 3-13 shows how 802.2 LLC encapsulation is used to carry an IP packet. In the figure, the "MAC headers" for 802.1H and RFC 1042 might be the 12 bytes of source and destination MAC address information on Ethernet or the long 802.11 MAC header from the previous section.

Figure 3-13. IP encapsulation in 802.11

Two different methods can be used to encapsulate LLC data for transmission. One is described in RFC 1042, and the other in 802.1H. Both standards may go by other names. RFC 1042 is sometimes referred to as IETF encapsulation, while 802.1H is sometimes called tunnel encapsulation.

As you can see in Figure 3-13, though, the two methods are quite similar. An Ethernet frame is shown in the top line of Figure 3-13. It has a MAC header composed of source and destination MAC addresses, a type code, the embedded packet, and a frame check field. In the IP world, the Type code is either 0x0800 (2048 decimal) for IP itself, or 0x0806 (2054 decimal) for the Address Resolution Protocol (ARP).

Both RFC 1042 and 802.1H are derivatives of 802.2's sub-network access protocol (SNAP). The MAC addresses are copied into the beginning of the encapsulation frame, and then a SNAP header is inserted. SNAP headers begin with a destination service access point (DSAP) and a source service access point (SSAP). After the addresses, SNAP includes a Control header. Like high-level data link control (HDLC) and its progeny, the Control field is set to 0x03 to denote unnumbered information (UI), a category that maps well to the best-effort delivery of IP datagrams. The last field inserted by SNAP is an organizationally unique identifier (OUI). Initially, the IEEE hoped that the 1-byte service access points would be adequate to handle the number of network protocols, but this proved to be an overly optimistic assessment of the state of the world. As a result, SNAP copies the type code from the original Ethernet frame. The only difference between 802.1H and RFC 1042 is the OUI used.

At one point, many products offered the option to switch between the two encapsulation standards, though this option is much less common. Microsoft operating systems default to using 802.1H for the AppleTalk protocol suite and IPX, and use RFC 1042 for all other protocols. Most access points now conform to the Microsoft behavior, and no longer have an option to switch encapsulation type. In fact, the Microsoft encapsulation selection is so widely supported that it was part of the Wi-Fi Alliance's certification test suite at one point.