A Field Guide to Wireless LANs for Administrators and Power Users

Introduction to IEEE 802.11's MAC Sub-layer Frame Structure

Besides defining access control and resource sharing capabilities for multiple stations contending for a common LAN medium, one of the other key operations performed by 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 Data Link layer headers, and appending a Data Link layer trailer, which is often simply an error-detecting sequence.

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

• MAC sub-layer Destination Address (i.e., MAC-DA)

• MAC sub-layer Source Address (i.e., MAC-SA)

Another common Data Link layer header element is a "type" or "protocol" field that indicates which kind of higher-layer packet is embedded in the frame, but strictly speaking, this is an LLC sub-layer^[20] function, and IEEE 802.11 only specifies a MAC sub-layer protocol. In fact, the IEEE 802.3 standard specifies two versions of Ethernet, wherein one variety interprets the two-octet field following the MAC-SA field as a "Type", and the other variety of the Ethernet header interprets this two-octet field as a "Length" value, which pushes the higher-layer protocol demultiplexing tasks up to the LLC sub-layer.

^[20] Note that the author did not say "LLC sub-layer protocol."

The LLC sub-layer protocol provides a form of demultiplexing for higher layer protocols via its two one-octet Destination and Source Service Access Point (DSAP and SSAP) fields. Due to the LLC sub-layer protocol's limited number of SAP values, there is another protocol in the LLC sub-layer known as the Subnetwork Access Protocol (SNAP), which has a full two-octet Type field and is layered on top of the LLC protocol. SNAP also has an OUI field, which means that everyone who acquires an OUI has access to the complete set of 2¹⁶ (i.e., 65,536) Type values, while setting the OUI to 0x000000 causes the Type field to be interpreted as if it were the Ethernet Type field.

IEEE 802.11 MAC Sub-layer Protocol's Idiosyncrasies

One cannot avoid comparing IEEE 802.11 to Ethernet. On one hand, Ethernet is the best-known example of a LAN technology, so it is a logical basis for comparison, and on the other hand, IEEE 802.11 is sometimes misleadingly referred to as "wireless Ethernet." The latter moniker is genuinely misleading. Other than the fact that IEEE 802.3 and IEEE 802.11 both define MAC sub-layer protocols and their associated PHYs, there is almost nothing in common between IEEE 802.3 and IEEE 802.11.

• The IEEE 802.11 MAC sub-layer protocol has no higher-layer protocol demultiplexing features of its own, and thus requires the presence of the IEEE 802.2 LLC sub-layer protocol headers to perform the higher-layer protocol demultiplexing function. The presence of the IEEE 802.2 SNAP sub-layer protocol's headers is optional.

• In wireless LANs based on IEEE 802.11, the stations must explicitly "join" the LAN, which is the equivalent of plugging in an Ethernet cable. The procedures that govern the joining and leaving processes have no exact equivalent in the world of the Ethernet MAC protocol.

  • To be fair, Ethernet does have an "autonegotiation" feature that allows two Ethernet peer entities to determine the best combination of speed (e.g., 10, 100, or 1000 Mbps) and duplex setting (e.g., full- or half-duplex). However, this negotiation happens below the MAC sub-layer.

• There is no implicit or explicit form of authentication in Ethernet as there is in IEEE 802.11.