IEEE 802 Network Technology Family Tree

802.11 is a member of the IEEE 802 family, which is a series of specifications for local area network (LAN) technologies. Figure 2-1 shows the relationship between the various components of the 802 family and their place in the OSI model.

Figure 2-1. The IEEE 802 family and its relation to the OSI model

IEEE 802 specifications are focused on the two lowest layers of the OSI model because they incorporate both physical and data link components. All 802 networks have both a MAC and a Physical (PHY) component. The MAC is a set of rules to determine how to access the medium and send data, but the details of transmission and reception are left to the PHY.

Individual specifications in the 802 series are identified by a second number. For example, 802.3 is the specification for a Carrier Sense Multiple Access network with Collision Detection (CSMA/CD), which is related to (and often mistakenly called) Ethernet, and 802.5 is the Token Ring specification. Other specifications describe other parts of the 802 protocol stack. 802.2 specifies a common link layer, the Logical Link Control (LLC), which can be used by any lower-layer LAN technology. Management features for 802 networks are specified in 802.1. Among 802.1's many provisions are bridging (802.1D) and virtual LANs, or VLANs (802.1Q).

802.11 is just another link layer that can use the 802.2/LLC encapsulation. The base 802.11 specification includes the 802.11 MAC and two physical layers: a frequency-hopping spread-spectrum (FHSS) physical layer and a direct-sequence spread-spectrum (DSSS) link layer. Later revisions to 802.11 added additional physical layers. 802.11b specifies a high-rate direct-sequence layer (HR/DSSS); products based on 802.11b hit the marketplace in 1999 and was the first mass-market PHY. 802.11a describes a physical layer based on orthogonal frequency division multiplexing (OFDM); products based on 802.11a were released as the first edition of this book was completed. 802.11g is the newest physical layer on the block. It offers higher speed through the use of OFDM, but with backwards compatibility with 802.11b. Backwards compatibility is not without a price, though. When 802.11b and 802.11g users coexist on the same access point, additional protocol overhead is required, reducing the maximum speed for 802.11g users.

To say that 802.11 is "just another link layer for 802.2" is to omit the details in the rest of this book, but 802.11 is exciting precisely because of these details. 802.11 allows for mobile network access; in accomplishing this goal, a number of additional features were incorporated into the MAC. As a result, the 802.11 MAC may seem baroquely complex compared to other IEEE 802 MAC specifications.

The use of radio waves as a physical layer requires a relatively complex PHY, as well. 802.11 splits the PHY into two generic PMcomponents: the Physical Layer Convergence Procedure (PLCP), to map the MAC frames onto the medium, and a Physical Medium Dependent (PMD) system to transmit those frames. The PLCP straddles the boundary of the MAC and physical layers, as shown in Figure 2-2. In 802.11, the PLCP adds a number of fields to the frame as it is transmitted "in the air."

Figure 2-2. PHY components

All this complexity begs the question of how much you actually need to know. As with any technology, the more you know, the better off you will be. The 802.11 protocols have many knobs and dials that you can tweak, but most 802.11 implementations hide this complexity. Many of the features of the standard come into their own only when the network is congested, either with a lot of traffic or with a large number of wireless stations. Networks are increasingly pushing the limits in both respects. At any rate, I can't blame you for wanting to skip the chapters about the protocols and jump ahead to the chapters about planning and installing an 802.11 network. After you've read this chapter, you can skip ahead to Chapters 17-23 and return to the chapters on the protocol's inner workings when you need (or want) to know more.