802.11 Wireless Networks: The Definitive Guide, 2nd Edition

Chapter 16. 802.11 Hardware

When writing a specification, it is important to leave some room for interpretation to allow innovation. A standard which is too rigid will drain the life out of implementation, while a standard that is too loose does not foster interoperability. This chapter is about what the standard does not exactly specify. How does hardware implement the standard? Where does the standard allow discretion in protocol implementation, and what does that mean for the administrator?

General Structure of an 802.11 Interface

Figure 16-1shows a block diagram of a generic wireless LAN interface. It is not representative of any particular manufacturer’s product, but is intended to serve as a general guide to the discussion of how cards are put together. Cards must implement the physical interface and the link-layer control expected by the operating system.

Like any other system based on radio technology, wireless LAN interfaces have antennas. Most 802.11 interfaces have two antennas for antenna diversity to improve reception in the presence of multipath interference. When a radio signal is received, the radio system selects the antenna with the strongest signal, and runs with it. Antenna diversity alleviates multipath because one of the two antennas should receive a signal if the weak signal is caused by self-interference of multiple signals. Diversity cannot help if the signal weakness is caused by distance because both antennas will receive equally weak signals. Many types ...