802.11 Wireless Networks- The Definitive Guide

How This Book Organized

Chapter 1 lists ways in which wireless networks are different from traditional wired networks and discusses the challenges faced when adapting to fuzzy boundaries and unreliable media. Wireless LANs are perhaps the most interesting illustration of Christian Huitema's assertion that the Internet has no center, just an ever-expanding edge. With wireless LAN technology becoming commonplace, that edge is now blurring.

Chapter 2 describes the overall architecture of 802.11 wireless LANs. 802.11 is somewhat like Ethernet but with a number of new network components and a lot of new acronyms. This chapter introduces you to the network components that you'll work with. Broadly speaking, these components are stations (mobile devices with wireless cards), access points (glorified bridges between the stations and the distribution system), and the distribution system itself (the wired backbone network). Stations are grouped logically into Basic Service Sets (BSSs). When no access point is present, the network is a loose, ad-hoc confederation called an independent BSS (IBSS). Access points allow more structure by connecting disparate physical BSSs into a further logical grouping called an Extended Service Set (ESS).

Chapter 3 describes the Media Access Control (MAC) layer of the 802.11 standard in detail. 802.11, like all IEEE 802 networks, splits the MAC-layer functionality from the physical medium access. Several physical layers exist for 802.11, but the MAC is the same across all of them. The main mode for accessing the network medium is a traditional contention-based access method, though it employs collision avoidance (CSMA/CA) rather than collision detection (CSMA/CD). The chapter also discusses data encapsulation in 802.11 frames and helps network administrators understand the frame sequences used to transfer data.

Chapter 4 builds on the end of Chapter 3 by describing the various frame types and where they are used. This chapter is intended more as a reference than actual reading material. It describes the three major frame classes. Data frames are the workhorse of 802.11. Control frames serve supervisory purposes. Management frames assist in performing the extended operations of the 802.11 MAC. Beacons announce the existence of an 802.11 network, assist in the association process, and are used for authenticating stations.

Chapter 5 describes the Wired Equivalent Privacy protocol. By default, 802.11 networks do not provide any authentication or confidentiality functions. WEP is a part of the 802.11 standard that provides rudimentary authentication and confidentiality features. Unfo rtunately, it is severely flawed. This chapter discusses what WEP is, how it works, and why you can't rely on it for any meaningful privacy or security.

Chapter 6 describes 802.1x, which is a new attempt to solve the authentication and confidentiality problem on LANs. 802.1x will serve as the basis for an authentication framework for 802.11, but the adaptation is currently being carried out.

Chapter 7 describes the management operations on 802.11 networks. To find networks to join, stations scan for active networks announced by access points or the IBSS creator. Before sending data, stations must associate with an access point. This chapter also discusses the power-management features incorporated into the MAC that allow battery-powered stations to sleep and pick up buffered traffic at periodic intervals.Chapter 8 describes the point coordination function. The PCF is not widely implemented, so this chapter can be skipped for most purposes. The PCF is the basis for contention-free access to the wireless medium. Contention-free access is like a centrally controlled, token-based medium, where access points provide the "token" function.

Chapter 9 describes the general architecture of the physical layer (PHY) in the 802.11 model. The PHY itself is broken down into two "sublayers." The Physical Layer Convergence Procedure (PLCP) adds a preamble to form the complete frame and its own header, while the Physical Medium Dependent (PMD) sublayer includes modulation details. The most common PHYs use radio frequency (RF) as the wireless medium, so the chapter closes with a short discussion on RF systems and technology that can be applied to any PHY discussed in the book.

Chapter 10 describes the three physical layers that have been used in 802.11 networks up through late 2001. These include the frequency hopping spread spectrum (FHSS) physical layer, the direct sequence spread spectrum (DSSS) physical layer, and the high-rate direct sequence spread spectrum (HR/DSSS) physical layer, which is defined by the 802.11b standard. Of these, the 11-Mbps HR/DSSS layer is most widely used at present.

Chapter 11 describes the 5-GHz PHY standardized with 802.11a, which operates at 54 Mbps. This physical layer uses another modulation technique known as orthogonal frequency division multiplexing (OFDM). OFDM is also the basis for a 54-Mbps standard known as 802.11g, which operates in the same frequency bands as the other 802.11 physical layers. 802.11a products started to appear in late 2001; 802.11g products will probably appear in late 2002. It's a good bet that one of these standards will supplant 802.11b, just as 100BaseT Ethernet has supplanted 10BaseT.

Chapter 12 describes the basic driver installation procedure in Windows. It also illustrates how some drivers allow reconfiguration of the 802.11 MAC parameters discussed in Chapters 3-7.
Chapter 13 discusses how to install 802.11 support on a Linux system. It discusses the Linux-WLAN-NG project, which provides support for cards based on Intersil's PRISM and PRISM2 chip sets. It also discusses the wireless driver that Lucent provides for their wireless cards (Lucent goes under many names, including WaveLAN, Orinoco, and Agere), and it discusses how to install PCMCIA support.

Chapter 14 describes the equipment used on the infrastructure end of 802.11 networks. Commercial access point products have varying features. This chapter describes the common features of access points, offers buying advice, and presents two practical configuration examples.

Chapter 15 suggests a process by which a wireless LAN could be installed. One of the key advantages of a wireless network is mobility. Mobility can be guaranteed only when all wireless stations reside on the same logical IP network. (This may require readdressing; it almost certainly requires renumbering to free a large contiguous addressspace.) Corporations deploying 802.11 must naturally be concerned with security. This chapter also discusses various aspects of network planning, including capacity management (how many users can you support, and what bandwidth can they expect?), site surveys, and physical details such as antennas and transmission lines.

Chapter 16 teaches administrators how to recognize what's going on with their wireless LANs. Network analyzers have proven their worth time and time again on wired networks. Wireless network analyzers are just as valuable a tool for 802.11 networks. This chapter discusses how to use wireless network analyzers and what certain symptoms may indicate. It also describes how to build an analyzer using Ethereal. Finally, AirSnort is a tool that allows recovery of WEP keys and is something that readers should be aware of, if only for its security implications when used by others.

Chapter 17 describes how network administrators can change commonly exposed 802.11 parameters. It revisits each parameter and discusses what changing the parameter will do to the wireless network.

Chapter 18 summarizes the standardization work pending in the 802.11 working group. After summarizing the work in progress, I get to prognosticate and hope that I don't have to revise this too extensively in future editions.

Appendix A is a description of the MAC MIB. A number of parameters in the MAC can be changed by the network administrator using standard SNMP tools. This appendix follows the style I have used in my T1 book to show the parameters and call out the important parameters.
Appendix B describes Apple's popular AirPort system. Apple's aggressive pricing of AirPort hardware was one of the most important events in the story of 802.11. AirPort base stations are fully compliant with 802.11 and can be used to build a network for any 802.11-compliant wireless device. Apple has also included a dedicated slot on all of their recent hardware for AirPort cards, which makes adding 802.11 interfaces to Apple hardware a snap. No book xabout 802.11 would be complete without a description of the AirPort.
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