11 MAC Fundamentals

MAC Fundamentals

This chapter begins our exploration of the 802.11 standard in depth. Chapter 2 provided a high-level overview of the standard and discussed some of its fundamental attributes. You are now at a fork in the book. Straight ahead lies a great deal of information on the 802.11 specification and the various related standards that it uses liberally. It is possible, however, to build a wired network without a thorough and detailed understanding of the protocols, and the same is true for wireless networks. However, there are a number of situations in which you may need a deeper knowledge of the machinery under the hood:

• Although 802.11 has been widely and rapidly adopted, security issues have continued to grab headlines. Network managers will undoubtedly be asked to comment on security issues, especially in any wireless LAN proposals. To understand and participate in these discussions, read Chapters 5 and 6. WEP with static keys should be considered fully broken. Solutions based on 802.1X and dynamic WEP keying are significantly stronger, with the full complement of protocols in 802.11i described in Chapter 7 stronger still.
• Troubleshooting wireless networks is similar to troubleshooting wired networks but can be much more complex. As always, a trusty packet sniffer can be an invaluable aid. To take full advantage of a packet sniffer, though, you need to understand what the packets mean to interpret your network's behavior.
• Tuning a wireless network is tied intimately to a number of parameters in the specification, as well as the behavior of the underlying radio technology. To understand the behavior of your network and what effect the optimizations will have requires a knowledge of what those parameters really do and how radio waves travel throughout your environment.
• Device drivers may expose low-level knobs and dials for you to play with. Most drivers provide good defaults for all of the parameters, but some give you freedom to experiment. Open source software users have the source code and are free to experiment with any and all settings.
• Wireless LAN technology is developing rapidly, and new protocol features are constantly being added. A solid understanding of the base protocol allows you to understand how new features will function and what they will mean for your network.

As with many other things in life, the more you know, the better off you are. Ethernet is usually trouble-free, but serious network administrators have long known that when you do run into trouble, there is no substitute for thorough knowledge of how the network is working. When the first edition of this book was out, wireless LANs had been given a "free ride." Because they were cool, users were forgiving when they failed; wireless connectivity was a privilege, not a right. And since there were relatively few networks and relatively few users on those networks, the networks were rarely subjected to severe stresses. An Ethernet that has only a half dozen nodes is not likely to be a source of problems; problems occur when you add a few high-capacity servers, a few hundred users, and the associated bridges and routers to glue everything together. As the typical 802.11 network grew up from an access point or two serving a dozen users into a much larger network designed to provide seamless coverage throughout a building, the stresses on the equipment and protocols has become much more apparent.

That is why you should read this chapter. Now on to the details. The key to the 802.11 specification is the MAC. It rides on every physical layer and controls the transmission of user data into the air. It provides the core framing operations and the interaction with a wired network backbone. Different physical layers may provide different transmission speeds, all of which are supposed to interoperate.

802.11 does not depart from the previous IEEE 802 standards in any radical way. The standard successfully adapts Ethernet-style networking to radio links. Like Ethernet, 802.11 uses a carrier sense multiple access (CSMA) scheme to control access to the transmission medium. However, collisions waste valuable transmission capacity, so rather than the collision detection (CSMA/CD) employed by Ethernet, 802.11 uses collision avoidance (CSMA/CA). Also like Ethernet, 802.11 uses a distributed access scheme with no centralized controller. Each 802.11 station uses the same method to gain access to the medium. The major differences between 802.11 and Ethernet stem from the differences in the underlying medium.

This chapter provides some insight into the motivations of the MAC designers by describing some challenges they needed to overcome and describes the rules used for access to the medium, as well as the basic frame structure. If you simply want to understand the basic frame sequences that you will see on an 802.11 network, skip ahead to the end of this chapter. For further information on the MAC, consult its formal specification in Clause 9 of the 802.11 standard; detailed MAC state diagrams are in Annex C.