Once authentication has completed, stations can associate with an access point (or reassociate with a new access point) to gain full access to the network. Association is a recordkeeping procedure that allows the distribution system to track the location of each mobile station, so frames destined for the mobile station can be forwarded to the correct access point. After association completes, an access point must register the mobile station on the network so frames for the mobile station are delivered to the access point. One method of registering is to send a gratuitous ARP so the station's MAC address is associated with the switch port connected to the access point.
Association is restricted to infrastructure networks and is logically equivalent to plugging into a wired network. Once the procedure is complete, a wireless station can use the distribution system to reach out to the world, and the world can respond through the distribution system. 802.11 explicitly forbids associating with more than one access point.
The basic association procedure is shown in Figure 8-9.
Figure 8-9. Association procedure
Like authentication, association is initiated by the mobile station. No sequence numbers are needed because the association process is a three-step exchange. The two frames are management frame subtypes defined by the specification. As unicast management frames, both steps in the association procedure are composed of an association frame and the required link-layer acknowledgment:
Reassociation is the process of moving an association from an old access point to a new one. Over the air, it is almost the same as an association; on the backbone network, however, access points may interact with each other to move frames. When a station moves from the coverage area of one access point to another, it uses the reassociation process to inform the 802.11 network of its new location. The procedure is shown in Figure 8-10.
Figure 8-10. Reassociation procedure
The mobile station begins the procedure associated with an access point. The station monitors the quality of the signal it receives from that access point, as well as the signal quality from other access points in the same ESS. When the mobile station detects that another access point would be a better choice, it initiates the reassociation procedure. The factors used to make that decision are product-dependent. Received signal strength can be used on a frame-by-frame basis, and the constant Beacon transmissions provide a good baseline for signal strength from an access point. Before the first step, the mobile station must authenticate to the new access point if it has not done so already.
Figure 8-10 depicts the following steps:
Reassociation is also used to rejoin a network if the station leaves the coverage area and returns later to the same access point. Figure 8-11 illustrates this scenario.
Figure 8-11. Reassociation with the same access point
What Is Roaming?
Roaming is not a word used in the 802.11 standard at all. (A task group recently formed to address roaming issues, but it is far from completing its work.) However, people use the word roaming informally a great deal when talking about 802.11. Generally speaking, most people are referring to the process of moving from one access point to another.
Roaming has suffered a bit from "buzzword overload" in the past few years, and now means different things to different speakers. At the most basic level, roaming is the process of moving a station between APs. How does a station decide to move between APs? The 802.11 standard has nothing to say on the matter. Decisions to move between APs are based entirely on the hardware and software, and up to each manufacturer. Some client devices will pick up on the strongest signal available when the interface is initialized, and hang on for dear life, only moving when the initial AP is no longer reachable. Some devices will always head for the strongest available signal, which may result in flip-flops between APs when two signals are evenly balanced. Still other devices will incorporate recent history into roaming decisions to avoid excessive movements between two nearby APs. How stations decide if and when to move between APs is entirely up to the manufacturer. With apologies to Milton Friedman, roaming is always and everywhere a client phenomenon.
Digging deeper into a second level of meaning, "roaming" is also used to discuss how a station can change APs while keeping active network connections open. If IP subnet boundaries are involved, this is an added level of complexity that needs to be addressed outside any standards. Chapter 21 discusses how to build a wireless LAN, and discusses how to achieve seamless roaming across arbitrary network topologies.
Roaming is sometimes given yet another meaning, referring to the process of moving a network session from one network (say, an 802.11 wireless LAN) to another disparate network (say, a 3G mobile telephone network). The IEEE has started another working group to define roaming operations to move network sessions and state between different types of IEEE 802 networks.
Introduction to Wireless Networking
Overview of 802.11 Networks
11 MAC Fundamentals
11 Framing in Detail
Wired Equivalent Privacy (WEP)
User Authentication with 802.1X
11i: Robust Security Networks, TKIP, and CCMP
Contention-Free Service with the PCF
Physical Layer Overview
The Frequency-Hopping (FH) PHY
The Direct Sequence PHYs: DSSS and HR/DSSS (802.11b)
11a and 802.11j: 5-GHz OFDM PHY
11g: The Extended-Rate PHY (ERP)
A Peek Ahead at 802.11n: MIMO-OFDM
Using 802.11 on Windows
11 on the Macintosh
Using 802.11 on Linux
Using 802.11 Access Points
Logical Wireless Network Architecture
Site Planning and Project Management
11 Network Analysis
11 Performance Tuning
Conclusions and Predictions