17
system. After all, they are connected to the distribution system medium. Wireless stations in an
infrastructure network depend on the distribution system to communicate with each other because they
are not directly connected to each other. The only way for station A to send a frame to station B is by
relaying the frame through the bridging engine in the access point. However, the bridge is a
component of the distribution system. While what exactly makes up the distribution system may seem
like a narrow technical concern, there are some features of the 802.11 MAC that are closely tied to its
interaction with the distribution system.
2.2.2.1 Inter-access point communication as part of the distribution system
Included with this distribution system is a method to manage associations. A wireless station is
associated with only one access point at a time. If a station is associated with one access point, all the
other access points in the ESS need to learn about that station. In Figure 2-5
, AP4 must know about all
the stations associated with AP1. If a wireless station associated with AP4 sends a frame to a station
associated with AP1, the bridging engine inside AP4 must send the frame over the backbone Ethernet
to AP1 so it can be delivered to its ultimate destination. To fully implement the distribution system,
access points must inform other access points of associated stations. Naturally, many access points on
the market use an inter-access point protocol (IAPP) over the backbone medium. There is, however,
no standardized method for communicating association information to other members of an ESS.
Proprietary technology is giving way to standardization, however. One of the major projects in the
IEEE 802.11 working group is the standardization of the IAPP.
2.2.2.2 Wireless bridges and the distribution system
Up to this point, I have tacitly assumed that the distribution system was an existing fixed network.
While this will often be the case, the 802.11 specification explicitly supports using the wireless
medium itself as the distribution system. The wireless distribution system configuration is often called
a "wireless bridge" configuration because it allows network engineers to connect two LANs at the link
layer. Wireless bridges can be used to quickly connect distinct physical locations and are well-suited
for use by access providers. Most 802.11 access points on the market now support the wireless bridge
configuration, though it may be necessary to upgrade the firmware on older units.
2.2.3 Network Boundaries
Because of the nature of the wireless medium, 802.11 networks have fuzzy boundaries. In fact, some
degree of fuzziness is desirable. As with mobile telephone networks, allowing basic service areas to
overlap increases the probability of successful transitions between basic service areas and offers the
highest level of network coverage. The basic service areas on the right of Figure 2-7
overlap
significantly. This means that a station moving from BSS2 to BSS4 is not likely to lose coverage; it
also means that AP3 (or, for that matter, AP4) can fail without compromising the network too badly.
On the other hand, if AP2 fails, the network is cut into two disjoint parts, and stations in BSS1 lose
connectivity when moving out of BSS1 and into BSS3 or BSS4.
Figure 2-7. Overlapping BSSs in an ESS