12.2.4 Wireless Roaming

12.2.4 Wireless Roaming Clients canroamwithin an extended service area (ESA). An ESA is created by installing APs with the same ESSID and security configuration connected by a wired network, ordistribution system (DS). The access points are configured with different channels so that where BSAs overlap, there is no interference. When the client detects that it is no longer receiving a good signal, it checks for another signal with the same ESSID on other channels or on a different frequency band, and if there is a stronger signal, it disassociates from the current AP. The station can then reassociate with the new AP. Depending on the roaming infrastructure and security type, the station may have to reauthenticate, or if 802.11r fast roaming is supported, it may be able to use its existing authentication status to generate security properties for the new association.Roaming is supposed to be seamless, but in practice reestablishing the connection can often cause time-out problems for applications. To improve mobility, there needs to be a balance between determining what constitutes a "good" signal and the rate at which a client tries to associate with different APs. Many adapters support a roaming "aggressiveness" setting that can be configured to prevent a Wi-Fi adapter "flapping" between two APs or (conversely) to prevent a client from remaining associated with a more distant AP when it could achieve better bandwidth through one closer to it. SSID Broadcast and Beacon Frame A WLAN is typically configured to advertise its presence by broadcasting the SSID. This allows a user to connect to a named network from a list reported by the client adapter. If SSID broadcast is suppressed, the user must configure the connection to the network manually. A beacon is a special management frame broadcast by the AP to advertise the WLAN. The beacon frame contains the SSID (unless broadcast is disabled), the BSSID, supported data rates and signaling, plus encryption requirements. The interval at which the beacon is broadcast (measured in milliseconds) can be modified. The default is usually 100 ms. Increasing the interval reduces the overhead of broadcasting the frame but delays joining the network and can hamper roaming between APs. Wireless Distribution System You can also configure access points to create wireless-only networks in areas where it is not possible to run cabling. This is referred to as a wireless distribution system (WDS). You must set the APs to use the same channel, SSID, and security parameters. The APs are configured in WDS mode. One AP is configured as a base station, while the others are configured as remote stations. The base station can be connected to a cabled segment. The remote stations must not be connected to cabled segments. The remote stations can accept connections from client stations and forward all traffic to the base station.