Feel The Load
One way to deal with the capacity issue is to add more APs, but veteran wireless administrators know Wi-Fi clients don't always play nice and distribute themselves evenly. As 802.11 was designed, APs exert very little control over clients; rather, a client independently chooses the AP to which it will associate, with no consideration as to traffic levels or the number of active clients on the device. Even if clients were evenly distributed among all available APs, traffic utilization could be lopsided, leaving Vo-Fi clients at a disadvantage.
With the advent of wireless controllers, the WLAN infrastructure gained network-wide insight into AP client counts and traffic utilization. Load-balancing clients and traffic among APs is one way to minimize the capacity problem.
But because there is, again, no standard, vendors have gotten creative.
One method is to have over-burdened APs ignore client probes and association requests for new clients. That's not ideal because as clients leave and traffic patterns change, one AP may still end up near capacity, while neighboring devices are almost idle.
A more proactive load-balancing method involves disconnecting associated clients and ignoring reconnection attempts. Problem is, some clients are aggressive. Rather than look elsewhere, they'll spin their wheels trying to reconnect, an obvious problem if done during a call. Fortunately, most WLAN infrastructure products are intelligent enough not to load-balance clients with active traffic.
Aruba Networks, Cisco, Trapeze Networks and other infrastructure vendors have--surprise!--proprietary load-balancing algorithms and methods. However, only Cisco can feed information to the client to assist it in making a better roaming decision. A CCX-enabled client can learn from the Cisco AP its network load and use that data in its roaming algorithm. On the infrastructure side, Cisco controllers can be configured such that the infrastructure will automatically move even non-CCX-compatible clients to adjacent and less-loaded APs when the clients are idle.
Standards to the rescue again. Two, in fact, play a role. The IEEE Radio-Resource Management Task Group k has spent about four years working on a standard to "define and expose radio and network information to facilitate the management and maintenance of a wireless and mobile LAN." Devices that follow the TGk standard measure utilization and share information about neighboring APs, including channel, signal strength, noise and hidden stations. That means that devices can learn about neighboring APs before roaming. We're hopeful a standard will be available by year's end.
While 802.11k provides clients and APs with the necessary information, Task Group v seeks to add management control at the PHY/MAC layers for attached devices. APs will be able to take a more definitive role in client access to the extent that standards-based load-balancing may be possible. There's much more to the work of TGv than management; it's also addressing troubleshooting and monitoring. And, the project document leans on 802.11k for some monitoring information. The target for completion is January 2009.
