The popularity and prevalence of wireless local area network (WLAN) deployments has increased dramatically in recent years. Contributing to this increase are communication standards such as the Institute of Electrical and Electronic Engineers (IEEE) 802.11 suite of standards, which define physical operating parameters, packet formats, and protocol behaviors for the exchange of data between mobile terminals and wireless access points. These standards also typically define the interactions between mobile terminals and access points to coordinate a handoff from one wireless access point to another. However, the standards do not necessarily dictate how handoff decisions are made. Typically, mobile stations assess a locally measured radio signal strength of beacons sent by nearby access points to select the access point with the strongest signal and highest available data rate. For most practical purposes, this metric boils down to choosing the physically closest access point. However, for a variety of network reasons, the closest access point is not necessarily always the best access point.
For example, offering high quality, reliable voice communication through access points of a WLAN service is challenging. Voice applications typically send small data packets (e.g. 40-60 bytes), yet send the data packets very frequently (e.g. every 20 milliseconds). Consequently, voice applications are sensitive to network latency, reliability, and capacity. Packets of voice data that do not arrive quickly, or “on time” relative to other voice data, are of little or no value to a listener. Delay, loss, and reordering of packets typically translates into “clicks”, “pops”, and general poor voice quality for the listener. Also, since voice data packets are small, a relatively high percentage of overhead data and processing is needed to identify and route voice data packets. In addition, since WLAN standards typically support a variety of data rates, the overall capacity of access points usually varies greatly with the number and location of mobile terminals, as well as the type of traffic being sent. Further, since WLAN connections typically use unlicensed radio frequency (RF) spectrum, transmitters can rightfully contend for the shared airways. The competition for air time can increase latency due to collision detection, retransmissions, and decreased data rates (hence increased propagation time and RF resource usage). All of these network performance factors contribute to the quality of a voice call or other communication using WLAN. However, a mobile terminal typically selects an access point based on locally available information such as radio signal strength, biasing mobile terminals to select the “closest” access point, which may not be the access point that will provide the best quality voice call or other communication.