Abstract
This chapter explains some of the existing issues with roaming in Wi-Fi networks. A mobile Wi-Fi phone, in most environments, will need to transition from one access point (AP) to another as it moves away from its current AP's range. Types of roaming include Intra-ESS, Intra-ESS-with-SubnetChange, Inter-ESS, and Inter-Network. The base standard supports Intra-ESS roaming implicitly through the association/deassociation procedures. The basic 802.11 approach works well for data applications because most such applications use a reliable transport layer protocol like TCP, which conceals the delay/packet loss due to the handoff by using retransmissions. Scanning maintains a list of available candidate APs that the station (STA) can connect to, if it is disconnected, or that are candidates to connect to if the current AP connection deteriorates. Roaming decisions should be based on running-average received signal strength indication (RSSI) rather than instantaneous RSSI, because this approach prevents having too many handoffs. Achieving seamless mobility, where a phone user does not experience a noticeable loss of service during the roaming process, is possible with a carefully tuned scanning and roaming algorithm.
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