Fast scanning schemes for IEEE 802.11 WLANs in virtual AP environments

摘要

Recently, provisioning a fast handoff in IEEE 802.11 Wireless Local Area Network (WLAN) has been an attractive research issue since the incurring latencies are unsatisfactory to support today's fast emerging realtime services with stringent Quality of Service (QoS) requirements. The 802.11 handoff consists of (1) scanning, (2) authentication, and (3) reassociation. Particularly, the scanning has been studied intensively since it accounts for a major portion of the handoff latency due to potential overheads in the scanning operation. That is, (1) a scanning STAtion (STA) should stay in a scanned channel waiting for Access Points (APs)' responses without any assurances of APs' existence after request frame transmissions; moreover, (2) per-channel waiting time is not explicitly defined in the 802.11 standard. In order to reduce the overheads, we propose novel scanning schemes composed of two phases: (1) channel selection phase; and (2) AP search phase. In the channel selection phase, a scanning STA identifies and selects the channels suitable for the 802.11 scanning by assessing all employed channels with Request-to-Send/Clear-to-Send (RTS/CTS) handshaking in a virtual AP environment. Then, in the AP search phase, it performs a unicast-based scanning in order to search for an AP suitable for its handoff in a selected channel. For a proper handoff decision depending on STA's requirement, we build two algorithms referred to as first-fit and near best-fit algorithms, respectively. We demonstrate the superiority of our proposed schemes to existing approaches via simulations with realistic time-varying channel models in various simulation environments. The simulation results demonstrate that the proposed scanning schemes provide relatively shorter handoff latencies as (1) a scanning STA has shorter channel switching time; (2) an 802.11 WLAN employs more channels; and (3) the APs are deployed more densely.