WLC24-3: Modelling the Impact of User Mobility on the Throughput in Networks of Wireless 802.11 LANs

The wireless LAN technology 802.11, also called Wi-Fi, offers high speed wireless Internet access for local area environments. WLANs provide much higher data rates than the mobile 2.5G and 3G networks and are relatively cheap and easy to install and maintain. Consequently, the popularity of WLANs has experienced tremendous growth over the past few years and WLAN technology has become a viable alternative to 2.5/3G mobile networks in providing Internet access in densely populated urban areas. The physical characteristics of a WLAN introduce range limitations and unreliable media, dynamic topologies where stations move about, interference from outside sources, and lack of the ability for every device to hear every other device within the WLAN. Due to these limitations service providers are faced with the problem to select and dimension the infrastructure of their WLAN networks caused by difficulties in performance evaluation. The main barrier, however, to the sustained growth of the use of WLANs is their limited capabilities to support user mobility, both in terms of coverage and handover control. Therefore, it is crucial for WLAN service providers to carefully design their networks in terms of the placement of hot spots, explicitly taking into account mobility patterns of the users. To address this design problem, in this paper we develop a simple model for the performance of WLAN networks, taking into account the limitations in coverage and handover control. The model captures the statistical behaviour of TCP packets at the MAC layer, taking into account the effects of initiation and completion of data transfers due to mobility of the stations when entering or leaving a cell. We develop closed-form expressions for the throughput of TCP flows in IEEE 802.11 WLANs in the presence of multiple classes of customers with different mobility patterns in multiple cells. The expressions are shown to be highly accurate when validated by simulation results.