Dependability Analysis of Wireless Local Area Networks

The complex behavior of wireless signal propagation, particularly indoors, is caused mainly by attenuation introduced both by distance from the transmitter and by stationary or moving obstacles blocking the signal's path to the receiver. This leads to frame corruption at the link-layer (layer 2), which in turn results in packet losses, and more variable packet latencies, at higher layers that will affect the wireless network dependability. An extreme form of attenuation is blocking of radio signals due to existing obstacles between senders and receivers. When a mobile station roams into a blocking region, it loses its network connectivity. IEEE 802.11 wireless local area networks (WLANs), has limited frequency spectrum, therefore it is not always possible to eliminate a blocking region by adding another cell at a different frequency as the case for cellular networks. WLANs are being used for military and multimedia applications, where high system dependability and the ability to stay in communication with access points (APs) are absolutely required. The main contribution of our paper is to provide a comprehensive dependability analysis of WLAN using a software tool for system dependability measurement, modeling and evaluation (MEADEP) academic version 1.8.1, and to suggest methods to increase the communication link reliability and availability of wireless LANs. An additional access point (AP) is placed in the blocking region to serve the mobile stations, which roam into that region. Since the additional AP operates on the same frequency as the existing AP, it will not form a separate cell. The two APs communicate via the same wired-network to allow medium access to wireless stations within the blocking region. This new configuration will be implemented using IEEE 802.11e MAC protocol. The dependability of the new WLAN infrastructure-based architecture with two APs in the same cell will be modeled, evaluated and compared with the basic (one AP) architecture. We demonstrate that the new configuration will have better coverage area, which leads to higher connection reliability, higher connection availability and lower link failure rate