Dependability Enhancement for IEEE 802.11 Wireless LAN with Redundancy Techniques
The presence of physical obstacles and radio interference results in the so called "shadow regions" in wireless networks. When a mobile station roams into a shadow region, it loses its network connectivity. In cellular networks, in order to minimize the connection unreliability, careful cell planning is required to prevent the occurrance of the shadow regions in the first place. In 802.11b/g wireless LANs, however, due to the limited frequency spectrum, it is not always possible to prevent a shadow region by adding another cell at a different frequency. Our contribution in this paper is to propose the alternate approach of tolerating the existence of "shadow regions" as opposed to prevention in order to enhance the connection dependability. A redundant access point (AP) is placed in the shadow region to serve the mobile stations which roam into that region. Since the redundant AP operates on the same frequency as the primary AP, it does not constitute a separate cell. In fact, the primary and the secondary AP communicate to grant medium access to stations within the shadow region. We consider two configurations, which differ in how the two APs communicate with each other. In the first, the secondary AP is connected to the same distribution system as the primary AP. In the second, the secondary AP acts as a wireless forwarding bridge for traffic to/from the mobile stations in the shadow region to the primary AP. The paper outlines the details of how redundancy may be implemented by making enhancements to the basic 802.11 channel access protocol. To evaluate the dependability of the network under study, we present the reliability, availability and survivability analysis of the two configurations and compare them with the scheme with no redundancy. With numerical examples, we show that the redundancy schemes demonstrate significant improvement in connection dependability over the scheme with no redundancy.
