Cross-layer design for interference mitigation and mobility support in wireless access networks

Over the past few years WiFi technology has emerged as the most widely used wireless technology for providing network access to end-users over single hop networks (WLANs) and multiple hop networks (Mesh Networks). Although WiFi-based access networks have numerous benefits such as use of inexpensive commodity hardware, operation in unlicensed spectrum and ease of deployment, they suffer from performance and reliability related problems that arise mainly due to the presence of wireless interference. The problems are exacerbated when there are mobile clients in the network. In this regard, we focus on two important issues: mitigating interference and supporting client mobility. We design novel cross layer solutions that utilize information available at the link layer and the network layer to effectively address these issues. Specifically, we contribute in four specific problem areas related to interference and mobility. First, interference and fading can cause sudden drop in wireless link quality, which can persist for hundreds of milliseconds. But, existing routing protocols for mesh networks are incapable of reacting quickly to such transient link quality fluctuations. To address this issue, we design a lightweight opportunistic protocol called Deflect that operates underneath the routing layer, and protects end-to-end routes from short-term link quality degradation. Deflect achieves this using a novel passive link quality measurement technique along with a low overhead path switching protocol. Second, in dense WLAN deployments interference between neighboring access-points operating on the same channel can be significant due to limited number of non-conflicting channels in WiFi networks, and this results in degradation of achievable network throughput. In this regard, we design a dynamic transmit power control technique called Contour that improves spatial reuse while avoiding link asymmetry problem associated with the use of multiple transmit power levels in the network. The Contour framework uses of a centralized slotted packet scheduling framework enforced on top of the existing IEEE 802.11 CSMA/CA protocol to achieve fine grained transmit power control and packet scheduling across all access-points in the network. Third, a malicious user can create interference and jam all communication on a channel. To counter such an extreme form of interference, we design a Channel Hopping protocol that improves the resilience of WiFi networks to jamming attacks. The hopping parameters are optimized to maximize the achievable throughput in presence of a smart jammer, who is aware of the use of a hopping protocol. Finally, client mobility causes temporary disruption in connectivity. This is particularly problematic in mesh networks. We design a mobility management architecture called iMesh that tracks the client’s location using link layer feedback information and quickly updates the routes on the mesh network for mobile clients. We design and implement system prototypes of our proposed solutions on commodity off-the-shelf hardware to show the feasibility of deploying these solutions. Further, through extensive experimentation on realistic wireless testbed deployments, we demonstrate the effectiveness of these systems in addressing the aforementioned issues.