Implementation-Based Evaluation of a Full-Fledged Multihop TDMA-MAC for WiFi Mesh Networks

Wireless mesh networks in general, and WiFi mesh networks in particular, offer a cost-effective option to provide broadband connectivity in sparse regions. Effective support for real-time as well as high throughput applications requires a TDMAbased approach. However, multihop TDMA implementations in wireless have been few and far-between, and for good reasons. These present significant issues in terms of time synchronization, TDMA schedule dissemination, multichannel support, routing integration, spatial reuse and so on. And achieving these efficiently, in the face of wireless channel losses presents a formidable challenge. In this work, we present an implementation of LiT MAC, a full-fledged multihop TDMA MAC, on commodity WiFi platforms. We undertake extensive evaluations using microbenchmarks as well as application level performance, using outdoor as well as indoor testbeds. We also present an integration of LiT MAC with various routing metrics, and a routing stability study of recently proposed routing metrics (ROMA, SLIQ). Our results show that we can achieve μs granularity time synchronization across several hops, and TDMA slot size as small as 2 ms. These imply low control overheads. Experiments over several days, on our nine-node outdoor testbed shows that LiT MAC's soft-state-based approach is effective in robust operation even in the presence of significant external interference.