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In this work, we consider the goal of scheduling the maximum number of voice calls in a TDMA-based multi-radio, multi-channel mesh network. One of the main challenges to achieve this goal is the difficulty in providing strict (packet-level) delay guarantees for voice traffic in capacity limited multi-hop wireless networks.In this context, we propose DelayCheck, an online centralized scheduling and call-admission-control (CAC) algorithm which effectively schedules constant-bit-rate voice traffic in TDMAbased mesh networks. DelayCheck solves the joint routing, channel assignment and link scheduling problem while satisfying the constraint. We formulate an offline version of this scheduling problem as an Integer Linear Program (ILP), the LP relaxation of which gives us an optimality upper bound. We compare the output of DelayCheck with the LP-based upper bound as well as with two state-of-the-art prior scheduling algorithms. DelayCheck performs remarkably well, accepting about 93% of voice calls as compared to LP-based upper bound. As compared to state-of-the-art algorithms, DelayCheck improves scheduler efficiency by more than 34% and reduces call rejections by 2 fold. We also demonstrate that DelayCheck efficiently exploits the number of channels available for scheduling. With implementation optimizations, we show that DelayCheck has low memory and CPU requirements, thus making it practical.

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In this work, we describe our experiences in building a low cost and low power wireless mesh network using IEEE 802.15.4 technology to provide telephony services in rural regions of the developing world. 802.15.4 was originally designed for a completely different application space of non-real-time, low data rate embedded wireless sensing. We use it to design and prototype a telephony system, which we term as Lo 3 (Low cost, Low power, Local voice). Lo 3 primarily provides two use cases; (1) local and broadcast voice within the wireless mesh network, and (2) remote voice to a phone in the outside world. A Lo 3 network can cost as less as $2K, and can last for several days without power "off the grid", thus making it an ideal choice to meet cost and power constraints of rural regions. We test deployed a full-fledged Lo 3 system in a village near Mumbai, India for 18 hours over 3 days. We established voice calls with an end-to-end latency of less than 120ms, with an average packet loss of less than 2%, and a MOS of 3.6 which is considered as good in practice. The users too gave a positive response to our system. We also tested Lo 3 within our department where it can be used as a wireless intercom service. To our knowledge, Lo 3 is the first system to enable such a voice communication system using 802.15.4 technology, and show its effectiveness in operational settings.