Venkata Rao Chimata: Master's Thesis, June 2005

Title: Implementation and Performance Issues of VoIP in Long Distance 802.11b Networks

Abstract: 802.11b seems to be a promising solution to bridge digital barrier between rural and urban areas. The main challenge in long distance 802.11b networks is to avoid the hidden node problem. Hidden node problem and contention based DCF (Distributed Coordination Function) protocol degrade QoS (Quality of Service) of VoIP. RTS/CTS may help to avoid hidden node problem, but the overhead associated with RTS/CTS mechanism is too high to use it for services like VoIP where the size of the packet is very small. 802.11b protocol also specifies a contention free protocol, PCF (Point Coordination Function) is not supported in any of the known existing cards. Our thesis work mainly focuses on two isssues, improving performance and implementaion (with low cost) of VoIP in point-to-multipoint networks. In the first part we have analyzed the performance of VoIP on a long distance 802.11b network and shown its bad performance under certain conditions. Later we have designed and implemented a driver based contention free protocol known as Master-Slave protocol for point-to-multipoint links. This protocol cleverly suppresses the behaviour of DCF on off-the-shelf (low cost) 802.11b cards. We have shown that with our protocol the performance from the perspective of QoS is improved significantly, eg jitter (A QoS metric) values with our protocol are around 4ms while with the original setup those values are around 80-100ms in the presence of cross traffic. In the second part we have engineered and implemented a VoIP system which is best suited to provide telephony/voicemail services to the rural areas with low cost using opensource software like asterisk (PBX software) and openh323 (VoIP client software) to reduce the overall cost.

Bhaskaran Raman
Last modified: Fri Jun 24 14:22:50 IST 2005