Sreekanth Garigala: Master's Thesis, July 2004

Title: Experimental Validation of Simultaneous Operation in an 802.11 Multi-hop Mesh Network

Abstract: Internet and cellular telephony has been growing exponentially from the past decade but, much of this growth has been confined to developed countries and metropolitan pockets in the developing world. A low cost access techchnology is necessary to develop a similar growth in rural areas. In the Digital Gangetic Plains (DGP) project, 802.11 technology is being used as a low-cost and long distance technology. As part of DGP, an extensive testbed has been built in the rural areas consisting of multi-hop 802.11 point-to-point links, the testbed spanning up to 80km at its longest. Since 802.11 was designed for indoor use only, trying to use it outdoor poses many challenges spanning several layers of the OSI networking stack.In this thesis, we address some of the issues at MAC layer. 802.11 uses CSMA/CA for medium access but, it is not suitable in networks where all the links are point-to-point and setup before-hand, since there is no arbitrary contention in the network. In contrary, Spatial-reuse Time Division Multiple Access (STDMA) Protocol where scheduling transmissions is done before-hand is more appropriate. STDMA performance depends on how much spatial reuse of the channel is possible. In the testbed, at a node we use multiple directional antennae each pre-mounted and pre-aligned towards a particular neighbour. 802.11 defines at least 11 channels out of which three (1, 6 and 11) are non-overlapping. We can schedule the links using these three channels independently of each other. If we use omni-directional antenna, we can setup at most 3 links with 3 independent channels simultaneously. Since we are using directional antennae, if we provide sufficient Signal-to-Interference ratio, we can transmit/receive independent information on different links at a node on the same channel. We call this as simultaneous ransmission/reception and collectively simultaneous operation. Now we can schedule the links such that a node alternates between transmit and receive modes for all its links imultaneously. This is called two-phase scheduling and is used in addressing the problem of medium-access in point-to-point multi-hop 802.11 network. We experimentally validated the simultaneous operation and calculated the Signal-to-Interference ratio required for error-free operation of the links.

Bhaskaran Raman
Last modified: Sat Jun 11 11:13:34 IST 2005