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Design and Evaluation of a new MAC Protocol for Long-Distance 802.11 Mesh Networks

August 4th, 2006

http://www.citeulike.org/user/scottr/article/526128

This paper presents a mesh network architecture and MAC protocol optimised for long-distance rural networks. Raman and Chebrolu expand on their previous work on SynRx and SynTx techniques, where with careful attention to the physical topology of the network, several radio interfaces on a node can transmit or receive at the same time without interferring with each other. That is, all interfaces on a node must be either transmitting or receiving, but not a mixture.
The 2P MAC protocol uses SynRx and SynTx by assuming point-to-point links between nodes that have multiple radio interfaces. There are several constraints put on the physical topology of the network to ensure correct operation of 2P. For example, the “mesh” must not contain loops with an odd number of edges. Also, we must pay attention to the angle between links to ensure that side-lobes have a minimal effect. Transmit power for each link is also constrained to ensure that links do not interfere with others. Even with all these constraints, the authors show that it is possible to create networks compatible with 2P. Whether or not it is always practical is something I am not yet convinced of.
Interestingly, 2P has been implemented using driver-level modifications to the Prism2 HostAP drivers. By using a combination of IBSS mode and broadcast frames, immediate ACKs are removed. This is a technique that I was going to use during development of the Atheros based MAC development framework. The authors also use the antenna diversity settings to remove CSMA/CA backoff. When an interface wishes to transmit, it sets it’s rx antenna to the connector that does not have an antenna connected to it. This way, it does not carrier sense the other interfaces transmitting and so does not back off.
The basic premise of 2P is that when a node is transmitting, all of the nodes that are connected to it are receiving. Once all of the interfaces have finished transmitting to their neighbours, the entire network swaps, and so all of the neighbours begin transmitting, and the node goes to receive mode. This requires communication between interfaces on a node, and has also been implemented in the driver.
Simulation results show that because all interfaces on a node can transmit at the same time, overall throughput is greatly increased over a CSMA/CA based network. Implementation results show that while the protocol does not live up to it’s simulation results, it is still on average better than CSMA/CA. The authors discuss the overheads involved in the implementation and the effect on the protocol, which is interesting as I had planned to argue during my research that development of MAC protocols requires more than just simulation to be effective, which this paper touches on.

Entry Filed under: Literature Review, Ph.D