Tuesday, September 30, 2008


Media access in wireless networks is a harder problem than wired networks. This paper gives a nice description of the lay of the land. CSMA senses the medium before transmitting but since collisions typically occur on the receiver, it suffers from the hidden and exposed terminal problems. MACA fixed this with a Ready-to-Send (RTS) and Clear-to-Send (CTS) control protocol. MACA used an exponential back-off timer which, as described in this paper, becomes a source for unfairness. The crux of the problem is that the back-off timer is a function of the number of tries that a node has made, bringing in inequality.

This paper proposes MACAW that proposes the following four modifications:

1. Fairness is achieved by nodes updating their back-off timer on hearing a packet.

2. DS and RRTS mechanisms to synchronize timing across senders

3. Location-tagged back-off timers, as congestion is more a function of a sender-receiver pair as opposed to a sender alone.

I vote to keep this paper in the reading list as it gives a good idea of the area stand-alone. I wasn’t happy about the experimental setting (base stations and pads) on two counts: One, it was unclear (and the paper doesn’t convince either) on how realistic this setting was to real wireless LANs. Second, I would have liked a description that helped me perform their experiments again, under different conditions. I didn’t understand what it would take to reproduce their experiments.

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