Review: A Media Access Protocol for Wireless LAN’s
Reviewer: Kevin Hofstra
The problem of media access must be dealt with differently on a Wireless LAN then on a typical Ethernet LAN. How can contention to the media be given out efficiently yet fairly when the congestion is at the receiver and is not uniform across the network?
Wireless LANs cannot use the same CSMA standard as Ethernet LANs because it is not possible to sense congestion at the sending end. It is not know where a packet may be sent, until a packet has reached the base station. Wireless LANs must therefore adjust the way it detects the bandwidth is being used and also use different methods in avoiding collisions. A balance must be reached between the efficient use of the bandwidth and the fairness of the media allocation. Much of this is decided by the backoff algorithm. “Fairness” is much harder to define in a Wireless environment because a single node may have access to multiple access points.
I believe this paper to be a major contribution (4) because it is the first study of its kind on Wireless LAN’s and its not only theoretical, but is also experimental with actual test data. The support of their ideas are not only backed up by theoretical evidence, but also by actual scientific data from their case studies. This means that the theories are extremely convincing (5), if not undisputable.
This paper was very early into Wireless research and thus does not take into account nodes traveling from one coverage zone to another seamlessly. This would be a limitation if the backoff information was different from zone to zone.
From this article I think we should take the idea that the idea of CSMA transfers easily to a different type of media, but the actual details and implementations must be drastically changed. The inability to sense the media at the sender transfers the responsibility of congestion control to a centralized access point. This is a limitation of wireless that forces an idea such as carrier-sensing to be much more difficult to implement.