Scott’s Weblog

An efficient polling MAC for wireless LANs

August 3rd, 2006

http://www.citeulike.org/user/scottr/article/692690

In this 2001 paper, Sharon and Altman describe a MAC protocol that exploits the capture phenomena in radio channels. The MAC is point-to-multipoint and uses polling techniques, however it uses the capture phenomena to overcome the fact that polling techniques perform poorly when only a small number of stations have data to transmit. When a base-station polls a station only to find that the station has no data to transmit back, that channel time is lost to protocol overhead. All stations must be polled however to ensure fairness of the medium - something that polling MACs are known for.
In this MAC, two radio channels are used. The first is for data and control frames being passed from the Base Station (BS) to mobile stations (Note that the paper refers to mobile stations throughout, however the authors state that they assume the stations are stationary when transmitting as the MAC relies on knowing the distance to the BS from the station). The second channel is for data and control frames being passed from stations to the BS. A circular list of stations present in the system is stored, and two separate circular lists are also stored - one contains stations that have frames to transmit, the other contains stations that do not have frames to transmit.
During its basic operation, a BS will send a control frame to the next active station giving it permission to transmit one frame’s worth of data. In the same control frame, the BS also queries a station in it’s inactive stations list. The active station will transmit its frame back to the BS and notify it whether it has more frames to transmit. If it does, the BS moves on to the next active station. If it does not, the BS moves the previously active station into the inactive list. When an inactive station is queried, if it does not have any data to transmit it will remain quiet, however if it does, it will transmit a “jam” signal at a low power. This is at the same time as the active station is transmitting it’s data frame. The capture phenomena allows the BS to correctly decode the signal from the active station while the inactive station is transmitting a jam signal. When the active station finishes sending its frame, if the BS detects a jam signal then it will add the inactive station to the active list.
This method allows the BS to poll inactive stations without stopping active stations from transmitting. The paper goes on to show that the MAC obtains better throughput, mean access delay and mean queueing delay when there are stations that are not transmitting than simple token passing does.
The paper is extremely interesting, as the MAC I have implemented in the last couple of months is a simple token passing MAC (with the idea that once we have a solid implementation we can use it as a base for improvements). This shows that with low-level access to the transmit hardware we can do interesting things at the MAC layer. However, the MAC presented in the paper requires two separate radio channels, and hence two separate radios - something that is very expensive to implement. So, it becomes a tradeoff between coming up with interesting theoretical solutions to problems, and coming up with realistic, implementable ones.

Entry Filed under: Literature Review, Ph.D