WiFi Access May Slow Down the Whole Network

[Russ Smith]

Using WiFi access may be slowing down any network that uses it. That's the finding of some Engineering experts at France's Centre Nationale de la Recherche Scientifique (CNRS) in Paris. The slowdown is due to the CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) channel access method (also called "listen before talk"). CSMA/CA waits for each device which "captures" the channel to send a packet before releasing the channel for other traffic. With devices that work at approximately the same speed, this process takes very little time and generally assures that the devices have equal access to the network. If, however, one of the devices is significantly slower, the network waits for the device to finish even though it could process other traffic while it's waiting. The result is that speed is degraded for every device on the network. Most networks operate at 100Mb/s and many now operate at 1Gb/s. When a WiFi device is well within range, the speed is 11Mb/s. In itself that will slow down network speed, but the speed falls to 5, 2, or 1Mb/s when the device gets near to the edge of the WLAN range. A device operating at 1Mb/s on a 1Gb/s network using CSMA/CA will create a major bottleneck.

The CSMA/CA channel access method is part the 802.11a, b, and g standards. That covers all current WLAN technologies. Some hardware has built-in methods of load-balancing or prioritizing traffic in relation to the speed of access but these are proprietary extensions to the standards. A better solution is the 802.11e standard which includes Quality of Service (QoS) mechanisms. Unfortunately 11e is still in the standards committees and so, won't be ready for implementation in the near future.

Until an overall solution, like 802.11e, can be implemented, this issue may forestall widespread creation of WiFi hot-spots. Wired users may legitimately complain that their access is degraded by allowing WLAN users.