* Finding hidden WLAN nodes
There are many aspects to managing unlicensed Wi-Fi spectrum to avoid interference and optimize wireless LAN application performance. Interference can be caused by traffic butting up against other traffic in overlapping channels, another operator’s 802.11 devices contending for your spectrum, non-802.11 devices operating in the spectrum, and environmental factors blocking or degrading signals, to name a few.
A less-obvious interference culprit is the “hidden node.”
Hidden nodes are basically client devices that are all within range of the WLAN access point (AP) but are not necessarily within range of each other. Picture an AP in the middle of a circle. Client A is at 9 o’clock, 50 meters (164 feet) from the AP. Client B is at 3 o’clock, 50 meters from the AP. The distance between the two clients – or the diameter of the circle – will be 100 meters (328 feet).
As you likely know, the 802.11b/a/g suite of standards uses a media access control (MAC) mechanism called carrier sense multiple access with collision avoidance, or CSMA/CA. Client nodes more than 300 feet apart are not likely to “hear” each other transmitting in order to avoid a collision. Two nodes transmitting on a common channel at once causes collisions, which results in interference and lowered throughput and response times.
Some of the Wi-Fi RF monitoring and management products can detect hidden nodes. AirMagnet, for one, just announced this capability in the latest version of its laptop analyzer product, Laptop Analyzer 7.0 PRO, last week. If performance is suffering, of course, one of the first things you have to do is find the source of the problem, so this type of capability can be a handy tool.
Once you realize you’ve got a hidden node situation, what can you do? A few suggestions next time.
