Remember what a ‘Collision Domain’ is?

by | Mar 17, 2010 | Blog

One of the things that a lot of  networking folks forget is the concept of a ‘Collision Domain’ – so for a little refresher – coming to you from our friends at Wikipedia:


Well in the example above, and from this graphic – it looks like Collision Domains are a only problem of a wired Ethernet system. But in addition, it is also very prevalent in Wireless Networks. Whenever devices, including Access Points as well as Client Stations, can ‘see’ each other on the same channel (with enough RSSI) they will be forced because of the protocol to wait for others on the same channel to stop transmitting before they can start their transmissions.

This process of a ‘Contention Window’ is a part of the 802.11 design specs.

As an example, here is a capture from the baggage claim area at a large airport. Note all the Access Points that are sharing the same channels. Adding more APs does not always add more capacity. Sometimes, when the APs can ‘see’ each other, they only add to the contention window and don’t ‘net’ any additional throughput.

In fact, many times this is the problem when clients are having ‘difficulties’ with their wireless connections!

This is easy to find. Just fire off your favorite tool that lets you see the access points, their channels, and their signal strengths. If from one location you can see more than two Access Points on the same channel with RSSI’s greater than -80dBm… then that channel has a ‘Collision Domain’. Plain and simple. Meaning when either AP talks, the other *can’t* talk–because it is waiting for the other to finish. (a definition of a Collision Domain)

Try it on your wireless networks and see what kind of Collision Domain’s you’ve designed into your network. What? You didn’t design in collision domains? Then why are there more APs showing up on the same channel at the same location? If you didn’t do it, then who did?

Many times folks who do ‘AP-on-a-stick’ site surveys and call them wireless network designs NEVER check this co-channel interference and you are left to deal with the results of having way too many devices sharing the limited capacity of a specific frequency!

Measure, Measure, and Measure your wireless network to *verify* not only where you have signal, but also where you have too much signal. (you measure your cable plant, why don’t you measure your wireless media?)

You also might want to check out my White Paper called ‘Want, Don’t Want, Don’t Care‘ and be able to define what your client stations use to determine those three levels.

As a side-note. My friend David Coleman, of CWNP, CWSP, and CWAP study guide fame, refers to this ‘Collision Domain’ as a ‘Contention Domain’… I agree with him.

Questions? Comments? Please leave your thoughts in the comments section below.