You may have heard of “Co-Channel Interference” (CCI) alongside explanations of what it means – One AP having to defer transmissions whilst another talks? Well, here’s a really clear example of CCI in action!
The Situation
I set up some alerts in Aruba Networks’ Airwave software in order to start to get a feel for areas where APs may be busy. These alerts track “Channel Utilization” – not in Megabits, but as a % of the available time an AP can hear transmissions or is transmitting something itself. (Megabits is BAD here – An AP could show low throughput in Megabits and yet still be really busy…)
I asked Airwave to tell me if an AP was “over 75% busy for more than 5 minutes”.
(I was really pleased with both the ability to alert on this and how easy it was to get these set up – well done Aruba!)
The Alert
I started to receive about a dozen alerts a day by e-mail. A few of these I checked out, including one from two APs which were located in close proximity to each other. The alert itself contained links to the Airwave graph showing how busy the AP was at the time.
The Graph.
I’ve included the graph in the infographic below (click for full view!) as it shows clearly what was happening.
Look at those traces – so very similar that the bulk of the traffic must be coming from a single transmission! Double checking proved that both APs are indeed on the same 2.4 GHz channel – Channel 6.
From the graph it looks like AP 18 is transmitting and AP 16 is experiencing this as CCI – it’s ‘Receiving’ line closely matching the ‘Transmitting’ line from AP 16.
Note also the ‘Receiving’ line from AP18 – constantly around 7%. This is the bandwidth which is lost from overheads such as beacons, etc.
