Fun with DFS

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We have a customer who complained that every day, just before noon, users would lose their WiFi. The customer was located on a flight path for a nearby airport and military installation. As it turned out, an interim wireless firmware release changed their 5 GHz channel plan to include some Dynamic Frequency Selection (DFS) channels. These DFS channels are shared with aviation and weather radar with the proviso that if an access point detects radar on the same 5 GHz channel it serves, then it must abandon the channel. So for this customer, every day at the same time, an overhead flight knocked out part of their WiFi!

Of the twenty-two, 20 MHz, 5 GHz WiFi channels in the United States, thirteen of them are DFS channels. Because DFS channels are subject to abandonment, WiFi equipment ships with DFS channels disabled. Most WiFi systems apportion the remaining nine non-DFS channels between access points with limited contention, but there are situations where the DFS channels can solve big problems.

For example, we have a scholastic customer with some lightly built dormitories of wood-frame and gypsum. The dorms are loaded with APs. Additionally, the dorms are situated in an open space with many scholastic buildings and green space-WiFi around them. Standing next the dormitories, one can ‘hear’ forty radios. The air is busy! Adding more APs offers diminishing returns as the infrastructure competes with itself for channel access; the nine 5GHz channels are oversubscribed.

In another case, a customer had some new LED lighting installed over the summer. In the fall, they complained WiFi was intermittent on the 5 GHz band. We took measurements. The new lighting appeared to be blowing raspberries on the unlicensed 5 GHz spectrum; whatever the communication method, the LED lighting wasn’t speaking 802.11 protocols, so the WiFi infrastructure couldn’t work with it. The result was a bad WiFi experience.

In both cases, what we did was turn up some DFS channels. In the second case, we also turned down non-DFS channels. Here’s the method for enabling DFS: enable a few channels at a time. Doing it in pairs makes sense. Choose channels that can be bonded for 40 MHz. Then, watch the logs for a day or so. Look for channel abandonment events. If you don’t see any, move on to the next batch of channels. In the end, you will have an expanded collection of 5 GHz channels for your area and more available WiFi bandwidth.

We’re cowboys, here, by the way. In our office, all we use are DFS channels. So far, no deleterious effects!

-Kevin Dowd

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