Tuesday, 7 May 2013

5GHz - 3 Missing Channels in Europe

Last year, I put up a posting which highlighted the fact that here in the UK (and I suspect all of Europe) we often have 3 channels missing from our allocation of unlicensed channels in the 5GHz band.  Looking at many manufacturer data sheets, channels 120, 124 and 128 are often shown as not being supported. This is despite the fact that they are allocated for use by local regulatory bodies (OFCOM here in the UK).

I recently posted a question about this on a partner forum of a major WiFi vendor that I deal with and finally got a definitive answer on this. In this post, I'll share my findings.

The reason that these particular channels (120 - 128) receive special treatment is that they occupy frequencies that are used by weather radar systems. WiFi systems have to be very careful not to interfere with those systems during their normal operation. Therefore, WiFi equipment has some additional checks and tests imposed on it to make sure that it does not inadvertently cause any interference.

In the ETSI region (Europe), the standard EN 301 893 dictates that any channels operating in the frequency range 5.6GHz to 5.65GHz must wait an additional period of time before using a channel. For most DFS-affected channels, a WiFi device must wait for 60 seconds to verify that no radar is present before commencing operation. However, on the channels in the 5.6GHz to 5.65GHz range, the device (i.e. Access Point) must wait 10 minutes! The table below (taken from Annex D of the standard) details this requirement:

Due to this 10 minute wait period, it seems that many manufacturers have chosen to withdraw support for the channels affected, which are 120, 124 and 128. This makes perfect sense now - who would want an AP to be allocated to a channel and then wait for 10 minutes before it can use it..?

I'm pleased to have finally got to the bottom of this particular grey area, but it seems like bad news for WiFi usage in Europe generally. With the anticipated sharp uptake in 5GHz usage around the globe, as 802.11ac starts to roll out, the loss of 3 channels is quite a chunk of spectrum to lose.

In the UK we have 19 unlicensed channels to use on 5GHz for WiFi. Losing 3 of those channels is a 15% loss in spectrum. This is at a time when we really need to be increasing spectrum availability to cope with the additional channel bonding opportunities that  802.11ac provides to increase WiFi speeds.

Hmmm....let's hope some additional spectrum turns up in the near future.

Update: I've now created a white paper which details 5GHz usage for WiFi in the UK. Find it here