Channels

The number of channels available for 802.11 b/g depends on the country you live in.  In the USA and Canada only the first 11 channels are available whereas in most of the rest of the world there are 13 usable channels.  In Japan they have an additional channel - Channel 14 - available.  Of course, various countries have various regulations and you’ll need to check the local regulations.  (Australia, out of interest, allows the use of channels 1 – 13 for both 802.11b and 802.11g.)

Now, this doesn’t (unfortunately) mean that you have a free choice amongst all of these channels.  This is due to the bandwidth of each channel being 22 MHz (at its -30dB points) but the channels being spaced only 5 MHz apart (except for channel 14 which is centered 12 MHz above channel 13).  This is where the misinformation that I spoke about comes into play.  A number of people don’t understand how Wi-Fi channels are designated, how they look when observed on a spectrum analyser, and how they interfere with adjacent channels.  The people who don’t understand technically how Wi-Fi works will often tell you that if you are in a building with neighbours using channels 1, 6 and 11 already, that using channels 3 or 9 will give you the best result.  Sadly, they are mistaken and if, for example, you chose channel 3, then the networks on channels 1 and 6, as well as your network on channel 3 will all suffer performance penalties more so than if you just chose channel 1, 6 or 11 even though they are already in use.  (Of course, for those users in Japan you have the added bonus of Channel 14 being available and non-overlapping with any other channel.)

OK.  That may sound rather weird logic to the uninitiated, but if you have a closer look into the characteristics of an average 802.11b transmitter (we’ll mention the differences between 802.11b and 802.11g transmitters in a minute) you’ll see that the radiation pattern isn’t a true bell curve, parabola or sinx/x curve that most people claim to be the case.  The 802.11b spec states that at 11 MHz either side of the center frequency, the radiated power must be 30 dBm lower, and at 22 MHz it must be 50 dBm lower.  The vast majority of transmitters will adhere to these specifications.

Now to get really nerdy, the 802.11b spectrum is roughly a “sinx/x” curve that has had a square root raised cosine (SRRC) filter applied to it.  This holds quite well for the first 11 MHz each side of center at least, but widens over the next 11 MHz each side.  OK, so this means nothing to you?  Fine – there’s no need to know this, but it is there for those who wanted to exercise the left side of their brain!

If you want some pretty pictures to explain all that, have a look at the top picture on this page at RF Cafe for the 802.11b spectral mask and Figure 1-1 in this PDF file from Texas Instruments.  Cisco has a nice write-up that can be found here about 802.11b, 802.11g and their research into the Four Channel System that some people claim is a good way to run multiple access points.

Now, if you thought 802.11g was the same as 802.11b, that’s also not quite the case. With 802.11b, the 5 MHz centres and requirement for signal strength to be -30 dBm at +/- 11 MHz and -50 dBm at +/- 20 MHz means that you have a pretty good chance of having decent separation between channels 1, 6 and 11. Unfortunately, with 802.11g there’s only a requirement for -20 dBm at +/- 11 MHz and -30 dBm at 22 MHz, means that this separation is less. This also means that anyone using a Four Channel System with 802.11g devices will suffer noticeable performance degradation.