Page
21
February 2010
Increasing frequency
1
6
11
2
7
3
8
4
9
5
10
3.2
Selecting a Channel
The WI-MOD-E conforms to the IEEE 802.11 Wireless LAN specification. The WI-MOD-E
supports 11 radio channels, each 5MHz wide, in the range 2412MHz to 2462MHz. Only one of
these channels is used for a connection. The desired channel is selected and configured at the
Access Point, and is then used for all beacon transmissions and connections. Clients scan all 11
channels for a suitable Access Point and then adopt the same channel as the AP when a connection
is established. Although each channel is only 5MHz wide, the radio transmission is a lot wider.
Hence the channels
overlap
. The following diagram shows the RF energy distribution for a WiFi
transmission:
Most of the energy is in a central 22 MHz wide “lobe”, centered around the channel frequency,
however there are also side-lobes extending either side.
If we ignore the side lobes and consider each WiFi message as a 22MHz wide transmission, then the
following diagram represents how transmissions in each channel overlaps.
If there is more than one WiFi AP within the same wireless range, then it is important that the AP’s
are on channels as far apart as possible. If there are only two AP’s, then set them to 1 and 11. If
there are three, set them to 1, 6, 11.
It is also important that correct channel is selected for region. Channels 1 to 11 are approved for
North America (FCC), Europe (ETSI), Canada (IC) and Australia (ACMA). Channels 10 and 11
are approved for use in Spain and France. Refer to the relevant regulatory authority for the region as
to which radio channels are approved for use.
f-33
f-22
f-11 f
f+11 f+22 f+33
RF power distribution in a WiFi transmission with channel central frequency, f
