Q: I was told to deploy my wireless network on channels 1, 6, and 11. Why can't I use other channels (e.g., channel 2, channel 3)?
2.4GHz frequency band and how Wi-Fi uses spectrum
A: There are two aspects to answering this question. First, you need to understand a little about the 2.4GHz frequency band. Second, you need to know how Wi-Fi uses spectrum.
The 2.4GHz frequency band is divided into 5MHz blocks. In other words, the central frequency of the channels is 5MHz apart, so channel 1 is 2.4GHz, channel 2 is 2.405GHz, channel 3 is 2.410GHz, and so on.
Avoid signal overlap and interference
The signal bandwidth required by an 802.11g radio is 20MHz. If you put one Access Point (AP) on channel 1 and another on channel 2, when they transmit the signals, the signals would overlap and interfere with each other. In this situation, Wi-Fi will continue to operate but the performance would be significantly degraded. The solution is to put your APs on channels 1, 6, and 11. These channels are far enough apart that the 802.11g signals won't overlap and hence interfere with each other. Configuring your network such that adjacent APs are on non-overlapping channels will give you the best throughput.
The newer 802.11n APs can operate in either a 20MHz or 40MHz frequency channel. In environments that have more than one AP, 802.11n deployments in 2.4GHz will use the 20MHz channel, the same as 802.11g.