Selecting Channel Frequencies for Wireless APs
Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2
Direct communication between an 802.11 wireless network adapter and an AP occurs over a common channel corresponding to a frequency range in the S-Band ISM frequency range. You set the channel in the AP, and the wireless network adapter automatically tunes to the channel of the AP with the strongest signal. The wireless network adapter continues communication with the AP until the signal gets weak, at which time it attempts to locate another AP with a stronger signal.
To reduce interference between wireless APs, ensure that wireless APs with overlapping signals use unique channel frequencies. The 802.11b standard reserves 14 frequency channels for use with wireless APs. Within the United States, the Federal Communications Commission (FCC) allows channels 1 through 11. In most of Europe, you can use channels 1 through 13. In Japan, you have only one choice: channel 14.
Figure 11.4 shows the 11 802.11b frequency channels available in the United States. Notice that the 802.11b signals overlap with adjacent channel frequencies. As a result, you can only use three channels (in the United States, channels 1, 6, and 11) without causing interference between adjacent APs.
Figure 11.4 Channel Overlap for 802.11b APs in the United States
To select the channel frequencies for the wireless APs:
Identify any wireless networks owned by other organizations in the same building. Find out the placement of their wireless APs and the channel frequencies assigned to the APs.
Radio waves travel through floors and ceilings, so APs located near each other on different floors need to be set to non-overlapping channels. If another organization located on a floor adjacent to your organization’s offices has a wireless network, the wireless APs for that organization might interfere with the wireless APs in your network. Contact the other organization to determine the placement and frequencies of their wireless APs so that you can ensure that any of your own wireless APs that provide overlapping coverage use a different channel frequency.
Identify overlapping wireless signals on adjacent floors within your own organization.
After identifying overlapping coverage areas outside and within your organization, assign channel frequencies for your wireless APs.
Assign channel 1 to the first wireless AP.
Assign channels 6 and 11 to any wireless APs that overlap coverage areas with the first wireless AP, to ensure that those APs do not interfere with one another.
Continue assigning channel frequencies to the wireless APs, ensuring that any two wireless APs with overlapping coverage are assigned different channel frequencies.
Example: An Enterprise Corporation Determines IEEE 802.11b Channels
An enterprise corporation occupies multiple floors in a building. Because of this, they had to pay attention to both the horizontal and vertical dimensions when determining which IEEE 802.11b channel to assign to each AP. For example, if a certain spot on the first floor used channel 1, they assigned channel 6 to the same location on the second floor, and assigned channel 11 to the same location on the third floor. They did not use channel 1 again until the fourth floor.
Figure 11.5 illustrates the selection of channels for the wireless APs on a building floor. The wireless AP channels were selected to ensure that no two overlapping areas of coverage have the same channel (frequency).
Figure 11.5 Example of 802.11b Channel Allocation