This article provides information on channel selection for an AirPort Base Station as well as the overlap of frequencies between channels.

AirPort operates in the Industry Scientific and Medical band (ISM). The ISM band is an internationally recognized band for radio LANs which operates in the 2.4 Ghz range. There are 14 allocated channels within this band, as shown in figure 1. The concept of a channel is similar to a television channel, it represents a band of frequencies over which information is transmitted or received. Not all countries support all channels in the ISM band.

Figure 1 ISM Band Chanel Allocation

In the AirPort Admin Utility, you can assign a base station's channel of operation. When setting up a wireless network with multiple base stations, it is vital to select the proper channel for each base station. Failure to do so could result in poor network performance (slow network traffic).

Figure 2 shows the required channel bandwidth beginning at the center frequency (Fc) and decreasing in amplitude as it extends outward. In this case, channel seven extends well into adjacent channels as indicated by the red area. If channel five were also in operation, its bandwidth would extend into the lower half of channel seven's band. This presents a problem because before a base station can transmit data it must detect that the medium is free of other carriers (channel frequencies). The presence of another carrier indicates that the medium is busy. If it senses that another device is using the medium, it must wait and retry until the medium is carrier free. This results in slower network traffic.

Figure 2 Channel Bandwidth

This situation can be prevented by assigning the proper channels to multiple base stations operating in the same area. For example, if two base stations are physically located near each other, they could both operate at maximum capacity if one was operating on channel one and the other was operating on channel nine. Because their respective bands would not interfere with each other, they could both operate at the same time providing maximum bandwidth for their respective networks.

By selecting the Network tab at the top of the AirPort Admin Utility window, the network tab window appears as shown in figure 3. Clicking the channel selection pop-up menu (indicated by the red arrow) displays the 11 channels allowed in the United States.

Figure 3 AirPort Admin Utility Configure Network Tab

Note: Wireless Internet access requires AirPort Card, AirPort Base Station, and Internet Access (fees may apply). Some ISPs are not currently compatible with AirPort, including America Online. Range may vary with site conditions.