Having a solid understanding of channel use with wireless LANs is imperative for any good wireless LAN administrator. As a wireless LAN consultant, you will undoubtedly find many wireless networks that have many access points, all of them configured for the same channel. In these types of situations, a discussion with the network administrator that installed the access points will divulge that he or she thought it was necessary for all access points and clients to be on the same channel throughout the network in order for the wireless LAN to work properly. This configuration is very common, and often incorrect. This section will build on your knowledge of how channels are used; explaining how multiple access points using various channels can have a detrimental impact on a network.
Adjacent Channel Interference
Adjacent channels are those channels within the RF band being used that are, in essence, side-by-side. For example, channel 1 is adjacent to channel 2, which is adjacent to channel 3, and so on. These adjacent channels overlap each other because each channel is 22 MHz wide and their center frequencies are only 5 MHz apart. Adjacent channel interference happens when two or more access points using overlapping channels are located near enough to each other that their coverage cells physically overlap. Adjacent channel interference can severely degrade throughput in a wireless LAN.
It is especially important to pay attention to adjacent channel interference when colocating access points in an attempt to achieve higher throughput in a given area. Colocated access points on non-overlapping channels can experience adjacent channel interference if there is not enough separation between the channels being used, as illustrated in Figure 9.16.
In order to find the problem of adjacent channel interference, a spectrum analyzer will be needed. The spectrum analyzer will show you a picture of how the channels being used overlap each other. Using the spectrum analyzer in the same physical area as the access points will show the channels overlapping each other.
There are only two solutions for a problem with adjacent channel interference. The first is to move access points on adjacent channels far enough away from each other that their cells do not overlap, or turn the power down on each access point enough to where the cells do not overlap. The second solution is to use only channels that have no overlap whatsoever. For example, using channels 1 & 11 in a DSSS system would accomplish this task.
Co-channel Interference
Co-channel interference can have the same effects as adjacent channel interference, but is an altogether different set of circumstances. Co-channel interference as seen by a spectrum analyzer is illustrated in Figure 9.17 while how a network configuration would produce this problem is shown in Figure 9.18.
To illustrate co-channel interference, assume a 3-story building, with a wireless LAN on each floor, with the wireless LANs each using channel 1. The access points’ signal ranges, or cells, would likely overlap in this situation. Because each access point is on the same channel, they will interfere with one another. This type of interference is known as co-channel interference.
In order to troubleshoot co-channel interference, a wireless network sniffer will be needed. The sniffer will be able to show packets coming from each of the wireless LANs using any particular channel. Additionally, it will show the signal strength of each wireless LAN's packets, giving you an idea of just how much one wireless LAN is interfering with the others.
The two solutions for co-channel interference are, first, the use of a different, nonoverlapping channel for each of the wireless LANs, and second, moving the wireless LANs far enough apart that the access points’ cells do not overlap. These solutions are the same remedy as for adjacent channel interference.
In situations where seamless roaming is required, a technique called channel reuse is used in order to alleviate adjacent and co-channel interference while allowing users to roam through adjacent cells. Channel reuse is the side-by-side locating of non-overlapping cells to form a mesh of coverage where no cell on a given channel touches another cell on that channel. Figure 9.19 illustrates channel reuse.
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