As a wireless LAN installer or administrator, you really have two choices when considering access point co-location. You can accept the degraded throughput, or you can attempt a workaround. Accepting the fact that your users will not have 5 Mbps of actual throughput to the network backbone on each access point may be an acceptable scenario. First, however, you must make sure that the users connecting to the network in this situation can still be productive and that they do not actually require the full 5 Mbps of throughput. The last thing you want to be responsible for as a wireless LAN administrator is a network that does not allow the users to do their jobs or achieve the connections that they require. An administrator's second option in this case is to attempt a workaround. Below, we describe some of the alternatives to co-location problems.
Use Two Access Points
One option, which is the easiest, is to use channels 1 and 11 with only 2 access points, as illustrated in Figure 9.11. Using only these two channels will ensure that you have no overlap between channels regardless of proximity between systems, and therefore, no detrimental effect on the throughput of each access point. By way of comparison, two access points operating at the maximum capacity of 5.5 Mbps (about the best that you can expect by any access point), give you a total capacity of 11 Mbps of aggregate throughput, whereas three access points operating at approximately 4 Mbps each (degraded from the maximum due to actual channel overlap) on average yields only 12 Mbps of aggregate throughput. For an additional 1 Mbps of throughput, an administrator would have to spend the extra money to buy another access point, the time and labor to install it, and the continued burden of managing it.
In certain instances, the extra 1 Mbps of bandwidth might still be advantageous, but in a small environment, it might not be practical. Don't forget that this scenario applies only to access points located in the same physical space serving the same client base, but using different, non-overlapping channels. This configuration does not apply to channel reuse, where cells on different non-overlapping channels are alternately spread throughout an area to avoid co-channel interference.
Use 802.11a Equipment
As a second option, you could use 802.11a compliant equipment operating in the 5 GHz UNII bands. The 5 GHz UNII bands, which are each wider than the 2.4 GHz ISM band, have three usable bands, and each band allows for four non-overlapping channels. By using a mixture of 802.11b and 802.11a equipment, more systems can be co-located in the same space without fear of interference between systems. With two (or three) colocated 802.11b systems and up to 8 co-located 802.11a systems, there is the potential for an incredible amount of throughput in the same physical space. The reason that we specify 8 instead of 12 co-located access points with 802.11a is that only the lower and middle bands (with 4 non-overlapping channels each) are specified for indoor use. Therefore, indoors, where most access points are placed, there's normally only the potential for up to 8 access points using 802.11a compliant devices.
Issues with 802.11a Equipment
802.11a equipment is now available from only a few vendors, and is more expensive than equipment that uses the 2.4 GHz frequency band. However, the 5 GHz band has the advantage of many more non-overlapping channels than the 2.4 GHz band (8 vs. 3), allowing you to implement many more co-located access points. You must keep in mind that while the 2.4 GHz band allows for less expensive gear, the 2.4 GHz band is much more crowded, which means you are more likely to encounter interference from other nearby wireless LANs. Remember that 802.11a devices and 802.11b devices are incompatible. These devices do not see, hear, or communicate with one another because they utilize different frequency bands and different modulation techniques.
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