2.3 Site Survey

The most efficient wireless network consists of a single client talking to a single access point a few feet away with an absolutely clear line of sight between them and no other noise on the channel being used (either from other networks or from equipment that shares the 2.4GHz spectrum). Of course, with the possible exception of the home wireless LAN, these ideal conditions simply aren't feasible. All of your users will need to "share the airwaves," and it's more than likely that they won't be able to see the access point from where they are located. Fortunately, 802.11b gear is very tolerant of less than optimal conditions at close range. When planning your network, be sure to look out for the following:

  • Objects that absorb microwave signals, such as trees, earth, brick, plaster walls, and people

  • Objects that reflect or diffuse signals, such as metal, fences, tinted windows, mylar, pipes, screens, and bodies of water

  • Sources of 2.4GHz noise, such as microwave ovens, cordless phones, wireless X-10 cameras and automation equipment, and other 802.11b networks

The more you can eliminate from the path between your access points and your clients, the happier you'll be. You won't be able to get rid of every obstacle, but you should be able to minimize their impact by working around them.

You may have total control over your own access points and other 2.4GHz equipment, but what about your neighbors? How can you tell what channels are in use in your local area?

While a spectrum analyzer (and an engineer to run it) is the ultimate survey tool, such things don't come cheap. Fortunately, you can get a lot of useful information using a good quality client radio and software. Take a look at the tools that come with your wireless gear. Lucent's Site Monitor tool (shown in Figure 2-1), which ships with Orinoco/Agere/Avaya/Proxim cards, is particularly handy. You should be able to get an overview map of all networks in range, and which channels they're using.

Figure 2-1. Lucent's Site Monitor tool shows who's using 802.11b in your area

A good client monitoring tool should get you started with simple surveying, but may not give you all the detail you need. See Chapter 7 for more tools that can show you exactly who's doing what on the airwaves in your area.

Other (non-802.11b) sources of 2.4GHz radio emissions show up as noise on your signal strength meter. If you encounter a lot of noise on the channel you'd like to use, you can try to minimize it by moving your access point, using a more directional antenna (see Chapter 6), or simply picking a different channel. While you always want to maximize your received signal, it is usable only if the ambient noise is low. The relationship of signal to noise is critical for any kind of communications. It is frequently abbreviated as SNR, for signal to noise ratio. As this number increases, so does the likelihood that you'll have reliable communications.