Setting up a wireless network (WLAN) is far from rocket science, but it takes a little thought if you’re going to make a decent job of it. In many ways, setting up a WLAN is a great deal simpler than building a cabled network (at least you don’t have to get cables to every desktop) but there are still plenty of complications with locating the access points to provide optimal service.
The first thing to consider is the extent to which you’re restricted by cabling requirements. You have the option of using mains-powered access points (which need power and network outlets nearby, but nothing special at the core of the network) or selecting access point kit that takes its power from the Ethernet cable. The latter is simpler to install, as you don’t have to worry about locating power outlets, but you’ll need to equip the core network with switches or add-on boxes that shove the power down the network cable.
You have three main considerations to make when placing your access point equipment. First, there’s the basic range of the radio kit through thin air. To pick a random card from the Web, the NetGear MA401 PCMCIA card (an 802.11b NIC) will shout up to 835’ through thin air, though the more walls that get in the way, the lower the range of the card (and the technology will automatically crank the speed down as the signal gets weaker, in an attempt to make transmission more reliable). Next is the amount of brick and concrete in your office – the more walls you have, the more the signal will be blocked. Third, you need to consider how many computers will be using a single access point – just like shared-media Ethernet, and its performance under load, once you get more than a dozen or so people using a single frequency range to an access point, the contention for the limited space on the network makes throughput drop like a stone.
The first thing you’ll probably do is take a plan of the office, get a copy of the datasheets for the equipment you plan to use (both access points and computers/PDAs) and make educated guesses at where the access points will go. It’s all a bit of a guess but you can actually take a reasonable stab (particularly in an open plan office) at what coverage an access point in a particular place will give you. By all means draw yourself a multi-coloured diagram of the coverage you expect your access points to provide.
The next thing to do is to place an access point somewhere close to where you planned it and see what signals you really get. To do this you can either get a wireless LAN analyser or you can simply walk around with a laptop watching the signal strength meter on the software that came free with the NIC (most have acceptable diagnostic packages, which usually include a signal strength and throughput meter).
Your findings at this stage can be retro-fitted to the guesses you made on your first diagram – you’ll probably have been either under- or over-optimistic with the amount of signal you expected the walls to soak up, for example. With a bit of fiddling, you should be able to gradually refine the placement of the various access points such that you get an acceptable balance of throughput versus equipment cost.
Tools to help you
Wouldn’t it be handy if you could be a bit more scientific about this process? Well, some vendors are starting to be a bit clever with the tools or services they provide for wireless LAN design. Some, such as Airespace have a design service, and will take a diagram of your building and use it to deduce the building’s RF (radio frequency) issues and thus imply the location of your access points. Others, notably Trapeze Networks go a step further and provide a design tool with their equipment. You give it a plan of the office space, tell it what kind of walls, windows, etc., you have and it shows you what the radio profile will look like (you can even move things around until you’re happy with the coverage). Of course, what you achieve is similar to the results of the manual labours described earlier, only rather faster and with less walking around.
Final non-placement issue
Once you’ve dealt with the placement of your access points, there are some further aspects to consider with regard to the delivery of signals. First off, we mentioned earlier that you may need to double up access points in areas with many workstations. In these cases you’ll want to choose carefully the channels that the various workstations use, in order to spread the load evenly among access points. More importantly, though, if you want people to be able to roam around the building, roaming from access point to access point, without continually refreshing their network settings, you’ll either have to have all of your wireless access points in the same IP subnet (which can be inefficient once you have more than a few dozen machines) or you’ll need to think about a roaming IP installation. Some vendors (Cisco, for instance) prefer to use the Mobile IP standard, othes (notably Trapeze, whom we mentioned earlier) do it their own, proprietary way. Such “roaming” IP systems allow you to split your LAN into subnets as you would with a cabled world but with no intervention on the user’s part as they roam from one subnet to the next.
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