Are Site Surveys Unnecessary?
August 04, 2003
The arguments for careful planning -- not to mention the problems you can still encounter by just adding cheap access points to dead spots in the network-- still point toward the continuing need for surveys.
It used to be Gospel truth that if you wanted to deploy a serious 802.11-based network you had to do a site survey. Now, as the street price for low-end access points drops below $100, some would-be network administrators are contemplating forgetting about time-consuming and expensive site surveys in favor of simply throwing more access points at any coverage problems.
It sounds good doesn't it? Can't get a reliable connection in the corner office? No sweat, just bring in a Wi-Fi access point or repeater near the trouble spot, and everything will be wonderful.
Well, no, not really.
First your 'dead spot' problem may not be caused by a lack of signal strength at all. 2.4GHz interference from devices like microwaves and wireless phones isn't a joke. I've seen cases where a noisy 2.4GHz phone killed off constant Wi-Fi access even when an access point was moved within ten feet of the computer.
Sometimes, Wi-Fi interference problems can catch you by surprise. For example, I was recently embarrassed to discover that my bedroom computer's Wi-Fi problem came from its Microsoft Wireless IntelliMouse for Bluetooth. I knew Bluetooth runs at 2.4GHz, but it took me days to realize that I was dealing with an interference problem caused by the mouse. Once the mouse was replaced by a conventional wired mouse, my interference problem cleared up immediately.
I've also found many troublesome connections to ultimately have nothing to do with Wi-Fi signal strength but with power management. You see, many 'green' desktops and laptops will try to save power by cutting off power to unused peripherals. I've seen several situations now where a Wi-Fi NIC appeared to be unable to get a signal when the problem really was that over-aggressive power-saving was cutting off its juice.
In general, when working on a troublesome connection, I find it extremely helpful to not only check the card's power settings, but the computer's settings as well. My rule of thumb is that if a PC spends most of its life attached to an electrical outlet, you should just turn off all power saving settings except for the monitor. Besides Wi-Fi problems, over the years I've seen far too many laptops and PC fall into sleep mode and then refuse to wake up except with the cold bucket of water of a cold reboot.
You're sure it's Signal Strength?You've checked out all the usual connection problems--bad IP settings, rogue DHCP servers, overly aggressive firewalls--and you've checked some of the more obscure ones above, and you're sure it's a signal strength problem. Now, can you solve it by putting in more access points?
No, I'm sorry to say, you can't always.
Simply popping access points in where ever you have a coverage dead spot can cause even more problems. By far the most common of these is one well known to radio engineers but barely heard of by network administrators: the multipath problem.
An omni-directional Wi-Fi transmission, like those you get from ordinary access point antennas, tends to bounce off walls and objects on their way to a receiver. The result is that a receiver, like the Wi-Fi network interface card (NIC) in your computer, gets multiple signals instead of just the one strong signal it needs. When you listen to a radio, you hear the results as echoes or static. The same things happen to your NIC but these devices are far worse at in listening for just the right signal than your ears.
If you add another access point to the mix, that can work -- if, and this is a big if, its signal is much stronger than the more distant access point signal and its multipath reflections. If, as is often the case, however, the signals and/or multipath signals are about the same strength all that will happen is that your dead spot will get even deader or you'll get NICs that become confused as to which is the right access point for them to hook into. Most Wi-Fi NICs are set to home in on their last best signal source, but I've seen them so confused as to which access point is the right one for them that they might as well not have any Wi-Fi connection at all for all the good they'll do their users.
Of course, a related problem with using multiple access points is that there are only so many effective channels. For example, 802.11b only has three channels for simultaneous data transfers so its throughput can plummet with multiple access points competiting for bandwidth.
Another solution to the multipath problem is to use a directional, aka Yagi, antenna to point the signal right at the access points. For antenna basics see: Antennas: The Key to Maximizing RF Coverage. You can, if you want to get fancy about it, even aim a directional antenna at a known multipath reflection object -- say a metal filing cabinet -- and bounce signals around corners.
Of course, all of these solutions have one thing in common. Instead of trying them on the fly, or simply buying access points and moving them around until everything works right, you could have just done a site survey in the first place.
It doesn't matter how cheap access points get, the real cost in any Wi-Fi installation is network administrator and technician time. Like any other job, you save more time and money by planning out what you're building first rather than simply starting building and hoping that you'll make it under budget.