I have a customer with a small office on three floors, with about a dozen PCs and laptops, and a a couple of PCs and printers hard-wired.
The heart of the network is a Netgear FVS114 firewall providing DHCP. There's a hub feeding a Thomson ST180 wireless access point (ground floor), and a cable link to the 2nd floor feeding a Netgear WG602v3 access point. The PCs have AT-WCP200G PCI wireless cards.
Whatever I've tried over the last few months (!) there always seems to be one PC or other which can't stay on the network. Some never seem to have a problem, but the problem seems to shift around the ones which do.
The firewall was replaced recently (identical unit) which improved reliability of the network overall. I've also found that one of the links from the ground to the 2nd floor (note there's a 1st floor in between - I'm in the UK!) has an intermittent open-circuit at one socket; the other seems ok.
Most PCs, downstairs and upstairs, can bind to the Thomson ST180 and keep a stable connection, except for one downstairs (which usually can't see any wireless networks) and one upstairs. I've tried reloading drivers, extension antenna, switching round the WAPs, bringing both downstairs - no improvement.
I thought I was onto something when I bought WirelessMon, which showed about 8 wireless networks, all only intermittently available except for the Thomson ST180 (about -45dB). The Netgear WG602v3 is much stronger upstairs (that's where it lives) at about -15dB, but WirelessMon shows it coming and going several times a minute, seen through the wireless card of one machine which manages a steady connection to the ST180 two floors below. Another nearby machine can't get a steady hold on the downstairs WAP, and can't bind to the upstairs one.
I replaced the Netgear WG602v3 with a US Robotics USR5416. No improvement: the signal in WirelessMon was still shown as coming and going while the downstairs ST180 was steady as a rock.
Changing channel on the downstairs ST180 allowed one PC to get a connection but another one lost it, so I changed back, restoring the previous situation.
I disabled the PCI card on the "unlucky" PC upstairs and fitted a US Robotics USR5420 adapter, which was able to bind to the weak signal from downstairs, but not the Netgear WG602v3 upstairs. Substituting the USR5416 again (configured so that it's possible just to switch the boxes) produced a loss of ability to bind to the ground floor in the PC with the USR5420 USB adapter, and it couldn't bind to the USR5416.
Today I've tried fiddling with the various settings on the (reinstated) Netgear WG602v3. While the downstairs ST180 remains on WEP (128), I switched the WG602v3 to WPA-PSK. No improvement. I've also tried reducing the data rate from "Best" to 24Mb/s, and changed from "g or b" to "g only". No improvement.
Meanwhile, a network in their other office with a single Thomson ST180 and 8 of the same PCI cards soldiers on without a blink. If I could get another ST180 I would!
So, the only thing I can think of is to start replacing the wireless PCI cards, inclining towards US Robotics. However, if you've been generous enough to read this far and can suggest ANYTHING else worth trying, I'll be greatly indebted to you.
PS: I've stumbled on NetStumbler, which is useful. Here's a screnshot:26 networks detected after an hour's running (up to 40 36 hours later!). I've sorted by max SNR (Signal to Noise Ratio) although I'm not certain the "noise" part of that value is being correctly reported. Coincides with max Signal, though.
Just one signal stays steady as a rock, and it isn't the strongest!