T40 wireless (Cisco PCI) keeps shutting down Wifi router

[atx-88]

For some reason, my T40's internal wireless unit often knocks out my wireless router when I download files at high speed. For example, I would try downloading a 50 MB file at speeds from 80kb/sec to 125 kb/sec and the download would just stop after only 1/4 of the file was downloaded and the wireless signal would go down for every computer in our office. I would need to re-start the wireless modem to restore the signal. I had always thought that our Roadrunner cable internet was just being flaky and that there was some sort of system limitation on download volume until I pulled out my old X21 (with a PC Card WIFI card installed) and found that it could download the same files at the same speed as my T40 without problems? Anyone ever experience this problem?

[bill bolton]

my T40's internal wireless unit

Which is.......?

Cheers,

Bill B.

[Johan]

... a hypothetical explanation, and suggestions for an empirical investigation:

If there are several WLAN systems operating in your vicinity (neighbour offices?), this could cause trouble to both you and the neighbour, if your systems are using the same WLAN channel/frequency. Try check which channels your wireless router or access point is set up using. It has a default setting, so if everybody uses that channel, some interference may occur (possibly resulting in a lower data rate!).

To check what frequencies "are in the air" in your vicinity, try download the free - and very useful! - spectrum analyzer program --> NetStumbler.

I have no sure explanation as to why the X21 would be able to download at high speed at the same time as the T40 would not... except that the X40 WiFi card might be better than the T40 ("better" in terms of having better sensitivity = better signal-to-noise-ratio, and/or better selectivity?). Also, beware that for indoor WiFi systems, you partly or completely live on what is called multipath; i.e. there is often no direct, visible signal path from the transmitter antenna to the receiver antenna, so the signals between the antennas are bounced off (reflected multiple times) by walls, floor etc. etc. Because of this, the signal strength in an office environment may be very different within the office - so if operating e.g. two laptops in the same office, they may experience different signal strength, simply owing to their different locations in the office. And signal strength is only one issue; [electromagnetic] signals (and antennas!) have what is called polarization - and an antenna radiating (or receiving) purely vertical polarization signals has infinite isolation towards an antenna radiation (or receiving) purely horizontal polarization signals! This means that the antennas (their polarization properties) in the X21 and T40 will also have some impact on the overall picture. Fortunately, for in-door installations, multipath reflections will significantly alter the signal polarization, so even if e.g. transmitting purely vertical polarization, the resulting signal at the receiver will not be of purely vertical polarization.

Now, if this was not enough, there might be two antennas in the X21 card, whereas there might only be one in the T40 (I don't know?). The reason for most modern WiFi systems are having two antennas, in stead of only one, is, that the two antennas are located at physically different positions, and hence will experience a different, resulting multipath scenario. If the multiple signals (which altogether will create the resulting signal) might tend to cancel at the position of the first antenna, they will - statistically - not do this at the other antenna, provided the antennas have some minimum spacing. So, there will be a diversity receiver in such a WiFi card, comparing the signal strength from these two antennas, and using the strongest signal at any time. Such an antenna arrangement is called "space diversity", and is implemented to combat multipath. Other similar diversity arrangements are frequency diversity and polarization diversity.

So, just for fun, try move the T40 around a little bit in the office, and see if the signal strength (read as download speed, if the server is not the bottleneck!) goes up and down - and how much?

Good luck - optimizing WiFi/WLAN indoor propagation/throughput can be very tricky!

Best regards,

Johan

[atx-88]

I tried moving things around but the same problem with the wireless router shutting down continues to occur. I can download at higher speeds using a direct hook up from the cable modem without the signal going down so the problem is localized to the wireless components. Still at a loss.