How is speed (rate) determined?

[bairdc]

I have a question stemming from some experimenting I did tonight. Specifically, I'm wondering how the rate on a link is determined if everything is doing auto rate. I was always under the impression that it was a signal thing. The better the signal, the faster the connection. Tonight I got playing with one of my APs. I switched it to another channel, which had less noise. On that channel, the association list showed practically identical signal levels as on the previous channel, and like I said before, it had better noise levels. Yet, several customers' rates started fluctuating between 1 and 11 megs. Keep in mind that neither their signal or their noise levels were fluctuating more than 1 or 2 dB. One of the worst ones had -75 signal and -92 noise, yet they would often drop to 1 meg. After observing this, I switched back to the previous channel, and the rates stabilized. It appears to me that there's more to the rate thing than just signal. I'm just wondering what it is... Anybody know?

Craig

[lonnie]

You might have noise on the channel that is near those customers. Are they in the same area?

[tony]

While signal has much to do with the overall throughput, both the client, and the AP will adjust it's rate depending on how many ACK misses or FCS (CRC) errors it sees at a given speed. Placing either the AP or Client to a fixed rate would sometimes improve throughput as the rate adjustment has more of an impact than a few missed packets. (not that this does not necessarily result in packet loss, as the radio will retransmit a few times before giving up).

Note that the rate you are seeing on the AP for a specific client represents what the rate the client is sending to the AP at, and not what the AP is sending to the client at.

Thanks!

[ Jan 24, 2003 ]
changed:
"both the card, and the AP"
-- to --
"both the client, and the AP"

[bairdc]

You might have noise on the channel that is near those customers. Are they in the same area?

Well, the two that seemed to have the worst trouble actually were a block or two from one another. Maybe that's it. So, am I correct in assuming then, that if a client "hears" noise, it will adjust the rate it's transmitting at? In other words, rather than simply adjusting the rx rate to compensate for the noise it's hearing, it will also adjust its tx rate?

Craig

[lonnie]

That and packets that did not get acknowledged. it slows down to try and get the data through.