Poor wireless performance, high speed metro

[AnttiLeppanen4062]

Hi, 

We have cisco iw 3702 wgb acting as a client in metro (max speed 80km/h). Trackside ap:s {cisco 1532). 2.4ghz freq.

Now I need some advice.

Associations are working great and data rate is m15-s2 when high speed roaming.

Problem is poor wireless performance.iPerf results: 0-5Mb/s. 802.11b data rates are disabled.  High speed roaming enabled.

Is it possible that channel utilization that is avg 50%-60% is causing this poor performance, even if ap signal is very strong and used data rate is high?

Thank you!

[Scott Fella]

It is one variable that can cause issues. Look at how often the device and ap keeps negating data rates. If you see that happening a lot, it might be best to only have a few supported rate along with one mandatory rate.

[AnttiLeppanen4062]

Rates we are using:

[Scott Fella]

Well when you have devices moving fast, you need to try to reduce the negotiation as much as possible. Maybe only have one or two enabled and leave the one still as required. You might even try to keep one enabled maybe like 12mbps and keep 11mbps required. Then after testing add another enabled rate like 24mbps. Roaming will also introduce poor speeds in your setup and that has to be accounted for.

[Scott Fella]

I guess I need to ask, what is your expectations that you should see? It looks like you are also using older access points in autonomous mode? These are not great if you are roaming multiple access points, especially at high speeds.

[AnttiLeppanen4062]

We know that 1532 ap is not the best choice available but that is what we have right now. There are 600 ap:s total (under wlc 8540). IPerf tests on metro-test-track gives 25-30Mb/s (ch util is lower thought).

[Scott Fella]

That screen looks different. What code are you running in the 8540? Again you need to use the least amount of data rates. Speed is because the vehicle is moving fast and probably end up connecting at 11mbps. So depending on how far or close the aps are, you need to test and figure out what is the lowest data rate that you can use to keep a connection. The higher it is, the better throughput.

[AnttiLeppanen4062]

Thank Scott, this screencapt is from our wgb ap, cisco iw3702 (metro). I have to disable those rates tomorrow...

[AnttiLeppanen4062]

Hi, please check this attachment.  @Scott Fella 

This debug is from trackside ap (cisco1532 ) when metro (iw3702 wgb) is passing by.

Is access point really changing data rates that quickly??? And is it normal way to operate?

Thank you.

[AnttiLeppanen4062]

See attachment. @Scott Fella 

Metro (iw3702) stats. I see a lot of CRC errors and Retries...?

-A

[Scott Fella · Accepted solution]

AP and device needs to negotiate and tries to negotiate at the highest rate you defined. Then they both will renegotiate when there are errors seen in that rate and will re-negotiate at a lower rate, typically the next available rate, then the next. So yes this is how data rate negotiation happens in wireless and maybe its best to only try one data rate and check for stability and then add one and test. This way you can determine what is happening and what is best for your environment.

[Scott Fella]

Yes it is normal, it’s because you are allowing many data rates.