Wireless bridge question....?

[mrunal_47]

Hi all,

I have a wireless bridge between two buildings with 54 Mbps link. However theres around 90% packet loss between the two sites. What would be the best way to take care of packet loss.Any wireless experts..?

Saurabh

[scottmac]

First, what have you tried so far?

Have you checked the wireless environment with something like NetStumbler (www.netstumbler.com - free)?

Have you tried other channels?

What is your signal strength, and what is your signal quality?

Do you have a good line-of-sight? (like, between trees would be bad ... when the wind blows the trees eat your signal). In that same ballpark, did you make sure you had a clear Freznel zone?

Have you checked the WIRED connection to your bridges? It's certainly possible that your cabling is causng the high failure rate.

Is it bad all the time, or just some week days, nights, weekends, etc? Over what period of time was the 90% measurement recorded? Did you try resetting the counters to see what *rate* the failures occurred? (i.e., under heavy traffic loads, was the rate higher than under normal / low traffic loads)?

Let us know ...

Scott

[mrunal_47]

Thanks for reply Scott,

We have line of sight between the wireless link and as such we dont think there should be any interference ( buildings are close to airport though). We did new cabling today and didnt solve. At smaller rates like 128 Kbps theres no loss, however if we go higher rate like 10Mbps...its miserable! What do you suggest as next step..?

Thanks

[scottmac]

Reduce your RTS time by half (as a starting point).

If you are getting a lot of interference, reducing the size of the transmission should (except in *extreme* cases) reduce the error count.

Unfortunately, it will also increase your overhead a bit ... but if you're only getting 10% through now, a little extra overhead shouldn't be noticed.

In the Web GUI, RTS is under the "Interfaces" page.

I suppose it's possible that the radar at the airport, or the radar altimiters in airplanes, or some other system (ILS, markers ...) could be interfering ... most of that is VHF (~118-136Mhz) ... but maybe it works out in the harmonics ...

Reducing the RTS time should reduce the length of each transmission, so the interfering signal will have a "smaller target." It's also less to re-transmit.

Give it a shot and let us know.

Scott

[mrunal_47]

Thanks Scott..your suggestion are really helpful. Today I went with some test equipment to the site and the result were not good. Signal strength varied between -80 dbm to -70 dbm, while noise level was

-90 dbm. The signal to noise ratio was from 10-20 which is very low. We tried to align antennas but that didnt help us to change signal strength. The crosstalk was arond 10%. Is it the surroundings causing the problem ot the equipment. Can we use any kind of signal amplifiers...? Thanks!

[scottmac]

It's looking like you need some good directional antennas. They should increase the signal strength and potentially reduce some of the interference.

I'm very surprised thats you are indicating full rate with a signnal strength and signal quality you describe.

In the meantime, you can try forcing the data rate down, you might ultimately get more throughput (lower rates are less sensitive to bad conditions).

Signal amps in your situation are 1) not certified and 2) probably not helpful: You have to have a decent signal to amplify, otherwise you are also amplifying the noise right along with the desired signal (higher signal levels, same SNR).

So, pick your poison; what have you got now (AP/Bridges) and what kind of antennas? What distance are you throwing the signals? What's your minimum acceptable data rate(s)?

Let us know ...

Scott

[mrunal_47]

Hi Scott,

We expect to atleast make out 20Mbps out of the wireless link. We are using using Cisco 1300 Aironet,

in root bridge and non-root topology. At lower rates of 128 Kbps the links works fine with 0% loss. Its only around 1Mbps that start creating problem. The distance between the building should be around 50-60 meters.The channel we are using is not congested either. We tried moving antennas for better signal strength but with no changes in signal strength. What could be the next step ? Thanks..