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Community Member

Large numbers of CRC and Holdoffs

We have 2 Aironet 340 bridges that have been working flawlessly for 18 months. During the last 3 weeks they started dropping connections periodically throughout the day while users were working I checked out the radio error logs and they are reporting large (100-3000) volumes of Receive CRC errors and large numbers of Transmit Retries and Holdoffs over a short period of time 3-4 minutes.

I found a document on CCO that states CRC errors could be caused by radio interference but to check the Line-of-Sight first. I checked the line of sight and the path appears very clear.

The Radio signal strength meter is around 90% with quality around 50-60%.

We have installed more bridges within the last month. All bridges Frequency setting was set to "auto". I discovered one pair was using the same requency as another pair so I turned off "auto" and set the frequencies manually.

Still large numbers of CRC, Tx Retries and Holdoffs. However the connectivity issues seem to have been fixed.

My question is are these large numbers normal, I don't remember seeing them in the past and if not what should I check?

Community Member

Re: Large numbers of CRC and Holdoffs

If not a line-of-sight issue, and not a radio inteference issue, you are down to the antenna and the cable run which I have seen cause this very problem.

Take a look at this, after this give TAC a shout.

Warmest regards,


Community Member

Re: Large numbers of CRC and Holdoffs

Are you in a multi-tenant building? It could be that someone else in your building has newly installed a WLAN and they are interfering with you.

Has there been a reconfiguration of your office? New or moved desks, bookcases, and whatnot could change interference patterns.

Also, remember that only chan 1, 6, 11 do not overlap.

If you've added APs, you need to make sure the same channels to not overlap. Did you try reducing the signal strength? -- which will reduce cell size and thus the overlap.

Community Member

Re: Large numbers of CRC and Holdoffs

Could you please describe how a cable run can create this type of


Thanks for your help,


Community Member

Re: Large numbers of CRC and Holdoffs

If I may inquire:

When you write "dropping" connections were your clients getting

"logged off" automatically? Was there a re-ocurring automatic logon

procedure with the end user completely unaware of this event

happening until the logon procedure is noticed?

Thanks for your contribution,


Community Member

Re: Large numbers of CRC and Holdoffs

I discovered a couple of the bridges were set to auto scan for frequencys and 2 separate links were using the same frequency.

When I said dropping I meant they lost connectivity. Several times each day.

When I changed all bridges to "manual" frequency selection I made sure each bridge pair had different frequencies. This has solved the loss of connectivity problem.

I guess the CRC errors and Holdoffs are normal. I had 2 bridges setup on a test bench with nothing connected and they were reporting a small number of CRC and Holdofffs so I guess some are normal.

Thanks for the suggestions.

Community Member

Re: Large numbers of CRC and Holdoffs

I had a similar problem.

Most people check the unused frequencies at the 'root' site, but forget that when the other radio attaches to it, there may be a frequency problem near that end that is not appearant at the root.. I would use the site survey function on each radio and find the best frequency for both ends. Then set that frequency manually.

Of course, as someone else pointed out, water in your waveguide could cause similar problems. This is hard to troubleshoot without replacing your cable (and don't think you can dry out your cable... water wicks its way down/up the cable). WATERPROOF...WATERPROOF...WATERPROOF!!!

Mike Wrobel

Bridges Communications, LLC

Community Member

Re: Large numbers of CRC and Holdoffs

Thanks for the reply Mike. Excellent suggestions.

I will give them a try.



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