UC520 quality fine internal, when dialing out bad quality

Unanswered Question
Aug 30th, 2007

Hi all. We've got a UC520 running over an ISDN2. when we call IP and analog phones internally, the quality is great. When someone dials in or out however, the line is very crackly. Theres no delay, but the quality is very bad. BT have plugged an ISDN phone into the line and the line was crystal clear.

Any ideas?

I have this problem too.
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bwilmoth Wed, 09/05/2007 - 13:16

Try "no battery reversal" into the "dial-peer voice" and make some calls to see if the situation is improved or not.

Please give me following sh commands:

show ephone, show ephone phone-load, show voice port summary, show voice dsp group all and show voice dsp.

Paolo Bevilacqua Wed, 09/05/2007 - 13:22


beside that "battery reversal" applies to voice port and not dial peer configuration, for sure it doesn't apply to ISDN BRI lines.

Davieshow: If the UC520 is like I think it is, please configure:

network-clock-participate wic x/y

network-clock-select BRI x/y 1

davieshuw Thu, 09/06/2007 - 00:51

Hi guys, ive got it sorted now, on my BRI port u-law was set. As Im in the UK it needed to be a-law, now works perfectly.

Cheers :)

Paolo Bevilacqua Thu, 09/06/2007 - 03:11

Well I would have suggested that, if you had said "voice is horribly garbled" - that is the usual result of companding law mixup :)

Still curious to know how clocks are handled with an UC520, can you send here output of "show network-clocks" and if you have avaiability of "networock-clock" commands ?


davieshuw Thu, 09/06/2007 - 03:28

'voice is horribly garbled' as opposed to 'bad quality'.. ill remember the difference next time ;)

heres my sh network-clocks output :

UC520#sh network-clocks

Network Clock Configuration


Priority Clock Source Clock State Clock Type

3 Backplane GOOD PLL

Current Primary Clock Source


Priority Clock Source Clock State Clock Type

3 Backplane GOOD PLL

and network-clock ?


network-clock-participate network-clock-select network-clock-switch

Paolo Bevilacqua Thu, 09/06/2007 - 03:36

Ok, the thing is that once you configure network-clock to take from BRI per above suggestion, you should not have perceptible change in voice quality, but if you have fax or modem connected via FXS ports or ATA, these may fail to connect unless you do such configuration.

Now my colleague here claims that on companding mixup you can still recognize the voice, thing that I contend fiercely :)

davieshuw Thu, 09/06/2007 - 03:41

well interestingly I typed network-clock-participate wic 1, soley because thats what I did on my Cisco IPTX course last week when using a 2811 with a PRI in it.. im suprised it worked to be honest.

Re the companding, we could talk to each other, but it sounded like terrible interference, like talking over really bad radios. I didnt think it would work at all with a companding mixup thats why I wasnt thinking compand issue.. we live and learn!

Paolo Bevilacqua Thu, 09/06/2007 - 05:40

Ok, then you need "network-clock-select BRI x/y 1" or something like that, and BRI will become the primary clock source for voice for the box, thing that is very beneficial for fax and modem especially. The only annoying effect is that it will likely fill up your log every time the BRI layer gets deactivated by telco (that is, all the time)

davieshuw Thu, 09/06/2007 - 06:24

I've just done a network-clock-select 1 bri0/1/0 and everything seems to be OK, cheers for the info!


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