Modem Passthrough: Clocking Issue

Unanswered Question
Feb 26th, 2008
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I am working on voip setup where i am trying to configure modem passthrough but i am encountering slips issue. My topology you can see below:

RAS----PBX1----Router1--------Router2-----PBX2-----Dialup User

Note: RAS is connected with PBX1 via PRI, PBXs with routers(c3800) via E1 PRI (Qsig). Routers are connected back-to-back via E1. All devices are in same testing lab.

when dialup user try tot connect with RAS, modem keeps negotiating and not gets connected. Something it gets connected but after a while gets disconnect.

I diagnose the problem, that is not because wrong configuration but it is due to clocking missmatch between Routers and PBXs. Thats why when i run command "show controller e1 0/0/0", i find slips there.

Please someone suggest me how to get this issue resolved. And also tell me the best prectice regarding clocking for the topology above.

Waiting For your positive immediate response.....

Best Regards


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teodorgeorgiev Thu, 02/28/2008 - 02:12
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For each link Router-PBX check at both sides

who is supposed to be the clock source.

Let say, you check Router1-PBX1.

PBX1 E1 link is configured to get clocking by line (means - from the other side)

Router1 E1 link is configured ALSO to get clocking by line.

This is obviously wrong. One side of a link must provide clocking and the other side "tunes" its clocking on it.

This is IMHO the most possible reason for having clocking issues.

Hope that helps.

paolo bevilacqua Thu, 02/28/2008 - 09:30
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you just need to configure:

network-clock-participate wic 0

network-clock-select E1 0/0/0 1

slips will cease and modems should be able to connect.

Hope this helps, please rate post if it does!


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