clocking on isdn line .. connection from call manager to pabx

Posted on 2010-08-23
Last Modified: 2012-06-21
Hi there,
Just wanted to confirm that in Call manager E1 configuration, do we set clocking to internal or external when the Cisco router (VG) is connected to PABX ?
Question by:nabeel92
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Expert Comment

ID: 33509336
External, it will revert back to internal if it can not get synced.

Expert Comment

by:Alex Bahar
ID: 33509629
You should always set it to external, this means VG is getting clock reference from the PSTN. Internal means, the VG is using its internal oscillator. If you set it to internal, you experience clock slips (show controller e1). Clock slips may cause your fax and modem (POS devices) fail.
Theoritically there is only one worldwide global master clock at the top. All equipment below (branch) gets the reference clock from the master, these are called slaves. Every slave is a master to its branches. It looks like a pyramide.
You use internal clock only if you are running on your own, without a master clock source. This may happen in an isolated environment, without any PSTN connections.

Author Comment

ID: 33518927
Ok, that explains !
I was under the assumption that if we are connected via QSIG link to a PABX, then we both (The VG & the PABX) are both slaving off PSTN so we'd be in sync either way; internal/externl won't matter ? The only time we set it to external is when we have an ISDN link to PSTN because clocking source ... But as you correctly pointed out that everything is in a pyramid sort of heirarchy. i.e.

Call Manager VG > PABX > PSTN > ... > Global Master Clock ?

In that case, I would chang my clocking from internal to external on qsig links ? Will that also cause any outage ! We had some fax transmission faults from pabx to iptel sites so am thinking this might have to do something with it, not sure
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Author Comment

ID: 33519103
luc roy, falling back of clocking from external > internal if it cant negotiate clocking, is that true for all isdn links or just with that specific device mentioned in the link. i.e. vg30

Accepted Solution

Alex Bahar earned 500 total points
ID: 33519444
That is correct, your PABX gets the clock from PSTN, and your VG gets the clock (external) from your PABX. This way all devices are synched to the same clock source.
Call Manager VG > PABX > PSTN > ... > Global Master Clock

You can change your clock source to external anytime. It won't cause any outage because the active clock will cut-over only after the VG will synch with the external clock. It is a smooth process, and it will complete in a couple of seconds transparently.
As I said in my previous post Clock slips may cause your fax and modem (POS devices) fail. Voice calls/quality are not affected by clock slips.

Expert Comment

by:Alex Bahar
ID: 33519476
Every chassis has one common reference clock (external or internal). ALL E1s in your chassis internally synch to your primary clock source if it is available. If your primary clock srouce is down (E1 gets disconnected) then your secondary clock source tries to synchup with external clock (from PSTN, PABX etc). If you do not have a reliable external clock source, then the last resort is to use internal oscillator of your device.
I used to develop device drivers for Cisco AS5400 AS5350 AS5850. I am quite familiar with those devices.

Author Comment

ID: 33519525
Thank you abahar for such accurate explanation ! much appreciated.

Author Closing Comment

ID: 33519528
thnx much !

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