[Asterisk-Users] ISDN master clock issue ?

[Rich Adamson]

Tristan wrote:
> The fact is that I have 2 different E1 (euroisdn) providers and an E1 
> (euroisdn) connection to a Matra PBX...
> 
> The PBX needs to be master and as far as I know the PSTN providers needs 
> it too...
> 
> So I want to be sure that the quad E1 card I'll buy will work without 
> troubles in this kind of setup...
> 
> Can I only put the synchro to the PBX and forget about the PRI providers ?

No.

It is highly unlikely the PBX truly needs to be master. If that were an 
actual requirement, the vendor would never be able to sell their PBX 
into "any" environment where they connected to a E1 provider. The telco 
providers "never" slave their equipment from a customer-owned PBX. You 
will need to find the option in the Matra PBX to define it as syncing 
from the E1. (In fact, I'd bet a small amount of money the default 
implementation in the PBX is to sync from any attached E1.)

There is a 99.99% probability the two E1 providers obtain their clock 
sync from a higher level (hierarchical source), and are already in sync 
with each other. If you use a digium card, you select one of the 
providers as your "first choice" sync source, and the second provider as 
your second choice sync source when the first choice provider's E1 is down.

You definitely want your digium/sangoma card to support the hierarchical 
design of the digital network, and that well known design requires you 
to sync from your upstream provider, and pass that sync along to your 
downstream PBX. If you don't do that, calls originating from the PBX and 
passing through the digium/sangoma card to the PRI network will incur 
clock slips (out of sync). If the clock slips are too great, you will 
experience clicks, etc, during a call. Also, if all the components are 
not in sync, any use of modems (eg, faxes or pc modems) will be 
significantly degraded if not impossible to use.