[asterisk-users] Clocking Multiple T1 Cards

[Rich Adamson]

Shaw Terwilliger wrote:
> Andrew Kohlsmith wrote:
>> What I was trying to state was that if you have two data streams that are 
>> solidly clocked but out of phase, you will not encounter any of these issues.  
>> If the clock period of either (or both) drifts then yes, you will run into 
>> trouble.
> 
> So it sounds like Asterisk can't synchronize the clocks between the
> Digium and Sangoma boards (or any two PCI boards), and this just may be
> a limitation of the T1-peripheral-on-PCI architecture.  But it really
> shouldn't matter because of the nature of my setup: errors caused by
> timing mismatch between the PRI and channel banks won't cause noticeable
> quality issues.  Do I have it right?

Yes, for voice there isn't any need to bit-sync the two T1's.  Both the 
digium and sangoma cards gather up bytes (or groups of bits from the 1.5 
mbs stream) and sends them via the pci bus to the drivers, then back 
across the pci bus to the second card. If a "group of bits" is delayed 
or lost in the process (due to significant clocking differences), the 
voice user isn't going to notice. However, as others have noted, it will 
make a difference for modem users (eg, fax).

Saying it a bit different, if the two T1 clocks were significantly out 
of sync, the flow may drop one out of a thousand bytes (or more) per 
voice channel, which the average human won't hear.