[Asterisk-Users] Asterisk - fax - spandsp

Steve Underwood steveu at coppice.org
Tue May 17 05:49:42 MST 2005


It doesn't help at all, since you are talking rubbish. Try to keep track 
of the subject matter. We are discussing modems, where not slipping is 
vital.

Regards,
Steve

Rich Adamson wrote:

>It doesn't make any difference. The pcm data that arrives from the telco
>is buffered in the zaptel and/or asterisk code, and sent out the second
>T1 card as soon as it can. That buffering reduces (or eliminates) the
>need to sync one T1 card to another.  However, if the clock on the second 
>card were "way off" frequency, there could be a missed pcm frame from
>time to time. The missed frame would not even be noticed by users in
>most cases. (There was some discussion about how great of an impact
>that really has several months ago. Personal opinion plus experience 
>says its a non-issue other then on some very extreme cases where a clock
>is way off frequency. That tends not to happen with today's electronics.)
>If you think about the variable delays that occur because of contention
>for the pci bus, interrupt latency, etc, there are likely larger swings
>in dropped packets resulting from that then there would be from multiple
>T1 cards not having the exact same clock frequency.
>
>The bigger issue with multiple T1's occurs with the Quad T1 cards, where
>a single on-card clock is used with all four T1's. Syncing that clock
>"to the pstn" is the only correct way (timing=1), making all other 
>downstream T1's (eg, channel banks, pbx's, other * boxes) subservant 
>to it (timing=0).
>
>If you had multiple asterisk boxes located in several cities, and each
>of those asterisk boxes had a pstn T1, then you better read and understand
>T1 clock syncing very well as it will make a signifcant difference how
>each box obtains clock sync since there is a potential for a circular
>timing loop if one doesn't think that process through very carefully.
>
>Hope that helps...
>
>Rich
>  
>



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