[Asterisk-Users] Asterisk - fax - spandsp

[Rich Adamson]

Steve, the stuff below was in direct response to a user question
regarding T1 timing sync and understanding why that might be 
important relative to spandsp and/or other modem use. So it does
apply directly. (Of coarse it slid into a tangent.)

------------------------
> 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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