[Asterisk-Users] Cannot get two TE410Ps to operate correctly in the same machine

[Rich Adamson]

> > > There is a 
> > > buffer but the buffering can only handle jitter, not compensate for 
> > > frequency difference. 
> > 
> > No, you're assuming a one-byte (or very small) buffer, and that's not 
> > what's going on in asterisk.
> 
> You misunderstand me. I know that the buffers are larger. However, even if 
> they are 1 second deep they will eventually empty / overrun. There is no 
> way about this except to either allow data to be invented/dropped or to 
> keep the source and sink in perfect sync.

I wouldn't doubt there is code in asterisk to handle some sort of buffer
overrun, but I'd guess its simply throwing away some bytes. Voice
users wouldn't even hear it. An 8,000 byte buffer should handle a
frequency differential of 4%. I don't know of any modern electronics
that can't hold better tolerances then that, including the digium cards.
So, the entire discussion of syncing two cards is irrelavent. Asterisk
users really don't care and don't need it; a large majority would be
very happy to have a single T1/PRI available to them. :)

> You do not have to have the pci bus in sync though, as long as the two 
> cards are in sync. 

And how would propose to keep two cards in sync if you didn't rely on
the only common existing path (pci bus)?

> The interrupt latency is a red herring. Extremely precise interrupt timing 
> is _not_ needed to communicate the clocking from one board to another.

You should probably ask any of the audio folks and a large number of
digium x100p/tdm04b folks if they think interrupt latency (and pci bus)
is a red herring. Think you'll find a strong difference of opinion.

> It is only a problem when you want to get data connections across the bus. 
> For modem communications you will get a failed crc and possibly a 
> retraining each time data is invented / dropped. For a fax the problems 
> may be worse.

No, its only a problem when trying to retrofit legacy analog technology
onto a digital backbone. So, why not spend some time finding a way to
convert those legacy thingies to digital before injecting into the rtp
stream. Think you might find some takers on that one. Build a analog
fax to rtp converter the size of a spa-3000 and you'll probably sell
them. 

> For Unrestricted Digital connection I wonder what happens. I guess they
> are not allowed across boards? (They do work very nice within a board
> though, this is very nice).

I don't have one of the digium T1 cards in front of me, but if they are
designed with the same type of pci chips used on the analog boards,
there are only tiny buffers (a few bytes, maybe eight). That's why
interrupt latency is such an issue. If the card can't dump its data
across the pci bus quickly (by raising an interrupt) and in a 
consistent manner, overruns or lost data will occur right on the card.

The digium boards don't have much intelligent controllers on them; all 
of the control intelligence resides in asterisk code running on linux.
So, there isn't any capability to send selected data from one card
directly to another across the pci bus. It all has to pass from card
to bus to OS to asterisk, and then back through that path to the next
card (if that's the destination for those bytes).