[Asterisk-Users] Interfacing T100P with Definity PBX

[Rich Adamson]

> > > If span 1 is the telco you want sync to be '1' not 0 -- you want the span
> > > to take clock from the PSTN.  span 2 with sync '0' is right 99% of the
> > > time -- most PBXes and KSUs don't have an option to be the sync source so
> > > they'll be trying to sync from *, which should have it span sync set to
> > > '0' to achieve this.
> >
> > Andrew, I think the words came out a little different in your response
> > from what you were actually thinking in the last sentence above.
> 
> ... I think I got it right.
> 
> > T1/E1 spans are really four wires; two transmit and two receive. The two
> > transmit wires (regardless of whether they come from a pstn, pbx, KSU,
> > or whatever) always has embedded clocking within the pcm stream. There
> > is no way to turn that off, so its always a possible "source". What you
> > probably meant was the clock (in a KSU as an example) has no options
> > other then to sync _from_ the receive side of the span (as it doesn't make
> > any sense generally to turn that off). It considers itself the end node,
> > not the kingpin in the sync hierarchy. Your "span 2 with sync 0" is still
> > correct, its just the wording of the sentence.
> 
> I understand the physical level of T1 (I used to design this stuff many moons 
> ago) -- I understand that regardless of what it is, it's embedding clock in 
> its transmit stream.  
> 
> Where does this clock come from?  You have several options, but I'll say 
> there's two: an internal clock or an external clock.  Now technically it 
> always comes from an internal clock but that clock is either self-timed or it 
> is phase-locked to an external clock.
> 
> This is what the span sync is doing, it is setting up the internal clock to 
> either free-run or sync to a recovered clock from a datastream the card is 
> connected to.
> 
> With span sync set to 0, the zaptel driver does not try to synchronize the 
> internal clock to the received clock from that span.   With a span sync set 
> to non-zero, it will attempt to lock the internal clock to the recovered 
> clock from the span.  You usually use a value of 1 but higher numbers just 
> define a priority -- a span sync of '2' will use the recovered clock from 
> this span if the span with a span sync of '1' is down.  
> 
> So yes -- that is what I meant -- KSU/PBX/Channel Banks typically assume they 
> are syncing their transmit clock to the recovered clock from the other side, 
> which is why when you're connecting to these things with Digium hardware you 
> typically set your span sync to '0' so the two sides aren't "chasing each 
> other's clocks", so to speak.  :-)

Right on. I wasn't disagreeing with you at all, just didn't want a T1
newbie reading archives and getting more confused. Good explanation!