> > > 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! _______________________________________________ Asterisk-Users mailing list [EMAIL PROTECTED] http://lists.digium.com/mailman/listinfo/asterisk-users To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
