[Asterisk-Users] Clock Sync

[Steve Underwood]

Don Pobanz wrote:

>When I was looking at timing before this is the conclusion that I have 
>come to.
>
>The T400P card has an internal clock that all four T1s of that card 
>will be timed off of. This internal clock can be free running (not 
>referenced to any other clock) or reference to another clock (the clock 
>on the other end of a T1 line). This is set up in Zaptel.conf
>span=<span num>,<timing>,<line build out>,<framing>,<coding>
>
>Important points.
>  1. All T1s of a T400P run off of the same internal clock.
>  2. The internal clock can be free running - not reference to any 
>other clock.
>
You can only use this for a single card within a private network, 
unconnected to the public network. In that case you can make that one 
card free run, and it becomes the network master. All the others must 
slave to it.

>  3. The only other reference the clock can use is timing from an 
>incoming T1. If the primary clocking T1 is down then it can be set to 
>use a secondary T1 to derive timing (or 3rd or 4th)
>
This is what 99% of people need. The public network will not sync to 
you. You must sync to them. Public exchanges actually use rhubidium 
atomic clocks, so their clock is darned accurate (just like the line 
scan rate of your TV signal, and for similar reasons).

>  4. There is no way to time off of an external clock be it stratum 3 
>or 2 or whatever (unless it is part of a T1 thereby burning up a T1 
>port in the T400P card).
>
Stratum 3 is a term I normally associate with NTP. This is a 
macroscopically accurate clock. A clock accurate at the cycle level is 
needed to keep a T1 flowing right.

>  5. If the system has 2 T400P cards the clocks will not be timed to 
>each other (unless each has an incoming T1 with the same timing.)
>  
>
If you are smart this will probably be the case. Make one of the T1s on 
each card a line from the public network, and use 1 to 3 of the others 
for private network lines. All those T1s comming in from the public 
network, even from different carriers, will be in isochronous, so you 
will never get slips this way. It would have been nice if Mark had 
provided a link wire facility between the cards, to keep them all in 
step under all conditions, but life is never perfect. :-\

Regards,
Steve