[Asterisk-Users] T1 Sync clarification

[Rich Adamson]

> > If you've got spans from different providers...you're in for an
> > adventure. You'll be able to do one of the following (which one is telco
> > and luck dependant):
> 
> So what you're saying is that the TE410P is not capable of *independently* 
> clocking each of the T1s.  Hell even the venerable old AS5248 can handle 
> that.  This is going to be fun... 
> 
> Is it possible to accept clock from the telco for one span and *generate* 
> clock on the other three spans (i.e. for internal channel banks and 
> whatnot) ?  Will I run into problems there?  I don't forsee it but I also 
> didn't forsee the problem being discussed in this thread...

Think we're trying to make this more difficult then what it really is.

Every T1 card has a clock, period. The card, regardless of whether it is in
a channel bank or a PC, runs at some frequency determined by the engineer
that designed the card. A specific card's clock might run at 15.44 
mega-units/sec, however the exact frequency at any point in time might be 
15.43999 or 15.44001, or some other variation.

Letting the clock slide around over time is not a cool thing in high speed
digital communications. Therefore, the person implementing the card usually
has to choose "a" source from which to sync his card's clock. There isn't
any need to attempt to sync the card's clock from multiple sources 
simultaneously.

The telephone company engineers have had to make the exact same engineering
decisions for "each" central switching office, however since many of these
offices have digital facilities from several external companies, they
simply coordinate with these other companies as to "who" is going to be
the source (for clock syncing) verses who will simply listen. Those 
decisions are based on a rather well understood hierarchical arrangement
that usually starts with a large carrier and an atomic clock. (The telco
will also engineer for a primary and one or more failover secondaries, etc.)

Since the digium card has a clock, you simply pick "one" source to sync
from.  If you just happen to have multiple T1's coming from different
companies, you can only hope/expect those companies have participated in
the effort to follow the hierarchical, historically well understood, 
syncing arrangements. If one of them happens to be a fly-by-night organization
that hasn't understood the international sync requirements, your only 
option is to either encourage them to participate or find a different 
provider. Period.

Once you've chosen a sync source, your card's clock should now be in sync
with master atomic clock via layers of this well understood hierarchy.

If you connect channel banks to this same card, the digital signals transmitted
by your card "to" the channel bank is going to be derived from your card's
in-sync clock. That says your channel banks should then be configured to 
sync "from" that card. If you don't do that, then "you" are breaking the 
hierarchical structure within your network.

If you are large enough to have many asterisk boxes all interconnected via
T1's in some sort of full mesh configuration, then as an engineer you have 
to design your systems in such a way as to pick "a" clock source to sync 
with (call it your Master), and design each component in your network to
sync from that Master via your own hierarchy. Its not that hard, but it 
really needs to be done.

Just like the telephone company engineers, you should think about what
happens if your primary source of sync fails. If you enjoy T1's from
multiple external sources, then pick a secondary (backup) for syncing. 
However you choose to do that is based on your exact network configuration, 
and not on how the digium card was designed, etc.

If your asterisk box interconnects with a traditional pbx that has T1
connections to the pstn, then whoever engineered that pbx had to make the
same "sync" decisions (even though they didn't tell you about it). In this
case, your asterisk machine should sync "from" the traditional pbx.

If you have a T1 from your pstn telco terminating on your asterisk, and
another T1 going from asterisk to your traditional pbx, then configure 
asterisk to sync from the telco and the traditional pbx to sync from 
your asterisk.

To complete this rather lengthy topic... what happens if you ignore all of
this and just slap a bunch of systems together with no regard to a master
sync source?  The quality and stability of your network will likely not be
as good as what it could be. If your clocks (in each device) happen to be
running very very close to what is expected, your network might run just
fine. But, if one of the clock's frequency drifts around, it could impact
quality via frame slippage and other unwanted events, and if off by a 
large amount could even be the source of failures. (Your milage will vary
directly with the stability of your clocks.)

Hope that helps someone....

Rich