[Asterisk-Users] PRI in and out

[Rich Adamson]

> I am wanting to front-end a legacy PBX with an asterisk box. I have done plenty 
> of asterisk work over the last 6 months to PRI circuits, but not with a PBX 
> being involved.
> 
> I know I can use asterisk and digium cards in this manner, but do I need 
> separate cards for the PRI -> Asterisk side to the Asterisk -> PBX side, or will 
> a 4-port PRI card do the job? (I already have a spare one of these).

The 4-port card will work just fine.

> In other words, can I use SPAN 1 as a timing source, then provide timing to the 
> PBX connected to SPAN 2 of the same card?

Yes. In fact, the 4-port card will be a slight advantage over two 
single port cards as all ports on the 4-port card will have their
clocks in sync with your external timing source.

Keep in mind that all T1/E1 spans having timing embedded in their
transmit legs; you can't turn that off even if you tried. The clock
timing source is always an engineering decision as to chosing which
"receive leg" to use for clock sync. (Obviously, the span from the
pstn would be your timing source and not the span to the pbx. If
you already are using the PRI with the PBX, then no changes required
on the PBX side for clock sync.)

The config examples in zapata.conf and the wiki are good. Not much
to configure really.

You will probably want to focus more on options that your pstn 
provider can/will impact such as the number of digits to be sent 
from them to you, which channel is the d channel, the digits they 
expect from you for each call (whether prefixed with "1", "0" or 
whatever), etc.

As sort of a side note, the 4-port card gives you another slight
advantage from an ongoing support perspective. The third (or forth)
port could be connected to a "test" asterisk box on which you can
stage/test future asterisk code before moving it into the production
box. Think about reserving a couple of DID numbers for the test
box if you'll be using DID.