The way I've been taught to view it, 'T1' is a generic term for any
physical and logical circuit that has a framing scheme that divides it
into 24 channels, some of which may be idle (fractional T1).
PRI (in the US) is served 'over' a T1 circuit. Thus PRI is a _type_ of T1,
with special provisioning at the switch, such that the 24th 64K channel is
taken over for 'D channel signalling'.
In most installations 64K is way more bandwidth than is needed for ISDN
call signalling, so they came up with an exception (that Livingston says
it won't support). Using NFAS, you can take a SECOND T1, configure all 24
channels as 'B channels' to carry voice/data, and and send all the
signalling over the _first_ T1's 'D channel'. If you do this across a
total of 3 T1 lines (more than that isn't recommended), you get an
extra two channels (phone lines) for free.
But again, this is an exception, and Livingston won't support it :-(
> >conecerning the PM3 Do you have to provision the t1 so that lets say 8
> >are for analog and 16 are for ISDN, so if 8 analog users are on and
> >another call will it get a ninth channel that is not used or do you have
> >to specify which are anolog and which are digital.
>
> The PM-3 can tell what kind of call it is and decide how to handle it.
While this is true, it should be noted that some clueless phone companies
are unable to implement a hunt group for both voice and data simultaneously,
there are ways to work around this by using extra DN's, but first you'll
need to find the one guy at your telco who knows how to do this.
The moral is, before you go ordering PRI circuits, you need expert advice
from somebody else who's dealt with your RBOC and knows their specific
pitfalls.