[Asterisk-Users] OT (maybe?) : TI - PRI Question

[Steven Critchfield]

Please learn about threads. It is nice to have new topics in new
threads, not buried in old threads unrelated to your question.

Answers inline.

On Thu, 2004-09-30 at 17:49, Steve Maroney wrote:
> Hey guys,
> 
> Im new to the telecom industry and I am having a hard time understanding
> the differences between a voice T1 and a PRI line ? I was taught by a
> friend that I met on this list that a voice T1 provides all 24
> channels for voice and/or data and uses inband signalling. PRI uses 1 channel for
> signalling and leaves the other 23 channels for either voice or data.
> 
> This makes sense but how else does T1 compare/contrast to a PRI ?
> 
> Doesn't a T1 carry the PRI signal(such as UDP over IP) ?
> Does the T1 cards from digium support both voice T1's and PRI lines ?
> What benefits and features does a PRI line have over a T1 and vice versa ?
> I learned a little about the use of DIDs with PRI. Does DIDs apply to
> voice T1s ?
> 
> HELP !

At the lowest level a T1 is just a serial line run at a specific speed.
A channelized T1 is just that serial line divided into 24 time slots and
those time slots represent a digitized form of a analog phone line. The
parts that aren't audio get sent in a special bit that is "robbed" from
the audio every couple of frames. So a channelised T1 is usually only
really 7 bits accurate with a little noise on the lowest 8th bit a few
times a second.

PRI still divides the channel into 24 sub channels based on time.
Difference being that all signalling is moved from the robbed bits into
a more featureful and robust protocol and it takes up a channel of it's
own. This leaves the bearer channels to be fully 8 bit clean. This means
clearer analog data calls and the chance to make 64kbps data calls on a
channel. 

Digium hardware supports Channelized T1, PRI, and even data modes over
PRI and full cisco HDLC over T1. The card itself doesn't care much about
what you send over it as it passes all the data up to the driver. If you
wanted to spend some effort, you could support anything that could be
sent via a T1 circuit.  
-- 
Steven Critchfield <critch at basesys.com>