[VoIP] T1 transmission protocol

[Martin Harriss]

Mad Mark wrote:
> I've just been spending an interesting hour browsing WiKiPedia, reading all the pages on telephone exchanges and trunking technologies.
> 
> I remember there was some talk a couple of days or so ago about T1 connections. I just have a few questions about T1 that someone may be able to help with.
> 
> I was wondering what data format is transmitted over a T1 line - Is it compressed audio & signaling, or is it PCM & signaling (ignoring bit-robbing for the moment)?

T1 consists of 24 channels of 8-bit u-Law companded audio PCM sent back 
to back, followed by a framing bit.  Signalling, where necessary, is 
done by robbing one bit from each channel on certain frames.

> Does anyone have any documentation of the framing and data format of T1?

T1 is an ANSI standard so if you had enough money you could purchase the 
standard from them.  I'm sure there are plenty of descriptions on the 
'net if you look hard enough.  I got quite a good description from an 
old Mitel data book, but I'm sure that's well out of print by now.  Note 
that a lot of the official spec covers things that (judging from your 
questions) you probably aren't interested in, such as pulse rise time 
and wave shape.

> Can a standard Asterisk system be set up to give T1 connections over Ethernet? Not to open a can of worms here or anything, but I'm talking of pure T1 data over and Ethernet connection, i.e. no IP framing or anything, just a direct cable connection between two Ethernet cards.

If what you mean by this is can you use an Ethernet card to connect to a 
T1 circuit, the answer is no.  The protocols are completely different on 
all levels.

> Basically I'm thinking of getting an Ethernet T1 connection set up between an Asterisk box and a Windows box, as I'm a Windows programmer, not a Linux programmer.

There is (used to be?) a channel driver in Asterisk that let you use a 
point-to-point Ethernet connection in a similar manner to a T1 by 
sending PCM frames as raw Ethernet datagrams.  If you want to do this, 
you'll need something on both ends of the circuit that speak this 
protocol.  I'm not aware of anything for Windows that would do this, 
apart from the rather questionable port of Asterisk to Windows.

> Thank you.

You're welcome.

Martin