[Asterisk-Users] Difference between E1 and PRI

[Alistair Cunningham]

Eric,

E1 is a physical layer protocol, like ethernet. It defines a 2Mbps pipe, 
which can be used for data, or can be split into 32 64Kbps telephone 
channels, or a mixture. If used for telephone channels, 30 of these 
channels can carry one telephone conversation each, and 2 carry 
signalling and timing information.

T1 is similar to E1. It is used in North America. It is 1.544Mbps, and 
can carry 24 telephone channels, each of which can carry a telephone 
conversation (but see below).

There are a number of protocols which can run on top of E1. Some of 
these are called CAS, Channel Associated Signalling. Examples are FXS 
loop start and E&M wink start. They provide information such as the 
number that was called, and what state the call is in. They're limited 
in what information they can carry, and are slow to set up.

A more modern protocol which overcomes these problems is ISDN. On E1, 
EuroISDN is the standard. On T1, there are different standards from 
different providers. DMS100, DMS250, NI1, and NI2 are common examples. 
ISDN uses one channel (called the D channel) for signalling call 
information. On E1, this is one of the 2 signalling channels, leaving 30 
channels for voice (called B channels). On T1, there aren't any spare 
signalling channels, so one of the voice channels is used, leaving 23 B 
channels for voice.

A PRI (Primary Rate ISDN) is simply an E1 or T1 with ISDN on top of it. 
ISDN gives fast, reliable call setup and hangup detection, and detailed 
information about the call. In the UK, PRI is also called ISDN30.

An important extension to ISDN is Q.SIG, which provides extra signalling 
information that is used when connecting PBX systems.

An alternative to PRI is BRI (Basic Rate ISDN), which is a cheaper 
system for small offices. It has 2 64Kbps B channels for voice, and 1 
16Kbps D channel for signalling. It is sold as an alternative to 
analogue telephone lines. IN the UK, it is also called ISDN2e.

I hope this answers your question! My company offers commercial support 
and installation services for PRI and Asterisk if you need help for 
specific scenarios.

This email may form the basis of a future Integrics Tip. See:

http://integrics.com/tips/

Alistair Cunningham,
Integrics Ltd,
Telephony, Database, Unix consulting worldwide
+44 (0)7870 699 479
http://integrics.com/


Eric Bishop wrote:
> Hi all,
> 
> I have seen the term E1 and PRI used interchangably when referring to
> a voice service with 30B channels and 1 D channel. Are they just
> different terms for the same thing or is there some technical
> difference. Even Newton's telco dictonary seemed a bit fuzzy on this
> topic. I have seen it said the PRi is a protocol that runs on top of
> E1. Is this true?
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