Re: E1Definition

[Brian Candler <B.Candler at pobox.com>]

On Sun, Jul 09, 2000 at 01:52:14PM +0000, Adiel AKPLOGAN wrote:
> Let put out what i need. I whant to know exatly how E1 link work and what 
> communication standard are used on it. The thing is that i got some E1 (by 
> Radio) link from the local Telecom. The are talking me about C.7 
> communications standard (protocol). As i will use these E1 for digital 
> dialup acces with a CISCO AS3500, i will need to know exactly what that C.7 
> is! As i know tha the Quad E1/PRI Interface i will use support G.703 protocol.

Not sure about on-line references - you may like to do a search on "CEPT"
which is an alternative name for E1 used in Europe.

However, I can tell you a few things:

- At the physical layer, it's G.703 as you know. This presents you with
either two coaxial cables (unbalanced) or two 120-ohm twisted pairs
(balanced) - one for send, one for receive. Note that you must know which
sort you have, to get the correct version of the interface card.

- The E1 link runs at a speed of 2,048,000 bits per second. It's divided
into 32 timeslots of 8 bits each:

    <------------------ 1/8000th sec --------------------->

    |--|--|--|--|--|--|--|--|--|--|--|--|.........--|--|--|
      0  1  2  3  4  5  6  7  8  9 10 11             30 31
    |  |
   /    \
  /      \
 |01010101|
 <-8 bits->

Timeslot 0 always carries a framing pattern (used by the receiver to detect
the start of the frame). That leaves 31 timeslots for calls or data. When
using an E1 as a leased line, all the timeslots just carry data bits. So
your "2M" leased line actually runs at 8000 * 31 * 8 = 1,984,000 bps of
useful data-carrying capacity.

When carrying telephony calls, each of the timeslots is a separate call.
(8 bits x 8000 frames per second = 64,000 bits per second).

Normally when a telco presents you (the end-user) with an E1 for voice
telephony, timeslot 16 is reserved for signalling. That gives you a 64,000
bps signalling channel, and 30 channels for voice (timeslots 1-15 and 17-31)

In Europe, the telco would use DASS2 or DPNSS as the signalling protocol to
you (messages such as "incoming call on channel 4", "call terminated" etc).
C.7 is only used for signalling when one telco interconnects to another
telco. Furthermore, when you use C.7, normally you run it over separate
cables: you have two whole separate E1's, one to carry the signalling, and
one as a backup.

I don't know if it is possible to carry C.7 within timeslot 16, or whether
that is even desirable. I certainly know that most end-user equipment does
not support it; if presented with C.7, you will need to buy a telco switch
(e.g. Nortel DMS100, Siemens EWSD, Okeford) - don't expect much change from
a million dollars, or $100K in the case of small switches like the Okeford.

- If you are just running a 2M leased line, you can buy a G.703 to serial
convertor, and use a V35 or X21 cable to plug into your router:

+--------+S0/0   X21+-----------+G703
| Router |----------| Convertor |-------------> Telco
|        |          |           |<-------------
+--------+          +-----------+

Suitable convertors are sold by companies like Black Box for a few hundred
dollars.

But if you are using the E1 to receive voice calls (e.g. to terminate
phonecalls into a bank of modems) then you will need to present the E1
directly to your terminal server equipment, e.g. an E1 card in a Cisco or
whatever, since a serial line doesn't support the concept of channels.

Note that the equipment you have may be able to terminate ISDN data calls
without extra hardware, but you may have to buy modem cards (with Digital
Signal Processing) to be able to terminate analogue modem calls; the DSPs
deal with the modulation (V90, V34, etc)

So, what happens with signalling depends on what the E1 line is going to be
used for:

- If it's a point-to-point leased line, there's no signalling involved. You
just plug the line in at each end. Ignore what your telco is telling you
about signalling :-)

- If it's to receive voice or modem calls, your telco should present you
with DPNSS, DASS2 or some similar signalling, on timeslot 16. Your modem kit
will expect this.

- If you have a million-dollar switch, and wish to connect it to your
telco's million-dollar switch, then you will use C.7 signalling on separate
bearers dedicated to signalling, plus an E1 bearer for every 30 voice
channels of interconnect capacity.

HTH,

Brian.

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