From: rpw3@rigden.wpd.sgi.com (Rob Warnock)
Newsgroups: comp.dcom.telecom
Subject: Re: ISDN Nitty-Gritty
Date: 1 Apr 92 09:05:15 GMT
Reply-To: rpw3@sgi.com (Rob Warnock)
Organization: Silicon Graphics Inc., Mountain View, CA
X-Submissions-To: telecom@eecs.nwu.edu
X-Administrivia-To: telecom-request@eecs.nwu.edu
X-Telecom-Digest: Volume 12, Issue 299, Message 1 of 9

Small clarification: In article <telecom12.281.2@eecs.nwu.edu> I
wrote:

| There is a protocol for contention resolution on the "D" channel,
| which depends on the fact that the NT "echos" each incoming "D" bit in
| the next available outgoing "E" ("D-channel Echo") bit. ...each TE monitors
| the "E" bits while it's sending to make sure that the echo matches
| what it sent -- if it doesn't, it presumes there was a collision.
| There's a whole priority and backoff scheme... similar to "Ethernet".

A subtle implication about the above (which was not immediately
obvious to me) has been pointed out to me by a co-worker, and I
thought I'd pass it on:

Since the "D" channel is an "inclusive-OR bus" (due to the way the "L"
bits adjust D.C.-balancing so that the "D" bits can all send in the
same polarity), when there is a "collision" only the station (TE) that
tried to drive a logical "1" (zero volts) has its bit "corrupted"; the
TE that drove a "0" ("negative" differential) will see its bit get
through.

And since the echoing "E" bit from a given "D" bit is seen before any
TE has to send the next "D" bit (due to the way the "D" and "E" bits
are interleaved into the total bit stream), any TEs that had their
bits corrupted (that is, they sent logical "1" but saw "0") have time
to drop out (cease transmitting) before damaging any later bits of the
"winner's" packet.

Therefore, unlike Ethernet or other similar multi-access protocols, on
the ISDN D-channel SOMEBODY ALWAYS WINS! It might possibly take
several bits into the packets before what's being sent differs enough
to shut up every TE but one, but *that* TE's packet will get through
unharmed. All the other TE's will go back into "listening for 1's"
state (which they can't exit until the "winner" finishes sending).

[There are more details having to do with how many 1's a TE has to
hear before starting to send, based on its priority and whether it has
sent recently, but you get the idea ...]

Again, this "collision-free contention bus" stuff only works on the
"D" channel (because of the "E" bits and the uni-polarity of the "D"
bits).  The "B" channels can only be driven by one TE at a time.

> If I were to simply take the 160 kbs data stream raw as is, and relay
> it via some other means of communications (such as full duplex radio)
> somewhere else where a reverse setup exists, do you see any reason why
> this would not basically work?  I want to be able to relay it
> somewhere without having to worry at the relay points about the B and
> D channels and the protocols dealing with them.

It's not quite that simple. You still need a real live NT1 (or NT2,
i.e., PBX) at your remote site, because the "D" channel echo protocol
has a fairly short distance limit over which it will work (~200m), and
it's the NT1/2 (not the CO) that echos the "D" bits as the "E" bits.

If you think you can build a conforming NT1 at your remote site, or if
you can find a vendor to sell you an NT1/2 that will work with your
"extender" media/encoding/transmission_means on the line side, then
fine.

Actually, you might consider this: Design/build/buy an "NT2" with an
internal extender. That is, the normal ISDN reference model is:

		        "S"           "T"           "U"
                     interface     interface     interface

     [Trm]---+-...--+----|---- NT2 ----|---- NT1 ----|------....--- CO
	     |      |
            TE ... TE

The "NT2" (i.e., ISDN PBX) is optional, which is why people call the
interface on a TE the "S/T" interface. But if you built a one with an
"internal" extender, that is:

                   "S"                      "T"       "U"
                 interface                interface interface

[Trm]---+-...--+----|---- NT2a---//---NT2b --|-- NT1 --|----....--- CO
	     |      |              <------>
            TE ... TE         radio, or whatever

You could hide the extender. Remember, your "NT2" still has to play
the "D"-bit/"E"-bit game at the remote end [*ignoring* the "E" bits
that come back from the NT1], and meet all the rules for coding,
framing, superframes, etc.

Looks like a job for a major ISDN vendor, not an amateur, but good
luck!


Rob Warnock, MS-9U/510	  rpw3@sgi.com
Silicon Graphics, Inc.	  (415) 335-1673
2011 N. Shoreline Blvd.   Mountain View, CA  94039-7311