From: rfranken@cs.umr.edu
Newsgroups: comp.dcom.telecom
Subject: Re: Telco Handling Of Cable Cut
Date: 10 Nov 92 21:12:13 GMT
Organization: TELECOM Digest
X-Submissions-To: telecom@eecs.nwu.edu
X-Administrivia-To: telecom-request@eecs.nwu.edu
X-Telecom-Digest: Volume 12, Issue 839, Message 1 of 4

>> On the other hand, if the trunks are connected to some kind of
                      ^^
>> multiplexor/ DACS, it may handle the rerouting without knowledge of
>> the switch.           ^^^

> Not true.  As soon as the circuit (circuit switched voice call) is
> lost, supervision on the circuit indicates it has gone idle.  I know
> of no switching system (I could be wrong here) that will automatically
> reconnect the ends of a failed circuit switched call.  Furthermore, I
> have not experienced voice trunks that are switched through a DCS
> (Digital Cross Connect System) after the End, Tandem, or Access Tandem
> machine finishes switching them.  There are multiple ways of routing
> (Hierarchical, Dynamic Non-Hierarchical-DNHR, and Real Time
> Non-Hierarchical-RTNR), but all of these are accomplished through
> routing tables contained in the switches themselves.

> Jack (John) Adams	Bellcore NVC 2Z-220	
> jadams@vixen.bcr.com	kahuna@attmail.com	

First, let me clarify exactly what I meant.  The point I was making is
that a switch need not know that anything has happened.  As an
example, lets assume there is a T1 line connecting Switch A and Switch
B (which could be any type of switches -- Local CO, Access Tandems,
etc).  The switch does not know or care how the T1 gets from Switch A
to Switch B.  As long as everything that switch A spits out on the T1
gets to switch B, they are happy.  If the T1 connecting them goes
through equipment that is able to very rapidly reroute the T1 around a
failure, then the T1 path between the switches will never actually
fail, the switch will not know what happened, and the call will stay
up.
 
Equipment to do this DOES exist.  I work for a railroad, in the
telecommunications department (although not the switched network
group), and we have equipment (I can't remember the name - I'll refer
to it as a DACS, although that is not really an accurate name) that
does this.  Basically, a network of DACS is created, with the nodes
connected by T1s.  Ports are then added to the various nodes (Ports
can be voice, data, T1s, etc).  You then instruct the system to
connect any port to any other port, and it finds available bandwidth
in the network and routes the circuit.  Should any Network T1 fail,
the connections that were routed over that T1 are dynamically, and in
real-time, re-routed.  Therefore, if we want a T1 trunk group to
connect switch A to switch B, we connect the T1 output of switch A to
a DACS port, the output of switch B to a DACS port, and instruct the
DACS to connect the ports.  Bingo -- a T1 trunk group that will stay
alive even if the actual transmission medium connecting them fails
(because the DACS will reroute it around the failure without the
switch noticing).  Of course, if there were an active call on the
trunk, you would hear silence while the reroute was taking place (so
modem calls might be dropped), but the call would stay up (assuming
that the DACS was able to reroute the connection, which assumes that
sufficient backup bandwidth is available).  Note that a connection
between a port on Node A and a port on node B need not necessarily be
routed over a T1 from Node A to Node B -- it may go via one or more
intermediate nodes on its way.
 
Notice here that the DACS is doing no circuit switching.  It is simply
maintaining a path between the switches (regardless of whether or not
the switch is using the data path).  (Actually, I have greatly
understated the capabilities of this "DACS", and it does do some
things such as not wasting bandwidth with Voice T1 channels that are
idle, etc, but the general idea is there).  NOTE: DACS is not an
accurate term here, but its the best one that I can come up with.

I an fairly certain that supervision on the trunk will not be lost
during the interval that the trunk is being rerouted.  If it was,
then, of course, the call would be dropped, but there are easy ways
around that.  (This post is long enough without the details).

Do any telephone companies actually use this for voice circuits?  I
don't know, but my point was that it could be done if they wanted to.
 

Brett   (rfranken@cs.umr.edu)

(As usual, I speak neither for the university via which I access Usenet
nor for my employer.)