From: deej@cbnewsf.cb.att.com (david.g.lewis)
Newsgroups: comp.dcom.telecom
Subject: Re: Telco Handling Of Cable Cut
Date: 10 Nov 92 22:35:53 GMT
Organization: AT&T
X-Submissions-To: telecom@eecs.nwu.edu
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X-Telecom-Digest: Volume 12, Issue 839, Message 2 of 4

In article <telecom12.836.11@eecs.nwu.edu> vixen!jadams@uunet.UU.NET
(22475-adams) writes:

> In article <telecom12.831.7@eecs.nwu.edu>, rfranken@cs.umr.edu writes:

>>> A recent Bell Canada TV commercial shows a raccoom chewing ...

>> ... In
>> general, the switches will not dynamically reroute a call should a
>> trunk fail.  That is, if a switch has multiple paths available, and
>> one fails, your call will be lost, but the next call will be routed
>> over a path that is still functioning.

>> 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.

Stepping back a pace: the original post referred to a physical cable
failure (namely, a racoon chewing through a cable).  If this physical
cable is carrying facilities which are 1+1 or 1:N protected by the
multiplexers, transmission systems (e.g. fiber optic terminals), or
wideband/broadband DCSs, and the protection switch occurs in less than
three seconds, the facilities will switch to the protection system
before Carrier Group Alarm (CGA) occurs.  From the point of view of
the switch, the "circuit" is never "lost", and the calls are not
cleared.

I don't know exactly what you mean by switching "after" the switch
"finishes switching" trunks, but it is certainly possible to have
interswitch trunks riding on top of protected facilities.  Whether or
not it's economically justified for voice traffic is a different
question.

AT&T uses FASTAR (Fast Automatic Restoration), which uses DCSs to
control rerouting of facilities; it's not fast enough to avoid CGA,
but does succeed in rerouting 100s of DS3s in minutes, which is pretty
good.  (And I know that's not proprietary cause I saw Frank Ianna
talking about it on TV...) (I wonder if a future version of FASTAR
which is fast enough to avoid CGA will be called FASTERAR?)

When I worked at Teleport, we used 1+1 protection with diverse
facilities (route diversity in the majority of cases; conduit
diversity where only one route was feasible, and sheath diversity
where there was only a single conduit, usually only at building
entrances).  Any facility failures would cause a roll to the
protection fibers in one second or less, which would be detectable as
a hit on a data line, possibly detectable as a brief dropout on a
voice line, and wouldn't cause any switches to go into CGA.  Any
equipment rolls we did had the procedures very carefully designed to
meet the "three second test" -- you don't cause any hits lasting more
than three seconds.


David G Lewis                    AT&T Bell Laboratories
david.g.lewis@att.com or !att!goofy!deej     Switching & ISDN Implementation