Date: Mon, 16 Nov 1992 22:50:30 GMT
From: deej@cbnewsf.cb.att.com (david.g.lewis)
Newsgroups: comp.dcom.telecom
Subject: Re: Telco Handling Of Cable Cut
Organization: AT&T
X-Submissions-To: telecom@eecs.nwu.edu
X-Administrivia-To: telecom-request@eecs.nwu.edu
X-Telecom-Digest: Volume 12, Issue 855, Message 5 of 8

In article <telecom12.848.7@eecs.nwu.edu> vixen!jadams@uunet.UU.NET
(22475-adams) writes:

> In article <telecom12.839.1@eecs.nwu.edu>, rfranken@cs.umr.edu
> writes:

>> First, let me clarify exactly what I meant.  The point I was making is
>> that a switch need not know that anything has happened.  
>> ...
>> I an fairly certain that supervision on the trunk will not be lost
>> during the interval that the trunk is being rerouted.

> The previous post (this issue) from Al Varney about signaling on DS1
> indicates a very short (less than 125 us) interval needed to maintain
> supervision.  Protection switching operates with switching times of at
> least two orders of magnitude higher. As such, it is highly unlikely
> that individual circuit supervision will survive such a switch.

I missed Al's article, but I don't think the time needed to maintain a
trunk in the active state is that short.  Notes (BOC Notes on the LEC
Networks -- 1990, SR-TSV-002275 -- the pfabulous repfrence with the
pfunny name), Section 6.21, Carrier Group Alarm, says:

"After a carrier failure occurs but before CGA trunk conditioning
begins, it is desirable to maintain the same supervisory states on
each trunk that existed before the failure.  If the carrier cannot be
restored in a reasonable time (for example, 2.5 seconds), trunk
processing should be initiated to remove the trunks from service.
However, since there is no fixed maximum in this case, the time could
be longer or shorter than the 2.5 seconds given above.  The time
should be long enough to maximize the possibility of restoring the
carrier before trunk processing begins, but short enough so that the
customers using the facility are not more annoted by the effects
caused by the delay in processing the failure than they are by the
effects of the carrier failure ..."

"Typical intervals between carrier failure and the beginning of trunk
processing vary from 300 ms to 2.5 seconds ..."

("Trunk processing" refers to the application of on-hook to clear the
call and stop charging, followed about ten seconds later by an offhook
to prevent the trunk from being selected for a call.)

So if a protection switch occurs in less than 300ms (not 125us), it is
virtually guaranteed to occur prior to any trunk conditioning; a
protection switch accomplished in less than a second is very likely to
not cause trunk conditioning.

> Moreover, in current digital facilties (FT3 comes to mind), this
> protection switching is built in to automatically switch to a spare
> line when the BER exceeds 10-6(?).

I don't know the FT3, but most new fiber optic systems that use
protection switching will switch on a BER threshold crossing or a loss
of signal, or potentially other parameters.

>> 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.

> Now that we've beaten this to death, I reiterate that I know of NO
> telephone companies (this doesn't mean that none exist!) that use this
> approach.

Increasing the cost of trunking by a factor of two is not a popular
thing to do.


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