Re: Sonet protection usage

[William Allen Simpson <wsimpson () greendragon com>]

smd () clock org wrote:
> Bill Simpson writes on the "expertise list":
>
> | APS is specified to switch a failing circuit over to a backup within 50
> | milliseconds.
>
> No.  APS/MSP is a simple protocl that uses the K1/K2 MSOH bytes
> *between* SDH section terminating devices to determine which
> of several parallel paths is active and should be used as the
> working TxInterface->RxInterface pair.
Yes, Sean, very good, I see that you've read the specification, too.
(Some of us try not to confuse issues with overly verbose terminology, 
especially as the terms used by SONET in the US differ from SDH used 
everywhere else that you just cited.)

> ...
> This is typically 50ms, but one has to add the one-way delay to that.
I haven't seen anything about adding one-way delay to make it longer, 
could you cite, please?  My reading was that the 50 milliseconds 
included delay, as 50 milliseconds would include a one-way trip 
around the world, and section regenerators are much closer together 
than that....


> If the sender knows there is an incipient failure on the
> working path, it can force a switch to a protect path using
> the K1/K2 protocol.  Otherwise, we have a timeout, and how quickly
> the sender realizes that the working path is malfunctioning depends
> on whether the signalling is bidirectional or not, although that's
> often not very important.
As you may note in your nit-pickery, I wrote "within" 50 milliseconds, 
which is how the specification is written.  We all agree it can be 
quicker.  If it's not very important, why did you bring it up?


> | APS (as sold) is a fraud on the uninformed.
>
> Those who live in glass houses should not confuse APS with
> restoration topology layout.
Let us look at the original question, to wit:

Steve Feldman wrote:
# 
# A quick poll:
# 
# Are many ISPs taking advantage of SONET APS protection
# to provide port or router redundancy on short (metro-area)
# circuits?  Or is it more typical to get two circuits and
# load-share?  Or just not bother?
# 
Now, it appears to me that the question was about metro-area circuits 
(recalling that these are usually called "intermediate" in the 
specification) -- not short circuits inside a single facility, and 
not transoceanic -- for "router redundancy".  OK?

In giving a cogent answer, I've noted the carrier problems, and given 
examples and rationale.  In my experience, multiple circuits with 
diverse topology is the best answer, because you also get additional 
bandwidth during the times that the circuit is not failing.  We live 
in hope that the circuit doesn't fail very often, don't we?

Neil says (in a later message) that his metro never overbooks.  And 
that may be a good thing.  But I'm not sure that it applies to 
"router redundancy".

WSimpson () UMich edu
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