Cell Relay Archive

[George Marshall <george@marshalls.org>]

marc langston wrote:
> 
> I have recently been in a discussion concerning how ATM would handle the
> case of a PVC/VCC reconnect in the event of an intermediate switch
> outage.  For example, if the PVC travelled from access switch A to
> intermediate switch B to intermediate switch C to access switch D, and
> switch B were to fail, what would happen to the PVC?  Would it "regrow,"
> or is that PVC just plain down?
>
> I have always been under the impression that ATM "the protocol" does not
> have any way to "rebuild" the PVC in the event of a switch outage.  (If
> there was a trunk outage, SONET would re-route to the switch.)  But my
> recent discussions have brought up points to the tune of "How can you
> have a modern transport protocol that isn't able to recover from that
> sort of outage?"
> 
> I haven't been able to find any information in ATM books about what
> happens in outage situations, so thought of asking the question here.
> Any thoughts?

Until the PNNI spec was completed, there was no "standard" way to do
this.  PNNI calls the facility a "soft PVC", and essentially one of the
switches signals for an SVC across the backbone, but with pre-specified
VCC's at the endpoints so the user sees it as a PVC.  If the VC is lost
due to trunk failure or whatever, the responsible switch signals for it
to be set up again.

In the carrier world, the same feature might more often be called a
"survivable PVC'.  Vendors have been offering this facility via
proprietary means for a while, but apparently the ITU hasn't seen fit to
define and specify it.

George Marshall

--
george@marshalls.org