Cell Relay Archive

[Ken Agress <kagress@linkline.com>]

On Tue, 08 Aug 2000 13:19:22 +0200, Thomas Frieling
<thomas.frieling@arcormail.de> wrote:

<snip>
>> 
>> Is this really an accurate description?  I'd have said "both an SPVC
>> and a PVC are established on a permanent basis, but an SPVC is capable
>> of re-routing through the ATM network due to congestion, outages, or
>> the creation of a more efficient path."  (...)
>
>I agree that it is as you say in practice - and that is what
>counts! But are SPVCs intrinsically capable of re-routing broken
>connections? What makes the PVC a smart/soft one is the fact,
>that it is not neccesary to hand-nail it. Re-routing is an option
>which makes the SPVC especially useful.
>
We agree on the above, but the distinction between an SVC and an SPVC
still sounds about as clear as mud.  :-)

Perhaps spelling the two out separately would be a better way to
illustrate the differences:

SVC - established on an "as needed" or "on demand basis" and may be
torn down once the existence of the VC is no longer required.  The SVC
will only be established if the specified minimum service parameters
can be met by the network according to the network management
software.  SVCs will re-route around failures or extreme congestion
within the network, so long as such re-routing is possible while
maintaining the required service levels.

SPVC - established through the network's provisioning system as a PVC,
but with capabilities not present in an ordinary PVC.  Rather than
following a static path through the ATM network, an SPVC shares the
re-routing capabilities of an SVC, but is a pre-configured, permanent
connection, and frequently receives priority over SVCs when re-routing
is required.  SPVCs will only be torn down through the intervention of
the provisioning system, and are intended to improve upon the
availability of PVCs by providing the re-routing capability.

I'll certainly agree that a SPVC will almost certainly use the same
routing algorithms as a SVC, but the behavior of the VC itself strikes
me as more akin to a PVC than a SVC, due to the inherent permanence of
the connection and the (frequently) priority that the SPVC may receive
when network outages cause re-routing to occur.