Subject: Re: [Ieprep] Architectural considerations

From: [email-masked]
Date: Tue, 3 Dec 2002 15:19:42 EST

In a message dated 12/3/2002 1:26:55 PM Eastern Standard Time, 
[email-masked] writes:

> The Internet has a fundamentally different architecture than the PSTN. To run
> voice over the Internet, one should adapt the voice application to the 
> Internet
> architecture rather than insisting that the Internet adopt the PSTN's broken
> (e.g. no dynamic routing of traffic around fibre cuts) architecture.
> 

[MAP] I'm always confused by statements like that last one that the PSTN 
can't do dynamic routing of traffic around fibre cuts. There are many lower 
layer schemes (SONET, etc) that do just that. I'm sure that a lot of the 
reliability of the Internet depends on such "rerouting" at the physical 
layer. At the "application" layer, every telephone switch recognizes when 
trunks to another switch are out of service and routes around them. Just like 
it routes around trunk groups that are congested (which the Internet 
apparently can't do).

I'd really be curious to understand what it is that you think the current 
PSTN can't do regarding routing that is a result of a "broken architecture".

MIke Pierce
Artel

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From: RJ Atkinson <[email-masked]>
Date: Tue, 3 Dec 2002 15:39:21 -0500

On Tuesday, Dec 3, 2002, at 15:19 America/Montreal, [email-masked] 
wrote:
> [MAP] I'm always confused by statements like that last one that the 
> PSTN can't do dynamic routing of traffic around fibre cuts. There are 
> many lower layer schemes (SONET, etc) that do just that.

SONET rings are limited to 16 nodes, which is not the same capability 
as IP
has to route around arbitrary fibre cuts whether or not the physical 
topology
is a ring and over diverse layer-2 technologies mixed together 
arbitrarily.

Apologies if I wasn't sufficiently clear.

> I'm sure that a lot of the reliability of the Internet depends on such
> "rerouting" at the physical layer.

s/physical layer/Internet Layer/

	Now there can also be layer-2 fault recovery, such as Ethernet APS or
SONET APS, or FDDI rings, but that's independent and additional bonus
beyond what IP does regardless of the choice of underlying layer-2 
technology.

> At the "application" layer, every telephone switch recognizes when 
> trunks to another switch are out of service and routes around them. 
> Just like it routes around trunk groups that are congested (which the 
> Internet apparently can't do).

We do route around stuff, but we have a layered architecture with 
different
layering than the PSTN.  So the layers at which re-routing can happen 
are
different.  This yields different behaviours and properties between the 
two
kinds of networks.  So the layer in our system that does routing is 
unaware
of which packets are voice and which aren't.  In the PSTN, everything 
is a logical
circuit even if it is carrying packets instead of voice.

The main point is:
	Different architectures have different properties and require 
correspondingly
different approaches to meet a given objective.  Trying to apply 
Internet solutions
to PSTN issues is just as unlikely to work as applying PSTN solutions 
to Internet
issues.  The applications need to adapt to the actual network 
technology underneath.

Ran

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From: Bill Woodcock <[email-masked]>
Date: Thu, 5 Dec 2002 04:48:17 -0800 (PST)

On Tue, 3 Dec 2002 [email-masked] wrote:
    > I'm sure that a lot of the  reliability of the Internet depends on
    > such "rerouting" at the physical layer.

No, it's _packet switched_.  Rerouting is principally done at layer
_three_.

                                -Bill