Subject: SONET/SDH Pointers in draft-ietf-pwe3-sonet-01

From: Carmine Daloia <[email-masked]>
Date: Tue, 24 Jun 2003 12:26:55 -0700 (PDT)

Hello,
 
I was reading over draft-ietf-pwe3-sonet-01 and I came across Section 6 which deals with SONET/SDH Pointer Management.
 
I do not understand why CEP needs to deal with SONET/SDH negative and positive pointer adjustments. The SONET/SDH pointer adjustment is needed to compensate for any differences in rates between the SONET/SDH path and the SONET/SDH line rate.
 
At the ingress side of the network, the SONET/SDH line overhead is terminated and the SONET/SDH path is fragmented and carried as packets.  At the egress side of the network, the fragments are reassembled into the SONET/SDH path and sent out over a SONET/SDH line interface. It's at the point where the path is mapped into the SONET/SDH line that pointer adjustments may or may not be needed.
 
So I don't see why any pointer adjustment information needs to be relayed across the packet network.
 
Can someone please clarify this point for me?
 
Thanks
Carmine

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From: "Ron Cohen" <[email-masked]>
Date: Wed, 25 Jun 2003 08:35:16 +0200

Carmine,

I think the best way to look at N,P PA indications in the CEP header as
indications on the difference between the system clock and the actual
SPE clock. CEP carries SONET SPE data. The difference between the SPE
and the System clock is mapped by SONET to Pointer adjustments, and this
information is carried across.

There are several scenarios in which this information is useful. The
most notable one is the use of the EPAR mechanism in common clock
scenarios:

EPAR: Explicit Pointer Adjustment Relay, described in this draft. Assume
a common clock scenarios, were two PEs have effectively the same system
clock. The SONET SPE rate may differ from this common clock. If the
egress PE would be aware of the difference in rate, it would be able to
play out the SPE data with this rate and make sure the jitter buffer
does not overflow or underflow. The difference in rate is encoded in
SONET by pointer adjustments. Therefore, if the ingress PE encode the
SPE pointer adjustment and the egress PE replay the pointer adjustment
on the egress SONET line, we are done, and the jitter buffer remains
fixed. 

Hope this helps
Ron

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On Behalf Of
Carmine Daloia
Sent: Tuesday, June 24, 2003 9:27 PM
To: [email-masked]
Subject: [PWE3] SONET/SDH Pointers in draft-ietf-pwe3-sonet-01

Hello,

I was reading over draft-ietf-pwe3-sonet-01 and I came across Section 6
which deals with SONET/SDH Pointer Management.

I do not understand why CEP needs to deal with SONET/SDH negative and
positive pointer adjustments. The SONET/SDH pointer adjustment is needed
to compensate for any differences in rates between the SONET/SDH path
and the SONET/SDH line rate.

At the ingress side of the network, the SONET/SDH line overhead is
terminated and the SONET/SDH path is fragmented and carried as packets.
At the egress side of the network, the fragments are reassembled into
the SONET/SDH path and sent out over a SONET/SDH line interface. It's at
the point where the path is mapped into the SONET/SDH line that pointer
adjustments may or may not be needed.

So I don't see why any pointer adjustment information needs to be
relayed across the packet network.

Thanks
Carmine

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