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RE: [PWE3] comments on draft-ietf-pwe3-requirements-03.txt
It's not uncommon to use PDH to map E1/T1 to T3 (e.g. G.747), then carry
3xT3 in an OC-3 or STM-1. T1 and E1 reprsent the majority of subscriber
needs for voice. T3 etc is either carrying multiple T1/E1 or data, and if
it's data I guess we are all assuming that the data services will migrate to
native Ethernet (eventually, about ten year before VoIP replaces E1 / T1 for
voice).
The inference being that the bulk of the application for PWE3 at the edge is
E1/T1 carrying voice over PSN, so may be we should focus on that. Carrier to
carrier probably starts with OC-3 / STM-1 rates.
Bob
-----Original Message-----
From: pwe3-admin@ietf.org [mailto:pwe3-admin@ietf.org]On Behalf Of Ron
Cohen
Sent: 08 October 2002 10:36
To: 'Yaakov Stein'; neil.2.harrison@bt.com; stbryant@cisco.com
Cc: pwe3@ietf.org
Subject: RE: [PWE3] comments on draft-ietf-pwe3-requirements-03.txt
Yaakov,
There is a different between the classification of TDM circuits into
plesiochronous digital hierarchy (PDH) and synchronous digital hierarchy
and the actual synchronization architecture deployed in the network, or
their internal multiplexing characteristics.
E1 and T1 are referred in the ITU specs as belonging to the
plesiochronous digital hierarchy (PDH). It does not matter that in most
deployments they are not operated in a plesiochronous timing
architecture, or that they carry DS0 synchronously.
To understand the difference, you might note that plesiochronous
synchronization is used in certain SDH deployments. This mode of
operation is typical for connections across administration boundaries.
SDH networks do not carry their PDH tributaries synchronously, but
rather asynchronous mapping are the dominant ones.
See an additional remarks below
Ron
> -----Original Message-----
> From: pwe3-admin@ietf.org [mailto:pwe3-admin@ietf.org] On
> Behalf Of Yaakov Stein
> Sent: Tuesday, October 08, 2002 10:17 AM
> To: neil.2.harrison@bt.com; stbryant@cisco.com
> Cc: pwe3@ietf.org
> Subject: RE: [PWE3] comments on draft-ietf-pwe3-requirements-03.txt
>
>
>
> > E1 and T1 belong to PDH and only PDH.
>
> Sorry Neil, but PLESIOchronous refers ONLY
> to multiplexing of sources with NEARLY (BUT NOT EXACTLY)
> the same clock (PLESIO is Greek for near or close).
> (See G.701)
Below is G.701 definition. Please see Note 1.
6019 plesiochronous
F: plsiochrone
S: plesicrono
The essential characteristic of time-scales or signals such that their
corresponding significant instants occur at nominally the same rate, any
variation in rate being constrained within specified limits.
NOTES
1 Two signals having the same nominal digit rate, but not stemming
from the same clock or homochronous clocks, are usually plesiochronous.
2 There is no limit to the time relationship between corresponding
significant instants.
> PDH thus refers to the hierarchy of multiplexing schemes
> that use with stuffing bits, e.g. E2,T2,E3,T3.
>
> E1 and T1 mandate a single clock for all the DS0s they multiplex.
> They are thus not PLESIOchronous by any stretch of the imagination.
> They are simply synchronous (SYN means SAME).
>
> Please do not fall into the same pit as someone did in
> a previous thread on this list, and claim that various ITU
> standards refer to E1 and T1 as PDH. They don't. They use
> the term BASED ON the 1544 kbps or 2048 kbps hierarchy. The
> PDH hierarchy is BASED ON the E1 or T1 clocks, but the E1/T1
> signals themselves are NOT PDH.
>
> Y(J)S
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> pwe3@ietf.org
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>
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