RE: [PWE3] comments on draft-ietf-pwe3-requirements-03.txt

["Ron Cohen" <ronc@ntear.com>]

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:  plésiochrone
	S:  plesiócrono
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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