Introduction To Pdh / Dsn - HP 37717C Operating Manual

HP 37717C PDH / DSn Features

Introduction to PDH / DSn

Introduction to PDH / DSn

The Plesiochronous Digital Hierarchy (PDH / DSn) is still the dominant technology
in most existing telecommunications networks throughout the world, although it is
being replaced in many networks by Synchronous Digital Hierarchy (SDH) or
SONET networks. PDH / DSn networks were developed at a time when point-to-
point transmission was the predominant network requirement. To support this
requirement, the standard approach to network management and maintenance was to
use manual distribution frames for access to individual signals. This is now
considered out of date and consequently SDH / SONET is now the preferred
network topology for new installations.
However PDH / DSn networks will exist for a long time to come even in networks
where SDH /SONET is the preferred technology. PDH is the ETSI international
standard, based on 2 Mb/s, defined by the ITU-T, and covers the hierarchal
transmission rates of 2 Mb/s, 8 Mb/s, 34 Mb/s and 140 Mb/s. DSn is the ANSI
standard covering transmission rates of 1.544 Mb/s (DS1) and 44.736 Mb/s (DS3).
PDH is asynchronous at 8 Mb/s, 34 Mb/s and 140 Mb/s. In order to access a signal,
for rerouting or test purposes, the whole line signal structure must be demultiplexed
step by step down to the 2 Mb/s level, because of the asynchronous nature of the
multiplexing.
At each multiplexing step, the bit rate of the individual tributary signals is controlled
within specified limits and is not synchronized with the multiplex equipment.
Because the bit rates of the individual tributaries are controlled within specific limits
this type of multiplexing is referred to as Plesiochronous i.e. nearly synchronous.
The individual tributaries are synchronized with the equipment at each multiplex
step by the process of positive bit stuffing justification.
In new SDH / SONET networks, PDH / DSn signals are mapped into virtual
containers / tributaries before being transported as part of the SDH / SONET
payload. The SDH / SONET payload must then be demapped into a PDH / DSn
tributary signal.
Therefore all PDH / DSn, SDH / SONET and mixed PDH/SDH DSn / SONET
networks require test sets which have PDH / DSn interfaces and PDH / DSn test
capability.
2