Plesiochronous Digital Hierarchy (PDH)

The PDH system effectively develops the idea of primary multiplexing using time
division multiplexing (TDM) to generate faster signals. This is done in stages by
first combining (multiplexing) E1 or T1 links into what are known as E2 or T2
links, and if required, going even further by combining (multiplexing) E2 or T2
links, etc.

This multiplexing hierarchy is known as the Plesiochronous Digital Hierarchy
(PDH). Plesiochronous, meaning “almost synchronous,” relates to the inputs that
can be of slightly varying speeds relative to each other and the system’s ability to
cope with the differences.

These groups of signals can be transmitted as an electrical signal over a coaxial
cable, as radio signals, or optically via fiber-optic systems. As such, PDH formed
the backbone of early optical networks.

The aggregate signal can be sent to line at any stage of the hierarchy, using the
appropriate transmission medium and modulation techniques.

PDH Network Operation — PDH network equipment is now quite physically
small, allowing for its deployment in locations other than a telephone exchange.

Network operators are able to house such equipment in street cabinets; enough
equipment to supply 120 telephone lines can be stored in an enclosure measuring
1.5 meter by 1 meter high. This has removed the need for many of the smaller telephone
exchange buildings that we used to see in fixed networks.

PDH systems are generally used only for point-to-point communications systems
because the signals must be fully demultiplexed to access a single information
channel. In addition, proprietary alarm configuration and management means that
equipment at either end of a PDH system must be from the same manufacturer.
PDH advantages include:

Equipment small enough for use in street cabinets
Good for point-to-point connections
Cost-effective support for access networks
PDH disadvantages include:
Manufacturer-specific systems
Multiplexer mountains
No integrated network management
Limited management available