A Tale of Three Standards

Plesiochronous Digital Hierarchy (PDH) was not a single, unified global standard. Due to independent technological development in different parts of the world, three major, incompatible "families" of PDH emerged. This divergence was a major challenge for international telecommunications, as it complicated connections between continents. The three main hierarchies are:

• The European international structure (E-carrier system).
• The North American structure (T-carrier system).
• The Japanese structure.

The European Hierarchy (E-carrier)

The European system, standardized by CEPT, is based on multiplexing 30 voice channels. It is known for its logical and consistent structure.

Hierarchy Levels:

• E1 (Level 1): This is the fundamental building block. It combines 30 digital voice channels (64 kbit/s each), plus one channel for signaling and one for framing/synchronization.
  Total Bit Rate: $32 \times 64 \text{ kbit/s} = 2.048 \text{ Mbit/s}$
• E2 (Level 2): Created by multiplexing four E1 streams.
  Total Bit Rate: $4 \times \text{E1} = 8.448 \text{ Mbit/s}$
• E3 (Level 3): Created by multiplexing four E2 streams.
  Total Bit Rate: $4 \times \text{E2} = 34.368 \text{ Mbit/s}$
• E4 (Level 4): The highest standardized level in this hierarchy, created by multiplexing four E3 streams.
  Total Bit Rate: $4 \times \text{E3} = 139.264 \text{ Mbit/s}$ (often rounded to 140 Mbit/s).

The key feature of the European hierarchy is its consistent multiplication factor of 4 at each subsequent level.

The North American Hierarchy (T-carrier)

The North American system, developed by AT&T, is based on multiplexing 24 voice channels and uses different data rates and multiplication factors.

Hierarchy Levels:

• T1 / DS1 (Level 1): The base stream, combining 24 voice channels (called DS0) plus an additional framing bit for synchronization.
  Total Bit Rate: $(24 \times 8 + 1) \times 8000 = 1.544 \text{ Mbit/s}$
• T2 / DS2 (Level 2): Created by multiplexing four T1 streams.
  Total Bit Rate: $4 \times \text{T1} = 6.312 \text{ Mbit/s}$
• T3 / DS3 (Level 3): Created by multiplexing seven T2 streams.
  Total Bit Rate: $7 \times \text{T2} = 44.736 \text{ Mbit/s}$
• T4 / DS4 (Level 4): Created by multiplexing six T3 streams.
  Total Bit Rate: $6 \times \text{T3} = 274.176 \text{ Mbit/s}$

Note the irregular multiplexing factors (x4, x7, x6), which made this hierarchy more complex to manage than its European counterpart.

Global Incompatibility and its Consequences

A third, distinct hierarchy was developed in Japan, which started similarly to the T-carrier but diverged at higher levels. The existence of these three separate, incompatible standards created significant challenges for global communication.

To connect a European E4 stream (140 Mbit/s) to the North American network, it was not possible to simply convert the signal. The E4 stream had to be demultiplexed all the way down to its 63 constituent 2 Mbit/s streams, which were then further demultiplexed into individual 64 kbit/s voice channels. These individual channels then had to be re-multiplexed according to the T-carrier hierarchy up to the desired level. This process required complex and expensive gateway equipment at every international interconnection point.

This inefficiency and complexity were major drivers for the development of a unified global standard in the 1990s: the Synchronous Digital Hierarchy (SDH) and Synchronous Optical Networking (SONET), which aimed to solve the problems of the plesiochronous world.