The Need for Speed: Building the Hierarchy

While the basic E1 (2.048 Mbit/s) and T1 (1.544 Mbit/s) streams were revolutionary, they were quickly insufficient for trunking large volumes of calls and data between cities and countries. The purpose of a digital hierarchy is to systematically combine these basic-level streams into progressively faster, higher-capacity streams.

This process of combining lower-rate tributary streams into a single higher-rate stream is called multiplexing. In PDH, this is accomplished through bit interleaving.

The Principle of Higher-Order Multiplexing

In the European PDH hierarchy, four lower-level streams are multiplexed to create the next level up. However, you cannot simply add their bit rates together. For instance, four E1 streams ($4 \times 2.048 \text{ Mbit/s} = 8.192 \text{ Mbit/s}$) are multiplexed into an E2 stream, but the E2 stream's actual bit rate is 8.448 Mbit/s.

Where Does the Extra Overhead Come From?

The difference in bit rates is the necessary overhead required to make the plesiochronous system work. This extra data, added by the multiplexer, consists of:

• Frame Alignment Words (FAW): Similar to the FAW in the E1 frame, the higher-level stream needs its own unique pattern so the receiving equipment can find the start of its larger frame.
• Service and Alarm Bits: Bits used for network management, performance monitoring, and signaling alarm conditions.
• Justification (Stuffing) Bits and Control Bits: This is the most crucial part of the overhead. Because each incoming E1 stream has its own independent clock and may be slightly faster or slower, the multiplexer must use justification to compensate. This involves adding "stuffing" bits and the control bits that tell the receiver whether to read or discard the stuffing bit.

Overview of the European and North American Hierarchies

Both hierarchies follow the same principles but use different building blocks, multiplexing factors, and bit rates.

The logical structure and consistent 4x multiplexing factor made the European system simpler to manage. Each of these levels has its own unique and complex frame structure designed to accommodate the interleaved tributary data and the necessary overhead. These specific frame structures for E2, E3, and E4 will be explored in subsequent sections.