Higher Order PDH Levels

Why higher PDH levels were needed

A basic E1 stream has a rate of 2.048 Mbit/s and a T1 stream has a rate of 1.544 Mbit/s. Larger trunks require these streams to be combined into successive levels with higher bit rates. A digital hierarchy defines those levels and the multiplexing relationship between them.

Combining lower-rate tributaries into one higher-rate stream is called multiplexing. PDH uses bit interleaving.

Higher-order multiplexing in the European hierarchy

In the European PDH hierarchy, four streams from one level form the next level. Four E1 streams have a combined rate of $4 \times 2.048 \text{ Mbit/s} = 8.192 \text{ Mbit/s}$, while the E2 line rate is 8.448 Mbit/s. The 0.256 Mbit/s difference provides capacity for framing, service information, and justification.

How the added rate is used

The rate above the sum of the tributaries is overhead capacity for the plesiochronous system. Justification positions can carry data when no stuffing is signaled, so only part of the difference is fixed overhead. The multiplexer uses this capacity for:

• Frame Alignment Signal (FAS): Each higher-level stream has its own pattern so the receiver can find the beginning of its frame.
• Service and alarm bits: Bits allocated to alarm indications and national-use functions. For example, the E4 frame has one alarm bit and three national-use bits.
• Justification and control bits: Each tributary has its own clock, whose frequency can differ slightly from the other tributaries. The multiplexer uses justification to compensate. In positive justification, it inserts a stuffing bit when needed and sends control bits that define how the corresponding position is interpreted.

European and North American PDH hierarchies

The European and North American plans use the same aggregation principle but differ in their base streams, multiplexing factors, and line rates.

The rate differences are 0.576 Mbit/s for E3 ($34.368 - 4 \times 8.448 = 0.576 \text{ Mbit/s}$) and 1.792 Mbit/s for E4 ($139.264 - 4 \times 34.368 = 1.792 \text{ Mbit/s}$). They provide capacity for FAS, service bits, C fields, and the designated justification positions. E4 can also be formed by directly multiplexing sixteen E2 tributaries, as specified in ITU-T G.751.

Each level has its own frame structure carrying interleaved tributary data and control fields. The E2, E3, and E4 frames use different block counts and justification fields.