What is PDH: Advantages and Disadvantages of Plesiochronous Digital Hierarchy

Plesiochronous Digital Hierarchy (PDH) is a digital transmission technology that multiplexes multiple low-speed digital signals into higher-capacity communication links using nearly synchronized clocks. Although largely replaced by SDH and SONET, PDH remains important for understanding legacy telecommunications infrastructure.

Key Features of PDH

• PDH stands for Plesiochronous Digital Hierarchy.
• It’s a technology associated with digital exchanges.
• PDH combines different hierarchies of digital signals, each having different data rates. Tributaries can contain 24 or 30 channels, or multiples thereof.
• PDH combines inputs that have the same data rates but originate from different clock sources.
• Europe uses 30 channels, while North America uses 24 channels. Japan utilizes different PDH levels.
• Bit interleaving is employed to combine these signals.

PDH level hierarchy in North America

Benefits or Advantages of PDH

1. It was designed to support the transportation of large amounts of data over digital equipment using various transmission mediums, such as microwave radio or fiber optic systems.
2. Network management is significantly enhanced.
3. PDH functions independently and effectively in North America, Europe, and Japan according to their respective standard specifications.

Drawbacks or Disadvantages of PDH

1. PDH uses different frames for transmission and in the data layer, leading to complex multiplexing and de-multiplexing processes.
2. Accessing a lower tributary requires de-multiplexing the entire system.
3. The maximum capacity for PDH is 566 Mbps, which can be a bandwidth limitation in modern applications.
4. Tolerance is allowed in bit rates.
5. PDH only supports point-to-point configurations.
6. PDH doesn’t support a hub architecture.
7. Different manufacturers have their own standards, and PDH has varying multiplexing hierarchies, which can make integrating interconnecting networks difficult.

Summary

Plesiochronous Digital Hierarchy (PDH) is a digital transmission technology that multiplexes multiple digital signals using nearly synchronized clocks. It served as the foundation of early telecommunication networks before SDH.