Introduction to PDH
Plesiochronous Digital Hierarchy (PDH) is a technology used in telecommunications networks to transport large quantities of data over digital transport equipment such as fibre optic and microwave radio systems. The term plesiochronous comes from Greek and means "almost synchronous", which describes the timing characteristics of PDH systems.
Key Concept: In PDH, different parts of the network are synchronized to clocks of similar accuracy, but not exactly the same frequency. This is different from Synchronous Digital Hierarchy (SDH) where all clocks are traceable to a highly accurate primary reference clock.
PDH was developed in the 1960s and became the standard for digital transmission in telecommunications networks throughout the 1970s and 1980s. It was eventually largely replaced by SDH/SONET, but understanding PDH is essential for comprehending the evolution of digital transmission systems.
PDH Hierarchy
PDH is based on a hierarchy of transmission levels, each with a specific bit rate. The hierarchy differs between North America/Japan and Europe/other regions:
PDH Hierarchy Comparison
| Level | North America/Japan | Europe/International | Channels (64 kbps each) |
|---|---|---|---|
| 0 | 64 kbps | 64 kbps | 1 |
| 1 (Primary) | 1.544 Mbps (T1) | 2.048 Mbps (E1) | 24 / 30 |
| 2 | 6.312 Mbps (T2) | 8.448 Mbps (E2) | 96 / 120 |
| 3 | 44.736 Mbps (T3) | 34.368 Mbps (E3) | 672 / 480 |
| 4 | 274.176 Mbps (T4) | 139.264 Mbps (E4) | 4032 / 1920 |
| 5 | N/A | 565.148 Mbps (E5) | 7680 |
Table 1: PDH hierarchy showing different regional standards
The multiplexing process involves combining multiple lower-rate signals into a single higher-rate signal. For example, in the European system:
- 30 E0 (64 kbps) channels + 2 signaling channels = 1 E1 (2.048 Mbps)
- 4 E1 signals = 1 E2 (8.448 Mbps)
- 4 E2 signals = 1 E3 (34.368 Mbps)
- 4 E3 signals = 1 E4 (139.264 Mbps)
E1 Frame Structure
The E1 frame has a duration of 125μs (8000 frames per second) and consists of 32 time slots (TS):
- Time Slot 0: Used for frame alignment and alarms
- Time Slot 16: Used for signaling (call setup, teardown, etc.)
- Time Slots 1-15 & 17-31: Carry user data (30 voice channels)
E1 Frame (2.048 Mbps)
| TS0 | TS1 | TS2 | ... | TS15 | TS16 | TS17 | ... | TS31 |
125μs duration = 256 bits = 32 time slots × 8 bits
Frame rate: 8000 frames/second × 256 bits/frame = 2,048,000 bps
T1 Frame Structure
The T1 frame (North America/Japan) has 24 channels + 1 framing bit:
- 24 channels × 8 bits = 192 bits
- + 1 framing bit = 193 bits per frame
- Frame duration: 125μs (8000 frames/sec)
- Bit rate: 193 bits/frame × 8000 frames/sec = 1.544 Mbps
- Cost-effective: PDH equipment was less expensive than synchronous alternatives during its peak adoption period
- Proven technology: Well-established with extensive field experience
- Flexible add/drop capability: Lower rate signals can be extracted and inserted without demultiplexing the entire high-speed stream
- Network resilience: Multiple vendors provided interoperable equipment
- Gradual migration: Allowed telecommunications providers to gradually digitize their networks
- Lack of standardization: Different hierarchies in North America, Japan, and Europe
- Complex multiplexing: To access a lower-rate channel, the entire high-rate signal must be demultiplexed
- Limited management capabilities: Poor operations, administration, and maintenance (OAM) features
- Clock synchronization issues: Plesiochronous operation requires bit stuffing, making it inefficient
- No inherent protection switching: Limited resilience compared to SDH/SONET
- Bandwidth inefficiency: Overhead for justification and framing reduces usable bandwidth
PDH vs. SDH/SONET
Comparison of PDH and SDH Characteristics
| Feature | PDH | SDH/SONET |
|---|---|---|
| Synchronization | Plesiochronous (almost synchronous) | Synchronous |
| Multiplexing | Complex, requires full demultiplexing | Direct access to tributaries |
| Standardization | Regional variations | International standard |
| OAM Capabilities | Limited | Extensive |
| Maximum Rate | 565 Mbps (E5) | 40 Gbps and beyond |
| Protection Switching | Not inherent | Built-in (MS-SPRING, etc.) |