Plesiochronous Digital Hierarchy

Description

Plesiochronous Digital Hierarchy (PDH) is a digital transmission system for multiplexing multiple tributary signals into a single higher-bit-rate stream for transmission over telecommunications networks. The term 'plesiochronous' means 'almost synchronous,' indicating that the individual input streams are nominally at the same rate but allowed to have slight timing variations (within a specified tolerance). The system works by bit-interleaving the tributaries. Each tributary signal (e.g., a 2.048 Mbps E1 stream) has its own clock source. At the multiplexer, justification (or stuffing) bits are added to compensate for the small frequency differences between the tributary clocks and the multiplexer's internal system clock. These justification bits allow the multiplexer to temporarily increase the bit rate of a slower tributary to match the system timing.

The PDH hierarchy is defined differently in different world regions. The primary base levels are: 2.048 Mbps (E1, 32 channels of 64 kbps) in Europe and most of the world, and 1.544 Mbps (T1, 24 channels of 64 kbps) in North America and Japan. These are multiplexed into higher levels: for example, four E1s are multiplexed into an E2 at 8.448 Mbps, four E2s into an E3 at 34.368 Mbps, and four E3s into an E4 at 139.264 Mbps. A similar but different hierarchy exists for T-spans. The multiplexing is synchronous at each stage but plesiochronous between stages, meaning to extract a single low-rate channel from a high-rate stream, the entire signal must be demultiplexed step-by-step back down to the desired level—a process known as 'back-to-back multiplexing.'

In 3GPP specifications, PDH is referenced primarily in the context of legacy transport interfaces for network elements, especially in early releases for connecting Node Bs to Radio Network Controllers (RNCs) in UMTS, or Base Transceiver Stations (BTS) to Base Station Controllers (BSCs) in GSM. Specifications like TS 25.411 (UTRAN Iu interface user plane protocols) and TS 29.332 (Media Gateway Control Protocol) include support for PDH physical layer framing (e.g., E1, T1). Its role in 3GPP is as a supported transport technology for legacy network deployments and interconnections, with newer networks predominantly using Ethernet and IP-based transport.

Purpose & Motivation

PDH was developed to address the need for efficient digital transmission and multiplexing in the public switched telephone network (PSTN) during the 1970s and 1980s. Prior to PDH, analog carrier systems were used, which were susceptible to noise and crosstalk. The transition to digital (Pulse Code Modulation - PCM) for voice required a method to combine multiple 64 kbps voice channels for long-distance transmission over coaxial cable or microwave radio links.

The purpose of PDH was to create a standardized hierarchy for aggregating these digital streams. It solved the practical problem of clock synchronization across a vast network where each central office had its own clock. The 'plesiochronous' approach, with justification, was a pragmatic solution that avoided the cost and complexity of requiring a perfectly synchronous network-wide clock source. This enabled the widespread deployment of digital trunking, dramatically improving voice quality and enabling the first digital data services. However, PDH had limitations: complex add/drop multiplexing, limited management capabilities, and lack of standardization between regions. These limitations motivated the development of the fully synchronous SONET/SDH standards, which eventually superseded PDH for core network transport, though PDH remains in use in some access and legacy network segments.

Key Features

• Multiplexes multiple lower-bit-rate digital signals into a single higher-bit-rate stream
• Uses justification (bit-stuffing) to handle slight timing differences between input signals
• Defines hierarchical levels (e.g., E1, E2, E3, E4 in Europe; T1, T2, T3 in North America)
• Supports digital transmission over copper, coaxial, microwave, and early optical fiber
• Requires step-by-step demultiplexing to access individual low-rate channels within a high-rate stream
• Provides a foundational digital transport layer for legacy telecom networks

Evolution Across Releases

Defining Specifications