PDH (Plesiochronous Digital Hierarchy), SONET (Synchronous Optical Network), and SDH (Synchronous Digital Hierarchy) are Time Division Multiplexing (TDM) technologies that have formed the backbone of telecommunications networks for decades. These hierarchical structures define how digital signals are multiplexed, transported, and managed across telecom networks.
T-Carrier (T1) introduced in North America at 1.544 Mbps
E-Carrier (E1) introduced in Europe at 2.048 Mbps
Higher-order PDH rates standardized (T3/E3, T4/E4)
First proposals for synchronous multiplexing
SONET standard approved by ANSI
SDH standardized by ITU-T as G.707/G.708/G.709
First commercial SONET/SDH deployments
Next-Gen SONET/SDH standards (GFP, VCAT, LCAS)
Transition from TDM to packet-based transport begins
| Feature | PDH | SONET | SDH |
|---|---|---|---|
| Synchronization | Plesiochronous (nearly synchronous) | Fully synchronous | Fully synchronous |
| Primary Signal | DS1 (1.544 Mbps) or E1 (2.048 Mbps) | STS-1 (51.84 Mbps) | STM-1 (155.52 Mbps) |
| Regional Deployment | North America (T-Carrier) and Europe (E-Carrier) | North America | Europe, Asia, Rest of World |
| Overhead Management | Limited, scattered throughout frame | Comprehensive, organized in section/line/path layers | Comprehensive, organized in section/path layers |
| Multiplexing/Demultiplexing | Requires full demultiplexing to access lower-order signals | Direct access to tributaries (add/drop) | Direct access to tributaries (add/drop) |
| Frame Structure | Not uniform across levels | Uniform, byte-interleaved | Uniform, byte-interleaved |
PDH was the first standardized digital transmission hierarchy, allowing multiple voice channels to be multiplexed into higher-rate signals. PDH systems are "plesiochronous," meaning they run at nearly—but not perfectly—the same rate, requiring complex bit-stuffing procedures for multiplexing.
| Level | Signal | Rate (Mbps) | Voice Channels | Structure |
|---|---|---|---|---|
| Level 0 | DS0 | 0.064 | 1 | 1 voice channel |
| Level 1 | DS1 (T1) | 1.544 | 24 | 24 DS0s + framing |
| Level 2 | DS2 (T2) | 6.312 | 96 | 4 DS1s + overhead |
| Level 3 | DS3 (T3) | 44.736 | 672 | 7 DS2s + overhead |
| Level 4 | DS4 (T4) | 274.176 | 4032 | 6 DS3s + overhead |
| Level | Signal | Rate (Mbps) | Voice Channels | Structure |
|---|---|---|---|---|
| Level 0 | E0 | 0.064 | 1 | 1 voice channel |
| Level 1 | E1 | 2.048 | 30 | 30 E0s + 2 signaling channels |
| Level 2 | E2 | 8.448 | 120 | 4 E1s + overhead |
| Level 3 | E3 | 34.368 | 480 | 4 E2s + overhead |
| Level 4 | E4 | 139.264 | 1920 | 4 E3s + overhead |
| Level 5 | E5 | 565.148 | 7680 | 4 E4s + overhead |
Note: Despite its limitations, PDH remains in use as a legacy technology in many networks. T1/E1 circuits are still common for enterprise connectivity, and DS3/E3 services continue to be used for backhaul and private line applications. Even in modern networks, PDH signals are often carried as tributaries within SONET/SDH or packet-based transport systems.
SONET (Synchronous Optical Network) and SDH (Synchronous Digital Hierarchy) are synchronous transmission standards that overcome PDH limitations through a structured, byte-interleaved multiplexing scheme with comprehensive overhead for management, monitoring, and protection functions.
| SONET Level | SONET Rate (Mbps) | SDH Level | SDH Rate (Mbps) | Payload Capacity | Common Applications |
|---|---|---|---|---|---|
| STS-1 / OC-1 | 51.84 | - | - | ~50 Mbps | 28 DS1s or 1 DS3 |
| STS-3 / OC-3 | 155.52 | STM-1 | 155.52 | ~150 Mbps | 84 DS1s, 3 DS3s, or ATM/Ethernet |
| STS-12 / OC-12 | 622.08 | STM-4 | 622.08 | ~600 Mbps | 336 DS1s, 12 DS3s, or ATM/Ethernet |
| STS-48 / OC-48 | 2,488.32 | STM-16 | 2,488.32 | ~2.4 Gbps | 1,344 DS1s, 48 DS3s, or Gigabit Ethernet |
| STS-192 / OC-192 | 9,953.28 | STM-64 | 9,953.28 | ~9.6 Gbps | 5,376 DS1s, 192 DS3s, or 10GE WAN PHY |
| STS-768 / OC-768 | 39,813.12 | STM-256 | 39,813.12 | ~38.8 Gbps | 21,504 DS1s, 768 DS3s, or multiple 10GE |
SONET/SDH provides a flexible framework for multiplexing various signals and accommodating different client payloads through standardized mapping procedures. The structured multiplexing approach allows for easy access to individual tributaries without complete demultiplexing.
| PDH Signal | SONET Mapping | SDH Mapping | Capacity Utilization |
|---|---|---|---|
| DS1 (1.544 Mbps) | VT1.5 | VC-11 in TU-11 | 28 per STS-1 / 84 per STM-1 |
| E1 (2.048 Mbps) | VT2 | VC-12 in TU-12 | 21 per STS-1 / 63 per STM-1 |
| DS1C (3.152 Mbps) | VT3 | Not commonly used | 14 per STS-1 |
| DS2 (6.312 Mbps) | VT6 | VC-2 in TU-2 | 7 per STS-1 / 21 per STM-1 |
| E3 (34.368 Mbps) | STS-1 (float) | VC-3 in TU-3 | 3 per STM-1 |
| DS3 (44.736 Mbps) | STS-1 (locked) | VC-3 in TU-3 | 1 per STS-1 / 3 per STM-1 |
| E4 (139.264 Mbps) | STS-3c (float) | VC-4 | 1 per STS-3 / 1 per STM-1 |
SONET/SDH networks incorporate robust protection switching mechanisms to ensure high availability and rapid recovery from failures. These standardized protection schemes operate at various levels of the network and offer differing trade-offs between recovery time, bandwidth efficiency, and complexity.
SONET/SDH includes extensive management and monitoring capabilities embedded within the overhead bytes. These functions enable comprehensive network visibility, performance monitoring, and troubleshooting capabilities.
The evolution from PDH to SONET/SDH represented a significant advancement in telecommunications transport technology. While SONET/SDH continues to be deployed in many networks, the industry is gradually transitioning to packet-based transport technologies.
| Feature | PDH | SONET/SDH | Packet Transport (OTN/MPLS/Ethernet) |
|---|---|---|---|
| Synchronization | Plesiochronous | Fully synchronous | Asynchronous or synchronous (depending on implementation) |
| Network Management | Limited, proprietary | Comprehensive, standardized | IP-based management, extensive OAM |
| Add/Drop Capability | Requires full demux/remux | Direct tributary access | Flexible, service-based provisioning |
| Protection | Limited, proprietary | Standardized APS, sub-50ms | Various options (MPLS FRR, G.8031/32, LAG) |
| Bandwidth Efficiency | Low, fixed hierarchy | Medium, VC/VT structure | High, statistical multiplexing |
| Data Handling | Circuit-oriented only | Extended for data (NGS/SDH) | Native data support |
| Equipment Cost | Low (legacy) | Medium-high | Initially high, now decreasing |
| Deployment Status | Legacy, declining | Mature, stable installed base | Growing rapidly |
Reference guide to common terms and acronyms used in TDM technologies.
| Term | Definition |
|---|---|
| AIS | Alarm Indication Signal - Notifies downstream equipment of upstream failures |
| APS | Automatic Protection Switching - Mechanism for protection switching in SONET/SDH |
| BLSR | Bidirectional Line Switched Ring - SONET protection ring architecture |
| DCC | Data Communications Channel - Management channels in SONET/SDH overhead |
| DS0 | Digital Signal Level 0 - Basic 64 kbps channel (single voice channel) |
| DS1 | Digital Signal Level 1 - 1.544 Mbps signal carrying 24 DS0s (T1) |
| DS3 | Digital Signal Level 3 - 44.736 Mbps signal carrying 28 DS1s (T3) |
| E1 | European PDH signal at 2.048 Mbps carrying 30 voice channels |
| E3 | European PDH signal at 34.368 Mbps carrying 16 E1s |
| GFP | Generic Framing Procedure - Encapsulation method for data over SONET/SDH |
| LCAS | Link Capacity Adjustment Scheme - Dynamic bandwidth adjustment for VCAT |
| MS-SPRING | Multiplex Section Shared Protection Ring - SDH protection ring architecture |
| MSOH | Multiplex Section Overhead - Management information for SDH multiplexer section |
| OC-N | Optical Carrier level N - Optical form of STS-N in SONET |
| PDH | Plesiochronous Digital Hierarchy - Early digital transmission hierarchy |
| POH | Path Overhead - End-to-end management information in SONET/SDH |
| RDI | Remote Defect Indication - Upstream notification of receive failure |
| RSOH | Regenerator Section Overhead - Management information for SDH regenerator section |
| SDH | Synchronous Digital Hierarchy - ITU-T standard for synchronous data transmission |
| SNCP | Subnetwork Connection Protection - SDH path-level protection |
| SONET | Synchronous Optical Network - ANSI standard for synchronous data transmission |
| SPE | Synchronous Payload Envelope - Payload-carrying area of SONET frame |
| STM-N | Synchronous Transport Module level N - Basic transmission format for SDH |
| STS-N | Synchronous Transport Signal level N - Basic transmission format for SONET |
| TDM | Time Division Multiplexing - Technique for combining multiple signals by allocating time slots |
| TU | Tributary Unit - SDH structure for carrying lower-rate signals |
| UPSR | Unidirectional Path Switched Ring - SONET path-level protection ring |
| VC | Virtual Container - SDH structure for transporting payload |
| VCAT | Virtual Concatenation - Technique for flexible bandwidth allocation in SONET/SDH |
| VT | Virtual Tributary - SONET structure for carrying sub-STS-1 signals |
PDH, SONET, and SDH have played crucial roles in the evolution of telecommunications networks. They established the foundation for reliable, manageable transport networks that have supported voice and data services for decades.
While packet-based transport technologies like Carrier Ethernet, MPLS-TP, and OTN are increasingly deployed in new networks, understanding the principles and architecture of TDM hierarchies remains valuable for telecommunications professionals. The concepts of synchronization, protection, and hierarchical multiplexing continue to influence modern network design, and many legacy TDM interfaces will remain in service for years to come.