Advantages of SDH over PDH

Introduction: SDH structure compared with PDH

Before SDH and SONET were standardized, high-rate digital transport used the Plesiochronous Digital Hierarchy (PDH). PDH levels had different bit rates and frame structures, while the clocks of individual streams were only close in frequency. Access to one low-rate channel inside a higher-rate stream required successive demultiplexing operations. The aggregate stream had to be opened through its hierarchy levels before a lower-rate tributary could be reached.

SDH and SONET organize these functions through common frame structures, container pointers, byte interleaving, and overhead fields. These elements support access to lower-order signals, through-traffic, and monitoring of sections and paths. Synchronous timing and standardized interfaces reduce conversion between network elements, while overhead carries information used to manage transmission. The examples below connect these mechanisms with concrete differences between PDH and SDH. G.707 also defines client mappings and overhead functions for the SDH node interface.

Advantage 1: add/drop multiplexing

SDH and SONET simplify access to lower-rate channels inside higher-rate streams. In PDH, that access required successive demultiplexing stages. A node dropping a T1 from a DS3 therefore processed the DS3, DS2, and T1 levels before it could release the selected channel.

PDH limitation: access to lower-order signals

Accessing one 1.544 Mbps T1 channel from a 44.736 Mbps DS3 signal, which can be part of a higher-rate stream, required stage-by-stage demultiplexing in PDH:

1. The entire higher-rate signal had to be delivered to the demultiplexer.
2. It was demultiplexed into constituent DS3 signals at 44.736 Mbps.
3. The selected DS3 signal was demultiplexed into seven DS2 signals at 6.312 Mbps.
4. The selected DS2 signal was demultiplexed into four T1 signals at 1.544 Mbps.
5. Only then could the desired T1 channel be extracted.

Forwarding the remaining traffic required the higher-rate stream to be assembled again after demultiplexing. Intermediate nodes therefore used sets of multiplexers and demultiplexers, increasing equipment count and maintenance effort.

The SDH/SONET solution: pointers and byte interleaving

SDH and SONET use two mechanisms for this function:

• Byte interleaving: Lower-rate signals are placed in a higher-order structure according to a defined byte order. The interleaving rules determine the position of each element in the frame.
• Pointers: The AU and TU pointers carry the offset of a Virtual Container start relative to the frame of its supporting layer. The receiving device uses this value to locate the container.

An Add-Drop Multiplexer (ADM) reads the pointers and can extract or insert a tributary at the appropriate layer. The remaining traffic is forwarded through the network element. This reduces successive demultiplexing stages and simplifies add/drop nodes, including ring topologies. The pointer identifies the VC offset in the supporting frame, while the ADM changes only the selected tributary.

Advantage 2: interworking between networks

PDH limitation: regional hierarchies

PDH evolved regionally, and the systems used different bit rates and frame structures:

• North America: T-carrier system, based on the T1 signal $(1,544 \text{ Mbps})$, with 24 voice channels. Japan used separate hierarchy variants at 1.544 Mbps.
• Europe & most of the world: E-carrier system, based on the E1 signal $(2,048 \text{ Mbps})$, with 30 voice channels.

Connecting these systems required equipment that converted bit rates and frame structures. Conversion was needed at hierarchy boundaries and when traffic crossed between networks operated with different regional standards. The framing rules also place signaling and service information in different positions.

The SDH/SONET solution: aligned rates

SONET defines a base STS-1/OC-1 rate of $(51.840 \text{ Mbps})$, and three times that rate is $155.520 \text{ Mbps}$. SDH STM-1 uses the same transport rate. OC-3 and STM-1 therefore align in line rate, while field names and some frame structures follow separate specifications. Interworking uses the mappings and interfaces defined for the relevant signal. Equal line rates support interworking, while frame and field mapping rules still determine compatibility.

Advantage 3: network management and monitoring

PDH limitation: monitoring and fault location

PDH networks provided limited information about transmission state. The lack of common monitoring fields made performance checks, fault location, and remote operation difficult. Finding a fault often required measurements on successive sections with external equipment.

The SDH/SONET solution: OAM overhead

SDH and SONET carry Operations, Administration, and Maintenance (OAM) functions in frame overhead fields. The available information depends on the layer. The overhead includes B1, B2, DCC, APS, MS-RDI, and B3 fields.

• Performance monitoring: B1 monitors the regenerator section, B2 the multiplex section, and B3 the higher-order path. BIP values provide error indications at the assigned layer.
• Communication channels (DCC): D1-D3 form the regenerator-section data communication channel, while D4-D12 form the multiplex-section channel. These channels can carry management communication between SDH elements. Routing depends on the network configuration and layer.
• Alarms and signaling: AIS (Alarm Indication Signal) and RDI (Remote Defect Indication) carry service and defect information to adjacent elements or a path endpoint. These indications can be used as protection criteria.

These fields let an element distinguish section errors from path defects and carry control data along the transport span.

Additional SDH framework properties