SDH vs SONET

SDH and SONET scope

SONET and SDH are related synchronous optical transport systems, but they are defined by separate standards. ANSI T1.105 describes the SONET signal family, while ITU-T G.707 defines the SDH network-node interface. Both address the timing, multiplexing, and supervision limits of PDH.

SONET (Synchronous Optical Network) was standardized for North American transport, while SDH (Synchronous Digital Hierarchy) was defined by ITU-T for international use, including the European E-carrier hierarchy. Their frame rates and transport principles align at several interfaces, but field names, payload structures, and mappings remain standard-specific.

In practice, the comparison also covers overhead, pointers, and protection. G.707 specifies AU, TU, and VC structures and their mapping into STM frames, while SONET terminology distinguishes STS signals, OC carriers, and Virtual Tributaries. Matching bit rates do not remove these structural differences. This distinction matters when equipment maps client signals or crosses a domain boundary.

Standardization context and PDH constraints

The parallel development of SONET and SDH responded to the limitations of the older Plesiochronous Digital Hierarchy (PDH). PDH was divided into two incompatible regional hierarchies.

• North American PDH (T-carrier): Based on the T1 signal, which multiplexed 24 voice channels into a $(1,544 \text{ Mbps})$ stream. Higher rates like T3 $(\approx 45 \text{ Mbps})$ were built by multiplexing T1s.
• European PDH (E-carrier): Based on the E1 signal, which multiplexed 30 voice channels into a $(2,048 \text{ Mbps})$ stream. Higher rates like E3 $(\approx 34 \text{ Mbps})$ and E4 $(\approx 140 \text{ Mbps})$ were built from E1s.

This incompatibility led to two separate standardization efforts in the 1980s:

1. SONET Emerges: In the United States, Bellcore (now Telcordia) and ANSI led the development of SONET, designing it specifically to efficiently transport the existing T-carrier signals (T1, T3) over a new, high-speed, and manageable optical infrastructure.
2. SDH defined: ITU-T defined SDH in G.707 with its own STM, AU, TU, and VC structures. The specification uses synchronous timing and pointers while supporting E-carrier mappings and interworking with other client signals.

Shared transport mechanisms

SDH and SONET use different field names and payload hierarchies, but they rely on related transport mechanisms. Both systems provide the following functions:

• Synchronous operation: Both systems use a clocking hierarchy traceable to a Primary Reference Clock (PRC), providing a common timing basis for network elements.
• Basic frame rate: Both use a frame duration of $(125 \mu \text{s})$, corresponding to 8,000 frames per second in digital voice transport.
• Pointer mechanism: A pointer locates the payload in the transport frame, supports add/drop operations, and accommodates phase or frequency differences between networks.
• Layered overhead (OAM): Both allocate overhead bytes to Operations, Administration, and Maintenance, including performance monitoring, fault handling, and protection switching.
• Byte-interleaved multiplexing: Higher-rate signals are formed by an ordered byte interleave of lower-rate signals.

Base rates and hierarchies

The main difference is the base signal and the data-rate hierarchy built from it.

Terminology

The transport functions are related, but SONET and SDH use different names for structural components and payload types. Distinguishing the terms is necessary when connecting domains or selecting mappings in equipment from different vendors.

Multiplexing structures

SONET developed its mapping around the T1 stream, while SDH developed around E1. Their intermediate structures for multiplexing low-rate signals into the main STM-1 or OC-3 payload therefore differ.

• In SONET: The smallest tributary is the Virtual Tributary (VT). A T1 stream is mapped into a VT1.5. Then, 28 VT1.5s are multiplexed into a Tributary Unit Group, which is then mapped into the main STS-1 payload (SPE). To build the OC-3 signal, three such STS-1 payloads are byte-interleaved.
• In SDH: The smallest tributary is packaged into a Tributary Unit (TU). An E1 stream is mapped into a VC-12 and then given a pointer to become a TU-12. These are then grouped into TUG-2s and TUG-3s before being mapped into a single, large VC-4, which then gets an AU-4 pointer and fills the entire STM-1 payload.

Both paths produce a $(155,52 \text{ Mbps})$ transport frame, while the internal payload arrangement follows the originating hierarchy.