PDH hierarchy limitations
Before SDH and SONET were deployed, digital telecommunications networks relied on the . PDH had limitations that made network expansion and access to individual channels inside higher-rate streams difficult. A lower-rate channel therefore had to be reached through the intermediate hierarchy levels, and the remaining traffic had to be rebuilt for onward transport.
Main PDH limitations
- Difficult add/drop multiplexing: Extracting a low-rate channel, such as one telephone call, from a higher-rate PDH stream required demultiplexing through successive hierarchy levels. After the channel was removed, the remaining traffic had to be assembled again into the higher-rate stream.
- Regional standards: Different regions developed their own PDH variants. Europe and much of the world used E-carrier with E1 at 2.048 Mbps, North America used T-carrier with T1 at 1.544 Mbps, and Japan used its own hierarchy variants. The different rates and frame structures required conversion at network boundaries.
- Clocking and justification: The "plesiochronous" operation gave each incoming stream a slightly different clock rate. Combining them required a process called . The inserted bits made individual channels hard to locate within the multiplexed stream.
- Limited management: PDH provided few functions for network monitoring, performance measurement, and remote configuration. Fault location required measurements on successive sections.
- Vendor-dependent optical interfaces: As fiber optics became common, interfaces with vendor-dependent parameters were used. This made direct interconnection between equipment from different suppliers difficult.
Synchronous SDH and SONET structures
SDH and SONET introduced synchronous transport structures, rate hierarchies, and overhead fields for management. They form two related sets of standards:
- SONET (Synchronous Optical Network): Developed in the United States by Bellcore, now Telcordia, and described by the ANSI T1.105 family. It includes transport of North American PDH signals such as T1 and T3.
- SDH (Synchronous Digital Hierarchy): A hierarchy defined by the ITU-T, including Recommendation G.707, for transport of European PDH signals such as E1, E3, and E4 and for interworking with other structures.
SONET and SDH use different field names and partly different structures, while their basic transport rates are aligned. STM-1 and OC-3 operate at 155.520 Mbps. Interworking uses mappings defined by the applicable specifications. ITU-T G.707 defines the SDH node interface using bit rates, frame structures, client-signal mapping, multiplexing, and overhead functions.
Core SDH and SONET elements
SDH and SONET use several elements that organize synchronization, multiplexing, and transmission supervision.
1. Network synchronization
SDH and SONET network elements operate in a synchronization hierarchy referenced to a precise master source, typically a . A common reference reduces the need for justification between network elements, while the defined frame structures let equipment determine byte positions during demultiplexing. G.803 describes master-slave distribution from the PRC through transit and local node clocks to the SDH network-element clock, with timing distributed without intermediate pointer processing.
2. Byte-interleaved multiplexing
SDH and SONET place bytes from component signals according to defined interleaving rules. The position of an element depends on its hierarchy level, so equipment can identify the relevant container and pass it to an add or drop function without dismantling the entire PDH hierarchy.
3. STM-1 frame structure
The basic SDH structure is the Synchronous Transport Module, level 1 (STM-1).
- Data Rate: .
- Duration: (microseconds). The frame repeats 8,000 times per second, matching the sampling period used by PCM voice telephony ().
- Structure: The STM-1 frame is visualized as a block of 9 rows by 270 columns of bytes.
The first nine columns contain the Section Overhead fields and the AU pointer row, while the remaining 261 columns form the STM-1 payload area.
Higher-rate signals such as STM-4 and STM-16 are created by byte-interleaving lower-rate STM structures. STM-1 has a SONET counterpart with the same rate, OC-3, while the field names and overhead follow separate specifications.
4. Pointers and VC position
When a plesiochronous PDH signal is mapped into SDH, frequency and phase differences remain. The pointer identifies the position of the container within the frame.
Client data such as an E1 or T1 stream is mapped into a and receives POH. The AU pointer gives the offset of the VC start relative to the multiplex-section frame. The VC can therefore change its phase position relative to STM-1, allowing the network to handle timing differences without rebuilding the hierarchy. The pointer reports the VC frame start relative to the supporting frame, so a phase change updates the offset while the VC size remains unchanged.
5. Overhead and OAM
STM-1 overhead carries Operations, Administration, and Maintenance (OAM) information. The fields are divided into two main groups:
- Section Overhead (SOH): Supports the section between adjacent network elements such as regenerators and multiplexers.
- Path Overhead (POH): Travels with the container across its path and carries state information for that path.
Overhead fields carry monitoring data, indicate defects, and can provide criteria for protection mechanisms. The available functions depend on the layer and network-element type. SOH also carries framing, parity, alarm, and automatic protection-switching fields. POH travels with the VC and carries path trace, signal-label, and status information. SOH functions cover the regenerator section and the multiplex section.