SDH Network Equipment

SDH Network Elements

An SDH transport network consists of optical spans and devices known as Network Elements (NEs). A network element terminates selected signal layers, performs mapping or switching, and passes the remaining layers to the next span. The functional model separates these tasks into blocks, but it does not require one particular physical partition of equipment.

The main SDH equipment categories are regenerators (REG), terminal multiplexers (TM), add-drop multiplexers (ADM), and digital cross-connects (DXC). They differ in which layers they terminate and at what VC level they expose or switch a transported service.

These functions may be combined in one device or divided among several network elements. Classification follows the terminated interface and signal layer, so the enclosure does not determine the role. An ADM, for example, may contain terminal-multiplexer functions for tributary ports while its line function continues transit traffic. Section-overhead communication channels carry management messages for the span, whereas POH describes an individual path.

1. The Regenerator (REG)

A Regenerator (REG) restores a transmission signal after a span has degraded its optical and timing parameters. It does not change the container assignment or provide a cross-connect function.

Why is it Needed?

Fiber causes attenuation, which lowers received optical power, while dispersion broadens and distorts pulses. The span between a regenerator and its neighboring element is set by the link budget, interface type, attenuation, dispersion, and required margin. Distances around 60 to 120 km occur in selected configurations, but they are not a universal limit for every route.

How it Works: 3R Regeneration

A regenerator performs O-E-O (Optical-to-Electrical-to-Optical) conversion and carries out the three 3R operations:

• Re-amplification: The received optical signal is converted to an electrical signal, and its amplitude is restored to the level required by the transmitter for the next span.
• Re-shaping: Decision circuits recover the logical levels from the distorted electrical waveform and limit the accumulation of pulse-shape errors.
• Re-timing: The recovered pulses are synchronized to a clock recovered from the data stream, limiting accumulated jitter.

The restored electrical signal drives the optical transmitter for the next span. The new signal does not carry the previous span's amplitude and shape degradation forward.

Overhead processing

A regenerator reads, processes, and terminates the Regenerator Section Overhead (RSOH), then generates RSOH for the next span. It does not terminate the Multiplex Section Overhead (MSOH) or Path Overhead (POH), so those layers and the payload pass through the regeneration function without local path termination.

2. The Terminal Multiplexer (TM)

A Terminal Multiplexer (TM) is the start or end point of a high-speed SDH line. Its primary function is to perform the multiplexing of multiple lower-rate tributary signals into a single STM-N signal, or the reverse process of demultiplexing. The TM sits at the boundary between the transport network and client interfaces.

• In the transmit direction: A TM receives client signals such as E1 or E3, maps them into C-n containers, and adds Path Overhead to form VC-n. It then creates TU, TUG, AU, or AUG structures according to the mapping, assembles the STM-N frame, and generates its section overhead.
• In the receive direction: A TM receives an STM-N frame, checks section overhead and pointers, separates TU or AU structures, and terminates the selected paths. It reconstructs the client signals from the recovered VC-n structures and delivers them to tributary ports.

A TM terminates the paths for its client signals, so it originates and terminates their Path Overhead (POH). On the line interface it processes the complete section overhead, including RSOH and MSOH.

3. The Add-Drop Multiplexer (ADM)

An Add-Drop Multiplexer (ADM) operates at a network node, usually between at least two line directions. It processes selected Virtual Containers without fully demultiplexing the STM-N signal, so it can terminate local traffic and insert new traffic into the same aggregate stream.

Operating principle

An ADM has at least two high-rate line interfaces, such as west and east directions in a ring, and tributary interfaces. The supported granularity depends on the equipment and can range from VC-12 to VC-3 or VC-4.

• Through traffic: The ADM reads line overhead and AU or TU pointers, then directs selected VCs to the other line interface. The transported path remains logically the same VC, while the equipment processes overhead and may regenerate the frame and pointers on output.
• Drop functionality: For selected VCs, the ADM uses the pointers, reads their POH, and terminates the path at the node. The content is reconstructed as a signal on a local tributary port, such as E1 from VC-12.
• Add functionality: A signal from a local port is mapped into C-n and VC-n. The ADM selects a configured position in the output structure, adds the required pointers and overhead, and inserts the VC into the STM-N frame.

An ADM provides access to individual paths without unpacking the complete signal. In a ring it can perform protection switching when monitored failure criteria and APS messages call for a move to an alternate route.

4. The Digital Cross-Connect (DXC or DCS)

A Digital Cross-Connect (DXC or DCS) establishes configured connections between many line and tributary ports. It switches selected Virtual Containers, so it can route traffic between network spans without terminating every client signal.

DXC functions

• Multi-port switching: A DXC provides line and tributary ports plus a switching matrix that realizes connections stored in its configuration. The available connection patterns and any blocking depend on the implementation, so not every DXC connects every input to every output.
• Switching granularity: A DXC can operate at different hierarchy levels:
  • High-order level: Switches VC-3 or VC-4 paths, including paths equivalent to E3 or E4 signals.
  • Low-order level: Switches individual VC-11, VC-12, or VC-2 paths. This allows lower-rate tributaries to be aggregated into larger output paths.
• Traffic grooming: A DXC can regroup partially used lower-order paths and place them on fewer higher-order paths. Consolidation frees capacity on other ports, but it requires available resources and a switching level that supports the transported VC.
• Provisioning and service restoration: A management system can remotely write long-distance connection assignments into a DXC. After a failure, a protection or restoration function can request a new assignment when an alternate route has sufficient capacity. A DXC can connect ring segments or nodes in a mesh, while the protection policy remains part of the network configuration.

Equipment selection follows the network position. A REG extends a section span, a TM terminates edge services, an ADM adds and drops selected VCs at a node, and a DXC establishes many connections between ports. The management system supervises configuration, alarms, and quality measurements, but it does not replace the switching matrix. Protection can move traffic to an alternate route only when capacity, interfaces, and the configured path are compatible.