SDH management functions
SDH places Operations, Administration, and Maintenance (OAM) information in the overhead (OH) bytes of its frames. The overhead reports defects, carries trace and status information, and provides communication channels for equipment management.
ITU-T G.784 separates management of SDH layer networks from management of their client layers. The same management model can therefore be applied when a network element transports an E1, Ethernet, or another client. Management functions cover fault handling, configuration, and performance monitoring, while protection switching remains a transport function coordinated through the overhead.
G.784 describes an organizational model in which an operations system in the element management layer communicates with the equipment management function inside the network element. Embedded control channel functions carry messages through the S.MSN. A network element may contain only SDH layer entities or both SDH and client layer entities, while the client entities remain in their own logical management domain.
The recommendation provides libraries of equipment management function building blocks for fault management, performance monitoring, and configuration management. An operator and supplier can select the subset needed for an application, so not every defined function is required in every network element.
Management scopes in an SDH signal
SDH supervision follows the transport layers. A section function is monitored on one physical span, a multiplex section between multiplex section endpoints, and a path from the source of a Virtual Container to its sink. The overhead field associated with each scope is terminated at the boundary where that scope ends.
- Regenerator section: This scope covers one fiber span between adjacent network elements. A regenerator processes the Regenerator Section Overhead (RSOH), including framing, trace, BIP-8, and the regenerator DCC.
- Multiplex section: This scope covers the AUG and its component paths between two multiplex section endpoints. It can include several regenerator sections. A terminates the Multiplex Section Overhead (MSOH) and processes BIP, protection, synchronization, and management fields.
- Path: This scope follows one client payload, such as an E1, from the path source to the path sink. The Path Overhead (POH) travels with the Virtual Container and carries path trace, signal label, and end-to-end performance information.
BIP monitoring by transport layer
One part of OAM is continuous transmission-error monitoring. SDH uses Bit Interleaved Parity (BIP) codes at the regenerator section, multiplex section, and path layers.
BIP calculation and scope
A BIP-N code places the checked bits in N conceptual columns and calculates one even-parity bit for each column. The receiver repeats the calculation and compares it with the value in the overhead. A mismatch reports an errored block or section event at the corresponding layer.
- B1 (BIP-8), regenerator section: The B1 byte in frame N+1 carries the even-parity result calculated over all bits of STM-N frame N after scrambling. It is placed before scrambling of frame N+1 and checks one regenerator section.
- B2 (BIP-N×24), multiplex section: The B2 bytes carry the even-parity result for all bits of the previous STM-N frame except its first three SOH rows. STM-1 uses BIP-24, while STM-4 uses BIP-96 (). The receiver uses this value for the multiplex section between its endpoints.
- B3 (BIP-8), path: The B3 byte in a high-order VC-4 or VC-3 carries an even-parity result calculated over all bits of the previous Virtual Container of the same type. It lets the path endpoints assess one client trail independently of other paths in the STM-N signal.
- V5 byte (BIP-2), low-order path: A VC-11 or VC-12 uses two BIP bits in V5 for low-order path monitoring. The associated path overhead also carries defect, label, and status information.
Fault reporting and maintenance signals
A detected defect is reported to the local management function and, where the SDH layer defines it, to the remote endpoint. This separates local detection from the action taken by an operator or a protection function.
Remote Error Indication (REI)
The receiving node, called the far end, can detect BIP violations in the incoming signal. It reports the result toward the transmitting node, called the near end, so the two ends keep separate near-end and far-end performance records.
For a multiplex section, the far end reports remote error information in M1. For a high-order path, the path status carried in G1 includes the path REI field. The near end can then associate the report with the section or path that it transmitted.
Alarm Indication Signal (AIS)
AIS is a maintenance replacement signal sent downstream after an upstream defect. A receiving element can report AIS as the cause instead of declaring the same physical failure at every following span.
- Scenario: A fiber span is interrupted.
- Detection: The first network node downstream from the interruption detects a .
- Action: The node generates AIS at the affected SDH layer and sends it downstream. The exact replacement format depends on whether the defect concerns a section, an administrative unit, or a path.
- Effect: Downstream nodes report the received AIS and preserve the location of the first observed physical defect. This avoids a chain of identical LOS reports along the remaining spans.
Remote Defect Indication (RDI)
RDI carries a defect indication toward the source of the affected signal. Multiplex section RDI uses bits 6 to 8 of K2, while path RDI uses the path status field in G1. AIS and RDI therefore describe the downstream and upstream views of the same defect at their respective SDH layers.
DCC channels and the SDH management network
The STM-N overhead contains two Data Communications Channels (DCC). They carry management messages separately from the client payload and form part of the SDH management subnetwork.
- Regenerator DCC (DCCR): Bytes D1 to D3 provide a channel. All SDH network elements can access this regenerator-section channel.
- Multiplexer DCC (DCCM): Bytes D4 to D12 provide a channel. The channel is part of the multiplex section and is not available at a regenerator.
G.784 defines an SDH management network, S.MN, which can be divided into subnetworks, S.MSNs. Every SDH network element terminates the embedded control channel and can forward management messages when its routing function is configured. A Q interface connects the S.MSN with the telecommunications management network, while an F interface provides an operations interface. The recommendation defines management functions and communication roles rather than one mandatory application protocol.