Sdh principle and alarm maintenance

Among various broadband fiber access network technologies, SDH (Synchronous Digital Hierarchy) technology is the most widely used. The introduction of SDH addressed the bandwidth limitations of home media, which previously caused bottlenecks in the connection between users and the core network. By increasing the transmission network's bandwidth, SDH has significantly improved the efficiency of data transfer. Since its introduction in the 1990s, SDH has become a mature and standardized technology, commonly used in backbone networks, with decreasing costs over time. Its application in access networks brings the bandwidth and technological advantages of the core network into the access domain, making full use of SDH’s synchronous multiplexing, standardized optical interfaces, powerful network management capabilities, flexible topologies, and high reliability. This article primarily explains the principles of SDH and alarm maintenance. **First, the Logical Function Blocks of SDH Equipment** The SDH transmission network consists of different types of network elements connected via optical cables. These network elements perform various functions such as uplink/downstream services, cross-connection, and network fault self-healing. Common SDH network elements include TM (Terminal Multiplexer), ADM (Add/Drop Multiplexer), REG (Regenerator), and DXC (Digital Cross Connect). The ITU-T defines a functional reference model for SDH equipment, breaking down the device’s functions into basic standard function blocks. These blocks are independent of physical implementation and can be flexibly combined to fulfill different device functions. Standardizing these function blocks ensures universal specifications and clear descriptions. To better understand this, we will describe the role of each basic function block using a typical TM device. It is important to pay attention to the alarm performance events and detection mechanisms monitored by each block. As shown in Figure 1. (Figure 1: Logical function of SDH equipment) In order to better understand the figure, the following function block names are explained: SPI - SDH Physical Interface TTF - Transfer Terminal Function RST - Regeneration Section Terminal HOI - High-order Interface MST - Multiplex Section Terminal LOI - Low-order Interface MSP - Multiplex Section Protection MSA - Multiplex Section Adapts PPI - PDH Physical Interface LPA - Low-order Channel Adaptation LPT - Low-order Channel Terminal LPC - Low-order Channel Connections HPA - High-end Channel Adaptation HPT - High-end Channel Terminal The signal flow in the TM device starts from the STM-N signal entering at point A, then passes through A→B→C→D→E→F→G→L→M to produce 140 Mbit/s PDH signals. It is split into 2 Mbit/s or 34 Mbit/s PDH signals after A→B→C→D→E→F→G→H→I→J→K. These functions are performed by each basic function block. Common network elements in the SDH network include TM, ADM, REG, and DXC. Their functional blocks are described below. Understanding the composition of these blocks helps grasp their respective functions. TM: Used at terminal stations, it multiplexes low-speed tributary signals into high-speed STM-N signals or separates them from STM-N signals. ADM: It cross-multiplexes low-speed tributary signals into east/west lines or splits them from line signals received from those ports. It can also cross-connect STM-N signals on the east/west line side. REG: It does not add/drop circuits but amplifies or regenerates optical signals. It processes optical signals on both sides via O/E, sampling, decision, regenerative shaping, and E/O. DXC: Primarily performs cross-connection of STM-N signals. It acts like a cross matrix, enabling signal cross-connections. Powerful DXCs can handle low-level cross-connections of high-speed signals. **Second, Main Alarm Signals and Overhead Bytes in SDH** Each function block in an SDH device generates main alarm and maintenance signals along with related overhead bytes. These include: - SPI: LOS - RST: LOF (A1, A2), OOF (A1, A2), RS-BBE (B1) - MST: MS-AIS (K2[b6 b8]), MS-RDI (K2[b6 b8]), MS-REI (M1), MS-BBE (B2), MS-EXC (B2) - MSA: AU-AIS (H1, H2, H3), AU-LOP (H1, H2) - HPT: HP-RDI (G1[b5]), HP-REI (G1[b1 b4]), HP-TIM (J1), HP-SLM (C2), HP-UNEQ (C2), HP-BBE (B3) - HPA: TU-AIS (V1, V2, V3), TU-LOP (V1, V2), TU-LOM (H4) - LPT: LP-RDI (V5[b8]), LP-REI (V5[b3]), LP-TIM (J2), LP-SLM (V5[b5 b7]), LP-UNEQ (V5[b5 b7]), LP-BBE (V5[b1, b2]) These alarms indicate various issues, such as signal loss (LOS), frame out of step (OOF), frame loss (LOF), background error blocks (BBE), and more. Each alarm has specific definitions and thresholds, helping operators identify and resolve problems effectively. **Third, Multiplexing PDH into SDH STM-N Signal Steps** ITU-T specifies a complete set of multiplexing structures that allow PDH signals to be multiplexed into STM-N signals in various ways. In China, the multiplexing route for a 2 Mbit/s signal is defined, with the AU-4 structure being used. This structure is shown in Figure 4. (Figure 4: China's SDH Basic Multiplexing Mapping Structure) **Fourth, SDH Frame Structure and Section Overhead** SDH uses a rectangular frame structure based on bytes. An STM-N frame consists of 9 rows and 270×N columns of bytes. The frame is divided into three areas: Section Overhead (SOH), Information Payload, and Administration Unit Pointer (AU-PTR). - **Section Overhead (SOH):** Contains bytes necessary for normal information payload and flexible transmission, used for network operation, management, and maintenance. It includes Regenerator Section Overhead (RSOH) and Multiplex Section Overhead (MSOH). - **Information Payload:** Stores information symbols from low-speed branches, encapsulated into STM-1. - **Administration Unit Pointer:** Indicates the location of the first byte of the payload in the management unit, allowing proper separation at the receiving end. **Fifth, Channel Overhead** - **High-order Path Overhead (HP-POH):** Includes J1 (channel trace byte), B3 (channel BIP-8 code), C2 (signal tag byte), and G1 (channel status byte). These monitor transmission performance and ensure correct signal mapping. - **Low-order Path Overhead (LP-POH):** Located in VC12, it monitors the transmission performance of the VC12 channel. It includes V5, J2, N2, and K4 bytes, which are spread across four frames. **Sixth, BIP-8 Error Detection Principle** BIP-8 is used for error detection in SDH. B1 checks the regeneration section, B2 checks the multiplex section, and B3 checks the channel. These bytes perform even parity checks on specific bits to detect errors in the transmission. **Seventh, Device Alarm Example** An example of an SDH device's alarm output shows metrics like LOF, LOS, BIP(B1), FEBE, and others. These help operators identify and address issues such as bit error rates, framing problems, and remote alarms. **Eighth, E1 Error Code and Circuit Alarm Detection** In E1 channels, 8 bits form a time slot (TS), and 32 frames make up a frame (F). TS0 and TS16 are considered overhead, while the rest carry the payload. Alarms like RFA (Remote Frame Alarm) and AIS (Alarm Indication Signal) are detected through TS0 and other signaling mechanisms. Switches like the Lucent 5ESS use commands to view error registers and detect issues such as slip, OOF, and CRC errors. This detailed explanation provides a comprehensive understanding of SDH technology, its components, alarm mechanisms, and practical applications in modern telecommunications networks.

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