Sdh principle and alarm maintenance

Among various broadband fiber access network technologies, SDH (Synchronous Digital Hierarchy) technology is the most commonly used. The introduction of SDH addressed the issue of bandwidth limitations in home media, which previously caused bottlenecks between users and the core network. By increasing the transmission network's bandwidth, SDH has significantly improved the utilization of network resources. Since its introduction in the 1990s, SDH has become a mature and standardized technology, widely adopted in backbone networks, with decreasing costs over time. Its application in access networks allows the SDH technology to bring its bandwidth and technical advantages into the access domain. This includes utilizing synchronous multiplexing, standardized optical interfaces, powerful network management capabilities, flexible network topologies, and high reliability, all of which benefit the development of access networks. 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 by optical cables. The functions of the SDH network are completed by these network elements: uplink/downlink services, cross-connection services, and self-healing for network faults. The main SDH network elements include TM (Terminal Multiplexer), ADM (Add/Drop Multiplexer), REG (Regenerator), and DXC (Digital Cross-Connect). The TU-T uses a functional reference model to standardize SDH equipment, decomposing the device’s functions into basic standard function blocks. These blocks can be flexibly combined to perform different device functions. Standardization of these blocks ensures universal specifications and clear descriptions. Below we use a typical function block of a TM device to explain the role of each basic function block. Special attention should be paid to the alarm performance events and detection mechanisms monitored by each function block. As shown in Figure 1. (Figure 1: Logical function of SDH equipment) To better understand the above figure, the function block names appearing in the figure are explained as follows: 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 HOA – High-order assembler MSA – Multiplex section adapts HPC – High-order channel connections PPI – PDH physical interface OHA – Overhead access function LPA – Low-order channel adaptation SEMF – Synchronization device management function LPT – Low-order channel terminal MCF – Message communication function LPC – Low-order channel connects SET – Synchronous device clock source HPA – High-end channel adaptation HPT – High-end channel terminal The figure above shows a block diagram of the TM function. The signal flow starts with the STM-N signal entering the device from point A, passing through A→B→C→D→E→F→G→L→M to split into 140 Mbit/s PDH signals. It then splits 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 of the SDH network include TM, ADM, REG, and DXC. Each element’s functions can be understood through their composition of function blocks. TM: Used at the terminal station of the network, it multiplexes low-speed signals from tributary ports into high-speed STM-N signals on the line port or separates low-speed branch signals from the STM-N signal. ADM: Cross-multiplexes low-speed tributary signals into east or west lines or drops them from received line signals. It also supports cross-connections of STM-N signals on the east/west line side. REG: Does not add or 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: Mainly performs cross-connections of STM-N signals. It acts like a cross matrix, enabling signal cross-connections. Powerful DXC can perform low-level cross-connections of high-speed signals. Second, the main alarm signals and overhead bytes of SDH. The main alarm maintenance signals generated by each function block of the SDH device and related overhead bytes are listed below. 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]) Each alarm signal has a specific meaning defined by ITU-T recommendations. For example, LOS indicates signal loss due to no optical power, too low or high optical power, or worse BER than 10^-3. OOF refers to frame out of step when searching for A1 and A2 bytes exceeds 625μs. LOF occurs when OOF lasts more than 3ms. Third, the steps to multiplex PDH into SDH STM-N signals. ITU-T defines a complete set of multiplexing structures, allowing PDH signals to be multiplexed into STM-N signals in various ways. The specified multiplexing route is shown below. (G.709 Multiplexing Mapping Structure) PDH Bandwidth: Although there are many routes for multiplexing signals into SDH STM-N signals, each country or region must choose one unique route. In China, the 2 Mbit/s signal is used as the basic PDH series, and the multiplexing route of AU-4 is selected. The structure is shown in Figure 4. (Figure 4: China's SDH Basic Multiplexing Mapping Structure) Fourth, the SDH frame structure and segment overhead. SDH uses a rectangular block-like frame structure based on a byte structure. STM-N is a code block consisting of 9 rows and 270×N columns of bytes. The frame structure can be divided into three areas: Segment Overhead (SOH), Information Payload (Payload), and Administration Unit Pointer (AU-PTR). (1) Segment Overhead (SOH): Attached to ensure normal information payload and flexible transmission. It is mainly used for network operation, management, and maintenance. SOH is divided into Regenerator Section Overhead (RSOH) and Multiplex Section Overhead (MSOH). (2) Information Payload Area (Payload): Stores information symbols from various low-speed branches in the frame structure. These symbols are encapsulated in different containers to reach the STM-1 rate. (3) Management Unit Pointer: Identifies the location of the first byte of the information payload in the entire management unit, allowing proper separation of the payload at the receiving end. Fifth, channel overhead. (1) High-order Path Overhead (HP-POH): J1 Channel Trace Byte: Indicates the starting point of VC4, helping the receiving end confirm continuous connection with the transmitting end. Channel BIP-8 Code B3 Byte: Monitors error performance of VC4 transmission in STM-N frame. If errors are detected, the device displays HP-BBE and HP-REI. C2 Signal Tag Byte: Indicates the multiplex structure and nature of the information payload. C2=00H means the VC4 channel is unloaded, triggering an HP-UNEQ alarm. (2) Low-order Path Overhead (LP-POH): Located at the first of each VC12 base frame. It monitors the transmission performance of VC12 channel level. Sixth, BIP-8 Error Detection Principle. BIP-8 parity checks are used for error detection in segments, multiplex sections, and channels. B1 detects regenerator section errors, B2 detects multiplex section errors, and B3 detects VC4 errors. The working mechanism involves calculating the number of 1s in each column to ensure even parity. Seventh, Device Alarm Example. An example of an alarm configuration is shown, including details about B1, B2, and B3 error counts, path alarms, and other relevant metrics. This provides insight into how alarms are triggered and managed in real-time. Eighth, E1 Error Code and Circuit Alarm Detection. In the E1 channel, 8 bits form a time slot (TS), and 32 frames constitute a frame (F). TS0 and TS16 are considered overhead, while the rest are payloads. Alarms are detected through FAS, CRC, and peer indications in TS0. Switches like the Lucent 5ESS use commands to view error registers and detect circuit issues such as RFA, LFA, and AIS. This detailed explanation covers the principles, components, and maintenance aspects of SDH technology, providing a comprehensive understanding of its role in modern telecommunications.

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