SDH Synchronous Digital series (sorting)

From: Communication encyclopedia

This technology is also a kind of optical fiber transmission system, which uses synchronous transmission module (STM-1, 155 Mbit/s) as the basic concept, its module consists of the Net Load of information (payload), segment overhead (SOH), Management Unit pointer (Au) composition, its outstanding characteristic is the use of virtual container mode compatible with various PDH systems. SDH transmission network has the advantages of intelligent routing configuration, convenient upstream and downstream circuits, strong maintenance and management capabilities, and unified optical interface standards.
 

Directory

· SDH Definition
· SDH background
· Advantages of SDH
· SDH devices and their networking principles
· SDH Network Management
· SDH network synchronization
· Future development and prospects of SDH technology

SDH Definition

Synchronous Digital Hierarchy-synchronous digital Series
It is a set of standardized digital signal structure levels that can be used for synchronous information transmission, multiplexing, plug-in and cross-connection. synchronous signal transmission is performed on transmission media (such as optical fiber and microwave.

It is a new digital transmission system. It will be called a revolution in the Telecommunications transmission system.

Information Highway has recently become a hot topic. By the 21st century, people can complete various daily activities at home with the help of the information highway. The road, the most basic unit of the information highway, will be composed of SHD devices.
-- SDH (synchronosdigitalhierarchy) is a new digital transmission system. It will be called a revolution in the Telecommunications transmission system.
-- We can make an analogy between the information highway and the expressway currently used in transportation: the highway will be an SDH Transmission System (mainly using optical fiber as the transmission media, microwave and satellite can also be used to transmit SDH signals. overpasses will be the upper-and lower-talk multiplexing (ADM) in the SDH series of Large ATM switches, which are some small overpasses or intersections, the "car" running on the "SDH highway" will be a variety of telecommunication services (voice, images, data, etc ).SDH background

The emergence of SDH technology has its inevitability. With the development of communication, the information required to be transmitted is not only voice, but also text, Data, images and videos. In addition, with the development of digital communication and computer technology, the T1 (ds1)/E1 Carrier System (1.544/2.048 Mbps) emerged in) multiple network technologies, such as X.25 frame relay, ISDN (integrated business Digital Network), and FDDI (Fiber Distributed Data Interface. With the advent of the information society, people hope that the modern information transmission network can quickly, economically and effectively provide various circuits and services. The aforementioned network technology is subject to the monotonicity and complexity of its services, the limitations of bandwidth cannot be modified or improved only within the original framework. SDH is developed in this context. Among various broadband optical fiber access network technologies, SDH-based access network systems are the most widely used. The emergence of SDH solves the problem that the bandwidth limit of the household media cannot keep up with the development of backbone networks and users' business needs, and leads to the access bottleneck between users and core networks, it also improves the utilization of a large amount of bandwidth on the transmission network. SDH has been a mature and standard technology since it was introduced in 1990s. It is widely used in backbone networks and its price is getting lower and lower, the Application of SDH technology in the access network can bring great bandwidth advantages and technical advantages of SDH technology in the core network into the access network field, taking full advantage of SDH synchronous multiplexing, standardized optical interfaces, powerful network management capabilities, flexible network topology capabilities, and high reliability brings long-term benefits in the development of access networks.

Advantages of SDH

Compared with the traditional PDH technology, SDH has the following advantages:
1. Unified bit rate:
In PDH, there are three speed levels in Europe, North America and Japan. SDH implements a unified bit rate. In addition, unified optical interface standards are defined, which makes it possible for devices from different manufacturers to connect to each other.
2. Strong network management capabilities:
The SDH frame structure specifies a wide range of Network Management bytes, which can meet various requirements.
3. Self-healing protection ring:
SDH devices can also form a ring network with self-healing protection capabilities. This effectively prevents the disconnection of transmission media and the termination of all communication services.
4. The Byte reconnection Technology Used in SDH technology:
If we compare the main difference between SDH technology and pDH technology with railway transportation, PDH technology is like a bulk train, and various goods (businesses) are piled up in the carriage, if you want to remove a specific package of goods (a certain Transmission Service) from a certain station, you need to first unload all the goods on the car and find the goods you need, then, the remaining goods and the new loaded goods on the station will be piled up on the vehicle one by one and transported away. Therefore, PDH technology must be equipped with a large number of reconnecting devices in various groups wherever the uplink and downlink circuits are needed. SDH technology is like a container train. All kinds of goods (businesses) are labeled (overhead) and loaded into containers. Then, the small boxes are loaded into large boxes, and the first level is set. Through labels at all levels, a certain package of goods can be accurately removed on a high-speed train, the whole train does not need to be "rummaging down the cabinet" (the label can be used to accurately know the number of trains and the number of containers in a certain package). Therefore, only in SDH, in order to achieve a simple up and down circuit.
Therefore, it is certain that the information highway to be implemented will basically be composed of SDH devices, and only the branches and cross roads connected to SDH will retain some PDH devices.
According to statistics, there are currently 17 telecommunications manufacturers in the world master SDH technology. With the launch of ATM-1/STM-4-level SDH Equipment developed by China Post and Telecommunications Industry Corporation and its four factories in cooperation with the Fifth Research Institute of the Ministry of Posts and Telecommunications, the company has become one of the 18th companies in the world that can provide SDH devices.

SDH devices and their networking principles

Various transmission devices developed based on the SDH system can fundamentally solve the capacity, quality, network management, and security problems faced by the network.
Due to the diversity of SDH devices, flexible circuit scheduling and management, and strong network management capabilities, we have made more choices in the network organization.
From the perspective of the entire network, we must reasonably establish the network and give full play to the advantages of SDH to ensure the uniformity and
Integrity and advancement.
SDH devices can be divided into terminal multiplexing TM, regenerative repeater Reg, plug-in Multiplexing ADM and digital crossover equipment dxc according to their types,
During networking, pay attention to the proper configuration of various interfaces of devices and the proper use of devices in the network.
Among the four devices mentioned above, ADM is an important device that represents SDH characteristics. ADM can be used to form a link and is suitable,
It can also be used in the access network in the environment where the circuit is required. If the TM at both ends of the link is changed to ADM and the ring is connected at the beginning and end
SDH Self-healing Ring with automatic protection and switching. This method is suitable for use in the primary network and has been developed for secondary backbone networks in recent years. With SDH
With the rapid development of technology, most ADM devices now have the branch-group-road, group-road, branch-branch connection capabilities, and the uplink and downlink circuits are equivalent
Flexible. In terms of functions, it is equivalent to a small dxc.
The network structure of the Self-healing ring can be divided into the following four types: one-way channel switching ring (1 + 1), two-way channel switching ring (), two-fiber Bidirectional
Reusable section public protection ring and four-fiber Bidirectional multiplexing section public protection ring.
An important indicator to measure the performance of a self-healing network is the duration of protection/recovery. Many important services can only tolerate extremely short Business Protection/restoration.
Recovery time, which is about 50 ms. In this regard, it takes several minutes to build a self-healing Network Based on dxc routing, while the self-healing Ring is generally better and reachable.
To (50-200) MS, with the continuous improvement of SDH technology, the protection/recovery time of the Self-healing Ring will be shortened to 50 ms.
Dxc is a device that connects all or part of the time slots of a port to any port. Common dxc methods include dxc4/4 and
Dxc4/1: the lowest rate of cross-connection is vc4 and vc12, respectively. The port types include 2 Mbit/s, 34 Mbit/s, 140 Mbit/s,
155 Mbit/s. The former is mainly used for backbone network nodes, and the latter is mainly used for backbone networks. Dxc device and corresponding Network Management System Configuration
When a fault occurs in the network, you can find a preset alternative route in a short time to restore the interrupted service.
One thing to mention here is that in addition to business volume classification and improved fill levels within the STM-N, a single dxc device is not available to play other
Benefits: Each dxc can be used in a circuit group environment with at least three directions. dxc applications are characterized by networking.

SDH Network Management

Improving service quality and reducing maintenance costs have always been the goal of the telecom operation department. The Telecommunications Department is flexible in operating new inbound Devices
And equipment reliability and maintenance automation. SDH Transmission Network is a complete telecommunications infrastructure network
The SDH management system is of profound significance to the service quality and maintenance costs of the entire network. Compared with the previous PDH transmission system, SDH technology is
A considerable number of overhead bytes are arranged for the OAM & P of the network. At present, because the ITU-T in the Network Element Level Management Standards are relatively perfect, such
G.784, g.774 series, q.811/q.812 and other suggestions, and the management standards at the network level, especially the information model, are still being improved.
Although the content of SDH network management system is quite rich, the management functions of SDH management system can still be carried out using the five management functions of TMN.
Description: fault management, performance management, configuration management, security management, and accounting management.
Due to the different products of different SDH Equipment Manufacturers, there is a problem of interoperability between different SDH management systems in terms of network management, that is, Management
SDH network resource modeling, and has a common external protocol transmission management information structure. Like TMN, the SDH management system
The objective-oriented (object) and customer/Server methods in ISO/OSI management are implemented, and the physical resources and logic of SDH Transmission Network are expressed in management object abstraction.
Resources.
In addition, the SDH Transmission Network is highly dependent on network management. Therefore, you must note that
Normalization of intended operations; During Software Version Upgrade, always pay attention to the running status of the network and prepare for handling emergencies.

SDH network synchronization

SDH network synchronization has the following features:
(1) The transmission equipment between SDH switching nodes should work simultaneously.
(2) SDH devices are sensitive to short-term clock instability.
(3) The SDH device clock must have three working modes: synchronous tracking mode, persistence mode, and free fluctuation mode.
(4) The 2 Mbit/S Signal carried by SDH transmission network should not be used as a synchronization reference timing signal due to its Pointer Adjustment jitter.
(5) the dynamic topology applications of dxc and ADM in SDH technology may lead to timing loops. Note that no matter which external reference is tracked
Source, no scheduled loop can appear in any situation. If the timing loop cannot be eliminated, it is better to identify only one external reference source.
Special attention should be paid to the networking.

Future Development and Prospects of SDH technology

1. SDH network management software development
SDH is a complex system and network controlled by software. It draws a lot from the latest research results of computer science.
Software design methods, UNIX operating systems, and the latest relational database structure. A well-considered and advanced flexible network management system is SDH
The key to the success or failure of the network technology. Therefore, after the hardware system is developed successfully, a lot of subsequent work will be focused on software development. Due to SDH technology
In the development stage, the ITU-T about SDH network level management is still in the perfect process; in the aspect of the horizontal compatibility of the network management system
The capability of multiple manufacturers is still in the research and development stage, and the software version must be upgraded in concert with the manufacturer to improve the performance on a daily basis.
2. Development of ultra-high speed Optical Fiber Transmission Technology
Due to the bottleneck effect of high-speed electronic circuits and optoelectronic devices, the transmission rate of the traditional TDM optical fiber communication system has reached
It is very difficult to develop the network at 2.5 Gb/s, and the development of broadband services puts forward higher requirements for the transmission network. Therefore, the latest OTDM is adopted.
(Optical time division multiplexing) and DWDM (Dense Wavelength Division Multiplexing) technologies will be imperative (dense refers to shorter wavelength intervals than ordinary wavelength division multiplexing, at 0.2nm
Or its integer multiple is the wavelength interval ). Some manufacturers are now able to provide commercial DWDM products, so we can consider the maturity and
When the price is reasonable, DWDM technology is used between some specialized exchange bureaus with large business volumes.
3. SDH application transmission media Extension
In most cases, the transmission network is dominated by optical fiber and supplemented by wireless. Microwave is an important channel for wireless communication.
Information means. The SDH microwave transmission system is compatible with the existing PDH microwave system by using the original channel interval of pdh140mb/ s system, that is, 30 MHz and
40 MHz, but the bit rate to be transferred is higher. Currently, the commercial systems are at a rate of 155 Mbit/s and 2 × 155 Mbit/s. We are studying the 622 Mbit/s series.
In addition to microwave, satellite communication will also transition to SDH in the future, so it is suggested to install dxc function on the satellite. In the future, satellite exchange may also be realized.
And processing on the star.
4. SDH application in Broadband Access Technology
SDH as a B-ISDN transmission technology must be applied to access networks, for example, FTTC based on optical fiber or
It can also be used in metal wire-based vhdsl or bdsl to transmit 51 Mbit/s or 155 Mbit/s/ssdh signals within a short distance. Future exchange
Devices will be able to provide optical trunk lines based on SDH standards; vswitch user lines will develop towards V5, and SDH-based v5.3 connections will be available in the future
In this way, the SDH signal can be directly sent from the switch to the user/Network Interface (UNI)