Overview of SDH and Time Gap

I:Technical Background of SDH Generation

What is the technical background of SDH generation?

We know that today's society is the information society, the highly developed information society requires the communication network can provide a variety of telecommunication services, the amount of information transmitted, exchanged, and processed through the communication network will continue to increase, which requires the modern communication network to digitalization, synthesis, intelligence and personalization direction.

Transmission system is an important part of the communication network, and the good or bad transmission system directly restricts the development of the communication network. Currently the world's countries vigorously develop the information superhighway, one of the focus is the formation of large-capacity transmission fiber optic network, constantly improve the transmission line signal rate, broaden the transmission band, as if a constantly expanding to accommodate a large number of traffic flow of the highway. At the same time, users hope that the transmission network can have a worldwide interface standard, can realize our global village in every user can communicate conveniently at any time and anywhere.

At present, the traditionaltransmission network formedby the PDH transmission system, due to its multiplexing approach is obviously not able to meet the requirements of the signal transmission of large-capacity, in addition tothe regional specifications ofthe PDHsystem also makes the network interconnections increase the difficulty of the network, which shows that in the communication network to the large-capacity, standardization of the development of the present day,the PDHtransmission system has increasingly become the bottleneck of the modern communication network, restricting the transmission network to a higher rate of development. development.

Second: SDH concepts and features

1 : What isSDH?

SDHiscalled synchronous digital transmission system, which shows thatSDHis a transmission system ( protocol) , just like PDH.

(Protocol), like the PDH quasi-synchronous digital transmission system,SDHthis transmission system regulates the frame structure of digital signals, multiplexing, transmission rate level, interface code type and other characteristics.

The core of the SDH concept is the formation of a digital communication network from the height of a unified national telecommunication network and international interoperability, which isan important part of theIntegrated Services Digital Network (ISDN), especially the Broadband Integrated Services Digital Network (B-ISDN).

2 :Main features and advantages ofSDH

1) Electrical interface

The standardization of the interface is the key to determine whether the equipment of different manufacturers can be interconnected. SDH systemhas made a unified specificationfor the network node interface (NNI). The content of the specification has digital signal rate level, frame structure, reconnection method, line interface, monitoring and management. So this makesSDHequipment easy to realize multi-vendor interconnection, that is to say, in the same transmission line can be installed on different manufacturers of equipment, reflecting the horizontal compatibility.

2) Multiplexing

Since low-speed SDH signals are multiplexed intothe frame structure ofhigh-speedSDHsignalsin a byte-by-byte interpolation manner, theposition oflow-speedSDHsignals inthe frame ofhigh-speedSDHsignals is fixed and regular, i.e., predictable, which makes it possible todivide/interpolate low-speedSDHsignalsdirectlyfrom high-speedSDHsignals such as2.5Gbit/s(STM-16),for example.155 Mbit/s(STM-1), which simplifies the multiplexing and taping of signals and makestheSDHregime particularly suitable for high-speed, high-capacity fiber-optic communication systems.

In addition, due to the synchronous multiplexing and the flexible mapping structure, the PDH low-speed tributary signals (e.g.2 Mbit/s)can bemultiplexed intothe frame of theSDHsignal (STM-N) so that the low-speed tributary signalsare also predictableinthe STM-Nframe, and thus thelow-speed tributary signalscanbesplit/inserteddirectlyfromthe STM-Nsignal.Note that this is not the same as thedirect splitting/insertion of low-speedSDHsignalsfrom high-speedSDHsignalsas described above, but refers to thedirectsplitting/insertion of low-speed tributary signals, e.g.,low-speed signals such as2 Mbit/s, 34 Mbit/s, and140 Mbit/s,fromthe SDHsignals.Thus, it saves a large number ofreconnection/tap equipment(back-to-back equipment),increases reliability,reduces signal damage,equipment cost,power consumption,complexity, etc., andmakes the service up anddown more simple.

This multiplexing of SDH makes the digital cross-connect (DXC) function easier to realize, so that the network has a strong self-healing function, which facilitates the user's on-demand dynamic networking, real-time flexible service deployment.

***What is network self-healing function?

Network self-healing means that when the service channel is damaged and causes service interruption, the network will automatically switch the service to the alternate service channel, so that the service can beresumed in a shorter period of time ( ITU-T stipulates that it isless than50ms) to resume normal transmission. Note that this only means that the service can be restored, while the faulty equipment and the faulty channel still need to be repaired by human beings.

3) Operation and Maintenance

The frame structure of SDH signals is rich inoverhead bytesfor operation and maintenance (OAM) functions, which greatly strengthens the monitoring function of the network, i.e., the degree of automation of maintenance is greatly enhanced, whilethe overhead bytes ofPDHsignals are not so many that when monitoring the performance of the line, it is still necessary to add redundancy bits in the line coding, and thisisaccomplished.structure has onlybits in theTS0time slot andTS16time slot that are used forOAMfunctions.

The rich overhead of SDH signaling occupies1/20of all the bits in the whole frame, which greatly enhancesthe OAMfunction. This makes the maintenance cost of the system is greatly reduced, and in the comprehensive cost of communication equipment, maintenance costs account for a considerable part of the comprehensive cost ofthe SDHsystem islowerthan thecomprehensive cost of thePDHsystem, it is estimated that only65.8%of thePDHsystem.

4) Compatibility

SDH has a strong compatibility, which means that when the formation ofSDHtransmission network, the originalPDHtransmission network will not be invalidated, the two transmission networks can coexist. In other words,SDHnetworkcan be usedto transmitPDHservices, in addition, asynchronous transfer mode signals (SDH/" target="_blank">ATM),FDDIsignals, and other systems of signals can also be usedto transmit SDH network. Sohow does theSDHtransmission network realize this compatibility?The basic transmission module (STM-1)forSDHsignalsin the SDH networkcan accommodatethe three digital signal series ofPDHand other digital signal series of various regimes- SDH/" target="_blank">ATM ,FDDI,DQDB, etc., which embodiesthe forward and backward compatibility ofSDH, and thus reflectsthe forward and backward compatibility ofSDH.SDHis anetwork of digital signals, SDH/" target="_blank">ATM, FDDI, DQDB and so on , thus reflecting the forward and backward compatibility of SDH, and ensuring thesmooth transition fromPDHnetwork toSDHnetwork and fromSDHtoSDH/" target="_blank">ATM.It is very simple,SDHmultiplexesthe low-speed signals of various systemsintothe frame structure ofSTM-1signalsat the network boundary (e.g.,the starting point ofSDH/PDH), and then splits them out at the network boundary (the end point), so thatdigital signals of various systemscanbe transmitted on theSDHtransmission network.

III:Defects of SDH

1. Low bandwidth utilization

2. Complicated mechanism of pointer adjustment

3. the impact of the extensive use of software on system security

IV:The basic network topology of SDH

SDH network isinterconnectedby theSDHnetwork element equipment through the fiber optic cable, network nodes(network elements)and the geometric arrangement of the transmission line constitutes the topology of the network,the effectiveness of the network(channel utilization), reliability and economy to a large extent related to its topology.

The basic structure of the network topology are chain, star, tree, ring and mesh.

Chained Networks - This network topology connects all the nodes in the network one by one in series, with the ends open. This topology is characterized as more economical and was used more in the early days ofSDHnetworks, mainly in private networks (e.g., railroad networks).

Star network --- this network topology is a network element in the network as a special node connected to the other network element nodes, the other network element nodes are not connected to each other, the network element nodes of the service should be transferred through this special node. This network topology is characterized by a special node to unify the management of other network nodes, which is conducive to the distribution of bandwidth and cost savings, but there are potential bottlenecks in the security and processing capacity of the special node. The role of the special node is similar to that of the switching network's convergence bureau, and this topology is mostly used in the local network (access network and subscriber network).

Tree network - This network topology can be viewed as a combination of a chain topology and a star topology, but there are also potential bottlenecks in the security and processing power of special nodes.

Ring Network - A ring topology is actually a form of network topology in which the chain topology is connected at the head and the tail so that no single network element node on the network is open to the public. This is currently the most used network topology form, mainly because it is highly survivable, i.e., self-healing. Ring networks are commonly used in local networks (access networks and user networks), inter-office relay networks.

Mesh shaped network - A mesh shaped network topology is formed by connecting all network element nodes two by two. This network topology provides multiple transmission routes between two network element nodes, making the network more reliable and free of bottlenecks and failures. However, due to the high redundancy of the system, the effectiveness of the system will be reduced, and the cost and complexity of the structure will be high.

Mesh shaped networks are mainly used in long distance networks to provide high reliability of the network.

The most used network topologies are chain and ring, which can be flexibly combined to form more complex networks.

The topology diagram is as follows:

V:PCM30/32-way system

PCM30/32-way system frame structure

PCM30/32-way system (n=32,30represents the number of channels) is called primary group or base group, each frame consists of32time slots, each time slot corresponds to1sample value, and1sample value is coded with8-bit code.

The role of each time slot:

( 1 )30voice path time slots:TS1toTS15,TS17toTS31

* TS1 toTS15transmit the1stto15th(CH1toCH15) voice signals, respectively.

* TS17 toTS31transmit the16thto30th(CH16toCH30) voice signalsrespectively

( 2 ) Frame synchronization time slot:TS0is used for frame synchronization.

Even frame TS0 transmits frame synchronization code (0011011), the receiving end receives the frame synchronization code and finds a way to achieve frame synchronization (for details, see Knowledge PointsPCM30/32-channel system synchronization implementation). Odd-frameTS0transmits a detection alarm code, which works with the frame synchronization code to reliably achieve frame synchronization.

( 3 ) Signaling and frame synchronization time slot:TS16

* The first4bits ofTS16of F0 frametransmit the compound frame synchronization code (0000), and the last4bits transmit the detection alarm code;

l The first4 bits ofthe TS16of the F1 frametransmit the1stchannel signaling code, and the last4bits transmit the16thchannel signaling code;

l The first4bits ofTS16of F2 frametransmitthe 2ndchannel signaling code, and the last4bits transmit the17thchannel signaling code;

* ...

l The first4bitsof TS16of F15 frametransmitthe 15thchannel signaling code, and the last4bits transmit the30thchannel signaling code;

The realization of compound frame synchronization: the realization method of compound frame synchronization is similar to the realization method of frame synchronization, the receiving end receives the compound frame synchronization code, and finds a way to do the compound frame synchronization