Abstract : The planning principles of local transmission networks, the advantages and disadvantages of various self-healing rings and the application occasions are briefly described, and finally the issues of scheduling, management and development of the network are elaborated.

Keywords: transmission network , planning , self-healing ring , compatibility

0 Preface

With the advent of the information age, the volume of communication services grows year by year, the level of technology continues to improve, and the network scale is constantly expanding. Especially, the broadband intelligent communication network integrating voice, data, image and other comprehensive services is developing rapidly, and the requirements for the basic transmission network are also increasing. SDH technologyhas become a hotspot of telecommunication developmentwith its superiority of large transmission capacity, equipment compatibility and self-healing ability, flexible upper and lower circuits, rich testing and management overheads, which cannot be compared withPDH.

SDHequipment configuration is diverse, the network topology is complex and dynamically variable, and the network is flexible, but it also increases the difficulty of network planning. Due to the unbalanced economic development of different places, the development ofSDHnetwork is faced with different environments and needs, so in the construction ofSDHnetwork should pay great attention to network planning and design.

1 Transmission network planning principles

(a) At present, the construction of the information superhighway has become a top priority for the development of telecommunications network. As the basic backbone of the information superhighway transmission network, the principle of its construction is "high-speed, safe, flexible", and can adapt to the future needs of the development of broadband integrated services digital network services.

(b) The planning of the transmission network should not only take into account the full utilization of the original resources, but also take into account the principle of economy and adaptability to new technologies and new services. In addition, taking into account the future development of the network, the transmission network must also be highly scalable, so as to maintain the technological advantage in future development and prevent the loss of potential market share.

c) The security and reliability of the network should be fully considered in the construction of the transmission network.the self-healing function of SDH ring network is very effective in preventing service interruptions due to line equipment failures, which greatly improves the availability and reliability of the circuits and enhances the security of the network. Therefore, in the construction of transmission network, most of them will adopt ring network with high reliability. In theSDHring network planning should be fully considered in the physical direction of the fiber, the ring network between the two nodes of the fiber optic cable should be used in a different physical route, so as to truly play the role of self-healing ring. In addition, the ring network should not be too long, the number of nodes in the ring should not be too many.

Considering the average available capacity of the nodes, the damage of timing transmission, the response time of automatic protection reversal and the limitation of automatic protection reversal algorithm, the ring network should not be longer than 1,200 km, and the actual number of nodes in the self-healing ring is also limited by the service flow in the ring, and the number of nodes in the ring network should not be more than10 in general.

From the security point of view, at least two common nodes are required for the interoperability of two neighboring rings, otherwise the failure of one of the common nodes may cause the two rings to be unable to interoperate.

(d) In order to better utilizethe advantages of SDH technology, in theplanning ofSDHnetwork, to obtain the most optimized program, not only to consider the transmission of service capacity, but also to consider the scheduling, management, development of the network and the existing transmission network to make up for each other, with the problem. In the selection of equipment should also adhere to the standards, so as not to affect the unity of the entire network, advanced and holistic.

2 local transmission network planning discussions

2.1 Market Survey and Business Forecast

In network planning, market survey and business forecasting is very necessary. The prediction of telecommunication business volume is an important basic basis for determining the scale of project construction and deciding the technical level of the network, and the prediction result should be in line with the objective actual needs as well as taking into account the future development trend. In the prediction of the transmission network, according to the program-controlled telephone exchange network, PHS, DDN, frame relay and other business traffic flow to the organization of the 2 Mbit / s circuit group, according to the development of broadband services to the needs of the organization of155,100 Mbit / s circuit group. In recent years, the telecommunication industry has been developing exceptionally rapidly, and various new services and new communication methods are constantly appearing, and the operation scope of telecommunication network is becoming more and more extensive. Therefore, it is necessary to take into full consideration the development trend of the entire telecommunication network and management network in the next few years, and to make good market surveys and business forecasts.

2.2 Selection of self-healing ring

There arefivetypes of self-healing rings forSDHtransmission network: 2-fiber unidirectional channel protection ring, 2-fiber bidirectional channel protection ring, 2-fiber unidirectional multiplexing segment protection ring, 2-fiber bidirectional multiplexing segment protection ring and 4-fiber bidirectional multiplexing segment protection ring. Each of them has its own characteristics and can be applied to different network applications.

2.2.1 Comparison of Self-Healing Ring Characteristics

The characteristics of thefivetypes of self-healing rings are compared in Table1.

InTable 1 , the two-fiber bi-directional multiplexed segment shared protection ring and the four-fiber bi-directional multiplexed segment protection ring can provide protection for node failure, and its protection mechanism is equivalent to the ring network, including the failure of the node including the fiber is cut off. However, the protection only applies to the transmission of the node's services, but can not provide protection for the services in and out of the node.

2.2.2 Comparison of Advantages and Disadvantages of Self-healing Ring

(1) Two-fiber unidirectional and two-fiber bidirectional channel protection ring

(a) Advantages: Simple realization of dual transmitter-receiver selection; no use of APS switching protocol, fast switching speed, switching time is generally less than30 ms.

(b) Disadvantage: Because the time slots between nodes cannot be reused, the transmission capacity of the ring is small, and the transmission capacity of the whole ring is STM-N .

2) Two-fiber unidirectional multiplexing segment protection ring

a) Advantage: the spare fiber can carry additional services, increasing the transmission capacity of the ring; if no failure occurs, the transmission capacity of the entire ring can be up to 2 xSTM-N.

b) Disadvantage: Because of the need to use the APS reversal protocol, its reversal speed is relatively slow (its reversal speed is slower than that of the channel ring, but faster than that of the two-fiber bi-directional multiplexed segment shared protection ring).

3) Dual-fiber bi-directional multiplexed segment protection ring

a) Advantages: time slots can be reused, increasing the transmission capacity of the ring (up to k/2xSTM-N); additional services can be transmitted using the protected channel.

b) Disadvantages: the use of bridging and inverting techniques is technically complex. Because of the need to use the APS protocol, the inversion speed is slow. When the transmission path of the ring is less than1200km, the protection inversion time is less than50ms; when the transmission path of the ring network is very long, the protection inversion time may be as high as100-200ms.

4) Four-fiber bi-directional multiplexed segment protection ring

a) Advantages: time slots can be reused, increasing the transmission capacity of the ring (up to k ×STM-N); additional services can be transmitted using the protection fibers; resistant to multi-node failure.

b) Disadvantages:slow inversion speed because of the need to use the APS protocol. When the length of the ring is less than1,200 km, its protection inversion time is less than50 ms; when the length of the ring is more than1,200 km, its protection inversion time may be as high as100-200 ms. The technology is complex and the node cost is high. The four-fiber bidirectional multiplexing segment protection ring puts forward higher requirements for synchronous multiplexing equipment: first, the equipment must havefouroptical line interfaces; second, it must be a dual-system design, i.e., not only must it havethe capacity of thetwosystems, but also must have stronger cross-connection capability.

2.2.3 Applications of various self-healing rings

In the five kinds of self-healing ring, the most commonly used is the two-fiber unidirectional channel protection ring and two-fiber bidirectional multiplexing segment protection ring.

(a) The channel protection ring (unidirectional or bidirectional) adopts the method of "double transmitter and selector", does not need to use the APSreversal protocol, is simple to realize, and has fast reversal speed, and is especially suitable for the application occasions where the service volume is relatively centralized, i.e., each node has business transactions with the central node and the service volume between each other is relatively small, which is the case of the general communication between county bureaus. The general communication between county bureaus belongs to this situation.

(b) The two-fiber bidirectional multiplexing segment protection ring has the advantage of "time slots can be reused", so it is especially suitable for applications with decentralized business volume, i.e., the business traffic between each node in the ring network, especially between neighboring nodes, is more and more evenly distributed, and the general inter-bureau communication belongs to such a situation.

(c) four-fiber bi-directional multiplexing segment protection ring has the advantages of "time slots can be reused", the ring capacity is larger, and synchronous multiplexing equipment is generally used in dual-system structure design, so it is suitable for large and decentralized service volume, complex network applications.

For the access network, because it is at the border of the network, the required service volume is small, and most of the service volume is pooled in a node, therefore, the use of simpler and more economical channel protection ring (unidirectional or bidirectional).

For the inter-office communication portion, a two-fiber bi-directional multiplexing segment protection ring with a large service capacity can be used since there is a large amount of service between all nodes and the nodes generally require a large service splitting capacity. However, if the service volume is concentrated in a certain node (hub office), it is more appropriate to use the channel ring. For grid-type and neighbor-type networks, the multiplexed segment protection ring is more suitable. As to whether to use two-fiber multiplexing segment protection ring, or four-fiber multiplexing segment protection ring, depends on the capacity requirements and economic considerations of the overall comparison. When the service volume is not too large, the two-fiber multiplexing segment protection ring is more suitable; when the service volume is large, the four-fiber multiplexing segment protection ring should be used.

3 Network scheduling, management and development

3.1 CompatibilitywithPDHSystem

In the SDH network planning, to obtain the most optimal program, not only to consider the transmission of business capacity, but also to consider the network scheduling, management, development and the existing transmission network to make up for each other, with other issues. Such asin 1998 in the formation of Nanping localSDHring network phase I project, taking into accountthe coexistence ofPDHequipment andSDHequipment in a certain period of time, in theplanning ofthe localSDHring network, take into full account thecompatibility and interoperabilitywith the existingPDHsystem. Since thePDHsystem oflocal network in Nanping Citymostly adopts140 Mbit/s rate,each up and down channel node oflocalSDHring network adopts140/155 Mbit/s interface-compatibleTPU140&155tributary disks in order to realizethe interoperability betweenPDHandSDH.At the same time, each up and down channel node connects16coaxial hops of155 Mbit/s electrical interfaces toDDF, so that thescheduling ofPDHhigh subgroupscan be convenientlycarried outonDDF.In recent years, a number ofobstaclesoccurred due to the local network cable blockage caused byPDHsystem interruption, the maintenance personnel directly usetheSDH155M channel for140M PDHsystem scheduling, quickly adjusted the blocked circuits, greatly reducing the obstacle calendar time.

3.2 Integration with Data Network

With the rapid development of the Internet and the rise of broadband metropolitan area networks, voice, data, images and other business transmission needs continue to grow, the business transmission environment has changed greatly, the trend of convergence of transmission and data technology has been very obvious.

Another development direction ofcurrent SDH technology is the application in the convergence part of MAN, i.e., onthe basis ofSDHtechnology, it integrates the support function for many kinds of services (mainly Ethernet services and ATM services) to realize the convergence of MAN services, and this kind of technology isMSTPtechnology. AsMSTPcan optimize the combination of multiple technologies and provide comprehensive support for multiple services, it enables telecom operators and service providers to provide new bundled services to subscribers at the network transmission layer, switching layer and routing layer. It improves the flexibility of adding, transferring or withdrawing customers while significantly reducing the time required to turn up new services. In addition,MSTPcan simplify the edge network architecture by consolidating the type and number of different devices required at the access function layer, thereby reducing the number of network management systems required and the resources needed to install, configure and maintain the network. SinceMSTPequipment is both technologically advanced and directly oriented to the current demand for MAN construction, and more importantly, it can save construction and maintenance costs and has a good return on investment, it has received widespread attention and is a hot spot for current engineering applications.

Huawei's OptiX Metro SeriesMSTPequipmentwas used in theNanping Local SDH Ring Network Phase III Expansion Project, realizingthe transmission, switching, and aggregation of Ethernet services for the10counties (municipalities) ofNanping's local network, and satisfying the applications of private line Internet access and enterprise private networks.

The following issues must be noted whenapplying MSTP technology:

(a)The relationship between the application mode of MSTP technology and the business network must be clarified;MSTPtechnology is a product of the development under the environment of the current high-speed growth of data services, and it is constantly developing towards the trend of business-oriented development, and this development trend will inevitably form a part of the overlap with the data network in terms of positioning. Therefore,whenintroducingMSTP, the relationship between the application mode ofMSTPtechnology and the service network should beclarified, and theintegrated planningwith the metropolitan IP network and localSDHnetworkshould be fully considered.In network planning, self-analysis and self-positioning should be carried out according to the existing network structure, current business needs and future business orientation, so as to give full play tothe business functions of theMSTPplatformthrough close integration with the data business networkand optimize the overall network performance and total cost.

(b) Do a good jobof selecting MSTP equipment. At present, most of the vendors ofMSTPproducts for data services have different support capabilities, some can only realize the transparent transmission of data services, while some have Layer 2 switching capabilities, some only support Ethernet services, while some support Ethernet,RPRandSDH/" target="_blank">ATMat the same time. Due to the different realization methods of Ethernet mapping and bandwidth management, the equipment of different manufacturers cannot be interconnected at present. Therefore, when selecting the equipment, we should give full consideration tothe wayMSTPproducts support different high-level services and the interconnection with the existing network, and testwhether theMSTPproducts are suitable for the operator's application environment. Through in-depth discussion of technologies and products, and balancing the relationship between the advancement and maturity of technologies and products, only then can the most idealcombination ofMSTPproducts and technologiesbe selectedto realize a low-cost but competitive MAN in a comprehensive sense.

3.3 About Microwave Communication

It is well known that telecommunication networks with only a single means of transmission are often vulnerable. Although fiber optic transmission networks have advantages in capacity that microwave networks cannot match, microwave systems are still an indispensable complement to and protection means for fiber optic networks, whether on communication trunk lines or branch lines. Experience over the years has shown that in the case of natural disasters, wireless communication is always the first way to restore telecommunication services. Therefore, while vigorously developing the fiber optic transmission network, attention should also be paid to the construction and maintenance of digital microwave network. Especially in areas prone to natural disasters, microwave communication systems should be developed appropriately to form a three-dimensional communication network on the ground (optical fiber) and in the air (microwave). Only by adopting mutual protection between multiple transmission media can the reliability of local network circuits be improved.

In short, the local transmission network should be built into a three-dimensional intersection, safe and reliable, adapted to the needs of business development of the basic backbone network. To re-examine the microwave trunk line, fiber optic trunk line network structure, to strengthen the fiber optic cable, microwave and other means of transmission of different means of comprehensive utilization, at the same time should ensure that the network of multi-channel and multi-route, to establish a "permanent anti-disaster project" concept, and further planning and construction of good communications network.

4 Conclusion

The focus of future competition is new services, high quality. Who has a high level of network, who has obtained the market pass. The battle for the future is won in the network, and the battle for the future is won today. How to build, operate and optimize this network, there are many strategic issues in front of us.