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As of the end of 2001, the data traffic in China's backbone optical transmission network has exceeded the voice traffic, reaching more than 50% of the total traffic. The characteristics of data services are the connection from any point to any point, and the dynamic change of business flow and direction. In order to adapt to the new characteristics of the data service of the transport network, the optical transport network has begun to develop toward an intelligent optical network that supports mesh networks and dynamic and flexible assignment of bandwidth. The emergence and rapid development of AutomaTIc Switched OpTIcal Network technology reflect this new trend in the development of optical networks.
ASON is a new-generation optical network that realizes a series of functions such as connection / disconnection, switching, and transmission in the optical transmission network controlled by signaling. ASON provides an architecture that quickly meets user needs, which can effectively solve the problems of network scalability, manageability, and how to quickly configure user bandwidth and provide end-to-end protection for user bandwidth. The concept of ASON represents the development direction of the next-generation optical network. It will completely change the network system, development model and service configuration mode of today's telecommunications transmission network, and make the optical transmission network intelligent.
2. Standardization progress
Since the concept of intelligent optical network was proposed in 1998, ITU-T formally confirmed the standardization of ASON in 2000. ASON's related technical research and standard work have made great progress. The major international standards organizations involved in ASON standardization work include ITU (ITU-T), Internet Engineering Task Force (IETF), and Optical Interconnection Forum (OIF).
(1) ITU-T puts forward the ASON architecture and overall requirements, as well as a series of recommendations for signaling, routing, and auto-discovery. Among them, standards related to architecture, signaling protocol, and routing structure have been passed. At present, ITU-T is further developing standardization work on routing protocols, automatic discovery, and ASON network management. According to the development progress of ITU-TSG15, it is expected that most of ASON's recommendations will be completed in 2005.
(2) ETF has proposed a general multi-protocol label switching (GMPLS) architecture for optical network control, including signaling, routing, link management and other specifications. Among them, the three draft GMPLS standards related to signaling have become official RFC. The IETF is currently discussing draft standards related to link management protocols LMP, network protection recovery, inter-domain routing protocols, etc. Most of the IETF GMPLS specifications are expected to be completed within this year.
(3) Based on the success of UNI1.0, OIF further developed the UNI2.0 draft, and added many new functions according to the needs of operators, including support for calling, dual homing, modified bandwidth Ethernet services, and extended security. . In January 2004, the ENNll.0 signaling specification was formally approved. An important work of OIF in 2004 is to carry out the interoperability test of UNI2.O and ENNI, with the main operators of the world as the main body, to carry out Ethernet adaptation (GFP). Experiments in VCAT / LCAS, UNIL.0R2, ENNI signaling and routing. Figure 1 shows the standardization progress of the control planes of various standardization organizations. 
Figure 1 Work progress of various standardization organizations
3. The situation of ASON equipment and its application at home and abroad
In recent years, optical network equipment manufacturers have invested heavily in the research and development of ASON equipment. The key technologies related to ASON, especially the control plane technology, have been breakthrough. ASON equipment has gradually moved from the laboratory stage to the practical stage.
In terms of transmission plane, the relatively mature high-capacity SDH cross-connect equipment based on electrical cross technology is currently adopted by most manufacturers. The cross-capacity can generally reach 320G or more and has the ability to smoothly expand to T-bit capacity. The cross particles can support VC-4 and VC-4-nc, and some manufacturers claim that they will further support the switching particles of wavelength, sub-band and fiber in the future. In addition to supporting traditional ring network protection and path protection, networking capabilities can also support mesh network recovery. In terms of control plane, the optical network is set up by the manufacturer based on the OIF UNIl.0 and IETF GMPLS specifications, and has developed an ASON control plane with distributed intelligent functions, which realizes automatic discovery of network topology, switched connections, and soft permanent Functions such as connection control, distributed routing, mesh network protection and recovery, support in-band or out-of-band signaling networks, multiple service level SLAs, and can provide OIF UNI 1.0 standard interfaces. However, because the relevant standards have not yet been completed, the current ASON products have only achieved some ASON functions, and the ASON products need to be further improved. The specific problems are as follows:
(1) The manufacturer supports fewer functions such as connection admission control and E-NNI interface for call and connection separation.
(2) ASON networks of different manufacturers cannot yet be interconnected through the E-NNI interface.
(3) There are few manufacturers with actual application of ASON network.
(4) The distributed network protection and recovery based on the control plane, the performance on the large-scale network still needs further examination by the test network.
In terms of ASON product application, some major foreign operators and equipment manufacturers started earlier. AT & T has pioneered the deployment of intelligent optical networks connecting about 100 cities across the United States; operators such as Vodafone, Telefónica, and Network have also applied ASON in trunk networks. In the past two years, China's major telecom operators have paid great attention to ASON technology. In 2003, China Railcom completed the provincial intelligent optical network test in Jilin. Beijing Communications adopted the intelligent optical network technology for the first time in the domestic metropolitan area network. China Telecom and China Unicom also actively carried out relevant technical research and economic analysis, and Started the planning and construction of the ASON trial network.
4. Strategies and problems of ASON network application and evolution
After several years of continuous development of ASON technology and active exploration of domestic and foreign operators in ASON networking, ASON technology has gradually become practical, and it is expected that 2004-2005 will become a watershed for ASON network applications. Discuss the strategies and problems of ASON network application and evolution from the following aspects.
1. Step by step evolution to ASON network
ASON technology is a huge change to the existing optical transmission network network structure, service provision, operation and maintenance system. At the same time, the urgent needs of the market and the imperfection of standards are the biggest problems operators face when deploying ASON networks. Therefore, the evolution of the optical transmission network to ASON must be carried out step by step, adopting a strategy of unified planning and step-by-step implementation.
From the perspective of the needs of domestic operators, it is possible to introduce ASON first in the backbone transmission network or metropolitan transmission network. The backbone transport network has an urgent need to evolve to a mesh network. The backbone network has high requirements for network resource optimization and requires rich protection and restoration mechanisms. Therefore, long-distance nodes can first use large-capacity cross-connect equipment based on OEO switching technology, and introduce ASON signaling, routing protocols, and NNI interfaces to realize the function of ASON. However, the introduction of ASON in the backbone network has a greater impact on services and the overall structure of the network, and operators are more cautious about this. Metropolitan area transport network has a large amount of traffic and has the characteristics of high requirements for dynamic service scheduling. In addition, the introduction of ASON in the metropolitan area network has less impact on the entire network. Therefore, some domestic operators consider firstly at the core level of the metropolitan area transport network Introduce ASON.
Because the current ASON interface standard is not yet mature, it is difficult for different manufacturers to achieve interconnection and intercommunication, so the existing ASON equipment cannot form a standard ASON network, which is generally called an intelligent optical network. The smart optical network deployed at this stage will inevitably use equipment from a single manufacturer, and only requires a single control domain networking application. In this networking mode, I-NNI interfaces are used to interconnect ASON nodes, and a standard UNI interface is used to connect to the customer network. Since the l-NNI interface belongs to the internal interface of the network and does not involve interconnection and interworking, it allows manufacturers to choose their own control protocols, and does not require standardization of protocols and signaling mechanisms.
With the gradual improvement and maturity of ASON standardization, by upgrading the software of ASON node equipment so that it has an E-NNI interface that conforms to the standard, it can realize the intercommunication with ASON equipment of other manufacturers, forming a layered control within a single operator The domain structure, and then interoperability between ASON networks of different operators, has evolved into a standard ASON network. To ensure that the intelligent optical network will be upgraded to the standard ASON network in the future, the following requirements should be considered when selecting equipment:
(1) Support or future support standard UNI, E-NNI interface.
(2) Intelligent functions and signaling, routing and link management protocols must comply with ITU-T / OIF / IETFASON related standards.
(3) Select widely used signaling and routing protocols to facilitate future interworking with other devices.
(4) The device must have a good smooth upgrade capability, and should be able to become a standard ASON network node only through software upgrade without replacing hardware.
2. Technology selection and networking
(1) Control mode selection. The ASON architecture supports flexibility in implementation, allowing centralized control, distributed control, or a mixture of the two. The control function of distributed control is realized by the control elements distributed in each network element. Completing any control function usually requires the interaction and coordination of the control elements of different network elements. In the centralized control mode, the intelligent control of the network is usually undertaken by a centralized network management system.
Because ASON needs to support a large number of dynamic connections, the use of distributed control can provide higher execution efficiency and reliability, and minimize the impact of local faults on the entire network. In the distributed control mode, each network element node has a complete signaling connection function, which makes network element organization and equipment from different manufacturers jointly realize simple networking, which is more in line with the characteristics of mesh networks.
The network performance of the centralized control mode is limited by the processing capacity of the management system, and the connection establishment and protection recovery time is relatively long. Network control is highly dependent on the management system, and the failure of the management system will affect the control of the entire network. However, since the management system grasps the dynamic information of the resources and status of the entire network, although the work of route optimization and restoration pre-calculation can be done independently by the control plane, under the guidance of the management system, the optimal network-wide range can be obtained. solution. Therefore, the distributed control method should be adopted for the newly built ASON network, which can be properly combined with the network optimization function of the management system to achieve optimized network intelligence.
(2) The organization of the control domain. According to the technical level of existing ASON products, intelligent optical network networking should use a single manufacturer's equipment, a single control domain networking, that is, all ASON equipment is in a control domain. In order to meet the scalability of the network, several routing areas can be divided as needed in the control domain, and I-NNI signaling interfaces are used to interconnect the devices. In the future network expansion, two methods can be adopted, namely, expansion within the original control domain, but the equipment of the same manufacturer must be used; a new control domain is formed, and the interconnection between the control domains is performed through the E-NNI signaling interface. This method allows the selection of equipment from different manufacturers.
The ASON network should provide a standard UNI interface to the customer network. Since there are relatively few customer-level network devices supporting UNI interface, ASON network should be required to provide UNI agent function. Customers can apply for the establishment, deletion and query of dynamic connection to DXC network through the terminal of UNI agent.
(3) Choice of network protection recovery method. Compared with the SDH linear and ring network protection mechanism, the dynamic connection recovery mechanism of the mesh network has a higher utilization rate of network resources. However, its recovery time is relatively long, generally in the order of hundreds of milliseconds to seconds. Since the transport network still carries a large number of voice services, protection switching time within 50ms is required. Therefore, the ASON network should adopt the traditional SDH protection and MESH network to restore the combined survivability mechanism.
· For voice and other services that are sensitive to service damage time, the traditional SNCP protection, multiplex section or ring network protection mechanisms are still used to ensure 50ms service switching time. At the same time, it can be combined with ASON's mesh recovery capability to improve the survivability of voice services under multiple failures.
· For IP services, data leased line services, etc., mesh sharing protection and dynamic recovery can be used. Take advantage of ASON's rich business-level service (SLA) functions to adopt different protection and recovery strategies and differentiated services for various businesses.
3. Compatible with existing networks
A large number of SDH ring networks and WDM systems have been built in existing networks. These networks do not have intelligence, which makes the end-to-end connection dynamic control of the network impossible to achieve, and becomes the bottleneck of the entire network intelligence. Therefore, the problem of how ASON is compatible with existing networks must be solved. For compatibility with old optical network equipment, there are the following two upgrade schemes:
(1) Add an intelligent control unit to each device. This solution requires complicated upgrades to the software and hardware of each device, which is technically difficult and uneconomical.
(2) Upgrade the management system so that it becomes an intelligent centralized control plane, and the original transmission network becomes a centralized control domain in the ASON network. The management system uses centralized control to realize the automatic establishment of connections in the control domain. The private control protocol is used inside the control domain, and external standard signaling interfaces (UNI and E-NNI) are added to the management system to achieve cooperation with other control domains, thereby finally achieving automatic exchange within the entire network.
The management system is upgraded to centralized control, taking into account the urgent needs of intelligent network construction and the current status of traditional network technology, which can achieve compatibility and smooth evolution of the old transmission network and ASON network.
V. Conclusion
I believe that with the acceleration of ASON standardization and the further maturity of ASON equipment, ASON technology is about to enter the practical stage. The evolution of the existing optical network to the ASON network is the development trend of the optical transmission network. The deployment of ASON network will profoundly change the system and functions of the optical transmission network. ASON's new network system will bring new business growth points to network operators and service providers, and will create huge market opportunities.
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ASON network application and progress
I. Introduction