Technical Analysis of optical transmission network protection and recovery

Network survivability refers to the ability of the network to maintain acceptable service quality levels in the case of various faults. It is a key factor in the design and operation of modern network planning and is also an important part of network integrity. The survivability of optical networks is based on shared resources and dynamic resource recovery. Among the various survivability technologies of optical networks, the survivability Technology of optical layers is fast and flexible, which can effectively improve the quality of service (QoS) of the network and reduce the loss of services, it is of great significance to study the survivability of the optical layer. The survivability of the optical layer includes two technologies: protection and restoration.

Protection refers to the protection resources reserved for the bearer services of optical networks. When a network failure occurs, the affected services are scheduled to be transmitted to the pre-allocated protection route to restore the affected services. Protection is usually under the control of local or remote network elements, so the Protection Switching time is very short without the intervention of the external network management system. However, the standby resources cannot be shared within the network, resulting in low network resource utilization, which is a disadvantage of the protection mechanism.

Recovery refers to the dynamic search for the remaining resources in the network (including the reserved dedicated idle backup capacity and the additional capacity that can be released for network-specific or even low-priority services) for the optical network bearer services ), by using the remaining resources, we can find alternative routes for the failed routes in the network, so as to quickly and accurately eliminate the congestion caused by the fault. The Restoration Technology can dynamically search for all idle capacity in the network, greatly saving backup resources, thus greatly improving the utilization of network resources. However, the recovery usually requires the intervention of the external network management system. The recovery time is slow, the recovery response is uncertain, and the service recovery time is relatively long, which is a shortcoming of the recovery mechanism. Restoration is usually used for mesh networks to make the best use of network capacity resources.

Both protection and recovery can adopt the reselect routing mechanism, but both have their own advantages and disadvantages. The protection technology has a short switching time, but the network resource utilization is low. The restoration technology greatly improves the utilization of network resources, but the service recovery time is relatively long. In practical applications, if we adopt a technical strategy that combines protection and recovery (PRC), we will be able to meet the operator's desired business protection level. Since the Ring Network and the mesh network are the two main network structures of the optical transmission backbone network, this article will analyze some important technical problems of protection and recovery in these two network structures.

Ring-shaped network protection

In Ring Optical Networks, protection is primarily used to recover services affected by failures. Currently, the protection of the ring network mainly includes two solutions: adjacent node loop back and source node rerunning. The following two protection schemes of the ring-shaped network are analyzed respectively.

1.1 neighboring node loopback mode

Nearby node loopback is a common protection method in the existing ring-shaped network. According to this solution, when the information is transmitted from the source node to the target node, if a line in the process fails, the system will reverse the protection on the adjacent faulty node, the affected services are switched to the protection channel for transmission. That is, the affected services are switched back to the adjacent nodes to avoid line faults and reach the target node through the protection channel.

Such as Xi'an? Lanzhou? Yinchuan? Hohhot? Beijing Shijiazhuang Zhengzhou seven major nodes constitute the ring network. When the information is transmitted from Xi'an to Shijiazhuang through Lanzhou, if the optical fiber between Shijiazhuang and Beijing is broken, the Beijing node adjacent to the accident will take protective measures to switch the business to the protection channel, that is, the business will be rolled back at the Beijing node and arrive at Shijiazhuang along the protection channel.

Providing protection for services in the neighboring node loop mode also raises another problem, that is, the transmission distance of information is much greater than normal. In this way, the transmission latency of information will be greatly increased.

Under the normal path, the path transmission delay of Shijiazhuang (Lanzhou) in Xi'an is t1 = 17.25 MS. When beijing? When an optical fiber fault occurs in Shijiazhuang, the transmission delay of the neighboring nodes is usually t2 = 36.35 ms.

 

Obviously, after switching is protected by the neighboring node loop, the transmission latency of information from the source node to the target node is greatly increased. The calculation here does not include the transmission latency caused by node devices. Generally, information is transmitted from the source node to the target node through many node devices. If the transmission delay caused by these node devices is included, the increasing transmission latency caused by neighboring node loopback will become more prominent, which will inevitably seriously affect the network system performance.

1.2 source node re-routing

Although the conventional adjacent node and ring protection in the ring network can provide effective protection for services, it also significantly increases the information transmission distance. The increase in transmission distance leads to an increase in information transmission latency, which seriously affects the performance of the network system. To solve the above problems, the industry has proposed the source node re-routing method. The above method solves the problem of increasing transmission latency caused by the neighboring node loop-back method.

When the information is transmitted from Xi'an to Shijiazhuang via Lanzhou, if Beijing? The optical fiber between Shijiazhuang is broken. At this time, the source node re-routing solution will re-route at the source node (Xi'an node), and the information to be transferred will be directly switched to the protection channel ., The information is directly transmitted through Zhengzhou to the destination node (Shijiazhuang node), and there is no need to go through Lanzhou or Beijing to complete the transmission. Compared with the neighboring node loopback mode, the source node re-routing solution greatly reduces the information transmission distance and has much lower information transmission latency. The transmission delay of information after switching is protected by source node re-routing as t3 = 35.85 MS, which has obvious advantages compared with the transmission delay after switching using the neighboring node loop back mode. In addition, if you further consider the latency introduced by the Intermediate nodes during transmission, the advantage of the source node re-routing mode will be more obvious.

The structure of the ring-shaped network has good survivability and short recovery time (can be less than 50 MS ). However, if a large long-distance backbone network uses a ring network structure, the transmission path is too long and the number of transmitted nodes is too large due to the fact that information is transmitted through a roundabout route in many cases. This will lead to a high end-to-end transmission latency of the entire information. Especially when the optical fiber line of the ring network fails and the network system uses the adjacent node loop back mode to protect the switching, the latency problem is particularly prominent. Excessive transmission latency not only seriously affects real-time services such as dialogs, but also negatively affects the IP address of the network. This will not only cause IP systems (TCP/IP systems) the rapid decline in throughput, resulting in a rapid decline in network utilization, but also increases the probability of loss of data groups, leading to serious deterioration of the overall network system performance.

In addition, the disadvantages of the ring-shaped network are also reflected in the difficulties in network planning. During network planning, it is difficult to predict future development. Therefore, a large capacity needs to be planned at the beginning. The idle capacity required for this structure is as high as 100% even when the business volume is relatively balanced, and some even require up to 150%. Practice has proved that the ring-shaped network has a great advantage in short distance and simple topology. in large-scale long-distance backbone networks, the application advantages of the mesh network are even more prominent.

Network protection and recovery

The multi-routing mode of the mesh network makes it much more reliable than other network topologies. The multi-route mode of the mesh network, the application of cross-connection devices, and the shortest path first function can ensure that the network still has the optimal transmission path even if the network fails.

In the mesh network, you can use the reselect Routing Mechanism for network protection and recovery. For protection, the channel protection technology can reserve a dedicated end-to-end protection channel for each working channel, or use 1 + 1 or 1: 1 protection switch. The Link Protection (or optical reuse segment protection) can use another dedicated parallel optical fiber or other optical fiber that is routed to a network that does not intersection the link, you can also use the share mode, such as the M: 1 structure. Protection Technology has the advantage of short switching time. However, due to the need to provide reserved protection resources for Optical Network bearer services, the network resource utilization rate is low.
In the mesh network, the restoration policy is highly survivability. The Restoration Technology can dynamically search for all the idle capacity in the network to find new routes for affected businesses, so as to best utilize network capacity resources. Under the same network survivability conditions, the additional idle capacity required can be much smaller than that of the Ring Network, Usually 30%-60%. Therefore, recovery ratio protection has an advantage in capacity efficiency. In general, when the network topology is complex, for example, in a highly interconnected long-distance network, the use of mesh network recovery is more economical and flexible than the ring network, and also facilitates planning and design. Because the mesh network provides recovery within the network range, it has a strong capability to deal with serious network faults and has the minimum limit on the network topology. However, mesh Network recovery also has the disadvantages of long service recovery time. It is worth noting that in the mesh network, the network-wide optimization recovery solution requires Optimization Analysis of resources throughout the network due to Network Protection Switching. Therefore, Network Protection Switching takes a long time, for the local optimization recovery scheme, the resource optimization analysis time can be greatly reduced, and Network Protection Switching and service recovery can be quickly implemented.

Based on their respective advantages and disadvantages of protection and recovery, in practical applications, the industry has proposed a technical strategy to adopt PRC. As mentioned above, helbel Alcatel took the lead in proposing and leading the highest level of PRC solutions in the industry. Shanghai Bell Alcatel's PRC solution combines protection and recovery. while giving full play to the protection and recovery advantages, it also overcomes their own shortcomings, it can meet the actual application needs of operators to the maximum extent.

In the optical mesh network, each node has multiple routes. Once a node fails or the optical cable is interrupted, the service can be easily switched to another path. Using the PRC solution of Shanghai Bell Alcatel, the mesh network can realize the automatic fault diagnosis of the network, the automatic recovery of the business and the optimization of routing, And the Protection Switching time can be less than 50 MS, to meet the requirements of telecom. In this way, the viability of the mesh network can best support the service protection level that the operator wants. In other words, with the PRC solution, operators can make full use of the excellent viability and reliability of the mesh network to flexibly manage various levels of services according to the SLA, provides different protection and recovery time for various levels of businesses.

Normally, services are transmitted through a master route between the source node and the target node. To ensure high-priority service transmission security, the network system uses the Shortest Path priority principle to calculate a preset protection route. When the primary route fails, the network management system automatically switches the service to the preset protection route. The client is not affected because the Protection Switching time meets the carrier-level requirements (less than 50 MS.

If the protection route fails again, the network management can also find the third available route based on network resources and switch the service to this route. Likewise, the client will not be affected because the protection SWAp time is less than 50 MS.

In actual situations, when PRC protection is used, if the third protection route also has problems, the network management system can further dynamically find the updated available routes.

Protection and Restoration of a combination of a mesh network and a ring-shaped network

At present, the backbone optical transmission networks of major carriers in China mainly adopt the network structure of "multiple Ring Networks and some branches". The service recovery types provided are ring/line protection and not protection. According to the advantages and disadvantages of the Ring Network and the mesh network, combined with the actual Backbone Transmission Network of major carriers in China, if the mesh topology is introduced in China's large-scale long-distance backbone network, building a large-scale long-distance backbone network in China into a network topology combining a mesh network and a ring network can provide ring network protection and 1 + 1/1 for services: multiple Combination protection and recovery methods, such as N linear protection and mesh PRC. In other words, using the network topology that combines the mesh network and the ring network, and using the SLA function, operators can flexibly provide different types of services for different types of customers.

Conclusion

Main Ring Networks