Prospect of MSTP in smart Optical Networks (1)

With the continuous development of the Internet, the rapid growth of voice, data and multimedia services poses higher requirements for the transmission network's transmission bandwidth, interface methods and operation and maintenance. With the development of 40g synchronous transmission technology, wavelength division multiplexing technology, and ultra-long optical transmission technology, optical transmission networks have the features of high speed, broadband, and excellent service, and become the preferred solution for transmission networks.

The introduction of multi-service transmission platforms (MSTP) greatly enriches the interface methods of optical transmission networks. MSTP can quickly and quickly access voice, data, multimedia, and other services, the data layer provides aggregation and switching functions, making it easier and more efficient to use optical transmission networks. The automatic switching Optical Network (ASON) is a disruptive change to the traditional optical network. It breaks through the traditional operation and management methods of centralized configuration and manual maintenance of optical networks, this greatly increases the speed for carriers to respond to customer needs and reduces maintenance costs. These two technologies have become two hot topics in the research and development of optical networks. How to effectively integrate these technologies, give full play to the characteristics of each technology, and generate benefits of 1 plus 1 and greater than 2, which is highly concerned by network technology researchers and network users [1-3].

1. MSTP Technology

MSTP technology is a new integrated transmission technology developed from SDH devices to meet the requirements of the comprehensive urban transmission network construction, its basic feature is to implement SDH-based integrated multi-service transmission by encapsulating Ethernet data frames and ATM cells [4].

With the development of switching technology, in order to overcome the defects that traditional SDH devices do not provide data service guidance and aggregation functions for voice services, L2 switching is introduced into MSTP devices, enable the device to have some layer-2 data exchange functions. However, QoS is weak for data services.

On this basis, various equipment manufacturers further introduce the RPR, MPLS, and LCAS technologies into MSTP devices, enhance end-to-end QoS Assurance for services. By dividing business levels and implementing traffic engineering control, access congestion and fair algorithm policy control are achieved, and the burstable and unbalanced features of data service transmission are solved, this enables SDH to transmit such businesses more efficiently and stably.

2. ASON technology

In the ASON standard formulation of ITU-T of Standardization Organization, optical transmission network is divided into three planes of management, control and transmission, and a large number of data products using various protocols and control technologies are introduced, make it a distributed smart optical network [5]. The new optical network overcomes the shortcomings of slow connection establishment and complicated configuration management. It has a series of new features, such:

Automatically discovers links, port capabilities, protocol entities, and the convergence of link transmission capabilities between adjacent devices.

Discover and spread network topologies for resource and topology management.

Use signaling and routing to quickly establish and maintain end-to-end connections in the network.

Local or end-to-end protection recovery is supported in the Mesh network topology, and recovery of different domain shards and partitions is supported.

Quickly locate faults at the device and network levels and assist in Fault Locating and analysis.

It has traffic engineering capability and end-to-end QoS Assurance capability.

Provides new business applications such as on-demand bandwidth adjustment, wavelength leasing, wavelength wholesale, and virtual private networks.

3. The development of intelligent MSTP Technology

From a technical point of view, MSTP represents the convergence of devices on the edge of the network, such as the exchange and transmission networks. ASON represents the intelligence and standardization of the transmission network, the two reflect the development trend of the next generation network, that is, to meet the basic requirements of operators to reduce the number of network devices, reduce operation and maintenance costs, quickly respond to customer needs, and provide quality and hierarchical services.

However, different layers of network devices correspond to different network features and function models in the OSI network model, integrate them into the same device, and establish a unified network platform, many problems need to be solved. Packet Switching, which features statistical multiplexing, is a dynamic exchange. MPLS technology provides the ability to establish a soft connection, however, the circuit exchange characterized by Time Division/Space Division Multiplexing establishes a hard connection. In order to achieve unified and effective management and maintenance, especially for MSTP devices at the junction of the two networks, there is a lot of work to be done. The first thing to do is to solve the network device construction model.

3.1 overlapping models

Currently, the Peer Model and Overlap model are usually used in the industry. The overlapping (or overlapping network) model belongs to the customer/Server (Client/Server) model. It has a clear and easily identifiable boundary between the customer network and the provider domain. The separation of customer/provider domains reflects the current situation of the network, that is, the ownership of devices on the third and first layers of the Network may belong to different organizations. This type of customer/provider domain separation requires that different routing protocols be run in their respective domains. Therefore, there is no need to share network topology information between the carrier and its customers. The optical layer in the smart optical network provides dynamic connection capabilities including SDH/SONET, wavelength and future optical fiber connections, and can arrange bandwidth as needed. Data services can be provided based on SDH/SONET, wavelength, and optical fiber connections, as well as new businesses (including virtual private networks) that provide better networking flexibility. To ensure end-to-end service QoS, the network needs to clearly define the service type and service level provided by the light layer for the data layer, so that operators at all levels can manage their services and improve service quality.

In this case, smart MSTP devices work in layers: Intelligent Optical Layer Networks use the automatic discovery and routing protocols to discover, aggregate, spread, and summarize Network Resources of optical networks, finally, it is provided to the data layer in the form of end-to-end port resources, hiding the topology structure and link information of the internal network. During the connection process, proposes connection applications and constraints to the service provider's optical network. The intelligent optical network runs routing policies and orders, creates connection paths that meet the constraints, and protects services, in addition, data streams are transparently transmitted in the optical network, and the data network works according to its network structure, traffic and switching capabilities. The upper and lower layers are the relationship between customers and servers. They do not interfere with each other's operation and maintenance. They only see a unified device management form at the network management level.

The overlapping model is relatively simple, with clear network layers and less changes to existing networks. The transmission network operator can flexibly lease the Network Element device port to the data network operator, we can make full use of the On-Demand bandwidth adjustment, Service Level Agreement, wavelength leasing, wavelength wholesale, virtual private network, and other new business methods provided by ASON to develop our own transmission network expertise. At this time, the MSTP device provides a variety of device interfaces and data aggregation and guidance in the access part, which undoubtedly makes the end-to-end connection of the Business smoother and more efficient. Currently, most devices use this model for evolution.

However, as a whole network, the separation of network resources at different levels will lead to a relatively low operating efficiency. First, service routing and traffic sharing are not taken into account in the end-to-end connection, which is not conducive to the allocation of network resources. Second, there is a protection recovery mechanism in different layers of networks, making collaboration difficult; once again, it does not meet the requirements of all service operators for unified planning, operation data, and transmission networks.

3.2 Peer Model

The use of MSTP embedded MPLS technology and the application of GMPLS protocol in ASON networks provide conditions for the emergence of Peer-to-Peer Model devices. The GMPLS protocol family developed on the basis of the MPLS protocol family fully utilizes existing signaling, routing, and discovery protocols, and fully considers the features of the software protocol control and hardware circuit transmission of the transport network, organically combines link discovery, link aggregation, route and resource diffusion, and restricted route selection. In this way, the data exchange layer and the transmission layer have a common "language", and the network interconnection and resource sharing are the foundation, however, the evolution of network hardware and software equipment is independent, and it is also in line with the principle of separation of the Next Generation Network bearer and control. However, integrating networks of different levels into a unified network peer-to-peer model still faces a series of complex problems.

(1) Resource and Topology Management

After the network device model is unified, all nodes in the network device, their ports, time slots, and channels will be fully displayed in front of network managers and users. Therefore, transfer resource management will become more complex, the links between service peer nodes have fixed channels for SDH (and WDM later) and group channels for statistics reuse. The characteristics of establishing connections between these channels vary greatly. At the same time, different types of links need to be aggregated, and the paths and channel labels at different levels need to be planned and identified to ensure that channels at different levels and at different granularities are fully and appropriately used. Theoretically, it is necessary to establish an atomic function model entity under a unified device model. through standardized definition and Management of functions and interfaces, in order to lay the foundation for real interconnection and intercommunication between devices, between the upper and lower layers.

On the other hand, with the expansion of the network scale, the merger of data and transmission networks will inevitably lead to a huge number of network nodes. When combined into a complex Mesh network topology, how to manage it will be an arduous task. It is generally believed that domain-based solutions need to be implemented based on regions, administrative zones, Technical Features of equipment, and network scale to reduce the scale and difficulty.

In addition, after the transfer node and control node are separated, address representation and translation will also become a problem. We need to design a globally unique transfer and control node identifier, ensure that data packets are routed between different nodes. In applications, IPv4 addresses are limited and can be differentiated by introducing new identifiers such as link ports and device identifiers.

(2) routing algorithms and Routing

After the network aggregates multiple sub-layers, you can select the path in a network, especially when selecting the optimal path becomes the NP problem of route computing, if the optical fiber, wavelength, VC channel, and tag channel are combined, the algorithm will become very complicated. with limited time and computing capability, the solution may not be completed. If it is simplified to computing different levels of paths, the optimization results will be greatly reduced. To what extent, the algorithm topology should be considered separately. Generally, the source route is easy to control traffic, but the control mechanism is complicated. The use of hop-by-hop routing is flexible and cannot solve the traffic control problem on its own. Hierarchical routing can be a good choice. In combination with the division of different network sub-layers, hierarchical source routing selection and optimization can simplify routing complexity and take into account the control of traffic balancing.

Limited by the cross capacity of the transport network node, if you want to extend to the WDM layer, there is also a problem of wavelength conversion. In the process of service path optimization, in addition to considering the link transmission capacity and quality, we also need to consider the node cross capability to ensure that the business connection can pass through the exchange/Cross node smoothly, reduce blocking.

(3) connection and signaling

For the usage characteristics of voice, data, multimedia, and other services, different strategies can be adopted in the multi-layer network connection process: for end-to-end voice services, it usually takes a short time to establish a connection from a call, and even the automatically connected optical network has a long time to establish a chain, large Capacity fixed channels such as SDH can be used to transmit data between backbone nodes, establish connections in advance, and maintain stability for a long time, the Group switching network can create a tag channel during the call connection process in real time. For data and multimedia services with relatively stable and high transmission capacity, connections are sudden, changeable, long-holding, high-bandwidth, and insensitive to distance. You can consider establishing connections in real time and using the bandwidth adjustment function to meet changes.

Signaling is converted and nested at different network levels. In a packet switching network, both signaling and load are exchanged in groups. The paths are usually the same. The router can forward data based on the information of the packet header and Shim. The signaling and routing in the circuit switching transmission network are usually separated. The label router forwards data by time slots, wavelengths, and physical ports. This requires the unified semantics and syntax of signaling, create Nested Tag interfaces at various layers of optical fibers, wavelengths, time slots, and packets to form a complete end-to-end tag switching path. When a business enters the optical transport network from a branch network, the signaling protocol can identify nested labels and create tunnels in the optical network. When the business comes out of the tunnel, tag nesting should be pop up to continue forwarding.