1 Introduction SDH technology has been a mature and standard technology so far. Using the advantages of SDH synchronous multiplexing, standardized optical interfaces, strong network management capabilities, flexible network topology capabilities and high reliability, the multi-service transmission platform MSTP implements TDM, ATM, Ethernet and other services based on traditional SDH Access, processing, and transmission provide multi-service nodes for unified network management. The functional model is shown in Figure 1. MSTP can introduce an intelligent intermediate adaptation layer such as RPR and MPLS technology between Ethernet and SDH, and combine multiple technologies to improve the data processing and QoS support capabilities of the device, and complete the Ethernet frame to the SDH virtual container through the GFP protocol Encapsulation mapping, while using virtual cascade and LCAS technology to enhance the flexibility and efficiency of virtual container bandwidth allocation. 2 Core technology of MSTP in metropolitan area network MSTP retains the traditional advantages of SDH and is constantly updating its technology. Since its development in 2001, it has experienced three major leaps. Below is a brief introduction to its current core technology. 2.1 VC cascade The concept of VC concatenation is defined in ITU-T G.7070, and is divided into two types: continuous concatenation and virtual concatenation. Continuous cascading is to combine several connected containers into a large container and transmit it through the SDH system. Each VC is continuous in the SDH frame structure and shares the same channel overhead (POH). Sometimes it is also simply referred to as a tier. United [3, 4]. In addition, he can enable operators to effectively adjust transmission capacity according to user business needs. Using VC virtual cascade technology can also achieve multi-path transmission. The characteristic of VC virtual concatenation is that the discontinuous SDH synchronous payload (data) is cascaded to form a virtual concatenated signal group (VCG) for transmission to achieve the purpose of matching service bandwidth. In the SDH network, the implementation of VC virtual concatenation is relatively simple. The only thing to pay attention to is to ensure the transmission of the serial number SQ of the virtual container participating in VCAT, and to ensure that the VC of the transmitted signal can be correctly queued and reassembled at the receiving end of the system.
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Application of Multi-service Transmission Platform in Metro Transmission Network
VC virtual concatenation means that each VC used to carry Ethernet services in SDH is independent in the frame structure of SDH, and its position can be handled flexibly. Through continuous cascading and virtual cascading technologies, rate adaptation between Ethernet bandwidth and SDH virtual channels can be achieved. In particular, the virtual cascade technology overcomes the disadvantage of lower transmission efficiency due to the larger bandwidth particles of the cascade. It can map each independent container to a virtual cascade link from VC-4 to VC-12, etc. The combined use of small containers of different rates provides finer bandwidth granularity and greatly improves transmission efficiency, as shown in Table 1.