Design Of Digital Dispatching Business Interface For Railway Optical Fiber Communication Systerm

According to the basic needs and special nature of China railway section communication, a set of railway section optical fiber communication systems have developed in our research center, which is based on optical time division multiplexing technology and passive optical network structure.An interface is needed between the main control unit of the optical fiber communication system station-side equipment and railway digital dispatching communication system in order to ensure the normal communication of the two systems.This interface is digital dispatching business interface. The staff of the railway section can get through to the dispatch telephone such as traffic dispatching and power dispatching through this interface in the necessary circumstances.This paper describes the design of digital dispatching business interface for the normal communication of the two systems.According to switch type of the railway digital dispatching communication system, this design adopts basic rate interface of ISDN users-network interface standard to realize digital dispatch business interface. The basic rate interface of ISDN user-network interface is an universal digital interface technology and include two B-channels and one D-channel, whose B-channels is intended to carry digital voice or data information and D-channel is intended to carry control information. So it is also known as2B+D interface. The2B+D data rate reaches to144kbit/s which can meet the basic voice communication and other dispatch communication.This paper begins with a brief analysis of the basic theory and relevant protocol of the ISDN user-network interface standards, afterward introduces a scheme of digital dispatching business interface board completely. The scheme follows the design principles which combine hardware and software. The function of network layer protocol is implemented by software in MCU. The D-channel control information is generated or parsed in this layer. The function of data link layer protocol is implemented by the HDLC protocol controller chip MT8952.This layer frames and formats data packets generated by the network layer according to LAPD frame format recommendations from the ITU-T.The functions of physical layer is implemented by the ISDN U interface transceiver chip MC145572.This layer sends2B+D data to the physical link and receives the standard encoded data sent from the network.2B-channels information is generated or received by the voice codec chip and the data is inserted into specify the time slot of ST-BUS or extracted from specify the time slot of ST-BUS.All operation about2B channel must be countroled by CPLD. Experiment results indicate that this design could carry out the physical connection in the U reference point of N-ISDN and ensure the line full-duplex digital communications.