Digital Data Network DDN

Digital Data Network Digital Data Network (DDN) is to provide users with dedicated high-speed digital data channel, so that users use it to organize their own computer communication network. Of course, it can also be used to transmit compressed digital voice or fax signals. Digital data circuit includes the user line, mainly by the digital transmission mode, it is different from the analog line, that is, the frequency division (FDM) mode of the multi-carrier telephone circuit. The traditional analog circuit can only provide the rate of 2400~96b/s, up to 14.4kb/s~28.8kb/s rate. While the digital data circuit can be a 64kb/s, if a plurality of speech paths together up to nx64kb/s, so the digital data network is to provide users with point-to-point, point-to-multipoint, high-speed circuit. Digital Data Network is a data transmission network which uses digital channel to transmit data signal. The digital channel should include the user-to-network connection line (user line). Digital data network is generally used to provide users with a dedicated digital data transmission channel, or to provide users access to public data exchange network access channel, but also for the public data Exchange network to provide the data transmission channel between the exchange nodes. Digital data network generally does not include switching functions, only the use of simple cross-linking and multiplexing devices, if the introduction of switching functions, constitutes a digital data exchange network.
1 Basic Concepts with the development of the data communication service, the relative fixed users are relatively large, and the delay is stable and the real-time is higher. Driven by market demand, the digital data Network (DDN)[1] , which is a semi-permanent connection between permanent and switched connections, generatesDigital Data network is based on digital transmission network, it must be based on optical cable , digital microwave, digital satellite circuit, in order to establish a digital transmission network. In the past, the traditional wire, cable, coaxial cable, analog microwave, short wave, etc., it is difficult to establish a digital transmission network. Compared with the traditional analog channel, the digital channel transmission data signal has a series of advantages, such as high transmission quality, fast speed and high bandwidth utilization ratio. DDN to the user is semi-permanent digital connection, along the way without complex software processing, so the delay is small, avoid the packet switching network transmission delay is large and not fixed shortcomings; DDN adopts digital cross-connect device, according to user needs, in the agreed time to connect the required bandwidth line, the allocation and continuation of channel capacity is carried out under the control of computer, which has great flexibility. 2 Transfer Rate the rate can be 2.4, 4.8, 9.6, 19.2, 64, nx64kb/s and even 2mb/s. 3 AdvantagesThe digital data network [2] has the following advantages over traditional analog data networks: (1) good transmission quality. The error rate of the general analog channel is between 1x10-5~1x10-6, and the quality decreases with the increase of the distance and the number of transfer, while the digital transmission is segmented regeneration does not generate noise accumulation, usually the ber of optical cable will be better than 1x10-8; (2) high utilization rate. The typical rate of a pulse code modulation (PCM) digital circuit is kb/s, which can be used to transmit data in real time up to 48kb/s or 56KB/S, which transmits 5 9.6kb/s or more low-speed data circuits via synchronous multiplexing. and a 300~3400hz standard analog channel can only transmit 9.6kb/s rate, even if the use of complex modems (modem) can only reach 14.4 kb/s and 28.8kb/s; (3) Expensive modems are not required. For the user, only a simple function baseband transmission of the modem, the price of only about One-third. 4 System Composition a digital data network mainly consists of four parts: digital data transmission system (1) The local transmission system refers to the transmission system between the end user and the local bureau of the digital Data network, that is, the user line, generally using the ordinary local telephone user line, but also using the data equipment (DOV) which is multiplexed on the phone line. (2) cross-connect and multiplexing system, multiplexing is less than kb/s of the data stream of multiple users on time division multiplexing principle into a set of KB/S data signal, commonly referred to as 0 Times Group Signal (DS0), Then, multiple DS0 signals are further reused in the architecture of the digital communication system into a one-time group of 2.048 MB/s or higher signals. Cross connection is the symbol of a certain format of the user data signal with 0 times the input of the cluster multiplexer or the output of one multiplexer and the input of another multiplexer cross-connect, to achieve a semi-permanent fixed connection, how to cross the operator of the Network Management Center to implement; (3) inter-office transmission and Synchronous Clock system, inter-office transmission is achieved by using existing digital channels. It is extremely important that each node in a DDN network must maintain clock synchronization. Usually adopt the digital communication network's all-network synchronous clock system, for example uses the Cesium atomic clock, its accuracy can reach nx10-12, next several rubidium clocks, its precision should be consistent with the master clock. can also be used multi-satellite coverage of the Global Positioning System (GPS) to implement; (4) network management system, whether it is the backbone of the national network, or an area network should be set up an Internet Management center, on-line transmission channels, user parameters, deletion and modification, monitoring, maintenance and scheduling of centralized management. 5 General Structural form based on the above digital data transmission system, it can form a digital transmission network. (1) DDN node type from the network function, the DDN node can be divided into 2 gigabit nodes, and three types of access nodes and user nodes.

• The 2 Gigabit node is used for the backbone node of the network, performs the conversion function of the Internet service, and provides the 2mbit/s interface (E1) to multiplex and cross-connect the n*64kbit/s signal;
• The Access node mainly provides the Access function for DDN all kinds of services, the multiplexing and cross-connection of less than 64kbit/s sub-rate signal, and provides the frame Relay service and the compressed voice/g3 the fax user network;
• The user node mainly provides the interface for the DDN user to access the network, and makes the necessary protocol conversion.

(2) network structure of DDN DDN is generally a grading net [3] . According to the network's business situation, DDN Network can set up two-level trunk network and local network. If the DDN network is separated from the vertical, its functional hierarchy can be divided into 3 layers, that is, the transport layer, the access layer and the user layer. The transport layer is responsible for transmitting the digital signal from the access layer, and the access layer adopts the bandwidth manager to realize the multi-service access of the user, provides the digital cross-connect and multiplexing function, and the user layer refers to the user terminal device and the function of the link in the network. 6DDN Network Business category DDN Network mainly for the user to provide dedicated circuit, including the specified rate of point-to-points or to multipoint digital dedicated circuit and specific requirements of the dedicated circuit, as well as Frame Relay service [4] and compressed voice/g3 fax business. The Frame Relay Service provides a permanent virtual circuit through the introduction of the Frame Relay Services Module, and provides the compressed voice/g3 Fax Service by introducing the Voice services module.

Digital Data Network DDN