Introduction to Digital Data Network (DDN) (1)

With the rapid development of computer communication technology, digital data network (DDN), as the basis for transmission of data communication networks, provides a large-capacity information transmission channel for point-to-point and point-to-point. It is a digital circuit that uses tens of thousands and hundreds of thousands of optical cables as the main body and uses digital circuit management devices to form a high transmission rate and good quality, basic Network for data transmission with low network latency, full transparency, and high traffic. Since 1994, China has officially opened public digital data networks. It has only been 10 years since then, DDN has been involved in meteorological, public security, railway, hospital, securities industry, banking, Gold Card engineering and other real-time data exchange industries.

1 DDN Definition

Digital Data Network is a data transmission network that uses digital channels (such as optical cables, digital microwave channels, and satellite channels) to transmit signals. He can provide users with a full digital, transparent, and high-quality network to connect and transmit various data services. He can also provide users with a dedicated digital data transmission channel, it provides conditions for users to establish their own private data networks. User private networks built using DDN can save investment, reduce daily maintenance costs, and shorten the construction cycle. Compared with traditional local leased lines, DDN features fast speed, low error rate, and high reliability, and is favored by users.

2 DDN Structure

2.1 Structure

DDN consists of a digital transmission circuit and a digital cross-multiplexing device. Among them, digital transmission is mainly based on optical fiber transmission circuit, digital cross-connection multiplexing equipment for digital circuit semi-fixed cross connection and sub-rate multiplexing. The network structure 1 is shown in.

(1) DTE: the user-end device connected to the DDN network can be a LAN device, connected to the peer through a router, or a general asynchronous terminal or image device, as well as fax machines, electronic transmission machines, telephones, etc. The DTE and DTE are completely transparent.

(2) DSU: The data business unit can be a modem or baseband transmission device, as well as time division multiplexing, voice/Digital Multiplexing, and other devices.

DTE and DSU are mainly used to access and receive services.

(3) NMC: the network management center can easily configure the network structure and business, and monitor the network operation in real time, collects and Reports Network Information, network node alarms, and line utilization.

Based on the basic functions of the network, the DDN network can be divided into the core layer, access layer, and user interface layer.

(1) The core layer uses a 2 m circuit to form the core of the Backbone Node and implements the network service transfer function, including the transfer function of the Frame Relay Service.

(2) The access layer provides Sub-rate multiplexing and cross-connection for various businesses of DDN, user access for Frame Relay services and local frame relay functions, and access for compressed voice/G3 fax users.

(3) The user interface layer provides the adaptation and transfer functions for users to access the network. For example, small-capacity reusable devices.

2.2 System Composition

A digital data network consists of four parts:

(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, which generally uses common local telephone user lines, you can also use a data device (DOV) That is reused on a telephone line ).

(2) the principle of cross-connection and multiplexing system multiplexing is to combine the data stream of multiple users under 64 kb/s into a set data signal of 64 kb/s, generally referred to as zero group signal (DS0), and then multiple DS0 signals are further reused into a group of 2.048 Mb/s or higher signals according to the architecture of the digital communication system. A semi-permanent and fixed connection is achieved by concatenating user data signals in a certain format with the input of the zero-time group multiplexing or the output of one Multiplexing with the input of another, it is implemented by operators in the network management center.

(3) Inter-Office Transmission and synchronous clock system inter-office transmission are mostly implemented through existing digital channels. In a DDN network, it is extremely important for each node to maintain clock synchronization. The clock synchronization system of the entire network of the digital communication network is usually used. For example, the accuracy of the clock with the accesskey atomic clock can reach n × 10-12, and the precision of the clock can be the same as that of the master clock. It can also be implemented using a global positioning system (GPS) that uses multi-satellite coverage.

(4) Whether it is a national backbone network or a regional network, a network management center should be set up for the online transmission channel, the addition, deletion, modification, monitoring, maintenance, and scheduling of user parameters are centrally managed.