DDN access method

As a data communication support network, DDN provides users with high-speed and high-quality data transmission channels and serves as a bridge for their network interconnection. If you leave the customer's access, it will promote the meaning of existence. However, due to the ever-changing turbidity of the customer, there are also differences in the access to the terminal devices or network devices. Here we will describe several common user access methods, it is not limited to the specific user terminal device.

User Access methods are generally divided into user terminal device access methods and user network and DDN interconnection methods.

User Terminal Device Access Method

Here, user terminals can be general asynchronous terminals, computers or image devices, as well as telephones, electronic transmission machines or fax machines. The way they access DDN depends on their interface speed and transmission distance. Generally, the client device is a certain distance from the network device of DDN. To ensure the sinking data communication and transmission quality, auxiliary means such as modem and user concentrator are required, the following are five aspects:

Access DDN through a modem;
Access DDN through DDN data terminal devices;
Access DDN through the user concentrator;
Access DDN through analog circuits;
Access DDN through a 2048kb/s digital circuit.

Access DDN through a modem

This access method is the most widely used in data communication, as shown in figure 4. This method is used to simulate Data Services opened on the private network and telephone network. This method is generally adopted by the Ministry of Civil Affairs of China, where the customer is far from the Access Point of DDN. In this access mode, the modem located in the DDN board extracts the timing standard from the receiving signal and generates the timing signal used by the local modem and the user terminal device. When the analog line is long and the loop delay changes, there will be a big difference between the receiving and output timing of the modem in the Access Board and the Improvement timing provided by the DDN device, therefore, a buffer memory must be added for compensation.

Figure 4 DDN access through a modem

The modulation and demodulation device can be divided into two types: baseband and band transmission. Baseband transmission is an important data transmission method. The basic function is to form an appropriate waveform, so that the data signal is out of service when the bandwidth is limited, inter-code interference is not produced due to multiple waveforms. The frequency band is used to stop production of data using the band in a given line, such as the band occupied by one or more telephones, it is more widely used than baseband, and the transmission distance is longer than baseband.

Depending on the number of cable cores occupied by the receiving and sending signals, the modem can be divided into two-line and four-line cables. When the transmission distance is long and the transmission speed is high, select four lines.

With the development of science and technology, the interface rate supported by modem is also getting higher and higher, not only can meet the ITU-TV.24, G.70364kb/s), V.35 and X.21 recommended port rate, it also supports G.703 2048kb/s high speed.

Access DDN through DDN data terminal Devices

In this way, the customer directly uses the data terminal device provided by DDN to connect to DDN without adding a separate modem, as shown in Figure 5. Advantages of this method include:

Figure 5 DDN access through data terminal Devices

First, you do not need to add a modem on the local terminal, but you need to place data terminal devices on the client;

2. The DDN Network Management Center can configure remote systems and perform routine maintenance and management for the data terminal devices it belongs to. The real-time devices themselves or the connected real-time connections increase the reliability of the system.

DDN provides data terminal device interface standard compliant with ITU-TV.24, 35, X.21 recommended, interface speed range between 2.4kb/s to 128kb/s.

Access DDN through centralized Devices

This method is suitable for scenarios where a large amount of user data interfaces is required or customers already have centralized user devices. A centralized user device can be a zero-group multiplexing device or a small-sized multiplexing device provided by DDN.

Zero-time group multiplexing devices reuse multiple kb/s, kb/s, the data rate of 9.6kb/s is reused into a 64 KB/s digital stream, which is connected to DDN by some means, as shown in figure 6. The format of sub-rate multiplexing can be X.50 or a format agreed by both parties.

The small-scale multiplexing provided by DDN is more flexible than the zero-time group multiplexing equipment. It not only supports 2. the data rate of 4 kb/s, 4.8kb/s, and 9.6kb/s, and supports a higher rate, as shown in figure 7. The customer can also provide voice and fax services as needed, applicable to V.24, V.35, X.21, and audio interfaces. In addition, DDN has detection, height, and management capabilities for its small multiplead.

Figure 6 Access DDN through zero group multiplexing devices

Figure 7 DDN access through small multiplead

Access DDN through Analog Circuit

This method is applicable to the scenario where the DDN audio interface is connected after analog circuit transmission by telephone, fax machine, and User Switch PBX. Here, analog transmission can be a local audio cable or a wireless analog frequency.

Access DDN through 2048kb/s Digital Circuit

In DDN, network devices are configured with standard G.703 2048kb/s digital interfaces that comply with ITU-T recommendations if the user device can provide the same interface can access the nearest DDN. In this access mode, the digital transmission circuit required by the business can be constructed in a unified manner with other communication services such as telephones, such as creating PCM cable systems and transmission systems ., In areas with poor line conditions, you can also use co-built digital microwave and digital special frequency.