Public data network

Public Data Network

Public Data Network (PDN) Public Data NetWorks (PDNs)

The establishment of a private network is a bit misleading, it usually refers to a A network established by public facilities, unless the customer provides all the switching equipment and leased lines are established between different locations. The fact that dedicated is more accurate: this organization can make full use of these lines and does not share these lines with anyone else. Another way to establish a private network is to use microwave installation or use metropolitan area network facilities provided by companies other than LEC. Many companies use the services provided by public data networks for wide area networks, but it is relatively easy to establish private network facilities for connecting local facilities. For example, it is relatively easy to route cables between buildings in a campus environment.

Public data network (PDN) is a packet-switched or circuit-switched service provided by local or long-distance telecommunications offices. These telecommunications offices include MCI, US Sprint, and AT&T. In addition, the above services are also available Provided by other organizations, these organizations first built the networks for their own use, and then enabled other users to use these networks. The packet switching services provided usually include X. 25. Frame relay, switched multi-megabit data service (SMDS) or asynchronous transfer mode. Circuit-switched services include dial-up lines, switched 56 lines, and Integrated Services Digital Network (ISDN).

The providers of packet switching and circuit switching services include:

US Sprint provides circuit switching services on its all-digital optical fiber network, X. 25. Frame Relay, Internet Protocol (IP), video services and ATM.

AT&T provides a series of circuit switching and packet switching services. Its ACCUNET circuit switching services, such as Exchange 56, have been used for many years. AT&T also provides frame relay, packet switching services and ATM.

CompuServe information service provides X. 25 and the access point for frame relay services.

GE Information Services provides packet switching and high-speed services, instantaneous asynchronous and synchronous services.

Infonet service company provides an array of international services.

Tymenet Global Networks has close to 5,000 global access points.

These services can be obtained on leased lines (paid monthly) or dial-up lines. A typical PDN network forms a global connection between PDN switching equipment. The circuits used include PDN's own dedicated lines or lines leased from major telecommunications companies, such as AT&T.

As shown in Figure P-19, access to the service is provided by the local telecom office (LEC) in the local access transport area (LATA). It is usually necessary to install a line between the customer site and the LEC switching facility. This line can be a dial-up line or a dedicated digital line. The PDN service provider connects to the local telecommunications office at a location called a point of access (POP). POP is where they visit customer lines. LEC must provide POP facilities as specified by the government.

Using PDN can reduce costs and reduce the need to lease expensive long-distance dedicated digital lines for the construction of private networks. PDN handles any issues with switching services and networks on its own. It can also guarantee better data distribution at a lower price.

Related items: Carrier Services telecommunications services; Circuit-Switching Services circuit-switched services, Packet Switching Networks packet-switched networks.

China Public Data Network

In recent years, China's public data communication network has been constructed very quickly. The telecommunications sector has established digital communication networks such as CHINAPAC, CHINADDN, and CHIANFRN, forming my country's public data communication network.

China Public Packet Exchange Data Network (ChinaPAC)

Opened in September 1993, it has covered cities above the county level and some towns and villages in developed regions at the end of 1996, and is in cooperation with 23 countries in the world. 44 data networks in the region are interconnected.

(1) Network status

The packet switching network is the earliest basic data communication network constructed and developed by the post and telecommunications department. The packet switching network is based on the CITTX.25 recommendation, which can meet the communication between terminals and computers of different speeds, different models, computers and computers, and computer local area networks. The packet switching network is a basic data communication network, and various value-added services can be constructed on its network platform, such as: electronic mail, electronic data exchange, fax storage and forwarding, etc.

CHINAPAC is composed of the national backbone network and the intra-provincial network of each province (cities, districts). At present, the backbone network covers all provincial capital cities, and the intra-provincial network covers all cities and developed towns that have business requirements. Through the interconnection with the telephone network, CHINAPAC can cover all areas reached by the telephone network. CHINAPAC has a primary switching center and a secondary switching center. An incomplete mesh structure is used between the primary switching centers, and a star structure is used between the primary switching center and the secondary switching center; CHIANIPAC has international offices in Beijing and Shanghai. For entrances and exits, Guangzhou has entrances and exits to Hong Kong and Macau to complete the network with international data.

(2) Network characteristics and business functions

The outstanding advantage of the packet switching network is that multiple virtual circuits can be opened on a physical circuit at the same time, which can be used by multiple users at the same time; It has a dynamic routing function and a complex and complete error correction function. The X.25 protocol was developed when the transmission quality of the physical link is very poor. In order to ensure the reliability of data transmission, it must perform error checking and error retransmission on each link; this kind of complex error Although the verification mechanism limits its transmission efficiency, it does provide a good guarantee for the secure transmission of user data.

The services provided by CHINAPAC are as follows:

l.Basic business functions

Basic business functions refer to the basic business functions provided by any digital terminal equipment (DTE) . It can meet the basic requirements of users for communication. There are two types of basic services, switched virtual circuit (SVC); permanent virtual circuit (PVC)

2. Optional business functions

Users can choose business functions to satisfy users Special needs, special service functions provided to users, such as incoming call blocking, outgoing call blocking, one-way incoming logical channel, one-way out logical channel, etc.

3. Other business functions

CHINANET also provides other business functions recommended by ITU-T, such as virtual private network (VPN), TCP/IP, packet multicast, call Change direction and so on.

User access methods

CHINANET provides two access methods.

1. Dedicated line method

Suitable for applications with large communication traffic, frequent use, high reliability, and lossless, but the dedicated line needs to be used and the cost is relatively high. The network access rate of the dedicated line is 9.6~64KBPS.

2. Telephone dialing

Suitable for applications with low business volume, long intermittent time, and call failure tolerance. Because it uses the existing telephone line, no additional investment is required, and the data can be shared with the voice line, so investment is greatly saved, and it is an ideal access method for scattered users. It can be divided into x.28 asynchronous dial-up network access or X.32 synchronous dial-up network access. The dial-up network access rate is l200-9600BPS

(4) tariff policy

CHINAPAC current two charging methods , One is the hourly metering fee, and the other is the monthly fee. Hourly metering charges.

Application field and business positioning

Compared with DDN and frame relay, packet service charges are cheaper, and it is the most economical choice for users to construct their internal WAN. In the case of simultaneous establishment of multi-point connections, the virtual circuit function of the packet-switched network can replace expensive multi-point DDN dedicated lines. However, due to the complexity of the X.25 protocol itself, packet services are used in low-speed applications where the rate is lower than 64K. For example, with the continuous advancement of the gold card project, the use of POS machines is becoming more and more popular. The POS business volume is small, but the real-time requirements are high. Non-packet network interconnection is a very good solution to realize the communication between POS machines and the host. .

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