When an organization builds a network across a wide area in the public area, it can have three choices: build its ownPrivate network；Use the existing public network;Or use a combination of the above two networks.
10. 25 network, namely public data network (PDN). Its main protocol is called CCITT X.25, so it is called X.25 for short.It is a packet switched data network based on analog system.It supports permanent virtual circuit (PVC) and switched virtual circuit (SVC). PVC is used for regular data transmission, and SVC supports burst packet transmission.
establishPrivate networkIt is a bit misleading. It usually refers to a network established by using public facilities, unless customers provide all switching equipment and establish a network between different locationsLeased line。Dedicated is more accurate: this organization can make full use of these lines and not share them with anyone else.establishPrivate networkAnother way is to use microwave installation or useLECProvided by other companiesMetropolitan area networkFacilities.Many companies areWANUse services provided by the public data network, but set up for connecting local facilitiesPrivate networkFacilities, such as cabling between buildings in a campus environment, are relatively easy.
Public data network (PDN) is a kind of network provided by local or long-distance telecommunication bureauspacket switchingOr circuit switching services. These telecommunications offices include MCI, US Sprint and AT&T. In addition, the above services can also be provided by other institutions. These institutions first build networks for their own use, and then enable other users to use these networks.Providedpacket switchingServices typically include X.25Frame relay、Switched multi megabit data service(SMDS) orAsynchronous transmission mode。Circuit switched services include dial-up lines, switched 56 lines and Integrated Services Digital Network (ISDN).
packet switchingAnd circuit switching service providers include:
US Sprint in its full numberOptical networkCircuit switching service, X.25Frame relay, Internet Protocol (IP), video services, andATM。
AT&T provides a range of circuit switching andpacket switchingServices.Its ACCUNET circuit switching services, such as Switch 56, have been used for many years.AT&T also providesFrame relay、packet switchingServices and ATM.
CompuServe information service is available in hundreds of locations throughout the United StatesFrame relayAccess point for the service.
GE Information Service Provisionpacket switchingAnd high-speed services, instant asynchronous and synchronous services.
Infonet Services provides an array of international services.
Tymonet Global Networks has nearly 5000 global access points.
These services can be accessed atLeased line(Pay monthly) or get it online.A typical PDN network forms a global connection between PDN switching devices.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 services is provided by the local telecommunications office (LEC) within the local access transmission area (LATA).A line is usually required between the customer site and the LEC switching facility.This line can be either a dial-up line or a dedicated digital line.The PDN service provider connects to the local telecommunications office at a location called the Access Point (POP).POP is where they visit customer routes.LEC must provide POP facilities as designated by the government.
The use of PDN can reduce costs andPrivate networkThe need to rent expensive long-distance dedicated digital lines.PDN handles any problems of switching service and network by itself.It can also ensure better data distribution at a lower price.
In recent years, the construction of public data communication network in China has been very fast.The telecommunications department has established digital communication networks such as CHINAPAC, CHINADDN, CHIANFRN, and formed China's public data communication network.
China Public Packet Switched Data Network (ChinaPAC)
It was opened in September 1993. By the end of 1996, it had covered cities above county level and some towns in developed regions, and was connected to 44 data networks in 23 countries and regions in the world.
(1) Network status
Packet switched networkIt is the earliest basic data communication network built and developed by the post and telecommunications department.Packet switched networkBased on the CITTX.25 recommendation, it can meet the communication between terminals and computers of different speeds and models, computers and computers, and computer LAN.Packet switched networkIt is a basic data communication network, on which various value-added services can be built, such as:E-mail、electronic data interchange, fax storage and forwarding, etc.
CHINAPAC is composed of national backbone network and provincial intranet of each province (city, district).At present, the backbone network covers all provincial capital cities, and the provincial intranet covers all cities and developed towns with business requirements.Through the interconnection with the telephone network, CHINAPAC can cover all areas reached by the telephone network.CHINAPAC is equipped with a primary exchange center and a secondary exchange center. Incomplete mesh structure is used between the primary exchange centers, and star structure is used between the primary exchange center and its secondary exchange center;CHIANIPAC has international entrances and exits in Beijing and Shanghai, and Guangzhou has entrances and exits to Hong Kong and Macao to complete the networking with international data.
(2) Network features and service functions
Packet switched networkThe outstanding advantage of the is that multiple virtual circuits can be opened on a physical circuit at the same time for multiple users to use at the same time;The network has dynamic routing function and complex and complete error correcting function.10. The 25 protocol was developed under the condition of poor transmission quality of the physical link. In order to ensure the reliability of data transmission, it must perform error checking and error retransmission on each link;This complex error checking mechanismtransmission efficiency It is limited, but it does provide a good guarantee for the safe transmission of user data.
The services provided by CHINAPAC are as follows
l. Basic business functions
The basic business function is to point to any numberterminal equipment(DTE).It can meet the basic requirements of users for communication.There are two basic services, switched virtual circuit (SVC);Permanent virtual circuit (PVC)
2. Optional business function
The user optional service function is a special service function provided to users to meet their special needs, such as inbound call blocking, outbound call blocking, one-way inbound logical channel, one-way outbound logical channel, etc.
3. Other business functions
CHINANET also provides other service functions that are recommended by ITU-T, such asvirtual private network(VPN)、TCP/IP, packet multicast, call redirection, etc.
User access mode
CHINANET provides two access modes.
1. Special line mode
It is suitable for applications with large communication traffic, frequent use, high reliability and no consumption. However, it requires dedicated lines and costs relatively high.The dedicated line access rate is 9.6~64KBPS.
2. Telephone dialing
It is suitable for applications with small business volume, long interval time and can tolerate call failure.Existing due to its useTelephoneThe line requires no additional investment, and the data can share the line with the voice, so the investment is greatly saved, and it is an ideal access method for scattered users.It can be divided into x.28 asynchronous dial in or X.32 synchronous dial in. The speed of dial in is l200-9600BPS
(4) Tariff policy
CHINAPAC currently has two charging methods, one is time measurement and the other is monthly fee.Time measurement charges.
Application fields and business positioning
And DDNFrame relayIn comparison, the packet service charges are relatively cheap. It is the internal part of the user architectureWANThe most economical choice.Set up at the same time when necessaryMultipoint connectionByPacket switched networkVirtual circuit function can replace expensive multi-point DDN 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 volume of POS business is small, but the real-time requirements are high. The non packet network interconnection is to achieve POS machines andhostA very good scheme for communication.
