Integrated Services Digital Network

History

In modern society, people have higher and higher requirements for communication. In addition to the original voice, data, and fax services, they also require comprehensive transmission of high-definition television and radio Broadband services such as TV and high-speed data fax. The development of computer technology, microelectronics technology, broadband communication technology and optical fiber transmission provides a good foundation for meeting these growing communication demands.

The earliest attempts at network synthesis began in the early 1980s. First, the concept and technology of Integrated Service Digital Network (ISDN) are proposed. In the ITU proposal, ISDN is a communication network developed on the basis of the digital telephone network IDN (the network can provide end-to-end digital connections). ISDN can support multiple services (including telephone services and non-telephone services). ). The most important feature of ISDN is that it can support end-to-end digital connections, and can realize the integration of traditional voice services and packet data services, so that data and voice can be transmitted in the same network. ISDN has a very close relationship with the digital public switched telephone network (PSTN), which can be considered to be formed on the PSTN to support the expansion of data services. The most basic function of ISDN is the same as that of PSTN, providing an end-to-end 64kbps digital connection to carry voice or other services. On this basis, ISDN also provides a higher bandwidth N*64kbps circuit switching function. The integrated switching node of ISDN should also have a packet switching function to support the exchange of data packets. The signaling structure is also the same as that of PSTN, using the No. 7 signaling system, and its user part is the ISUP protocol.

1. The development of ISDN abroad

There were 21 countries in the world that opened ISDN commercial services in 1993, and it developed to 30 in 1995. Many countries around the world have been able to Provide ISDN services. The number of ISDN subscriber lines in 1994 was 1.7 million, and in 1995 it increased to 3.95 million. According to data, in 1998, ISDN users in Germany reached 2.8 million, with a penetration rate of about 6%; the number of subscriber lines in the United States, Germany, and Japan ranked among the top 3 in the world, with 3.5 million, 3 million, and 3.6 million basic rate interfaces respectively. Small businesses and residential users in France are the main users of ISDN, accounting for 74% of the market. Initially, Deutsche Telekom mainly provided ISDN services for commercial users, but later turned to small merchants and residential users. France is the first country in the world to provide a national ISDN connection, and its number of users ranks second in Europe. Japan’s ISDN users have reached more than 3 million, and the ISDN penetration rate has reached 5% to 6%, but all are below 10%. According to statistics in 1999, there are 14.2 million basic rate interface (BRI) users in the world.

The development of ISDN in Asian countries is also exciting. In March 1995, Japanese ISDN services spread to all cities across the country, and 98% of telephone users already have the network conditions to use ISDN. Due to the rapid development of the Internet, the growth momentum of ISDN services abroad has been very strong. The development strategies adopted by foreign operators mainly include: (1) Using preferential service prices, ISDN voice channels are charged the same as ordinary telephone channels; (2) Speeding up and increasing ISDN network coverage capabilities; (3) Paying attention to system integration, Develop ISDN services and applications, and provide convenient ISDN services.

"European IT Watch" is the most optimistic about the development of ISDN: In 1999, it predicted that the number of ISDN users in the world would reach 104 million in 2001, exceeding 100 million for the first time; by 2002, the number of ISDN users would be 1.23 Billion, 139 million in 2003. However, the development of ISDN is restricted by other competing technologies, such as ADSL, which makes it difficult to achieve this goal. Table 1 is the ISDN newsletter reported the global ISDN user statistics in July 1999

Table 1 The global ISDN user statistics in July 1999

(Source: Operators, industry and FT Media &Telecoms estimate)

Portugal

Bell Canada

2. Development of ISDN in China

The earliest in China The city where ISDN commercial trials are conducted is Shanghai. In 1995, Shanghai began the construction of a commercial trial network, which was independent of PSTN. After that, Beijing and Guangzhou began to build small-scale commercial trial networks. In the absence of an ISDN local network, Beijing boldly adopted the international bureau, setting up independent ISDN bureaus and 8 modular bureaus, and was the first to open international ISDN services for domestic foreign companies and commercial users. In 1996, the ISDN service was officially named "one-line communication", which vividly summarized the basic characteristics of ISDN. During the 1996 Olympic Games, it provided 384kbit/s ISDN conference TV service, which was well received by the society. However, due to the impact of their coverage and business functions, these trial networks have slow initial business development.

In the "Seventh Five-Year Plan" and "Eighth Five-Year Plan" period of China, ISDN is a key scientific and technological research project. From 1985 to 1995, my country established ISDN model network and test network to conduct research on ISDN practical technology. Based on these research results and with reference to international recommendations, the former Ministry of Posts and Telecommunications Research Institute of Telecommunication Transmission has formulated a series of technical standards related to ISDN, laying the foundation for the commercialization of ISDN in my country.

After gaining ISDN operating experience, the original China Telecom has continuously strengthened its ISDN construction. The original China Telecom has been transforming its domestic long-distance network and international bureaus since 1996. By 1998, there have been 26 nationwide The 26 original PSTN long-distance switches in the provincial capital cities have been equipped with ISDN functions through version upgrades and hardware transformations. Among them, long-distance ISDN services were opened between Beijing, Guangzhou, Nanjing and Shanghai, and between Zhengzhou, Jinan, Chengdu and Tianjin in June 1998. Joining the national network in January, ISDN services in other provincial capitals have also begun to gradually open. At the same time, three international offices in Beijing, Shanghai and Guangzhou opened ISDN international services. Among them, the Beijing International Bureau opens ISDN services with the United States, Germany, Singapore, Japan, the United Kingdom, Australia, South Korea, Switzerland, Finland and other countries, as well as China's Hong Kong, Macau and Taiwan. At the end of 1998, ISUP signaling was opened with Belgium, Italy, France, Sweden, Spain and other countries. ISUP signaling has been opened between the Shanghai International Bureau and Japan, Taiwan, Hong Kong, Macau and other countries and regions in China, and ISUP signaling has also been opened between the Guangzhou International Bureau, Taiwan, and the United States.

The fastest-growing city of ISDN in mainland China is Shanghai. In 1997, Shanghai built an ISDN covering the urban area that overlaps with the PSTN and put it into formal commercial use. Since then, ISDN has entered a healthy and rapid development track. In 1997, there were only 645 ISDN users in Shanghai; by 1998, ISDN users reached 17,000, an increase of 25.3 times over the previous year; in 1999, Shanghai ISDN users reached 78,000, an increase of 3.59 times over the previous year; in 2000, Shanghai’s ISDN net increase The number of users reached 100,000, exceeding the total number developed in the previous three years. The cumulative number of users reached 179,000, and the ISDN penetration rate reached 3.26%. Germany had the highest ISDN penetration rate in the world in 1996, with 3.5%. In 2000, the ISDN penetration rate in Shanghai reached 3.26%, indicating that the level of ISDN development in Shanghai is close to that of some developed countries and regions.

At the same time, ISDN services in other parts of the country are also developing rapidly. The successful development of the former China Telecom "One-line" has greatly promoted the development of my country's narrowband integrated services. In 2000, the number of ISDN users nationwide (excluding China's Taiwan, Hong Kong and Macau) reached nearly 690,000, a cumulative increase of 520,000 from the end of the previous year, three times the number of previous years, and good social and corporate benefits have been achieved. As of June 2001, the number of ISDN basic interface users in China (excluding Taiwan, Hong Kong, and Macau) reached 937,838, and the number of primary group interface users reached 2,830.

Significance

ISDN is such a network, developed from IDN, providing end-to-end digital connections to support a series of services (including voice and non-voice services) , Provide users with a multi-purpose standard interface to access the network. The integration of communication services is to use one subscriber line to provide multiple services such as telephone, fax, videotext and data communication.

In addition to being used for making calls, the integrated service digital network can also provide multiple services such as videophone, data communication, conference TV, etc., so as to integrate multiple services such as telephone, fax, data, and images. Transmission and processing are carried out in a unified digital network, which is the origin of the name "Integrated Services Digital Network".

Since the opening range of ISDN is much wider than that of ADSL and LAN access, for those users who do not have broadband access, ISDN seems to be the only solution for high-speed Internet access. After all, 128kbps The speed of ISDN is much faster than dial-up; ISDN is charged by time like the phone, so for some users who have relatively little online time (such as users under 20 hours a month), it is much cheaper than using ADSL. In addition, because ISDN lines are digital lines, the effect of using it to make calls (including Internet calls) is much better than ordinary telephones.

It transmits voice and data at a higher rate and quality through ordinary copper cables. ISDN is a popular form of telephone network in Europe. The GSM mobile phone standard can also transmit data based on ISDN. Because ISDN is a fully digitized circuit, it can provide stable data services and connection speeds, unlike analog lines that are as obvious as interference. It is easier to carry out more digital information services that cannot or are more difficult to prove quality on digital circuits. For example, in addition to basic phone calls, it can also provide video, image and data services. ISDN requires an all-digital network to carry digital signals (only two states of 0 and 1). The biggest difference from ordinary analog phones is here.

In addition, ISDN also specifically refers to the use of this technology to establish a protocol suite for maintaining and disconnecting circuit switching, or isosorbide dinitrate, an abbreviation for isosorbide dinitrate.

(1) Realize high-reliability and high-quality communication. Since the channel between the terminal and the terminal has been completely digitized, noise, crosstalk, and signal fading distortion are very small due to the increase in distance and the number of links, so the communication quality is very high.

(2) Easy to use. The information channel is separated from the signal channel. 8 terminals can be connected to a subscriber line of about 2B+D, and 3 can work at the same time.

(3) Low cost.

Characteristics

1. Analyze the characteristics of ISDN from the perspective of definition

(1) ISDN is an all-digital network that realizes end-to-end digital connections. Modern telephone networks have adopted digital program-controlled switches and digital transmission systems. The processing inside the network has all been digitized, but analog signals are still used to transmit voice services on the user interface. In ISDN, the user loop is also digitized. Regardless of whether the original information is voice, text, or image, the terminal device first converts the information into a digital signal, and then transmits it over the network.

(2) Because ISDN realizes the end-to-end digital connection, it can support various services including voice, data, and image, so it is an integrated service network. Theoretically speaking, any form of original signal, as long as it can be transformed into a digital signal, can be transmitted and exchanged using ISDN to realize information exchange between users.

(3) Various service terminals use a standard interface to access ISDN. The same interface can be connected to multiple user terminals, and different terminals can be used at the same time. In this way, users can use all kinds of different services with just one interface.

2. Analyze the characteristics of ISDN from the perspective of application

(1) Multiplexing. For most users, one of the biggest advantages of ISDN is its multipath. ISDN users can provide two B channels (64kbit/s each) and one D channel (16kbit/s) on a pair of twisted pairs, while using multiple services.

(2) High transmission quality. ISDN uses an end-to-end digital connection, unlike analog lines that will be disturbed by static electricity and noise, so the transmission quality is very high. Due to the use of error correction coding, the error characteristic of ISDN transmission is at least 10 times better than the error characteristic of data transmitted over the telephone network.

(3) Comprehensiveness. ISDN provides various services. Users only need a network access interface to use various services provided by the network. For example, users can access equipment such as telephones, computers, conference TVs, and routers on a basic rate interface.

(4) High-speed data transmission. Using ISDN, the highest data transmission rate can reach 128kbit/s, and it is full-duplex, which is more than twice the theoretical uplink rate of a general V.90 modem.

Main categories

ISDN has two types: narrowband and broadband. Narrowband ISDN has two interfaces: basic rate (2B+D, 144kbps) and primary group rate (30B+D, 2Mbps). The basic rate interface includes two independently working B channels (64kbps) and a D channel (16kbps). The B channel is generally used to transmit voice, data and images, and the D channel is used to transmit signaling or packet information. B stands for bearer and D stands for control.

Broadband can provide users with communication capabilities above 1.55Mbps. However, due to the complex technology of the broadband integrated service digital network and the huge investment, it is unlikely to be put into a large amount of use. The narrowband integrated service digital network is already very mature and fully equipped with the conditions for commercialization. Therefore, ISDN refers to integrated services opened in various places. The digital network is actually a narrowband ISDN. Due to the use of digital lines, the bit error characteristics of data transmission are at least 10 times better than that of telephone lines. ISDN has a wide range of uses, but for ordinary people, only Internet access is the most commonly used.

ISDN functional group

User-network interface is the entrance for ISDN users to access ISDN. This interface must meet the requirements of service integration, that is, the interface is required to be versatile and able to accommodate circuit-switched services and packet services of different rates. One ISDN user-network interface can support multiple terminals, and the system model for user access to ISDN can be defined by the reference model of the user-network interface. The concept of user function group is used in the reference configuration. A function group is a combination of a set of functions on an interface, which can be a physical function component required on the interface, or an abstract concept. The ISDN user network interface includes the following functional groups:

(1) Terminal equipment (TE). There are two types of terminal equipment: TE1 is a digital terminal that meets the requirements of ISDN user-network interface standards, and a terminal that does not meet the requirements of user-network interface is TE2, such as analog telephones and X.25 terminals.

(2) Terminal adaptor (TA, Terminal Adaptor). Its function is to connect the non-ISDN terminal (TE2) to the ISDN network. TA's functions include rate adaptation and protocol conversion.

(3) Network terminal equipment (NT). Network equipment is also divided into two categories: NT1 and NT2. NT1 is a subscriber line transmission service, and its functions include line maintenance, monitoring, timing, power feeding, and multiplexing. NT2 performs the functions of a private branch exchange (PBX), local area network and mid-range control equipment.

(4) Line terminal equipment (LT). LT connects the user's ring line with the interface equipment of the exchange office to realize the interface function between the exchange equipment and the transmission end of the line.

The connection point between different user functional groups is called the access reference point. The reference points defined in the ISDN user network interface include R, S, T, U, etc., among which the S and T reference points are the standard reference points of the ISDN user-Wangkuo interface, which can be directly connected to ISDN standard terminals. Usually when the PBX is not used, the S and T references can be combined, which is called the S/T reference point. On the user-network interface, ISDN defines different channels for transmitting information. The B channel is used to transmit user information, and the channel bandwidth is 64kbps. The D channel is used to transmit control signaling required for circuit switching, and is also used to transmit packet switching information. The H channel is used to transmit high-bandwidth user information greater than 64kbps, and can be divided into H0, H1 (2.048Mbps), H3, H4 (130.264Mbps), etc. according to its transmission rate. The standard interfaces defined in ISDN mainly include the basic rate interface (PRI) and the group interface. The basic rate interface consists of two 64kbps B channels and one 16kbps D channel, usually called 2B+D. The B channel is used to transmit voice or other types of data services; the D channel is used to transmit signaling or packet data. The group interface is used for the communication of high-traffic users, and is usually composed of multiple B, D, and H channels, such as 30B+D (where the D channel bandwidth is 64kbps).

From the perspective of application promotion, ISDN has not achieved the results expected in advance. ISDN's main business is still aimed at the voice telephone exchange business, and the support for data services is limited by the 64kbps channel bandwidth. Therefore, ISDN actually provides a narrowband switching service, which cannot yet meet the requirements for higher bandwidth data communications, such as high-definition image data transmission. Compared with the later proposed bandwidth ISDN based on ATM, ISDN is usually called narrowband ISDN (N-ISDN). ISDN is not really integrated in the structure, because it uses circuit switching technology and packet technology at the same time, which are used for voice services and data services, respectively. The so-called integration is only realized on the user interface and adapts to new services and new technologies. Poor ability.

Access methods

ISDN has two access methods: Basic Rate Interface (BRI) consists of 2 B channels, each with a bandwidth of 64kbps and a D channel with a bandwidth of 16kbps. The three channels are designed as 2B+D.

Prime rate interface (PRI)-consists of many B channels and a D channel with a bandwidth of 64Kbps. The number of B channels depends on different countries:

North America, Hong Kong and China Japan: 23B+1D, total bit rate 1.544 Mbit/s (T1)

Europe, China and Australia: 30B+D, total bit rate 2.048 Mbit/s (E1)

Voice calls are transmitted through the data channel (B), and the control signal channel (D) is used to set up and manage the connection. When a call is set up, a 64K synchronization channel is established and occupied until the call ends. Each B channel can establish an independent voice connection. Multiple B channels can be combined into a single high-bandwidth data channel through multiplexing.

The D channel can also be used to send and receive X.25 data packets and access the X.25 message network. (In fact, it is rarely widely used).

Analysis of advantages and disadvantages

Analysis of advantages

1. Comprehensive communication services: Using one subscriber line, you can make calls and send and receive faxes while surfing the Internet. Just like two telephone lines.

2. High transmission quality: Due to the end-to-end digital transmission, the transmission quality is significantly improved.

3. Flexible and convenient to use: only one network access interface and a unified number can be used to obtain the various services that need to be used from the network. Users can connect multiple different types of terminals on this interface, and multiple terminals can communicate at the same time.

4. The Internet speed can reach 128kbps.

Disadvantage analysis

1. The speed is not fast enough compared to the access methods such as ADSL and LAN.

2. Long-term online fees will be very high.

3. Equipment costs are not cheap.

Application development

Since ISDN is not satisfactory in practice, since the mid-1980s, people began to seek a new network architecture to overcome the existence of ISDN Problem. In the design of the new network architecture, it is hoped that it can truly realize the integration of various services (voice, data, image, and even new services that will appear in the future), and can support the different characteristics of various existing services and future services; It can flexibly support the efficient transmission of services with different transmission rates, bursts and time characteristics in a unified network. Obviously, because broadbandization and service integration are the main features of this new network architecture, it is named Broadband Integrated Services Integrated Digital Network (B-ISDN) to distinguish it from the original ISDN.

The development from narrowband ISDN to broadband ISDN can be divided into three stages.

The first stage is to further realize the integration of voice, data and image services. Three independent networks constitute a preliminary integrated B-ISDN. The broadband switching network formed by ATM realizes the comprehensive transmission of voice, high-speed data and moving images.

The main feature of the second stage is that B-ISDN and user-network interfaces have been standardized, optical fiber has entered the home, and optical switching technology has been widely used, so it can provide high definition including multi-channel Broadband services including HDTV (High Definition Television).

The main feature of the third stage is the introduction of an intelligent management network in the broadband ISDN, and the intelligent network control center manages the three basic networks. The intelligent network can also be called an intelligent broadband ISDN, in which smart phones, intelligent switches, and various intelligent expert systems for engineering design or fault detection and diagnosis may be introduced.

The transmission modes adopted by B-ISDN mainly include high-speed packet switching, high-speed circuit switching, asynchronous transfer mode ATM and optical switching.

High-speed packet switching uses the basic technology of packet switching, simplifies the X.25 protocol, uses connection-oriented services, has no flow control, error-free control on the link, and centralizes packet switching and synchronization time division. The advantages of the exchange, a number of trial networks have been put into operation.

High-speed circuit switching mainly uses multi-speed time division switching (TDSM), which allows channels to be allocated according to time, and its bandwidth can be an integer multiple of the basic rate. Since this is a fast circuit switching, the management and control of its channel are very complicated, and there are still many problems that need to be studied further.

The main equipment of optical switching technology is the optical switch, which introduces optical technology into the transmission loop to realize the high-speed transmission and exchange of digital signals.

There is no doubt that the combination of ATM and Synchronous Optical Network (SONFT) will form the main body of communications in the 21st century.

B-ISDN (Broadband Integrated Service Digital Network) broadband integrated service digital network.

B-ISDN refers to ISDN with a subscriber line transmission rate above 2Mbit/s. It is developed on the basis of ISDN and can support various types of services at different rates, including not only continuous services but also burst broadband services. Its business distribution range is extremely wide, including narrowband services (such as voice/fax) with a rate of no more than 64kbit/s, broadband distribution services (broadcast TV, high-definition TV), and broadband interactive communication services (video telephony, conference phone) ), broadband burst service (high-speed data), etc.

Application of B-ISDN

In several high-speed wide area network technologies: SMDS (Switched Multimegabit Data Service), frame Following (frame relay) and B-ISDN (broadband integrated services digital network), B-ISDN uses a new network to replace the existing telephone network and various private networks. This single integrated The network can transmit all kinds of information. Compared with the existing network, it must provide a very high data transmission rate, and it is possible to provide a large number of new services, including on-demand TV, TV broadcasting, dynamic multimedia e-mail, video phone, CD quality Music, local area network interconnection, high-speed data transmission for scientific research and industry, and many other services that have not even been thought of today.

Summarizing these services can be divided into two categories: interactive services and dissemination services. From the characteristic point of view, there are three points:

(1) Not all services require very high transmission. Rate and bandwidth, but moving image transmission services and high-speed data communications have such requirements. For example, TV broadcasting requires 30 Mbps and HDTV broadcasting requires 130 Mbps.

(2) Some services have high burstiness. For example, connection-oriented data transmission is 1Mbps~50Mbps, text transmission is 1Mbps~20Mbps, and video conference is 1Mbps~5Mbps.

(3) The requirement for the network is to support various services at different rates, and to support burst communication. It is necessary to consider information loss sensitive applications and delay sensitive applications.

ATM and B-ISDN

A technology that makes B-ISDN possible is called asynchronous transfer mode (ATM). The development of technology is to comply with the requirements of multimedia transmission. The transmission characteristics of multimedia (voice/image) are different from traditional data transmission. The characteristic of data transmission is to allow delay, but there must be no errors. Data errors will lead to different meanings of the data and cause wrong results; voice/image transmission It is characterized by a large amount of information and high real-time performance, but a small amount of errors are allowed, and errors can only affect the quality of the voice/image at the time. Although various compression technologies can be used, the amount of multimedia information is still amazing, especially the real-time requirements of multimedia transmission make it difficult to adapt to other technologies, so a new switching technology: ATM switching technology has emerged.

Superiority

The integration of communication services can provide multiple services such as telephone, fax, videotext and data communication by using one subscriber line.

Realize high-reliability and high-quality communication.

Easy to use.

Low cost. Compared with separate communication networks, the cost of integrating services into one network is much lower.

ISDN has a very wide range of applications, the main application areas are local area network, multi-point screen sharing, video, voice/data integration, file exchange, remote communication, image, multimedia file access, computer-based Caller number identification, etc.

Industry forecast

There are two main views on ISDN. The most common view is that users want a digital connection from the home to the telephone and data network that has better performance than ordinary analog modems. The typical end-user’s Internet connection is based on this point of view, and comparisons of various modems and operators’ products and prices (performance, price), etc. are all based on this point. Most of the discussions in this area are based on this view, but in fact, as a data connection service, ISDN has in fact been eliminated by DSL technology.

However, there is another point of view: ISDN has not been fully sentenced to death for the telecommunications industry. A telephone network can be regarded as a collection of wired connections between different switching systems. It is also used as an intelligent network technology to provide more new services to the public switched telephone network (PSTN) through end-to-end circuit-switched digital services.

ISDN has not been widely used on the telephone network in the United States from beginning to end, and is already an outdated technology. However, it has some uses in recording studios, especially when the voice actor and the director and producer are not in the same place. ISDN highlights the advantages of real-time non-over-the-Internet services at this time, and its realistic language quality is comparable. POTS service.