Computer Network Technology and Application

Chapter 13 Internet Network Protocols and Technologies (1)
The Internet is widely used in various industries and fields, provides a large number of resources, and brings a lot of convenience to human life and study.

The Internet network is a complex network that spreads all over the world and is interconnected by multiple networks, also known as the Internet.This chapter will introduce the development of the Internet network and the TCP/IP network system and so on.

【Main content of this chapter】

The development of the Internet.

TCP/IP protocol of each layer.

Internet network access technology.

4.1 Internet everywhere
4.1.1 The development history and current situation of Internet
In 1969, the Advanced Research Projects Agency (ARPA) of the U.S. Department of Defense began to build a network called ARPAnet, which connected several mainframe computers used by the U.S. military and research institutes.At the beginning, ARPAnet was only connected to 4 hosts, and it was placed under the protection of the top secrets of the US Department of Defense in terms of military requirements, and it did not yet have the conditions for promotion to the outside world in terms of technology.

In 1983, ARPA and the Communications Agency of the US Department of Defense successfully developed the TCP/IP protocol for heterogeneous networks. The University of California, Berkeley used this protocol as part of its BSD Unix, making the protocol popular in the society. Thus was born the real Internet.

In 1986, the National Science Foundation (NSF) used the TCP/IP communication protocol developed by ARPAnet to establish the NSFnet wide area network on the basis of five supercomputer centers for scientific research and education services.

Due to the encouragement and funding of the National Science Foundation of the United States, many universities, government-funded research institutions, and private research institutions have incorporated their own LANs into NSFnet. The military part of ARPAnet was separated and established its own network - Milnet, and the transformed ARPAnet was gradually replaced by NSFnet.

In 1989, CERN successfully developed the World Wide Web (WWW), which laid the foundation for the Internet to realize information retrieval and services, making it easier for people to access Internet resources, which greatly promoted the development of the Internet.

In 1990, ARPAnet withdrew from the stage of history.Before the 20s, the use of the Internet was limited to research and academic fields.In the early 90s, the Internet has actually become a large network, and each subnet is responsible for its own construction and operation costs. These subnets are interconnected through NSFnet——NSFnet has become one of the important backbone networks of the Internet.People who use the Internet gradually regard the Internet as a tool for communication and communication.

In 1991, three companies operating CERFnet, PSInet and Alternet networks respectively provided Internet networking services to customers to a certain extent.They formed the "Commercial Internet Association" (CIEA), announcing that users can use their Internet subnets for any commercial purposes. The emergence of Internet commercial service providers has enabled industrial and commercial enterprises to finally enter the Internet in a dignified manner. Countless enterprises and individuals from all over the world have poured into the Internet, bringing a new leap in the history of Internet development.

The Internet has now connected more than 160 countries and regions, tens of thousands or more subnets, and tens of millions of computer hosts, becoming the public network with the richest information resources in the world.

Taking China, the fastest growing country, as an example, a statistical report from CNNC in July 2008 stated: "By the end of June 7, the number of Chinese netizens had reached 2008 million, ranking first in the world. The scale of Chinese netizens continued to show sustained and rapid development. The trend. An increase of 6 million people over the same period last year, a year-on-year increase of 2.53%. In the first half of 9100, the net increase in the number of Internet users in China was 56.2 million. The proportion of Chinese Internet users accessing broadband is 2008%, and the number of broadband Internet users has reached 4300 million people.” According to the report:

"The number of IPv4 addresses is 1.58 million. In June 2008, the number of IPv6 addresses in China has surpassed that of Japan and jumped to the second place in the world. The total number of domain name registrations in my country is 4 million, an increase of 1485% year-on-year. The number of CN domain names in China It is 61.8 million, a year-on-year increase of 1190%, accounting for 93.5% of the number of domain names in my country, which is the main driving factor for the growth of the number of domain names in China. The number of Chinese websites is 80.1 million, with an annual growth rate of 191.9%. Among them, the number of websites under CN 46.3 million, accounting for 137% of the total number of websites.

中国互联网国际出口带宽数达到493729Mbps，年增长率为58.1%。目前人均拥有水平为20Mbps／万网民，比2007年12月增长了2Mbps，中国互联网国际出口连接能力不断增强。”

4.1.2 Network structure
The Internet is a large-scale network, but it is not a wide area network, because it is a network connected by several networks belonging to different organizations. These connected networks are managed by different organizations, and the network structure, equipment, and protocols used may be different.

Taking the Internet within a country as an example, the connection between the networks relies on routers. The network structure is a three-layer structure. The bottom layer is the campus network and enterprise network, the middle layer is the regional network, and the top layer is the national backbone. network.

4.2 Internet Protocol Architecture
4.2.1 TCP/IP architecture
The OSI seven-layer reference model is a traditional open system interconnection reference model, which is a seven-layer abstract reference model, each of which performs a specific task. The TCP/IP protocol does not fully conform to this model, and adopts a four-layer structure, but follows the same rule, that is, the next layer provides services for its upper layer.

For this part, please refer to section 1.5.3.

4.2.2 Protocols in the Internet
In Internet network, use TCP/ IP network system structure, the agreement of different levels finishes different work.

1. IP protocol
Internet Protocol (Internet Protocol, IP) is the core of TCP/IP and the most important protocol in the Internet network layer.

On the receiving side of data transmission, the IP protocol receives the data packet sent by its lower layer (network interface layer, such as Ethernet device driver), and sends the data packet to the TCP protocol or UDP protocol in the transport layer; On the sending side, the IP layer protocol transmits the data packets received from the TCP or UDP protocol to the lower layer.

The transmission of IP data packets does not adopt a connection-oriented method, and the IP protocol does not confirm whether the data packets are sent in order or whether there is an error. The IP data packet contains the address of the host sending the data (source address) and the address of the host receiving the data (destination address).

The IP protocol is the best possible protocol, but there is no quality assurance.

2. TCP protocol
If there is already an encapsulated TCP packet in the IP packet, then IP will pass it up to the transport layer.

The TCP protocol sorts data packets and performs error checking, and at the same time realizes the connection between virtual circuits. Sequence numbers and acknowledgments are included in TCP packets, so packets received out of sequence can be sequenced and damaged packets can be retransmitted.

At the receiving end, the TCP protocol layer sends its data to higher-level applications, such as Telnet's server and client programs.At the sending end, the application sends the data back to the TCP protocol layer in turn, and the TCP layer transmits them down to the IP layer, as well as the device driver, and through the physical medium (wire), to the receiving end.

Connection-oriented services (such as Telnet, FTP, HTTP, and SMTP, etc.) require a high degree of reliability, so these services use the TCP protocol to transmit data.In some cases, the DNS service also uses the TCP protocol to send and receive domain name databases.

3. UDP protocol
Both the UDP protocol and the TCP protocol are located at the transport layer. UDP does not provide connection-oriented services. UDP is mainly used for those query-oriented or response-oriented services, such as NFS. The amount of data exchanged by these application services is small.

4. ICMP protocol
The ICMP protocol is at the same layer as the IP protocol, and it is used to transmit the control information of the IP protocol layer.Its main task is to provide information about the path to the destination address. For example, the "Redirect" information of the ICMP protocol informs the host of a better path to other systems, and the "Unreachable" information indicates that there is a problem with the path. PING is the most commonly used ICMP-based Agreement services.

5. Application layer protocol

For application layer protocols, please refer to Chapter 6.

4.3 Internet network access technology

In the 20th Internet report of CNNIC, it is pointed out that ADSL access and leased line access account for a relatively large proportion of the large number of broadband Internet users in my country at present.

The changing trends of several mainstream access methods and the size of Internet users are different.At present, the number of broadband (including leased line) Internet users has increased significantly, increasing by the order of tens of millions every year.Among them, dedicated line users are developing steadily, with an annual increase of one million; dial-up users continue to decline, but there are still more than 3000 million Internet users using dial-up.

Wireless access①, especially the wireless access using mobile phones as terminals, has developed rapidly in China, meeting the special needs of some netizens, and the netizens have already reached a certain scale.Currently, more than 5000 million Internet users use wireless access, of which more than 4000 million use mobile phones as terminals.

The following mainly introduces several common Internet access technologies at present.

4.3.1 ADSL Technology

Digital Subscriber Line (Digital Subscriber Line, xDSL) is a subscriber line high-speed transmission technology developed by the Bell Communications Research Institute of the United States in 1989 to promote the video on demand (VOD) service, but was shelved for a long time due to the setback of the VOD service.In recent years, with the rapid development of the Internet and Intranet, the demand for fixed-connection high-speed subscriber lines is increasing day by day. The xDSL technology based on twisted pair has risen again because of its low-cost realization of high-speed subscriber lines, breaking the trend of high-speed communication dominated by optical fibers. situation.

1. Classification of xDSL technology
xDSL is a combination of transmission technologies based on copper telephone lines as the transmission medium. It is divided into two types: rate symmetric and rate asymmetric according to whether the uplink (user to switch) and downlink (switch to user) rates are the same.Including ordinary DSL, HDSL (symmetrical DSL), ADSL (asymmetrical DSL), VDSL (very high bit rate DSL), SDSL (single-wire DSL), CDSL (ConsumerDSL), etc., generally known as xDSL.

For ordinary users who need high-bandwidth access, the ADSL downlink rate is very high, which is suitable for ① wireless access with a large amount of downlink data: including wireless access terminaled by mobile phones and wireless terminals terminaled by notebooks and other devices. access method.

Internet business.The downlink rate from the telecom network provider to the user generally ranges from 1.5Mbps to 8Mbps, while the reverse uplink rate is from 16kbps to 640kbps, corresponding to a maximum transmission distance of 4.5km. This method is currently the best way to access the Internet at a high speed. One of the promising means.

2. ADSL system structure
ADSL使用一对电话线，在线两端各安装一个ADSL调制解调器。这些调制解调器采用了频分复用（FDM）技术，将带宽分为三个频段部分：最低频段部分为0kHz～4kHz，用于普通电话业务；中间频段部分为20kHz～50kHz，用于速率为16kHz～640Mbps的上行数据传递；最高频段部分为150kHz～550kHz或140kHz～1.1MHz，用于1.5Mbps～6.0Mbps的下行数据传送。

(1) Function of the separator

ADSL technology can provide telephone and high-speed data services at the same time. A splitter should be connected at both ends of the telephone line to separate the low frequency band below 4kHz and the high frequency band used for ADSL Modem modulation.Ordinary users often refer to splitters as junction boxes.

(2) ADSL modulation method

The modulation technology of ADSL is the key of ADSL. Generally, high-speed digital signal processing technology and transmission pattern with better performance are used to obtain high speed and long distance in transmission.In signal modulation technology, ADSL modem adopts CAP and DMT technology respectively.

①CAP. CAP is a modulation method proposed by AT&T. The data signal is compressed before being sent, then sent along the telephone line, and reassembled at the receiving end.Under the same transmission rate, less bandwidth is occupied and the transmission distance is longer.

② DMT.Using multi-carrier modulation technology, the available frequency band is divided into multiple (typically 256) sub-channels, each sub-channel has a bandwidth of 4kHz, corresponding to different frequency carriers, and the data is allocated to each sub-channel according to the ability of the sub-channel to send data , the sub-channels that cannot carry data are turned off. DMT has become the modulation standard T1.413 of ADSL formulated by ANSI.

3. ADSL installation steps

ADSL installation is divided into central office line installation and user end equipment installation.The central office installation is to connect the user's original telephone line string to the ADSL central office equipment by the ISP service provider; the ADSL installation at the user end connects the telephone line to the splitter, and a two-core telephone line is used between the splitter and the ADSL MODEM. The ADSLModem is connected with the network card of the computer with a crossed twisted pair.

If there are many users who need to share the connection, such as enterprises, communities, families, etc., adopt the networking form of ADSLModem plus broadband router, and the broadband router will perform the dial-up function and undertake the routing work.This mode only needs to add another switch and router, and connect the ADSLModem with a twisted pair.

4. Advantages of ADSL

(1) The backbone network of telecommunications companies has adopted ultra-high-speed optical fibers of 2.5Gbps to 10Gbps, but most of the subscriber lines connecting users and switches are still twisted-pair lines for telephones. Using ADSL technology, they can be transmitted on twisted-pair lines Digital signals up to several Mb; a splitter (commonly known as a junction box) that separates audio frequency bands and high frequency bands is configured on the client side to provide telephone and high-speed data transmission at the same time.

(End of this chapter)