Computer Network Technology and Application

Chapter 5 A Brief Introduction to Computer Networks (3)
1. OSI seven-layer model
The OSI seven-layer model consists of Physical Layer-PH, Data Link Layer-DL, Network Layer-N, Transport Layer-T, and Session Layer from bottom to top. (Session Layer‐S), presentation layer (Presentation Layer‐P) and application layer (Application Layer‐A).

2. A brief introduction to the functions of the OSI seven-layer protocol

(1) Physical layer
The physical layer defines the mechanical, electrical, functional, and procedural characteristics required to establish, maintain, and tear down physical links, and its role is to enable the transmission of raw data bit streams on physical media.

(2) Data link layer
The main function of the data link layer is to upgrade the unreliable physical link to an error-free data link for the network layer by means of verification, confirmation and retransmission, and shield the physical layer.The data link layer also coordinates the data transmission rate of the sender and receiver, that is, performs flow control to prevent the receiver from overflowing the buffer and blocking the line because it is too late to process the high-speed data from the sender.

(3) Network layer
The network layer mainly solves the problem of how to transmit data across different communication networks from the data source to the destination, that is, to solve the interconnection between networks and the routing between networks.In order to avoid network congestion caused by excessive packets in communication, it is necessary to control the number of incoming packets.

(4) Transport layer
The transport layer is a host-host (end-end) layer, providing end-to-end transparent data transport services, so that high-level users do not need to care about the existence of communication subnets, the purpose is to provide good data for application processes (running programs) Transport services; the transport layer also deals with end-to-end error control and flow control issues.

(5) Session layer
The session layer is a process-process communication layer protocol, and its main function is to manage communication (also called dialogue) between various processes on different hosts.The session layer is responsible for establishing and dismantling the dialogue connection between two session layer entities; the session layer also provides a better mechanism, so that after the data transmission is interrupted due to a network failure, the interrupted transmission can be resumed.

(6) Presentation layer
The presentation layer provides common data syntax representation transformation for the upper layer users.In order to allow computers using different encoding methods to understand the content of data in communication, an abstract standard method is used to define the data structure, and a standard encoding representation is used.The specific functions are representation, compression and encryption of data.

(7) Application layer
The application layer is the highest layer of the open system interconnection environment, mainly providing network services.File transfer between different hosts, e-mail, virtual terminal (VT) protocol, etc. all belong to the category of application layer.In other words, the application layer provides corresponding protocols specifically for providing services.

1.5.3 TCP/IP protocol
1974年，Kahn定义了最早的TCP／IP参考模型；20世纪80年代，Leiner、Clark等人对TCP／IP参考模型进行了进一步的研究。TCP／IP协议共公布6个版本，后3个版本是V4、V5、V6。目前使用的是V4，它的网络层IP协议一般记作IPv4；V6的网络层IP协议一般记作IPv6（或Ipng，IPnext generation），IPv6被称为下一代的IP协议。

1. The overall structure of TCP/IP
The TCP/IP protocol does not fully comply with the seven-layer reference model of OSI. TCP/IP adopts a four-layer hierarchical structure, and each layer needs the services provided by its next layer to realize functions.

2. Four-layer protocol of TCP/IP

(1) Application layer
The application layer is the layer that provides user services and is the communication layer between application processes. The protocols include Simple Mail Transfer Protocol (SMTP), File Transfer Protocol (File Transfer Protocol, FTP) and Network Remote Access Protocol (Telecommunication Network Protocol, Telnet), etc.

(2) Transport layer
The transport layer provides data transmission services between nodes, including Transmission Control Protocol (Transfer Control Protocol, TCP), User Datagram Protocol (User Datagram Protocol, UDP), TCP and UDP add transmission data to the data packet and transmit it to the next layer, this layer is responsible for delivering data and making sure that data has been delivered and received.

(3) Internet network layer
The Internet network layer is responsible for providing basic data transmission functions, including Internet connection (IP), control message (ICMP) and other protocols.

(4) Network interface layer
In the TCP/IP system, there is actually no real management of the actual network media and definition of how to use the actual network (such as Ethernet, etc.) to transmit data, that is, it entrusts the actual work to any type of local network.

1.5.4 Other Protocols
1.X.25
The X.25 protocol is a packet-switching data network protocol formulated by ISO and ITU-T for wide area network (WAN) communication. It defines data terminal equipment (DTE, usually an end user's computer) and data circuit terminal equipment ( Exchange of data and control information between DCEs, such as Modems used by dial-up networks.This protocol is usually used in packet switching networks, such as the public telephone network, and the network using the X.25 protocol is often called an X.25 network.

2. Frame Relay

Frame relay is a data communication technology developed in the early 20s. Its English name is Frame Relay, or FR for short. It evolved from the X.80 packet communication technology.Frame relay technology is mainly used to transmit data services. It uses a set of procedures to effectively transmit data in the form of frames (referred to as frame relay protocol).

The frame relay protocol is a simplification of the X.25 protocol, so the processing efficiency is very high, the network throughput is high, and the communication delay is low. The access rate of frame relay users is between 64kbit/s and 2Mbit/s, and can also reach 34Mbit /s.

【chapter summary】

The main content of this chapter is to introduce the concept, function, and classification of computer networks; protocols and architecture; and the significance of protocol layering according to functions.On the basis of introducing the general system and layered protocols, this chapter also introduces OSI and TCP/IP, so that readers can understand the relationship between the protocol layers.

【Difficulties in this chapter】

(1) Understand several key points of the definition of computer network.

(2) "Topological structure" is a method of designing on paper. No matter where the node switching devices are or how far the devices are, the devices and connections must be represented by dots and lines.

(3) Grasp the classification method of using the scope to divide the types of computer networks.

(4) The concept of protocol layering, peer-to-peer layer, peer-to-peer entity, virtual communication and real communication, and service access point.

Exercise 1
[-]. Multiple choice questions

1. In the ISO/OSI reference model, the rules that must be followed when exchanging information between peer entities on the same layer are called ().

A. Interface
B.Agreement
c. service
D. Relationship
2. The basic functions completed by the computer network include data transmission and ().

A. Data processing
b. Data transmission
C. Message sending

D. Message storage
3. Regarding the OSI architecture, which of the following statements is false? ()
A. is the set of network protocols actually used
B. Support powerful network functions

C. Support data compression and encryption
D. Open network interconnection mechanism
4. Regarding the WAN, which of the following statements is incorrect? ()

A. Geographic scope can span cities and countries

B. Owned by an organization

C. Must use the X.21 protocol
D. The main task of the network is the exchange of data
5. Regarding the Internet, which of the following statements is false? ()

A. From the perspective of network design, the Internet is a computer Internet

B. From the perspective of users, the Internet is a network of information resources
C. Clients and servers connected to the Internet are collectively referred to as hosts

D. The Internet uses hubs to realize the interconnection of networks and networks
6. In the OSI model, the relationship between the Nth layer and the N+l layer above it is ()

The AN layer provides services for the N+1 layer

B. The N+1 layer will add a header to the information received from the N layer
The CN layer utilizes the services provided by the N+1 layer
The DN layer has no effect on the N+1 layer

Two, short answer questions
1. Find an example of network application, and use this example to explain the concept of computer network.

2. How do you currently connect your computer to the Internet and become a legal user?

3. Give an example to illustrate the role of protocols in daily work and life, and give another protocol used in the Internet.

4. Referring to the data flow diagram of the TCP/IP protocol, try to explain the concept of virtual and real communication.

(End of this chapter)