Basic knowledge of communication network


1. What is a communication network

There are no networks around us, such as telephone networks, telegraph networks, TV networks, computer networks, etc.; Even if there are many network systems inside our body, such as the nervous system and digestive system. The most typical representative is the computer network, which is a combination of computer technology and communication technology.

Second, the evolution of computer networks

Simple links are based on the host architecture low -speed serial links, X.25 and IBM’s SNA

3. The main characteristics of computer networks

Resource Sharing

Information transmission and centralized processing

Load balancing and distribution processing


Comprehensive information service

Fourth, the definition of LAN, Urban Division, WAN

LAN (LOCAL Area Network)

Usually, within a few kilometers, a collection of computers, printers, modem or other devices that can be interconnected by a medium.

Man (Metropolitan Area Network)

The coverage of MAN is medium -sized, between the LAN and the wide area network, and is usually a network connection in a city (the distance is about 10km).


WAN (Wide Area Network)


The distribution distance is far, and it can access through various types of serial connections to achieve access in larger geographical areas.

5. Common network topology structure

6. Circuit exchange and group exchange

Circuit exchange: Circuit exchange based on telephone network

Advantages: small, transparent transmission


Disadvantages: bandwidth fixed, low network resource utilization, slow initial connection establishment slow

Group exchange: Storage and forwarding in groups as a group


Advantages: multi -road reuse, high network resource utilization rate


Disadvantages: Large delay, poor real -time, complicated equipment functions


The unit of group exchange, Xinyuan, two -layer frames.

7. Performance standards for computer networks

Bandwidth. The amount of data that can be transmitted from one node to another within a certain period of time is described. Usually, BPS is used as a unit, such as Ethernet band width of 10Mbps and fast Ethernet as 100Mbps.


Delay. Describe the time that data on the network is transmitted from one node to another node.

Agreement, Standard


What is the network protocol?

The network protocol is a set of formats and agents that are pre -formulated by different devices in the network to communicate with data communication in advance.

The network protocol is a series of rules and agreed normative descriptions that define how to exchange information between network devices.

Data communication standards are divided into two categories: facts and legal ones

Fact standard: What has been widely used and accepted in the application is de Facto Standard without organizing groups.


Legal standards: The standards formulated by the official group are called the statutory standard (de jure state)

Standardized organization

International Standardization Organization (ISO)

Electronic Electrical Engineer Association (IEEE)

National Standards Agency (ANSI)

Electronic Industry Association (EIA / TIA)

International Telecom Alliance (ITU)

Internet engineering task committee (IETF)

Research Mission Committee IRTF

Number allocation committee Iana

OSI reference model

OSI RM: Open System Interconnection Reference Model

OSI reference model has the following advantages

Simplified related network operations

Provide compatibility and standard interface between equipment

Promote standardization work


Can be separated in structure

Easy to achieve and maintain


The first to the third layer becomes the bottom layer (LOWER LAYER), also known as the medium layer (Media). The underlying layer is responsible for the transmission of data in the network. accomplish. The fifth to seventh layers of the OSI reference model becomes high -level (Upper Layer), also known as host layer. The high -level is used to ensure the correct transmission of data and achieve it by software.

OSI reference model hierarchical structure, OSI reference model is divided into seven layers from bottom to top:

8. Overview of TCP/IP protocol stack


TCP/IP originated in a group exchange network research project funded by the U.S. government at the end of the 1960s. By the 1990s, it has developed into the most common form of networking between computers. TCP/IP is a real open system, because the definition of the protocol cluster and its various implementation can be obtained free of charge. TCP/IP is the basis of “Global Internet” or “Internet” Internet.

Like the OSI reference model, the TCP/IP -to -peer model is also divided into different levels, and each layer is responsible for different communication functions. The five -layer peer model is the synthesis of OSI and TCP/IP models.

TCP/IP protocol stack

The IP protocol is the network protocol that is trying to transmit, and the data transmission services provided by it are unreliable and unconnected. The IP protocol does not care about the content of the data packet, cannot guarantee whether the data packet has successfully reached the destination, nor does it care about any status information about the front and rear data packets. The connected reliability service is implemented by the upper TCP protocol. All data such as TCP, UDP, ICMP, and IGMP are finally encapsulated in IP packets.

Physical layer function

The physical layer is mainly: specifying the type of medium, interface type, and signaling type; specifications to activate, maintain and close the requirements of electrical, machinery, processes and functions of the physical links between terminal systems; The characteristics of maximum transmission distance and physical connector;

Synchronous serial ports can be used as DCE or DTE, supporting a variety of physical layer protocols: v.24/v.35/x.21 and other protocols. Asynchronous serial port supports the RS232 protocol with a maximum rate of 115.2kbit/s. G.703 E1/T1 interface type.

Physical layer medium and physical layer equipment

Physical layer medium: coaxial cable, twisted wiring, fiber fiber, radio waves;

Physical layer equipment: relay, hub;

Data link layer function

Mac Sub-Layer: Media Access Control Sub-Layer Medicine access control subclase. How to transmit data through physical lines and communicate with the physical layer.

LLC Sub-Layer: Logic Link Control Sub-Layer Logic Link Link Control Subtime. The type of identification protocol is transmitted through the network.


Data link layer protocol

Data link layer LAN and wide -area network protocol

Data link layer equipment: Ethernet switch

Network layer function and equipment

Function: forward the data packet between different networks

Equipment: router, three -layer switch

The task of the network layer is to choose the right path and forward the data packet so that the packet can be transmitted from the sender to the receiver correctly.

The main functions of the network layer include:

Site: The network layer is the allocation logo for each node, which is the address of the network (address). Address distribution also provides the foundation for path selection from source to purpose.

Routing selection: A key role of the network layer is to determine how the data transmission from source to destination should be selected. After the network layer equipment is calculated, the data packet is forwarded according to the routing information. The device selected by the network layer routing becomes a router.

Congue management: If the network transmits too much data packet at the same time, it may produce congestion, resulting in data loss or delay, and the network layer is also responsible for controlling the congestion on the network.

Different network interconnection: The type of communication link and the medium is diverse. Each link has its special communication regulations. The network layer must be able to work in a variety of links and medium types in order to cross multiple more The network segment provides communication services.

The network layer is between the transmission layer and the data link layer. It is responsible for providing services to the transport layer, and at the same time translates the network address into the corresponding physical address. The network layer protocol can also coordinate the imbalance of the processing capacity of sending, transmission, and receiving equipment. For example Support maximum data frame length.

Network layer protocol

When a host application of a network needs to send a message to the destination located in another network, a data frame of a router on the same network with the host will receive the data frame. The data type of the network layer is carried, remove the link layer frame head, and send the network layer data to the corresponding network layer for processing;

The network layer inspection newspaper head to determine where the destination address is located, and then find the route table to obtain the corresponding output interface;

The link layer of the output interface is added to the chain layer frame head to this packet, encapsulated into a data frame and sent to the next jump;

The forwarding of each message must be carried out. When reaching the network where the goal is located, the packet is encapsulated into the link layer data frame of the ground -to -earth network to send it to the corresponding destination host. After receiving the message, the target host receives the link layer and network layer, remove the link layer frame head and network layer post head, and give it to the corresponding protocol.


The router can support multiple independent routing protocols (such as IP RIP, OSPF, IPX RIP, etc.), and can maintain their respective routing tables for different network protocol stacks (such as TCP/IP, IPX). This capacity of the router allows routers to support multiple network layer protocols at the same time to forward messages.

Transmission layer function

The ultimate goal is to provide effective and reliable services to users (generally referring to the process of application layer). The transmission layer mainly defines the connectivity of the end -to -end end -to -end of the host application, which generally contains four basic functions.

Squading upper data

Establish end -to -end connection

Transfer the data from one end host to the other end host

Guarantee data in order, reliable, and correctly transmitted

The transmission layer is located on the fourth layer of the OSI reference model. The ultimate goal is to the user -generally referring to the process of the application layer and provide effective and reliable services.

The transmission layer mainly defines the connectivity of the end -to -end end -to -end of the host application. It generally contains four basic functions:

Send data segments to the network layer or merge the data segment of the network layer to the application layer.

Establishing end -to -end connections are mainly to establish logical connections to transmit data streams.

Send the data segment from one host to another. During the transmission process, the accuracy of the data can be guaranteed by calculating the verification and the method of flow control, and the flow control can avoid the buffer overflow.

Some transport layer protocols ensure the correctness of data transmission. It is mainly to ensure that the same data is not transmitted nor loss during the data transmission process. At the same time, it is necessary to ensure that the receiving order of the packet is consistent with the sequence of sending.

The transmission layer protocol mainly includes the TCP/IP protocol stack of TCP protocols and UDP protocols, and the SPX protocol of the IPX/SPX protocol stack. Among them, the TCP protocol and the SPX protocol provide a reliable, connected service for applications; the UDP protocol provides unreliable and unconventional services.

The main protocol comparison of the transmission layer

Application layer function

The main function of the application layer

Provide users with interfaces and processing specific applications.

Data encryption, decryption, compression, and decompression.

Define the standards for data representation.

Source: network security and informatization