synonymComputer network(Computer network) generally refers to computer network (a system that connects distributed computer equipment and communication equipment to realize information transmission)
Computer network is mainly composed of some universal and programmable hardware interconnected.These programmable hardware can be used to transmit a variety of different types of data, and can support a wide range of and growing applications.
Computer networkComputer communication network。The simplest definition of computer network is: some interconnectedshared resourceA collection of purposeful, autonomous computers.According to this definition, the early terminal oriented networks are not computer networks, but can only be calledOnline system(Because many terminals at that time could not be regarded as autonomous computers).But with the decline of hardware price, many terminals have certain intelligence, so "terminal" and "autonomous computer" gradually lose their strict boundaries.If usedMicrocomputerAs a terminal, according to the above definition, the early terminal oriented network can also be called computer network.[2]
In addition, from the logical function, the computer network is based on the purpose of transmitting informationCommunication lineA collection of computer systems that connect multiple computers. A computer network consists ofTransmission mediumandCommunication equipment。[2]
From the perspective of users, computer networks are defined as follows:Automatic managementThe network operating system of.It calls the resources called by users, and the whole network is transparent to users like a large computer system.[2]
A more general definition is: usingCommunication lineGeographically dispersed and functionally independentcomputer systemIt is a system connected with communication equipment in different forms to realize resource sharing and information transmission with fully functional network software and protocols.[2]
On the whole, computer networks are distributed in differentGeographical regionComputer and specializedperipheralThe communication lines are interconnected to form a large scale and powerful system, so that many computers can easily transfer information to each other and share hardware, software, data information and other resources.In short, a computer network is a collection of many autonomous computers connected by communication lines.[2]
The simplest computer network has only two computers and a link connecting them, that is, two nodes and a link.[2]
By Connect
Computer network is an autonomous collection of computers interconnected by lines. Specifically, computers, terminals andaccessory equipmentA system that uses communication equipment and communication lines to connect and configure network software to realize computer resource sharing.[2]
On demand
Computer networks are systems in which a large number of independent but interconnected computers work together to complete computer tasks.[3]
development history
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Since the emergence of computer network, itsDevelopment speedAnd its application is amazing.Looking at the development of computer network, it has roughly gone through the following four stages:[2]
Birth stage
Before the mid-1960s, the first generation computer network was a remote online system centered on a single computer. The typical application was an aircraft booking system composed of a computer and more than 2000 terminals across the United States. The terminal was the peripheral equipment of a computer, including a display and keyboardCPUAnd memory.along withRemote terminalThe front-end machine is added in front of the host（FEP)。At that time, people defined computer networks as "systems connected for the purpose of transmitting information, realizing remote information processing or further achieving resource sharing"communication system It has the rudiment of network.[2]
Formation stage
From the mid-1960s to the 1970sSecond generation computer networkMultiple hosts are interconnected through communication lines to provide services for users. It rose in the late 1960s. Typical examples areDefense Advanced Research Projects AgencyAssisted in developmentARPANET。Hosts are not directly connected by lines, but by interface messagesprocessor(IMP) Connected after transfer.The IMP and the communication lines connected between them are responsible for the communication tasks between hosts, which constitutesCommunication subnet。The host of communication subnet interconnection is responsible for running programs, providing resource sharingResource subnet。During this period,Network conceptThe basic concept of computer network is formed for "the collection of computers with independent functions interconnected for the purpose of sharing resources with each other".[2]
Interconnection stage
The third generation computer networks from the late 1970s to the 1990s are unifiedNetwork architectureAnd an open and standardized network complying with international standards.After the rise of ARPANET, computersNetwork developmentRapid, major computer companies have launched their own networksArchitectureAnd the software and hardware products to realize these structures.Because there is no unified standard, it is difficult to interconnect products from different manufacturers. People urgently need aOpennessStandardization and practicality ofnetwork environment In this way, two most important international architectures emerge, namelyTCP/IPArchitecture andInternational Organization for StandardizationOfOSI Architecture。[2]
High speed network technology stage
The fourth generation computer network since the 1990s, due to LANtechnological development Mature, optical fiber and high-speed network technology appear. The whole network is like a large computer system transparent to users, and has developed into the Internet represented by the Internet.[2]
form
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Classification of Computer Networks and General Thingsclassification method Similarly, things can be classified according to their different characteristics (i.e., their attributes).Computer network is generally defined as a network composed of multiple computers (or other computersnetwork equipment ）ViaTransmission mediumComposed of physical (or logical) connections with software.Generally speaking, the composition of computer network basically includes: computerNetwork operating systemTransmission medium (can be tangible or intangible, such aswireless networkAnd the corresponding application software.[3]
function
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data communication
data communicationIt is one of the most important functions of computer network.Data communication is based on certaincommunication protocol, usingData transmission technologyA method of transferring data information between two terminalscommunication mode And communication services.It can realize data information transmission between computers, computers and terminals, and between terminals. It is the third largest communication service after relay message and telephone service.The information transmitted in data communication isbinary dataAnother characteristic of data communication is that it is always connected withTelematicsConnected, it includesScientific computing, process control, information retrieval, etc.[2]
resource sharing
resource sharingIt is one of the main purposes for people to establish computer networks.Computer resources include hardware resources, software resources andData resources。The sharing of hardware resources can improve theUtilization, avoidReinvestment, such as using computer network to establishNetwork printer；The sharing of software resources and data resources can make full use of existing information resources to reduceSoftware development processLabor in, avoidingLarge databaseRepeated construction.[2]
network techniqueThe development ofDistributed ComputingIt's possible.For large projects, they can be divided into many small projects, which are completed by different computers, and then concentrated to solve problems.[2]
Load balancing
Load balancing means that the work is evenly distributed to each set on the networkcomputer system。networkcontrol centerIt is responsible for distribution and detection. When a computer is overloaded, the system will automatically transfer the load to a lighter computer system for processing.[2]
althoughNetwork typeThere are a variety of criteria for the division ofUniversal NetworkClassification criteria.According to this standard, various network types can be divided into LAN, MANWANAnd the Internet.Generally speaking, the LAN can only be in a small area, and the MAN is in different areasNetwork interconnectionHowever, it is important to note that the network division here does not strictly distinguish the geographical scope, but can only be a qualitative concept.These kinds of computer networks are briefly introduced below.[4]
LAN
LAN
（Local Area Network；LAN）Usually, our common "LAN" refers to the local area network, which is our most common and widely used network.With the development and improvement of the whole computer network technology, LAN has been fully applied and popularized. Almost every unit has its own LAN, and some even families have their ownSmall LAN。Obviously, the so-called LAN isLocal areasThe network within the range covers a small area.There is no too many restrictions on the number of computers configured in the LAN. The number of computers can be as few as two, and as many as hundreds.Generally speakingEnterprise LANIn China, the number of workstations ranges from dozens to 200.Generally speaking, the geographical distance involved in the network can be within a few meters to 10 kilometers.The LAN is generally located in a building or unit, and there is no path finding problem, not includingnetwork layerApplication of.[4]
This network is characterized by narrow connection range, few users, easy configuration and high connection speed.At present, the fastest speed of LAN is today's10G EthernetHas.IEEEThe 802 Standards Committee ofEthernet（Ethernet）、Token Ring Network（Token Ring）Optical fiber distributed interface network（FDDI）, Asynchronous Transfer Mode Network (ATM) and the latestWireless LAN（WLAN）。These will be described in detail later.[4]
Metropolitan area network
Schematic diagram of metropolitan area network
(Metropolitan Area Network; MAN) Generally speaking, this kind of network refers to the interconnection of computers in a city but not in the same geographical area.The connection distance of this network can be 10~100km, and it adopts IEEE802.6 standard.Compared with LAN, MAN extends a longer distance and connects more computers, which can be said to be an extension of LAN network in geographical scope.In a large city or urban area, a MAN network is usually connected to multiple LAN networks.Such as the LAN connecting government agencies, hospitals, telecommunications, companies and enterprises.becauseOptical fiber connectionThe introduction of makes the high-speed LAN in MANinterconnectionIt's possible.[4]
MAN is mostly usedATM technologydobackbone network。ATM is a technology used for data, voice, video and multimediaapplication programHigh speed ofnetwork transmissionmethod.ATM includes an interface and a protocol, which canBit rateSwitch between constant and changing traffic.ATM also includes hardware, software andATM protocolMedia with consistent standards.ATM provides a scalable backbone infrastructure to adapt to different sizes, speeds andAddressing technology's network.The biggest disadvantage of ATM is that the cost is too high, so it is generally used in government MAN, such as postal services, banks, hospitals, etc.[4]
WAN
Wide area schematic diagram
（Wide Area Network，WAN）This kind of network, also known as remote network, covers a wider range than MAN. It is generally interconnected between LANs or MAN networks in different cities, and its geographical range can range from hundreds to thousands of kilometers.Because of the distance,Information attenuationIt is very serious, so this kind of network generally needs to rent a special line, and connect it with the line through the IMP (interface information processing) protocol to formReticular structureAnd solve the routing problem.This kind of MAN is connected to many usersExit bandwidthLimited, so the terminal connection rate of users is generally low, usually 9.6 Kbps - 45 Mbps, such as:CHINANET，CHINAPAC, and CHINADDN network.[4]
The above mentioned several classifications of networks. In fact, in real life, we really encounter the LAN most, because it can be large or small, and it is relatively easy to implement in both units and families. It is also the most widely used network, so it is necessary for us toAccess equipmentMake a further understanding.[4]
Wireless network
Wireless network
along withNotebook computer（notebookcomputer）AndPersonal digital assistant(Personal Digital Assistant, PDA) and other portable computers are increasingly popular and developed, and people often have to be on the roadAnswer the phone, sending a fax andE-mailRead online information and log in to remote machines.However, it is impossible to connect with the network of the unit through wired media on the car or aircraftWireless networkI'm interested.Although the wireless network andmobile communicationThey are often linked, but the two concepts are not identical.For example, when a portable computer passes throughPCMCIA cardConnect to the telephone socket and it will becomeWired networkPart of.On the other hand, the location of some computers connected through wireless networks may be fixed. For example, computers in the two buildings can be connected together through wireless networks between buildings that are not convenient to be connected through wired cables.[4]
Wireless network, especially WLAN, has many advantages, such as easy installation and use.But there are many WLANdeficiencies: like itsData transmission rateGenerally low, much lower thanWired LAN；In addition, the wireless LANerror rateIt is also relatively high, and the interference between the stations is relatively severe.There are different ways to implement user wireless networks.Some foreign universities have installed many antennas on their campuses, allowing students to sit under trees and view library materials.This situation is realized by digital communication between two computers directly through wireless LAN.Another possible way is to use traditionalAnalog ModemadoptCellular telephone systemCommunication.Honeycomb has been available in many foreign citiesDigital informationGrouping data（ Cellular Digital Packet Data，CDPD）So that you canCDPD systemDirectly establish wireless LAN.wireless networkIt is currently a research hotspot at home and abroad, and the research of wireless network is made up ofmarket demandDriven.The characteristic of wireless network is that users can access computer network at any time and anywhere, which makes it have a strong application prospect.At present, there are many wireless network based products, such asPersonal communication system（ Personal CommunicationSystem，PCS）Telephone, wirelessData terminalPortableVideophone, personal digital assistant (PDA), etc.The development of wireless network depends onWireless communication technologySupport.wirelesscommunication system Mainly: low-powerCordless Telephone System , analog cellular systemDigital cellular system、Mobile satellite system, wireless LAN, wireless WAN, etc.[4]
performance
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The performance of computer network generally refers to its several importantperformance index 。But in addition to these important performance indicators, there are also some non performance characteristics that have a great impact on the performance of computer networks.[5]
1. Performance index of computer network
Performance indicators measure the performance of computer networks from different aspects.[5]
(1) Rate
The signals sent by the computer are all in digital form.BitIt is the unit of data quantity in the computerinformation theoryUsed ininformation contentUnits of.The English word bit comes from binary digit, which means a“Binary digit”, so a bit is a 1 or 0 in a binary number.In network technology, the speed refers to that the host connected to the computer networkDigital channelThe rate at which data is uploaded, also known asdata rate(data rate) orBit rate（bit rate）。Rate is the most important performance index in computer network.The unit of rate is bit/s（Bits per second）(i.e. bit per second).[5]
(2) Bandwidth
"Bandwidth" has the following two different meanings.[5]
①bandwidthIt originally refers to the bandwidth of a signal.The bandwidth of a signal refers to the bandwidth occupied by various frequency components contained in the signalfrequency range 。For example, in the traditionalCommunication lineThe standard bandwidth of the uploaded telephone signal is 3.1kHz (from 300Hz to 3.4kHz, that is, the frequency range of the main components of voice).The unit of bandwidth in this sense is Hz (orKilohertz，Megahertz，GihEtc.).[5]
② In a computer network, bandwidth is used to indicate the ability of the communication line of the network to transmit datanetwork bandwidth It refers to the "highest data rate" that can be passed from one point to another in the network in unit time.Generally speaking, "bandwidth" means this.The unit of bandwidth in this sense is "bits per second", which is recorded as bit/s.[5]
Throughput represents the amount of data passing through a network (or channel, interface) in a unit time.Throughput is more often used as a measure of the network in the real world, so as to know how much data can actually pass through the network.Obviously, the throughput is limited by the bandwidth of the network or the rated rate of the network.For example, for a 100Mbit/sEthernetThe rated rate is 100Mbit/s. This value is also the absolute upper limit of the throughput of the Ethernet.Therefore, the typical throughput of 100Mbit/s Ethernet may only be 70Mbit/s.Sometimes the throughput can also be determined by the number of bytes transferred per second orFramesTo represent.[5]
Delay refers to the time required for data (a message or packet, or even bits) to be transmitted from one end of the network (or link) to the other.Delay is a very important performance indicator, which is sometimes called delay or delay.The delay in the network is composed of the following different parts.[5]
The transmission delay is sent by the host or routerData frameThe time required is the time from the first bit of the data frame to the last bit of the frame.[5]
It can be seen that for a certain network, the transmission delay is not fixed, but is related to theFrame length(in bits) is proportional toChannel bandwidthInversely.[5]
Propagation delay iselectromagnetic waveThe time it takes to propagate a certain distance in the channel.The calculation formula of propagation delay is:[5]
Propagation delay=channel length (m)/electromagnetic wave on the channelPropagation rate（m/s）[5]
Electromagnetic wavefree spaceThe propagation rate of is the speed of light, namely 300000km/s.Electromagnetic wave in the networkTransmission mediaThe propagation rate in is slightly lower than that in free space.[5]
③ Processing delay.
When a host or router receives a packet, it needs to spend a certain amount of time to process it, such as analyzing the header of the packet, extracting data from the packet, performing error detection or finding an appropriate route, which results in processing delay.[5]
Grouping afternetwork transmissionYou have to go through many routers.However, after entering the router, packets must queue up in the input queue for processing.After the router has determined the forwarding interfaceOutput QueueQueue for forwarding.This results in queuing delay.[5]
Thus, the total delay experienced by data in the network is the sum of the above four delays:[5]
Total delay=sending delay+propagation delay+processing delay+queuing delay[5]
(5) Delay bandwidth product
Multiply the two measures of network performance discussed above propagation delay and bandwidth to get another very useful measure: propagation delay bandwidth product, real-time delay bandwidth product=propagation delay × bandwidth.[5]
(6) Round trip time (RTT)
In the computer network, the round trip time is also an important performance indicator, which indicates that fromSenderThe total time from the start of sending data to the time when the sender receives the confirmation from the receiver (the receiver sends the confirmation immediately after receiving the data).[5]
The utilization rate includes channel utilization rate and network utilization rate.Channel utilization refers to how many percent of the time a channel is used (with data passing through), and the utilization rate of a completely idle channel is zero.The network utilization rate is the channel utilization rate of the whole networkweighted average。[5]
2. Non performance characteristics of computer network
These non performance characteristics have a lot to do with the performance indicators introduced earlier.[5]
(1) Fees
That is, the price of the network (including the cost of design and implementation).The performance of the network is closely related to its price.Generally speaking, the higher the network speed, the higher the price.[5]
(2) Quality
The quality of the network depends on the quality of all components in the network and how these components form the network.The quality of the network affects many aspects, such as network reliabilityNetwork managementThe simplicity of, and some network performance.However, the performance of the network is not the same as the quality of the network. For example, some networks with good performance fail after running for a period of time and become unable to continue working, indicating that their quality is poor.High quality networks often cost more.[5]
Reliability is closely related to the quality and performance of the network.The reliability of the network with higher speed is not necessarily worse.However, it is often more difficult and costly for higher speed networks to operate reliably.[5]
When constructing the network, we should take into account that it may need to expand (that is, expand the scale) and upgrade (that is, improve the performance and version) in the future.The higher the performance of the network, the higher the expansion cost, and the difficulty will increase accordingly.[5]
(6) Easy to manage and maintain
If the network is not well managed and maintained, it is difficult to achieve and maintain the designed performance.[5]
Related applications
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In the 21st century, mankind will enter the information age in an all-round way.The important characteristics of the information age are digitalization, networking and informatization.To realize informatization, we must rely on a perfect network, because the network can transmit information very quickly.So the network has now becomeinformation societyThe lifeblood and development ofknowledge economyThe important foundation of.Network paircivil lifeMany aspects of and have had an immeasurable impact on social and economic development.[3]
The network here refers to "three networks", that istelecommunication network、Cable TVNetwork and computer network.These three networks provide different services to users.Users of telecommunication networks can receive telephone, telegraph, fax and other services;Users of cable TV network can watch various TV programs;Computer networks enable users to transmit quicklydata file, as well as finding and obtaining various useful materials from the network, including images andvideo file 。These three networks all play a very important role in the process of informatization, but computer networks are the fastest growing and play a central role.With the development of technology, telecommunications networks and cable television networks are gradually integrated into modern computer networks (also known asComputer communication network）The technology of“Network convergence”The concept of.[3]
Since the 1990s, the computer network represented by the Internet has developed rapidly. It has gradually developed from the initial educational and scientific research network to a commercial network, and has become second only to the global networktelephone networkOfSecond in the worldLarge network.The Internet is changing all aspects of our work and life. It has brought great benefits to many countries and accelerated the globalinformation revolutionThe process of.The Internet is humanTypographyThe biggest change in communication since its invention.Nowadays, people's life, work, study and communication are inseparable from the Internet.[3]
The computer network provides users with two most important functions, namelyConnectivityAnd share.[3]
The reason why so many computer networks are established is mainly because the use of computer networks is favored by individuals and companies.[3]
1、 Commercial application.
1. Mainly to realize resource sharing and finally breakgeographical positionTyranny of geography, mainly using the client server model-servermodel）。[3]
2. Provide powerfulCommunication media（communication medium）。For example:E-mail(E-mail), video conference.[3]
3. E-commerce activities.For example: purchase by different suppliersSubsystemAnd then assemble these parts.[3]
4. Do various transactions with customers through the Internet.For example, bookstores, audio and video stores buy goods or services at home.[3]
2、 Family use
1. Access remote information.For example, browse the Web page to get information about art, business, cooking, government, health, history, hobbies, entertainment, science, sports, tourism, and so on.[3]
