foreword
Today, I also want to enter the entire content of the computer network. Although the content of the computer network accounts for a relatively small proportion of the part of the postgraduate entrance examination, some people do not regard this part as the key point. This kind of thinking is wrong. I think these four contents of the postgraduate entrance examination are very important. We need to go all out to treat each content, with improving our own quality as the main task.
In this way, this blog may be difficult to understand. You can be confused after reading it. There will be a lot of nouns in this blog, so be prepared!
【Syllabus content】
1. Overview of computer network
- The concept, composition and function of computer network; classification of computer network
- Performance Indicators for Computer Networks
2. Computer network architecture and reference model
- Computer network layered structure; concept of computer network protocols, interfaces, services
- ISO/OSI reference model and TCP/IP model
【pay attention】
This chapter mainly introduces the basic concepts of computer network architecture , readers can memorize properly on the basis of understanding. Focus on mastering the layered structure of the network (including 5-layer and 7-layer structure) , especially the functions of each layer of the ISO/OSI reference model and the concepts of related protocols, interfaces and services . Master various performance indicators of the network . Especially the calculation of delay , bandwidth , rate and throughput .
1. Overview of computer network
1.1 The concept of computer network
When it comes to computer networks, the first term that catches our eyes is: network . What is the "network" ? A network is a "net-like thing or mesh system".
So what are the commonly used networks in daily life? Commonly used networks in daily life include: neural networks in the human body , telecommunication networks , cable TV networks , power grids , and computer networks . Now there is a hot word on the Internet: triple play (four play integration) . After the integration of the three networks, there was the integration of the four networks. From this we can see the importance of computer network .
【Key point: the concept of computer network】 It is generally believed that a computer network is athat connects scattered computer systems with independent functions through communication equipment and lines , and uses fully functional software to realize resource sharing and information transmission. In short,a computer network is a collection of interconnected , autonomous computer systems.
To summarize some of the terms involved in the above :
- Triple play: In today's society, we can convert videos, pictures, etc. into 0/1 bit streams, so that we can ingeniously integrate the cable TV network with the telecommunication network through the computer network, so as to watch TV on the computer, Make phone calls on the computer.
- Communication equipment: routers, repeaters, etc.;
- Circuit: Logical circuit (wired and wireless)
- Interconnection: Interconnection through communication links;
- Autonomy: no master-slave relationship
1.2 Functions of computer network
There are many functions of computer network, and many applications are related to network. But the main five functions of the network will be introduced one by one below:
1.2.1 Data communication
It is the most basic and important function of computer networks (be sure to remember that there may be multiple choice questions) .
| Function | It is used to realize the transmission of various information between networked computers , and connect computers scattered in different geographical locations for unified deployment, control and management. |
| example | For example, applications such as file transfer and e-mail cannot be realized without a computer network. |
1.2.2 Resource Sharing
Resource sharing can be software sharing, data sharing, or hardware sharing.
| Function | The behavior of other computers on the same computer network using the computer resources of a certain computer can share hardware, software, and data. |
| advantage | It enables the resources in the computer network to communicate with each other and cooperate with each other, thus greatly improving the utilization rate of hardware resources, software resources and data resources . |
1.2.3 Distributed processing
Multiple computers each taking on different tasks of the same job.
| Function | When a certain computer system in a computer network is overloaded , it can distribute a complex task it handles to other computer systems in the network, thereby utilizing idle computer resources to improve the utilization rate of the entire system. |
| example | For example: Hadoop platform |
1.2.4 Improve reliability
Each computer in the computer network can replace each other through the network . It is equivalent to that if one computer fails at work, another computer can be used as an alternative computer to continue working through the network.
1.2.5 Load Balancing
| Function | Evenly distribute work tasks to computers in the computer network |
| advantage | This allows for more intimacy between the computers |
In addition to the above major functions, the computer network can also realize functions such as electronic office and service, distance education, entertainment, etc., which meets the needs of the society, facilitates people's study, work and life, and has huge economic benefits.
1.3 The development stage of computer network (understand)
The first stage:
The US Defense Advanced Research Projects Agency (ARPA) designed a decentralized command system --> ARPAnet (Arpanet) --> hoped to realize the interconnection of different networks --> accepted TCP/IP in 1983 and selected the Internet as the main computer communication System --> Internet (Internet)
The network connects many computers together, and the Internet connects many networks together. The Internet is the largest Internet in the world .
The second stage - three levels of structure:
In 1985, the US National Science Foundation (NSF) built a computer network around six large computer centers, the National Science Foundation Network (NSFNET).
The third phase:
Multi-level ISP structure, divided into backbone ISP, regional ISP, local ISP
1.4 Standardization of computer networks (not the focus)
We know that state-owned laws, families have family rules, cars have car dealerships, and the same is true for computer networks. Although this part is not the focus of the postgraduate entrance examination, standardization work is very important to computer networks. The standardization of computer networks has played an extremely important role in the development and promotion of computer networks. This is like a rule, and compliance with this rule can make it more orderly. In order to realize interconnection between hardware and software of different manufacturers, a unified standard must be followed.
All Internet standards are published on the Internet in the form of RFC (Request For Comments), but not every RFC is an Internet standard. RFCs need to go through the following four stages to become an official Internet standard :
- Internet-Draft - This stage is not yet an RFC document.
- Proposed Standards - from this stage onwards are RFC documents.
- Draft Standard (Canceled)
- internet standards
The current standard classification is: legal standard - OSI and de facto standard - TCP/IP .
In the world, there are many standardization organizations responsible for formulating and implementing relevant network standards, mainly as follows:
- International Organization for Standardization (ISO) - OSI reference model, HDLC, etc.;
- International Telecommunication Union (ITU) - develops a large number of standards related to telecommunications;
- Institute of Electrical and Electronics Engineers (IEEE) - 802 standard, 5G, academic institutions;
- Internet Engineering Task Force (IETF) - responsible for the development of Internet-related standards.
1.5 Composition of computer network
From different perspectives, the composition of computer networks can be divided into the following categories:
1) From the perspective of components , a complete computer network mainly consists of three major parts : hardware , software , and protocol , all of which are indispensable.
- Hardware: It is mainly composed of host (also called end system), communication link (such as twisted pair, optical fiber), switching equipment (such as router, switch, etc.) and communication processor (such as network card).
- Software: mainly includes various software for realizing resource sharing and various tool software for users' convenience (such as network operating system, mail program, FTP program, chat program, etc.). Most of the software part belongs to the application layer .
- Protocol: It is a collection of a series of rules and conventions , and is the core of a computer network . Just like traffic rules restrict car driving, the protocol stipulates the norms that the network follows when transmitting data.
2) From the perspective of working methods , the computer network (here mainly refers to the Internet, that is, the Internet) can be divided into the edge part and the core part .
- Edge part: It consists of all hosts connected to the Internet and directly used by users for communication (such as transmitting data, audio or video) and resource sharing. There are two ways for users to use directly: C/S way and P2P way .
- Core part: Composed of a large number of networks and routers connecting these networks , it provides connectivity and switching services for the edge part.
3) In terms of functional composition , a computer network consists of a communication subnet and a resource subnet .
- Communication subnet: It is composed of various transmission media, communication equipment and corresponding network protocols. It enables the network to have data transmission, exchange, control and storage capabilities, and realizes data communication between networked computers.
- Resource subnet: It is a collection of devices and software for resource sharing/data processing, and provides network users with the service of sharing hardware resources, software resources and data resources on other computers.
1.6 Classification of Computer Networks
1.6.1 Classification by distribution range (not only by distance, but also by what technology is used)
[Key points] 1. Wide area network (WAN).
- The task of the WAN is to provide long-distance communication and transport the data sent by the host.
- The WAN is a core part of the Internet .
- The links connecting the switches of each node of the WAN are generally high-speed links with large communication capacity.
- Traditionally, WANs use switching technology .
2. Metropolitan area network (MAN).
- The coverage of the metropolitan area network can span several blocks or even the entire city.
- Most of the metropolitan area network adopts Ethernet technology, so sometimes it is often incorporated into the discussion of the scope of the LAN .
【Key points】 3. Local area network (LAN).
- The local area network is generally connected by a computer or a workstation through a high-speed line.
- There are not too many restrictions on the number of computers configured in the LAN, as few as two, and as many as hundreds.
- Traditionally, LANs use broadcast technology .
4. Personal Area Network (PAN).
- A personal area network refers to a network that connects consumer electronic devices (such as tablet computers, smartphones, etc.) using wireless technology in the place where individuals work, also known as a wireless personal area network (WPAN).
Why does the editor write "not just look at the distance, but also look at what technology is used" in front of it?
The reason is as follows: Xiao Ming and Xiao Hong are a pair of good neighbors, one day the two of them chatted on the mobile phone. Although the distance between the two of them is very small, within the scope of the local area network, because the two people use switching technology when chatting, the two people still use the wide area network to chat.
1.6.2 Classification by Transmission Technology
1. Broadcast network.
- All networked computers share a common communication channel. When one computer sends a packet of messages over a shared communication channel, all other computers "listen" to the packet.
- A computer that receives the packet will check the destination address to decide whether to accept the packet.
- Local area networks basically use broadcast communication technology, and wireless and satellite communication networks in wide area networks also use broadcast communication technology.
2. Peer-to-peer network.
- Each physical line links a pair of computers. If there is no direct connection between the two communicating hosts, the packet transmission between them will be received, stored and forwarded through the intermediate node until reaching the destination node.
- The point-to-point network adopts the packet storage and routing mechanism, and the wide area network basically belongs to the point-to-point network.
1.6.3 Classification by topology
Network topology refers to the network structure represented by the geometric relationship (bus shape, ring, etc.) between nodes (routers, hosts, etc.) and communication lines (network lines) in the network, mainly referring to the topology of the communication subnet.
According to the topology of the network, it is mainly divided into bus, star, ring, mesh, etc., as shown in the figure below. Star, bus, and ring networks are mostly used for local area networks, and mesh networks are mostly used for wide area networks.
1) Bus network: connect computers with a single transmission line. Advantages: easy to build a network, convenient to add/reduce nodes, and save lines. Disadvantages: The communication efficiency is not high under heavy load, and any part of the bus is sensitive to faults.
2) Star network: Each terminal or computer is connected to the central device with a separate line. The central device was a computer in the early days and is now generally a switch or router. Advantages: Ease of centralized control and management, since communication between end users must go through a central device. Disadvantages: high cost, central equipment is sensitive to failure.
3) Ring network: All computer interface devices are connected into a ring. The ring can be a single ring or a double ring, and the signal in the ring is transmitted in one direction.
4) Mesh network: In general, each node has at least two paths connected to other nodes, and it is mostly used in wide area networks. It has two kinds of regular type and irregular type. Advantages: high reliability. Disadvantages: complicated control and high line cost.
1.6.4 Classification by User
1) Public network - refers to the large-scale network funded and constructed by telecom companies. "Public" means that all people who are willing to pay the fee according to the regulations of the telecommunications company can use this kind of network, so it is also called the public network.
2) Private network - refers to the network built by a certain department to meet the needs of the unit's special business. This network does not provide services to people outside the organization. For example, private networks of railways, electric power, military and other departments.
1.6.5 Classification by switching technology (will be added later)
Switching technology refers to the data format and the way of switching devices used to exchange information between hosts, communication devices, or hosts and communication devices.
1) Circuit switched network. A dedicated path is established between the source node and the destination node for data transmission, including three stages of connection establishment, data transmission and disconnection.
2) Message switching network. User data is added with auxiliary information such as source address, destination address, and check code, and then encapsulated into a message. The entire message is transmitted to the adjacent node, and after all is stored, it is forwarded to the next node, and this process is repeated until reaching the destination node. Each message can independently choose the path to reach the destination node.
3) Packet switching network, also known as packet switching network.
1.6.6 Classification by Transmission Media
Transmission media can be divided into wired and wireless two categories, so the network can be divided into wired network and wireless network. Wired network is divided into twisted pair network, coaxial cable network and so on. Wireless networks can be divided into Bluetooth, microwave, radio and other types.
1.7 Performance Indicators of Computer Networks
Common performance indicators can be roughly divided into two categories: one is related to speed, and the other is related to time. Therefore, bandwidth, rate, and throughput are in one category; delay, delay-bandwidth product, round-trip delay, and channel utilization are in one category.
Rate-related metrics
1.7.1 Rate
Let's introduce velocity first. The word velocity is not uncommon, and it is found in mathematics, physics, and chemistry. The rate in the network is not the same as them. The rate in the network refers to the rate at which the host connected to the computer network transmits data bits on the digital channel, also known as the data transmission rate, data rate or bit rate, and the unit is b /s (bits per second) (or bit/s, also sometimes written bps). When the data rate is high, it can be represented by kb/s, Mb/s, and Gb/s. In computer networking, the highest data transfer rate is often referred to as bandwidth.
1.7.2 Bandwidth
In rate, we know that bandwidth is the highest data transfer rate. It originally indicates the signal frequency band range that the communication line allows to pass through, and the unit is Hertz. In a computer network, bandwidth represents the ability of the communication line of the network to transmit data, and is a synonym for the "highest data transmission rate" that can be transmitted by a digital channel, and the unit is bit/second.
1.7.3 Throughput
Refers to the amount of data passing through a certain network (or channel, interface) per unit time. Throughput is limited by the network bandwidth or the rated rate of the network. To give an easy-to-understand example:
Latency related indicators
1.7.4 Latency
Refers to the total time required for data (a message or packet) to be transmitted from one end of the network (or link) to the other. It consists of 4 parts: transmission delay, propagation delay, processing delay and queuing delay .
| name | describe | Calculation formula |
| sending delay | The time taken for data to travel from the host to the channel | Packet Length/Channel Width |
| propagation delay | The time it takes for data to propagate on the channel | Channel length/speed at which electromagnetic waves propagate on a channel |
| processing delay | The time for data to wait for previous data processing in front of the router | no calculation method |
| queuing delay | Time for data to process demand in the router | no calculation method |
Note: When doing a question, the queuing delay and processing delay are generally negligible (unless the question requires it). For high-speed links, it is only the data transmission rate that is increased rather than the propagation rate of bits on the link. Increasing the data sending rate is just to reduce the data sending delay.
1.7.5 Delay-bandwidth product
| describe | Refers to the number of bits that the pipe can hold |
| official | Propagation Delay* Channel Bandwidth |
1.7.6 Round trip time delay RTT
| describe | From the time the sender sends data to the time when the sender receives an acknowledgment from the receiver (the receiver sends an acknowledgment immediately after receiving the data), the total time delay experienced |
| official | RTT= propagation delay x2+processing time (sometimes it may be ignored directly) |
1.7.7 Utilization
1.7.7.1 Channel utilization
Channel utilization = data passing time /with + no data passing time
1.7.7.2 Network Utilization
Network utilization = weighted average of all channel utilization
1.7.7.3 Relationship between delay and utilization
The higher the utilization, the greater the latency
Summarize
This article mainly introduces the overview of computer network, and understands the concept, composition, function, classification and performance indicators of computer network. Thank you for your praise and attention. I will work harder to simplify my notes.