Computer Network Technology

Chapter 1: Overview of Computer Networks

Section 1: Overview of Computer Networks

Knowledge Essentials

I. Definition of Computer Networks

Computer network refers to a system that connects a number of computers distributed in different geographic locations with independent functions using communication devices and communication links to realize resource *** enjoyment, information transfer (data communication) under the management and coordination of network operating systems, network protocols, and network management software. Communication links connected, in the network operating system, network protocols and network management software management and coordination, to achieve resource **** enjoyment, information transfer (data communication) system.

Computer network is a product of the combination of modern computer technology and communication technology.

Second, the development

The first generation: terminal-oriented computer communications network, using circuit switching, essentially a star-shaped network centered on the host.

The second generation: packet-switched network, centered on the communication subnet, multiple computers and terminals constitute the periphery of the resource subnet, the data exchange method using packet switching.

The third generation: "open systems interconnection as a reference model" as the standard framework, the mid-80s, the emergence of the Internet (TCP/IP).

The fourth generation: broadband integrated services digital network (B-ISDN), 93 years the U.S. government proposed "information superhighway". The "information superhighway" is characterized by wide-area, high-speed and interactive.

Three, the system composition

Computer network is a product of the close combination of computer technology and communications technology, but also after the newspaper, radio, television, the fourth media.

From the point of view of network topology, computer network is composed of some network nodes and communication links connecting these network nodes.

In terms of logical function, a computer network is composed of a communication subnetwork and a resource subnetwork. Communication subnet is the part of the computer network responsible for data communication, mainly to complete the transmission of data between computers, exchange, and communication control, which consists of network nodes, communication links; resource subnet to provide access to the network and the ability to process data, is composed of the host system, terminal controllers and terminals, the host system is responsible for the data processing of the local or the entire network, running a variety of applications or large-scale databases to provide a variety of hardware and software resources to the network users. The host system is responsible for local or network-wide data processing, running various applications or large-scale databases, and providing various software and hardware resources and network services to network users. The terminal controller connects a group of terminals to the communication subnet and is responsible for controlling the terminals as well as receiving and sending terminal information.

From the point of view of system composition, computer network is composed of network hardware system and network software system.

1, network hardware system

Network hardware system generally refers to the hardware equipment that constitutes a computer network, including various computer systems, terminals and communication equipment.

(1) host system: the main body of the computer network, according to the different functions and uses in the network can be divided into servers and workstations.

Server is through the network operating system for online workstations to provide services and *** enjoy the resources of computer equipment; network workstations are connected to the network of computers, also known as the client, it is the network data is the main place of occurrence and use of the place in addition to maintaining the original function of the user services, but at the same time in accordance with the authority granted to access the server, the user is mainly through the use of the workstation for the use of network resources and complete the workstation. Workstation for the use of network resources and to complete their work. Workstations can be divided into diskless workstations and workstations with disk two kinds of workstations with disk is a microcomputer with a hard disk, itself has an independent function, with local processing capabilities. While diskless work is a microcomputer without hard disk, its boot program is stored in the network adapter EPROM, automatically executed after power-up, and connected to the server in the network. This type of workstation not only prevents computer viruses from infecting the server through the workstation, but also prevents illegal users from copying data in the network.

(2) terminal: itself does not have the ability to process, can not be directly connected to the network, only through the network host connected to the network and play a role in the common display terminal, print terminal, graphics terminal.

(3) transmission medium: between the network equipment constitutes a physical path in order to realize the exchange of information. The most common are coaxial cable, twisted pair, optical fiber.

(4) network interconnection equipment: used to realize the interconnection between the network, mainly repeaters, hubs, routers, switches and so on.

(5) network access devices: used to connect computers and computer networks for the equipment, common network cards, modems and so on.

2, network software system

Network software mainly includes network operating systems, network communication protocols and various network applications.

Operating system: including server operating system and workstation operating system.

Server operating system: generally multi-tasking, multi-user, it is installed on the server, mainly bear the network scope of resource management and distribution, access to network equipment, access to support communication between network users. Common windowsNT/windows server2000/netware/unix/linux, etc..

Workstation operating system: a strong support for the machine's processing power, responsible for the normal management of the machine's resources. The common ones are window98/windows2000/dos, etc..

Communication protocols: the network between computers, network devices and computers, network devices to communicate between the two sides to follow the rules of communication agreement. Commonly, there are packet switching protocol IPX, Transmission Control Protocol/Internet Protocol (TCP/IP), Ethernet protocol and so on.

Network management software: software systems used to provide information statistics, reports, warnings, and monitoring of network operating conditions.

Four, Classification

1, according to the scope of network coverage: wide area network (WAN), local area network (LAN), metropolitan area network (MAN). MAN). Comparison of the three types of networks is shown in the following table:

Table Comparison of the three types of networks

Type Distribution Range Transmission Rate Applications

LAN 1KM or so 1M or more a unit

MAN 5-50KM 1M a city

WAN dozens of thousands of KM or more than a couple of M a country or an intercontinental network

2, according to the topology of the network: star network (WAN), local area network (LAN), and MAN. Network topology: star, tree, bus, ring, mesh, hybrid.

3, according to the transmission medium: coaxial cable network, twisted pair network, fiber optic network, satellite network, wireless network.

4, according to the bandwidth and transmission capacity: baseband network (narrowband network) and broadband network (multimedia).

5. According to the nature of the use of the network: public network, private network.

6, according to the switching function of the network: circuit-switched network, message-switched network, packet-switched network, frame relay network, ATM network.

7, according to the control mode: centralized, decentralized, distributed.

Note: intranet, also known as intranet, serves the enterprise network, integrating LAN, WAN and data services as a whole, adopting the relevant technology of internet, and the same use of TCP/IP communication protocol for data communication.

V. Functions

The basic purpose of establishing a computer network is to realize data communication and resource **** enjoyment. Its main functions are:

1, data communication

Fax, e-mail, electronic data interchange (EDI), electronic bulletin board (BBS), video on demand (VOD), remote login and information browsing.

2, resource **** enjoy

*** enjoy the resources mainly refers to the computer system software, hardware and data; **** enjoy refers to the network users can enjoy all or part of the resources of the network of each computer system, and the user does not need to know the physical location of the resources.

3, improve the reliability and availability of the computer system

Each computer in the network can become a backup machine through the network, once a computer failure, its task can be completed by other computers; balanced load, thus improving the availability of each computer.

4, support for distributed information processing

Through the algorithm will be a large-scale integrated problem, to different computers to deal with the user can be based on the need to reasonably select the network resources, close to the rapid processing; on the other hand, the use of network technology will be connected to multiple computers into a high-performance computer system to solve large-scale problems, but also than the use of medium-sized and large-scale computer with the same performance to save money. computer to save money.

Knowledge

I. Basic Concepts

1, data communication: refers to the combination of computer technology and communication technology to realize the transmission, exchange, storage and processing of information.

2. Data: there are two types of data available in the network, namely, analog data with continuous values and digital data with discrete values.

3. Signal: electromagnetic wave or electrical coding of data, which is a concrete representation of data.

Based on the form of electrical signals are divided into two categories: analog signals with continuous values and digital signals with discrete values.

4, channel: is a signal transmission channel, can be divided into physical channel and logical channel, the transmission medium is generally known as the physical channel, according to the different transmission medium can be divided into wired channel and wireless channel, usually the channel is more focused on the meaning of the logic of the logical channel.

5, data transmission rate: usually measured in bit rate, that is, the number of bits of binary data transmitted per unit of time, usually b/s. Digital signals modulated transmission rate, that is, the number of electrical signals transmitted per unit of time, also known as the baud rate, which is also used as a physical channel performance is good or bad, the data transmission rate, the unit is baud.

6, channel capacity: the maximum data transmission rate allowed by the channel.

7. Throughput: the total amount of information that the entire network can process per unit of time, in bytes/second or bits/second.

8, channel bandwidth: is the channel can transmit the frequency width of the signal, its value for the channel can transmit the signal on the highest frequency and the lowest frequency difference. The larger the bandwidth, the greater the transmission speed that can be achieved.

9, BER: refers to the data transmission error data accounted for the proportion of the total number of data being transmitted.

The performance indicators of a channel are channel capacity, channel bandwidth, throughput, BER and so on.

Two, physical channel classification

1, according to the transmission medium: wired channel (such as twisted pair, coaxial cable, fiber optic cable), wireless channel (microwave, infrared, laser).

2, according to the form of the transmitted signal: analog channel, digital channel.

3, according to the use of: dedicated channel, public channel.

Three, transmission technology

1, baseband transmission and frequency band transmission

(1) baseband transmission

Baseband refers to the basic frequency band inherent in the electrical signal.

Baseband transmission refers to the transmission of digital signals from digital devices into the channel as they are.

(2) Band transmission

The digital signal from a digital device is modulated into an analog signal and then sent and transmitted, and when it reaches the receiving end, the analog signal is demodulated into the original digital signal for transmission.

(3) broadband transmission

Multi-way baseband signals, audio signals and video signals are modulated and put into the same cable at different frequency bands for transmission. The broadband transmission system can realize the integrated transmission of text, sound and image.

2, asynchronous transmission and synchronous transmission

(1) Asynchronous transmission

The transmitter adds a start bit and two stop bits at the beginning and end of each character when sending a character to indicate the beginning and end of the character, and can only send and receive one character at a time. The terminal device that has data to send can send a signal to the channel at any time, regardless of whether the receiver knows that it has begun to send the operation, and because the time interval between sending each character is arbitrary, the characters are asynchronous, so it is called asynchronous transmission.

(2) Synchronous transmission

In the synchronous mode, the transmitter sends a string of characters (or data blocks) one character immediately after another, only one byte of synchronous character SYN is appended before and after each data block, and the receiver relies on the character only to identify the data to be received. Synchronous transmission is a process of receiving and transmitting at the same speed, that is, the receiving end according to the frequency of the signal sent by the sender of the pound end of the signal to receive the signal, the receiving end of the receiving end to calibrate their own receiving time and repetition frequency, in order to with the sender of the signal in line with the process.

Table Asynchronous transmission compared with synchronous transmission

Transmission mode Transmission unit Advantages Disadvantages

Asynchronous transmission Characters Simple control, inexpensive, low efficiency, slow rate

Synchronous transmission Telegrams or packets Transmission efficiency Higher BER, control complexity

3, Simplex, half-duplex, full-duplex

Three communication methods Comparison of the three types of communication

Table Comparison of the three types of communication

Communication mode Transmission direction Number of channels Receiver and sender restrictions Advantages and disadvantages Application

Simplex Fixed unidirectional 1 One party can only send and the other party can only receive Simple structure, low efficiency, can only transmit information unidirectionally Broadcasting, television

Semi-duplex Time-limited bi-directional 2 Both parties can send or receive information at different moments respectively. or receive information at different moments Low efficiency Walkie-talkies, etc.

Full-duplex Bidirectional 2 Both sides of the communication can send and receive information at the same moment Structural complexity, high cost, the best performance Between computers

Simplex: allows data to be transmitted in a specified direction only, only one channel is needed, simple structure.

Half-duplex: data is transmitted in only one direction on the channel at any given moment, and in the opposite direction at another moment. In less demanding occasions, this communication method is mostly used, such as aviation and marine radio and walkie-talkie and most of the computer network in the data communication, etc., two channels are required.

Full-duplex: Allows data to be transmitted in both directions simultaneously. This method is the most efficient and easy to use, and is commonly used for computer-to-computer communication, which requires two channels to transmit signals in both directions, with each end sending information while also receiving it.

Full-duplex requires two independent channels, which can be realized using two sets of transmission lines or by multiplexing. This communication method has the best performance, requires the most complex equipment, and has the highest cost of realization.

Four, data exchange technology

In computer communications, two computers using communication lines, through a number of intermediate nodes or transit nodes of the computer network for transmission, intermediate nodes of the computer or computer network in the transmission of information does not care about the specific content of the information, is only responsible for the transmission of information from one node to the other node computer, until the information is transmitted to the destination. Transmission to the destination, we will participate in this communication by the intermediate nodes called "data exchange". Traditional data exchange can be divided into: circuit switching and storage switching, storage switching can be divided into message switching and packet switching, currently used frame relay, asynchronous transfer mode (ATM) belongs to the fast packet switching technology.

1, circuit switching technology

Also known as line switching technology, it is a channel-based **** enjoyment, similar to the telephone, must go through the establishment of the connection (channel establishment), the transmission of data, dismantling of the connection (release the channel) of the three communication processes, suitable for long-distance batch transmission of data.

Table Comparison of Data Switching Technologies

Switching Technologies Feature Working Position Advantages Disadvantages

Circuit Switching A physical channel or sub-channel is established before communication, and the channel is released after communication is finished, in units of bits Physical Layer Simple equipment and operation Good real-time performance Low utilization of static allocation of channels and lines

Store Switching Message Switching Store-and-forward Transmission of messages in units of bits, and the transmission of data in units of messages, and the transmission of data in units of bits, and the transmission of data in units of bits. Message-based transmission, and unlimited length Network layer High channel utilization, high reliability, both sides can not work at the same time Poor real-time, packet switching nodes must have high processing power

Packet switching Packet-based transmission, the length of which is generally 1~nKbit Network layer High line utilization, fast rate (64Kbps), high throughput, low BER, and good flexibility

Table Comparison of Two Typical Packet Switching Technologies

Switching Technology Working Position Characteristics Disadvantages

Frame Relay Link Layer Frame-based, receive-and-forward with routing function High transmission rate 2.048Mbps

Asynchronous Transfer Mode (ATM) Link Layer Unit of credits, credits are fixed at 53B, sender doesn't occupy the time slice Real-time performance is good. BER low transmission rate of 2.2Mbps delay is small

2, storage and switching technology

Communication between the two sides do not have to occupy a complete physical channel, the data unit being transmitted contains the destination address, the intermediate node is always the first to the node will be transmitted to this node of the data unit stored in the node, and then look for a free path forwarded to the next node.

Storage switching can be divided into message switching and packet switching, the main difference between the two is that the data unit of transmission is different, the data unit of message switching is a message without any limitation on its length, while packet switching has a clear regulation on the length of the data unit, which is generally from 1,000 to a few thousand bits, so in the packet switching mode, the long message has to be divided into several short packets first, and then transmitted as a packet. packets are transmitted.

Knowledge

I. Basic Concepts of Network Architecture

1. Network Protocols

A communication language used to communicate between entities in a computer network, which is a set of conventions or rules about the order of information transmission, information format, and information content. A network protocol contains three elements namely semantics, syntax and timing.

Semantics: refers to the meaning of the protocol elements that make up the protocol, different types of protocol elements specify different content to be expressed by the communicating parties, while the protocol elements refer to the control messages or commands and responses.

Syntax: refers to the form or format of the data structure of data or control information.

Timing: also known as rules, the order in which events are executed.

2, network hierarchical structure

For complex computer network protocols, the top-down approach is usually used to gradually refine the use of layered network structure, there is the use of layered approach to the practice, you can make each layer to achieve a relatively independent function, so that a difficult to deal with the complexity of the problem is broken down into a number of easier to deal with the small problem, and each layer is to its previous layer to provide services. Each layer provides services to the layer above it. The advantages of using a layered structure include:

(1) Each layer is independent of the other

(2) Flexibility

(3) Easy to standardize

(4) Each layer can choose the most appropriate implementation technology

3. Network Architecture: The hierarchy of a computer network and its set of protocols.

4. Network topology: also known as network structure, refers to the geometric location of network nodes and links. Junction refers to the network units that make up the network, such as: hosts, hubs, routers, etc., according to the different functions can be divided into endpoints and transfer points, end node refers to the source or host node of the communication, also known as the access point, such as hosts or terminals; transfer junction poof refers to the process of network communication with the role of the node of the control and forwarding of information, such as hubs, switches, etc..

5, link: the line between two nodes.

Two common network topology

The choice of network topology is closely related to the choice of transmission and output media and media access control methods. Common topologies are:

1, star

All computers are connected to a central node, the central node is usually the host or hub. The center node is responsible for receiving information from workstations and then forwarding it to the corresponding workstations, which has relay and data processing functions.

2, ring type

Connected into a closed loop network nodes, each node and its left and right neighboring nodes to connect and ultimately form a "ring", one-way transmission of information point by point, the nodes into the network of computers connected to the ring through the repeater signals to the same speed, the same direction to the next node. direction to the next node.

In this type of network, the token delivery method to solve the access control of the ring, the token is a kind of pass sign, it can be one or more binary digits composed of the code, only to obtain the token of the site can send data, because the token is only one, so there will not be a collision. More typical is IBM's token ring network.

3. Bus type

A way of connecting all user nodes using the same media or cable. Bus topology with a passive communication line as the backbone, the network computer through the appropriate interface (such as T-header) connected to the line, the backbone cable is known as the bus.

Because all sites **** with a cable, so there can only be one device at a time to transmit information, prone to collision, in order to prevent signal reflection, all the computers connected to a communication transmission line in the line at both ends of the device must be installed to prevent the signal reflection of the device that is the end of the connector. Commonly used Ethernet that is the use of bus-type network topology, in order to prevent collisions, the use of IEEE802.3 of the CSMA/CD for media access control methods.

The above three types are the most basic types of network topology, and are also the three commonly used network topologies in LANs, in addition to tree-type, mesh-type. In practical applications often use some combination of them. Comparison of several types of network structure is shown in the following table:

Table Comparison of several types of network structure

Network type Features Advantages Disadvantages Applications

Star From the node to the node must pass through the center of the node to communicate with the structure of a simple protocol is easy to detect and isolate the failure of the cost of the center of the node failure will result in the paralysis of the entire network Intelligent Building from the node to the node of the network with fewer exchanges of data

Bus type Only one channel, only one node can send data at a moment Low cost Easy wiring Easy maintenance Difficulty in detecting faults Competitive use of bus LAN or distributed processing, such as Ethernet

Ring type Unidirectional transmission along the ring Simple structure, good performance, suitable for fiber optic connection Poor reliability Difficulty in reconfiguration LANs such as FDDI, IBM Token Ring Network

Tree type Extension of the star type, the root node and subtree nodes can be used as a transfer node Performance of the same star type, the cost is lower than the star type Large latency Hierarchical management of the network

Mesh type Each node has at least two links to the other nodes Performance, high reliability, complex structure, control of complex and cumbersome large wide area networks

The performance of the network is largely determined by the network topology, the network topology, the topology, the topology, the topology, the topology, the topology, the topology, the topology, the topology, the topology, and the topology. When choosing a network topology, you should generally consider three main factors: reliability, expandability and cost. (To be continued)