home / Programs/ Hardware. Switching (computer networks) A device that switches several communication channels is called a hub

Hardware. Switching (computer networks) A device that switches several communication channels is called a hub

Independent work : pp. 646–651, 720–722, pp. 67–79, 542–544, –651, pp. 48–58; pp. 408-431

Repeater (repeater) – transmits electrical signals from one section of the cable to another, pre-amplifying them and restoring their shape. It is used in local networks to increase their length. In terminology OSI functions at the physical level.

Switches - multiport repeaters that read the destination address of each incoming package and transmit it only through the port that is connected to the receiving computer. Can function on different OSI layers... (another version - channel level)

Concentrator (hub) is a multi-port device for amplifying signals during data transmission. It is used to add workstations to the network or to increase the distance between the server and the workstation (the total bandwidth of the input channels is higher bandwidth output channel). Works like a switch, but in addition can amplify the signal.

Multiplexer (device or program) - allows you to transmit several different signals simultaneously over one communication line.

Gateway - transfers data between networks or application programs using different protocols (encoding methods, physical environments for data transfer), for example, connecting a local network to a global network. Functions on applied level.

Bridge - connects two networks with the same protocols, amplifies the signal and passes only those signals that are addressed to the computer on the other side of the bridge. Another edition : a computer with two network cards for connecting networks.

Router - (connects different LANs, like a bridge, passes only the information that is intended for the segment with which it is connected.) Responsible for choosing a route for transmitting packets between nodes. Route selection is based on: - routing protocol containing information about the network topology;

- a special routing algorithm.

Functions on network OSI layer.

Unclear questions :

A computer interface device with several communication channels is called:

- hub / repeater / multiplexer / modem

A device that switches several communication channels is called:

- data transmission multiplexer / hub / repeater / modem

Basic cryptography concepts

Independent work : pp. 695-699

Cryptography (encryption) - encoding the data sent to the network so that only the parties involved in a particular operation can read them. The reliability of protection depends on the encryption algorithm and the key length in bits.

Encryption method - an algorithm describing the order of converting the original message into the resulting one. Example ... Method gamming - replacing letters with notes according to a certain algorithm.

Encryption key - a set of parameters required for applying the method. Another revision: - a sequence of characters stored on a hard or removable disk.

Static key - does not change when working with different messages.

Dynamic key - changes for each message.

Types of encryption methods .

– Symmetrical : The same key is used for both encryption and decryption. Inconvenient in ecommerce, since the seller and the buyer must have different rights to access information. The seller sends the same catalogs to all buyers, but buyers return confidential credit card information to him, and orders and payments cannot be mixed for different buyers.

– Asymmetric (asymmetrical ): rely on special mathematical techniques that create a key pair so that what is encrypted with one key can only be decrypted with another, and vice versa. One of the keys is called open , anyone can get it. The developer of the key keeps the second key, it is called closed (secret) .

Orders, contracts encrypted with the public key, but can only be read by the owner of the private key. If a client received a file that his key does not match, then it was not sent by his company.

Independent work : pp. 646–651, 720–722,
pp. 67–79, 542–544, –651, pp. 48–58; pp. 408-431

Repeater (repeater) - transmits electrical signals from one section of the cable to another, pre-amplifying them and restoring their shape. It is used in local networks to increase their length. In terminology OSI operates at the physical layer.

Switches- multi-port repeaters that read the destination address of each incoming packet and transmit it only through the port that is connected to the receiving computer. Can function on different OSI layers. (another version - channel level)

Concentrator(hub) is a multi-port device for amplifying signals during data transmission. It is used to add workstations to the network or to increase the distance between the server and the workstation (the total bandwidth of the input channels is higher than the bandwidth of the output channel). Works like a switch, but in addition can amplify the signal.

Multiplexer(device or program) - allows you to transmit several different signals simultaneously over one communication line.

Gateway- transfers data between networks or application programs using different protocols (encoding methods, physical media for data transmission), for example, connecting a local network to a global one. Functions on applied level.

Bridge- connects two networks with the same protocols, amplifies the signal and passes only those signals that are addressed to the computer on the other side of the bridge. Another edition : a computer with two network cards for connecting networks.

Router- (connects different LANs, like a bridge, passes only the information that is intended for the segment with which it is connected.) Responsible for choosing a route for transmitting packets between nodes. Route selection is based on:
- a routing protocol containing information about the network topology;

- a special routing algorithm.

Functions on network OSI layer.

Unclear questions :

A computer interface device with several communication channels is called:

- hub / repeater / multiplexer / modem

A device that switches several communication channels is called:

- data transmission multiplexer / hub / repeater / modem

XXXIII. Basic cryptography concepts

Independent work : pp. 695-699

Cryptography (encryption)- encoding the data sent to the network so that only the parties involved in a particular operation can read them. The reliability of protection depends on the encryption algorithm and the key length in bits.

Encryption method- an algorithm describing the order of converting the original message into the resulting one. Example ... Method gamming - replacing letters with notes according to a certain algorithm.

Encryption key- a set of parameters required for applying the method. Another revision: - a sequence of characters stored on a hard or removable disk.

Static key- does not change when working with different messages.

Dynamic key- changes for each message.

Types of encryption methods .

– Symmetrical : The same key is used for both encryption and decryption. Inconvenient in e-commerce, since the seller and the buyer must have different rights to access information. The seller sends the same catalogs to all buyers, but buyers return confidential credit card information to him, and orders and payments cannot be mixed for different buyers.

Lecture number 8

Characteristics of information channels

The information channel can also be characterized by three corresponding parameters: the channel usage time T to, bandwidth of frequencies passed by the channelF k, and dynamic range channelD kcharacterizing its ability to transmit different levels signal.

The value is capacity channel.

Undistorted signal transmission is possible only on condition that the volume of the signal "fits" into the channel capacitance.

Consequently, the general condition for matching a signal with an information transmission channel is determined by the relation

However, the ratio expresses a necessary but insufficient condition for matching the signal with the channel. A sufficient condition is agreement on all parameters:

For information channel use the concepts: speed of information input, speed of information transfer and channel bandwidth.

Under the speed of information input (information flow) I ( X ) understand the average amount of information entered from the source of messages into the information channel per unit of time. This characteristic of the message source is determined only by the statistical properties of the messages.

Information transfer rate I ( Z , Y ) - the average amount of information transmitted over the channel per unit of time. It depends on the statistical properties of the transmitted signal and on the properties of the channel.

Throughput C - the highest theoretically achievable information transfer rate for a given channel. This is a channel response and is independent of signal statistics.

The bandwidth of the information channel is determined by two parameters: bit depth and frequency. It is proportional to their product.

Bit depth is the maximum amount of information that can be simultaneously placed in the channel.

Frequency shows how many times information can be placed in the channel during a unit of time.

The bit depth of the mail channel is huge. So, sending by mail, for example, a laser disk, you can simultaneously place more than 600 MB of information in the channel. At the same time, the frequency of the mail channel is very low - the seizure of mail from boxes occurs no more than five times a day.

The telephone channel of information is one-bit: at the same time, either one (current, impulse) or zero can be sent via the telephone wire. The frequency of this channel can reach tens and hundreds of thousands of cycles per second. This property of the telephone network allows it to be used for communication between computers.

With the aim of the most effective use information channel, it is necessary to take measures to ensure that the information transfer rate is as close as possible to the channel capacity. At the same time, the speed of information input should not exceed the bandwidth of the channel, otherwise not all information will be transmitted over the channel.

This is the main condition for dynamically reconciling the message source and the information channel.

One of the main issues in the theory of information transmission is to determine the dependence of the information transmission rate and bandwidth on the channel parameters and characteristics of signals and interference. These questions were first deeply investigated by K. Shannon.

1. Methods for increasing noise immunity

The basis of all methods of increasing the noise immunity of information systems is the use of certain differences between a useful signal and interference. Therefore, to combat interference, a priori information about the properties of the interference and the signal is required.

Currently, a large number of ways to improve the noise immunity of systems are known. It is convenient to divide these methods into two groups.

Igroup - based on the choice of the message transmission method.

IIgroup - associated with the construction of noise-immune receivers.

A simple and applied way to increase noise immunity is increase in signal-to-noise ratio by increasing the power of the transmitter. But this method may not be economically viable, since it is associated with a significant increase in the complexity and cost of equipment. In addition, an increase in the transmission power is accompanied by an increase in the interfering effect of this channel on others.

An important way to improve transmission noise immunity continuous signals is an rational choice of the type of modulation signals. By using the types of modulation that provide a significant expansion of the signal bandwidth, it is possible to achieve a significant increase in the noise immunity of the transmission.

A radical way to improve transmission noise immunity discrete signals is the use special anti-jamming codes ... In this case, there are two ways to increase the noise immunity of codes:

1. Selection of such transmission methods that provide a lower probability of code distortion;

2. Increase of correcting properties of code combinations. This way is associated with the use of codes to detect and eliminate distortions in code combinations. This coding method is associated with the introduction of additional, redundant symbols into the code, which is accompanied by an increase in the transmission time or the transmission frequency of the code symbols.

An increase in transmission noise immunity can also be achieved by repeating the transmission of the same message. On the receiving side, the received messages are compared and those that have the largest number of matches are accepted as true. To eliminate uncertainty in the processing of the received information and to ensure selection by the majority criterion, the message should be repeated at least three times... This method of increasing noise immunity is associated with an increase in the transmission time.

Repetitive transmission systems discrete information They are divided into systems with group summation, in which comparison is made by code combinations, and into systems with symbol-by-symbol summation, in which comparison is carried out by symbols of code combinations. A character-by-character check is more efficient than a group check.

A variety of systems in which an increase in noise immunity is achieved by increasing the transmission time are systems with feedback... In the presence of distortions in the transmitted messages, the information arriving through the reverse channel provides a repetition of the transmission. The presence of a return channel leads to the complication of the system. However, unlike systems with repetition of transmission, in systems with feedback, repetition of transmission will occur only if distortions are detected in the transmitted signal, i.e. the overall redundancy is less.

Interference-immune reception consists in the use of redundancy, as well as a priori information about signals and interference for solving the problem of reception in an optimal way: detecting a signal, distinguishing signals, or recovering messages. At present, the apparatus of the theory of statistical decisions is widely used for the synthesis of optimal receivers.

Receiver errors decrease with increasing signal-to-noise ratio at the receiver input. In this regard, pre-processing of the received signal is often performed in order to increase the ratio of the useful component to the interference. Such methods of signal preprocessing include the SHOW method (a combination of a broadband amplifier, a limiter and a narrowband amplifier), signal duration selection, interference compensation method, filtering method, correlation method, accumulation method, etc.

2. Modern technical means data exchange and channel-forming equipment

The receiver can be a computer, terminal or some digital device.

To ensure the transfer of information from a computer to a communication

It can be a database file, table, query response, text, or image.

Various types of communication channels are used to transmit messages in computer networks. The most common are dedicated telephone channels and special channels for transmitting digital information. Radio channels and channels are also used satellite communications.

LANs stand apart in this regard, where twisted-pair wires, coaxial cables and fiber-optic cables are used as the transmission medium.

To ensure the transfer of information from the computer to the communication environment, it is necessary to coordinate the signals of the internal interface of the computer with the parameters of the signals transmitted through the communication channels. In this case, both physical matching (shape, amplitude and duration of the signal) and code matching must be performed.

Technical devices that perform the functions of interfacing a computer with communication channels are called adapters or network adapters ... One adapter to provide interfacing with a computer of one communication channel. In addition to single-channel adapters, multichannel devices are also used - data transmission multiplexers or simply multiplexers.

Data transmission multiplexer - a device for interfacing a computer with several communication channels.

Data transmission multiplexers were used in teleprocessing systems - the first step towards creating computer networks... In the future, when networks with a complex configuration and with big amount subscriber systems for the implementation of interface functions began to use special communication processors.

As mentioned earlier, in order to transmit digital information through a communication channel, it is necessary to convert the bit stream into analog channels, and when receiving information from the communication channel to the computer, perform the opposite action - convert analog signals into a stream of bits that can be processed by a computer. Such transformations are performed by a special device - modem.

Modem- a device that performs modulation and demodulation of information signals when they are transmitted from a computer to a communication channel and when received in a computer from a communication channel.

The most expensive component of a computer network is the communication channel. Therefore, when building a number of computer networks, they try to save on communication channels by switching several internal communication channels to one external one. To perform switching functions, special devices are used - hubs.

Concentrator- a device that switches several communication channels in one way frequency division.

In a LAN, where the physical transmission medium is a cable of limited length, special devices are used to increase the length of the network - repeaters.

Repeater- a device that ensures the preservation of the shape and amplitude of the signal when transmitting it to a distance greater than that provided for by this type of physical transmission medium.

There are local and remote repeaters. Local repeaters allow you to connect fragments of networks located at a distance of up to 50 m, and remote- up to 2000 m.

Read also:

Topology System of relationships between components Windows networking... When applied to Active Directory replication, topology boils down to a set of connections used by domain controllers to communicate with each other.

(1) Computer networks realize information processing. М204, М205

parallel

local

● distributed

bidirectional

(1) The URL of a web page to be viewed in a browser starts with:

LAN COMBINATION

Reasons for LAN merging

Over time, a LAN system created at a certain stage of development ceases to meet the needs of all users, and then the problem of expanding it arises. functionality... It may be necessary to unite within the company various LANs that have appeared in its various departments and branches in different time, at least for organizing data exchange with other systems. The problem of expanding the network configuration can be solved both within a limited space and with access to the external environment.

The desire to gain access to certain information resources may require LAN connection to higher-level networks.

In the simplest version, LAN connection is necessary to expand the network as a whole, but the technical capabilities existing network are exhausted, new subscribers cannot be connected to it. You can only create another LAN and combine it with an existing one using one of the following methods.

LAN Connection Methods

Bridge. The easiest way to connect a LAN is to combine the same networks within a limited space. Physical transmission medium imposes length restrictions network cable... Within the permissible length, a network segment is built - a network segment. To combine network segments, use bridges.

Bridge- a device connecting two networks using the same data transmission methods.

The networks that are connected by the bridge must have the same network levels of the interaction model open systems, the lower levels may have some differences.

For the network personal computers bridge - a separate computer with special software and additional hardware. The bridge can connect networks of different topologies, but running under the control of the same type of network operating systems.

Bridges can be local or remote.

Local bridges connect networks located in a limited area within an existing system.

Deleted bridges connect geographically dispersed networks using external communication channels and modems.

Local bridges, in turn, are divided into internal and external.

Internal bridges are usually located on one of the computers of a given network and combine the function of a bridge with the function of a subscriber computer. Expansion of functions is carried out by installing an additional network card.

External bridges provide for the use of a separate computer with special software to perform their functions.

Router (router). A complex network configuration, which is a connection of several networks, requires a special device. The task of this device is to send a message to the addressee on the desired network. Such a device is called m router.

A router, or router, is a device that connects networks different types, but using the same operating system.

The router performs its functions on network layer, so it depends on the communication protocols, but does not depend on the type of network. Using two addresses - the network address and the host address, the router uniquely selects a specific station on the network.

Example 6.7. It is necessary to establish a connection with a telephone network subscriber located in another city. First, the address of the telephone network of this city is dialed - the area code. Then - the host address of this network - telephone number subscriber. Functions of the router is performed by the PBX equipment.

The router can also choose the best path for transmitting a message to a network subscriber, filters the information passing through it, sending only the information that is addressed to it to one of the networks.

In addition, the router provides load balancing in the network by redirecting message flows over free communication channels.

Gateway. To combine a LAN, completely different types operating according to significantly different protocols, special "devices are provided - gateways.

Gateway - a device that allows you to organize the exchange of data between two networks using different communication protocols.

The gateway performs its functions at the levels above the network. It does not depend on the used transmission medium, but depends on the used communication protocols. Typically, a gateway converts between the two protocols.

Using gateways, you can connect a local area network to the host computer, as well as connect a local area network to the global one.

Example 6.8. It is necessary to unite local networks located in different cities. This task can be solved using a global data transmission network. Such a network is a packet switching network based on the X.25 protocol. The gateway connects the local area network to the X.25 network. The gateway performs the required protocol conversions and enables communication between networks.

Bridges, routers and even gateways are structurally made in the form of cards that are installed in computers. They can perform their functions both in the mode of complete separation of functions, and in the mode of combining them with the functions of a workstation of a computer network.

(1) A computer that has 2 network cards and designed to connect networks, called:

Router

Amplifier

Switch

(1) A device that switches several communication channels into one by frequency division is called ...

repeater

● hub

data multiplexer

HARDWARE IMPLEMENTATION OF DATA TRANSMISSION

Methods for transmitting digital information

Digital data on the conductor is transmitted by changing the current voltage: no voltage - "O", there is voltage - "1". There are two ways of transmitting information over a physical transmission medium: digital and analog.

Notes: 1. If all subscribers of a computer network transmit data over a channel at the same frequency, such a channel is called narrowband(passes one frequency).

2. If each subscriber operates on his own frequency on one channel, then such a channel is called broadband(passes a lot of frequencies). The use of broadband channels allows you to save on their number, but complicates the process of managing data exchange.

At digital or narrowband transmission(Figure 6.10) data is transmitted in its natural form on a single frequency. The narrowband method allows you to transmit only digital information, provides each this moment time, the possibility of using the transmission medium only by two users and allows normal operation only at a limited distance (the length of the communication line is not more than 1000 m). At the same time, the narrow-band transmission method provides a high data exchange rate - up to 10 Mbit / s and allows you to create easily configurable computer networks. The vast majority of local area networks use narrowband transmission.

Rice. 6.10. Digital transmission method

Analog the method of digital data transmission (Fig. 6.11) provides broadband transmission due to the use of signals of different carrier frequencies in one channel.

At analog way transmission, the parameters of the carrier frequency signal are controlled for transmission over the digital data communication channel.

The carrier signal is a harmonic waveform described by the equation: "

A r = A r max sin (atf + 9 0),

where X max is the vibration amplitude; co - vibration frequency; t- time; ф 0 - the initial phase of oscillations.

You can transmit digital data over an analog channel by controlling one of the parameters of the carrier frequency signal: amplitude, frequency or phase. Since it is necessary to transmit data in binary form (a sequence of ones and zeros), the following control methods can be proposed (modulation): amplitude, frequency, phase.

The easiest way to understand the principle is amplitude modulation: "O" - no signal, i.e. no carrier frequency fluctuations; "1" - the presence of a signal, i.e. the presence of carrier frequency oscillations. There are fluctuations - one, no fluctuations - zero (Fig. 6.11 a).

Frequency modulation involves the transmission of signals 0 and 1 at different frequencies. When you go from 0 to 1 and from 1 to 0, the carrier signal changes (Fig. 6.116).

The most difficult to understand is phase modulation. Its essence is that during the transition from 0 to 1 and from 1 to 0, the phase of the oscillations changes, i.e. their direction (fig. 6.11 v).

In networks of a high level of the hierarchy - global and regional, it is also used broadband transmission, which provides for the operation for each subscriber at its own frequency within one channel. This ensures the interaction of a large number of subscribers at a high data transfer rate.

Broadband transmission allows combining in one channel the transmission of digital data, image and sound, which is a necessary requirement modern systems multimedia.

Example 6.5. A typical analog channel is a telephone channel. When the subscriber picks up the phone, he hears a uniform sound signal- this is the carrier frequency signal. Since it lies in the audio frequency range, it is called a tone signal. To transmit speech over a telephone channel, it is necessary to control the carrier signal - to modulate it. Sounds perceived by the microphone are converted into electrical signals, and these, in turn, modulate the carrier signal. When transmitting digital information, control is performed by information bytes - a sequence of ones and zeros.

Hardware

Technical devices that perform the functions of interfacing a computer with communication channels are called adapters or network adapters. One adapter provides interfacing with a computer of one communication channel.

Rice. 6.11. Methods for transmitting digital information using an analog signal: a- amplitude modulation; b- frequency; v- phase

Except single channel adapters multichannel devices are also used - data transmission multiplexers or simply multiplexers.

Data transmission multiplexer- a device for interfacing a computer with several communication channels.

Data transmission multiplexers were used in teleprocessing systems - the first step towards the creation of computer networks. Later, when networks with a complex configuration and with a large number of subscriber systems appeared, special communication processors began to be used to implement interface functions.

As mentioned earlier, to transmit digital information over a communication channel, it is necessary to convert the bit stream into analog signals, and when receiving information from the communication channel to the computer, perform the opposite action - convert the analog signals into a bit stream that the computer can process. Such transformations are performed by a special device - mod eat.

Concentrator- a device that switches several communication channels into one by means of frequency division.

In a LAN, where the physical transmission medium is a cable of limited length, special devices are used to increase the length of the network - repeaters.

Repeater- a device that ensures the preservation of the shape and amplitude of the signal when transmitting it over a distance greater than that provided for by this type of physical transmission medium.

There are local and remote repeaters. Local repeaters allow you to connect network fragments located at a distance of up to 50 m, and remote- up to 2000 m.

Communication network characteristics

The following characteristics can be used to assess the quality of a communication network:

■ speed of data transmission over the communication channel;

■ bandwidth of the communication channel;

■ reliability of information transfer;

■ reliability of the communication channel and modems.

Baud rate over a communication channel is measured by the number of bits of information transmitted per unit of time - a second.

Remember! The unit of measurement for the data transfer rate is bits per second.

Note. A commonly used unit of measure for speed is baud. Baud is the number of changes in the state of the transmission medium per second. So how each state change can correspond to several bits of data, then real speed in bits per second can exceed the baud rate.

The data transfer rate depends on the type and quality of the communication channel, the type of modems used and the adopted synchronization method.

So, for asynchronous modems and a telephone communication channel, the speed range is 300 - 9600 bit / s, and for synchronous modems - 1200 - 19200 bit / s.

For users of computer networks, it is not abstract bits per second that matter, but information, the unit of measurement of which is bytes or characters. Therefore, a more convenient channel characteristic is its throughput, which is estimated by the number of characters transmitted over the channel per unit of time - a second. In this case, all service symbols are included in the message. The theoretical bandwidth is determined by the data transfer rate. The actual throughput depends on a number of factors, including the transmission method, the quality of the communication channel, and the conditions of its operation, and the structure of messages.

Remember! The unit of measurement of the communication channel throughput is a character per second.

An essential characteristic of the communication system of any network is credibility transmitted information. Since, on the basis of processing information about the state of the control object, decisions are made about a particular course of the process, then the fate of the object may ultimately depend on the reliability of the information. The fidelity of information transmission is assessed as the ratio of the number of erroneously transmitted characters to the total number of transmitted characters. The required level of confidence should be provided by both the hardware and the communication channel. It is impractical to use expensive equipment if the communication channel does not meet the necessary requirements with respect to the level of reliability. *

Remember! Validity unit: number of errors per sign - errors / sign.

For computer networks, this indicator should be in the range of 10 -6 - 10 ~ 7 errors / sign, ie. one error per million characters transmitted or ten million characters transmitted is allowed.

Finally, reliability a communication system is determined either by the fraction of uptime in the total operating time, or by the average uptime. The second characteristic allows you to more effectively assess the reliability of the system.

Remember! Reliability unit: MTBF - hour.

For computer networks, the MTBF should be large enough and be at least several thousand hours.

226 CHAPTER 6. COMPUTER NETWORKS

6.3. LOCAL COMPUTER NETWORKS

Features of the organization of a LAN

Typical LAN Topologies and Access Methods

LAN bonding

FEATURES OF A LAN ORGANIZATION

Functional groups of devices in the network

The main purpose of any computer network is to provide information and computing resources to users connected to it.

From this point of view, a local area network can be viewed as a collection of servers and workstations.

Server- a computer connected to the network and providing its usefulness providers of certain services.

Servers can carry out data storage, database management, remote processing of jobs, printing jobs and a number of other functions, the need for which may arise from network users. Server is the source of network resources.

Work station- a personal computer connected to the network through which the user gains access to its resources.

Work station network functions both in network and in local mode. It is equipped with its own operating system (MS DOS, Windows, etc.), provides the user with all the necessary tools for solving applied problems.

Particular attention should be paid to one type of server - the File Server. In common terminology, an abbreviated name is adopted for it - file server.

The file server stores the data of network users and provides them with access to this data. This is a high capacity computer random access memory, high-capacity hard drives, and optional tape drives (streamers).

It runs under the control of a special operating system that provides simultaneous access of network users to the data located on it.

The file server performs the following functions: data storage, data archiving, synchronization of data changes by different users, data transfer.

For many tasks, using a single file server is not enough. Then several servers can be connected to the network. It is also possible to use minicomputers as file servers.

Managing the interaction of devices on the network

Information Systems, built on the basis of computer networks, provide the solution of the following tasks: data storage, data processing, organization of user access to data, transfer of data and data processing results to users.

In centralized processing systems, these functions were performed by the central computer (Mainframe, Host).

Computer networks implement distributed data processing. Data processing in this case is distributed between two objects: client and server.

Customer- a task, workstation or computer network user.

In the process of data processing, the client can form a request to the server to perform complex procedures, read a file, search for information in a database, etc.

The server defined earlier fulfills the request from the client. The results of the query are passed to the client. The server provides storage of public data, organizes access to this data and transfers the data to the client.

The client processes the received data and presents the processing results in a form convenient for the user. In principle, data processing can also be performed on the server. For similar systems terms accepted - systems client-server or architecture client-server.

The client-server architecture can be used both in peer-to-peer local area networks and in a network with a dedicated server.

Peer-to-peer network. In such a network, there is no single control center for the interaction of workstations and there is no single storage device. The network operating system is distributed across all workstations. Each station on the network can perform the functions of both a client and a server. It can serve requests from other workstations and route its service requests to the network.

All devices connected to other stations (disks, printers) are available to the network user.

Advantages of peer-to-peer networks: low cost and high reliability.

Disadvantages of peer-to-peer networks:

■ dependence of the network efficiency on the number of stations;

■ the complexity of network management;

■ the complexity of ensuring the protection of information;

■ difficulties in updating and changing software stations. The most popular are peer-to-peer networks based on network

operating systems LANtastic, NetWare Lite.

Network with dedicated server. In a network with a dedicated server, one of the computers performs the functions of storing data intended for use by all workstations, managing the interaction between workstations, and a number of service functions.

Such a computer is commonly referred to as a network server. A network operating system is installed on it, all shared external devices - hard drives, printers and modems.

Communication between workstations on a network is usually done through a server. The logical organization of such a network can be represented by the topology star. The server acts as a central device. In networks with centralized control, it is possible to exchange information between workstations, bypassing the file server. You can use NetLink software to do this. After launching the program on two workstations, you can transfer files from the disk of one station to the disk of another (similar to the operation of copying files from one directory to another using the Norton Commander program).

Advantages of a network with a dedicated server:

■ reliable information security system;

■ high performance;

■ no restrictions on the number of workstations;

■ ease of management compared to peer-to-peer networks. Disadvantages of the network:

■ high cost due to the allocation of one computer for the server;

■ the dependence of the speed and reliability of the network on the server;

■ less flexibility compared to a peer-to-peer network.

Dedicated server networks are the most common among computer network users. Network operating systems for such networks are LANServer (IBM), Windows NT Server versions 3.51 and 4.0, and NetWare (Novell).

(1) Local area networks cannot be connected using ... M232

locks, bridges

● hubs, modems

servers

routers

(1) BBS is ... M745

navigator

Intranet program

● system of electronic bulletin boards on the Internet

organization server maintainer

(1) Client-server processing of data is processing. M227

parallel

localized

bidirectional

● distributed

(1) The Bat program allows ...

load web pages

● download and edit email

archive email

(1) One of search engines on the Internet is ...

(1) Internet Explorer allows ...

chat on IRC protocol

● download web pages by protocol http and files on FTP protocol

download newsgroups using NNTP protocol

(1) Telephone cable is an option ... M228

optical - high-frequency

coaxial cable

fiber optic

● twisted pair

(1) Usenet is used by ... M239

registration of users on the network

● to move news between computers around the world

information processing in the network

creating a workstation on the network

(1) A discussion group that is part of Usenet is called ... M239

server group

group on the network

● by teleconference

(1) The message flow in the data network is determined ...

message channel memory

● traffic

6.1. COMMUNICATION ENVIRONMENT AND DATA TRANSFER

Purpose and classification of computer networks

Characteristics of the data transfer process

Hardware implementation of data transfer

Data links

PURPOSE AND CLASSIFICATION OF COMPUTER NETWORKS

Distributed data processing

Modern production requires high speeds of information processing, convenient forms of storage and transmission. It is also necessary to have dynamic ways of accessing information, ways of searching for data at specified time intervals; implement complex mathematical and logical data processing. The management of large enterprises and the management of the economy at the country level require the participation of large enough teams in this process. Such groups can be located in different parts of the city, in different regions of the country and even in different countries. To solve management problems that ensure the implementation of the economic strategy, the speed and convenience of information exchange, as well as the possibility of close interaction of all those involved in the process of making management decisions, become important and relevant.

In the era of centralized use of computers with batch processing of information, computer users preferred to purchase computers on which they could solve almost all classes of their problems. However, the complexity of the tasks being solved is inversely proportional to their number, and this led to ineffective use of computing power Computer at significant material costs. One cannot but take into account the fact that access to computer resources was hampered due to the existing policy of centralizing computing resources in one place.

Principle centralized data processing (Fig. 6.1) did not meet the high requirements for the reliability of the processing process, impeded the development of systems and could not provide the necessary time parameters for interactive data processing in multi-user mode. A short-term failure of the central computer led to fatal consequences for the system as a whole, since it was necessary to duplicate the functions of the central computer, significantly increasing the cost of creating and operating data processing systems.

Rice. 6.2. Distributed data processing system

The emergence of small computers, microcomputers and, finally, personal computers required a new approach to organizing data processing systems, to creating new information technologies... There was a logically justified requirement for the transition from the use of separate computers in centralized data processing systems. to distributed data processing (Fig. 6.2).

Distributed data processing- data processing performed on independent but interconnected computers representing a distributed system.

To implement distributed data processing, multi-machine associations, the structure of which is being developed in one of the following directions:

■ multi-machine computing systems (MCC);

■ computer (computing) networks.

Multi-machine computing complex- a group of computers installed side by side, united with the help of special interface means and performing jointly a single information and computing process.

Note .. Underprocess is understood as a certain sequence of actions for solving a problem, determined by the program.

Multi-machine computing systems can be:

local provided that computers are installed in one room, which do not require interconnection special equipment and communication channels; remote, if some computers of the complex are installed at a considerable distance from the central computer and telephone communication channels are used for data transmission.

Example 6.1. To a mainframe-type computer providing the mode batch processing information connected by means of a mini-computer interface device. Both computers are located in the same computer room. The minicomputer provides preparation and preprocessing of data, which are then used to solve complex problems on the mainframe. This is a local multi-machine complex.

Example 6.2. Three computers are combined into a complex for the distribution of tasks coming for processing. One of them performs a dispatch function and distributes tasks depending on the employment of one of the other two processing computers. This is a local multi-machine complex.

Example 6.3. A computer that collects data for a certain region performs their preliminary processing and transmits them for further use to a central computer via a telephone communication channel. This is a remote multi-machine complex.

Computer (computing) network- a set of computers and terminals connected using communication channels in unified system that meets the requirements of specific data processing.

Note. Under system is understood as an autonomous set consisting of one or more computers, software, peripheral equipment, terminals, data transmission facilities, physical processes and operators, capable of processing information and performing functions of interaction with other systems.

Generalized structure of a computer network

Computer networks are the highest form of multi-machine associations. Let's highlight the main differences between a computer network and a multicomputer complex.

The first difference is dimension. The structure of a multicomputer complex usually includes two, maximum three computers, located mainly in one room. A computing network can consist of tens or even hundreds of computers located at a distance from each other from several meters to tens, hundreds and even thousands of kilometers.

The second difference is the division of functions between computers. If in a multicomputer complex the functions of data processing, data transmission and system control can be implemented in one computer, then in computer networks these functions are distributed between different computers.

The third difference is the need to solve the message routing problem in the network. A message from one computer to another in the network can be transmitted along different routes depending on the state of the communication channels connecting the computers with each other.

Combining computer technology, communication equipment and data transmission channels into one complex imposes specific requirements on the part of each element of a multi-machine association, and also requires the formation of a special terminology.

Network subscribers- objects that generate or consume information on the network.

Subscribers networks can be separate computers, computer complexes, terminals, industrial robots, machine tools with numerical control, etc. Any subscriber of the network connects to the station.

Station- equipment that performs functions related to the transmission and reception of information.

The set of the subscriber and the station is usually called subscriber system. To organize the interaction of subscribers, a physical transmission medium is required.

Physical transmission medium - communication lines or space in which electrical signals propagate and data transmission equipment.

Based on the physical transmission medium, communication network, which provides information transfer between subscriber systems.

This approach allows us to consider any computer network as a set of subscriber systems and a communication network. The generalized structure of a computer network is shown in Fig. 6.3.

Rice. 6.3. Generalized structure of a computer network

Classification of computer networks

Depending on the territorial location of subscriber systems, computer networks can be divided into three main classes:

■ global networks (WAN - Wide Area Network);

■ regional networks (MAN - Metropolitan Area Network);

■ local area networks (LAN - Local Area Network).

The global the computer network unites subscribers located in different countries, on different continents. The interaction between the subscribers of such a network can be carried out on the basis of telephone communication lines, radio communication and satellite communication systems. Global computer networks will solve the problem of combining information resources of all mankind and organizing access to these resources.

Regional a computer network connects subscribers located at a considerable distance from each other. It can include subscribers within a large city, economic region, or a separate country. Typically, the distance between subscribers of a regional computer network is tens - hundreds of kilometers.

Local a computer network unites subscribers located within a small area. Currently, there are no strict restrictions on the territorial distribution of subscribers of a local computer network. Usually such a network is connected to a specific place. The class of local computer networks includes networks of individual enterprises, firms, banks, offices, etc. The length of such a network can be limited to 2 - 2.5 km.

Combining global, regional and local computer networks allows you to create multi-network hierarchies. They provide powerful, economically viable means of processing huge amounts of information and access to limited information resources... In fig. 6.4 shows one of the possible hierarchies of computer networks. Local area networks can be part of a regional network as components, regional networks can be combined into a global network, and, finally, global networks can also form complex structures.

Rice. 6.4. Hierarchy of computer networks

Example 6.4. The Internet computer network is the most popular global network... It includes many loosely connected networks. Within each network of the Internet, there is a specific communication structure and a specific management discipline. Within the Internet, the structure and methods of connections between different networks for specific user don't make any difference.

Personal computers, which have now become an indispensable element of any control system, have led to a boom in the creation of local area networks. This, in turn, gave rise to the need for the development of new information technologies.

The practice of using personal "computers" in various branches of science, technology and production has shown that the greatest efficiency from the introduction of computer technology is provided not by separate autonomous PCs, but by local computer networks.

(1) The network subscribers are .. М205.

network administrators

personal computer users

● objects generating or consuming network information

communications equipment

(1) Network subscribers cannot be ... М205

● computer complexes (can)

Terminals (can)

separate computers (can)

end users

(1) A network server is a computer ... M226 (a server is a source of network resources)

with the highest processor frequency

providing access to keyboard and monitor

with the most memory

● providing access to resources

(1) FTP server is ... M240

the computer that contains files intended for the network administrator

a computer that contains information for organizing the work of teleconferences

corporate server

● a computer that contains files intended for open access

(1) SMTP is for ...

(SMTP Protocol A component of the TCP / IP suite of protocols; this protocol manages the exchange of messages Email between messaging agents.

POP3 Protocol A popular protocol for receiving e-mail messages. This protocol is often used by Internet service providers. POP3 servers only allow access to one mailbox as opposed to IMAP servers, which provide access to multiple folders on the server.

A set widely used on the Internet network protocols that supports communication between interconnected networks consisting of computers of different architectures and with different operating systems... TCP / IP includes standards for communication between computers and conventions for connecting networks and rules for routing messages.)

Chatting

● Sending emails

Web browsing

Receiving email

(1) Most effective way communications for the transmission of computer traffic are ...

● M220 packages

messages

everyone is equally effective

Hardware. Switching (computer networks) A device that switches several communication channels is called a hub

Basic cryptography concepts

Password recovery