Today, networks and network technologies connect people in every corner of the world and provide them with access to the greatest luxury in the world - human communication. People can communicate and play with friends in other parts of the world without interference.

The events taking place become known in all countries of the world in a matter of seconds. Everyone is able to connect to the Internet and post their piece of information.

Network information technologies: the roots of their origin

In the second half of the last century, human civilization formed its two most important scientific and technical branches - computer and About a quarter of a century, both of these branches developed independently, and within their framework computer and telecommunication networks were created, respectively. However, in the last quarter of the twentieth century, as a result of the evolution and interpenetration of these two branches of human knowledge, what we call the term “network technology” arose, which is a subsection of more general concept"information technology".

As a result of their appearance, a new technological revolution occurred in the world. Just as several decades earlier the land surface was covered with a network of expressways, at the end of the last century all countries, cities and villages, enterprises and organizations, as well as individual homes found themselves connected by “information highways.” At the same time, they all became elements of various data transfer networks between computers, in which certain information transfer technologies were implemented.

Network technology: concept and content

Network technology is a sufficient set of rules for the presentation and transmission of information, implemented in the form of so-called “standard protocols”, as well as hardware and software, including network adapters with drivers, cables and fiber-optic lines, various connectors (connectors).

“Sufficiency” of this set of tools means its minimization while maintaining the possibility of building an efficient network. It should have the potential for improvement, for example, by creating subnets in it that require the use of protocols of various levels, as well as special communicators, usually called “routers.” After improvement, the network becomes more reliable and faster, but at the cost of adding add-ons to the main network technology that forms its basis.

The term “network technology” is most often used in the narrow sense described above, but it is often broadly interpreted as any set of tools and rules for building networks of a certain type, for example, “local computer network technology.”

Prototype of network technology

The first prototype computer network, but not yet the network itself, became in the 60-80s. last century multi-terminal systems. Representing a set of monitor and keyboard, located at great distances from mainframe computers and connecting to them via telephone modems or dedicated channels, the terminals left the premises of the computer information center and were dispersed throughout the building.

At the same time, in addition to the operator of the computer itself on the computer information center, all users of the terminals were able to enter their tasks from the keyboard and observe their execution on the monitor, carrying out some task management operations. Such systems, implementing both time-sharing and batch processing algorithms, were called remote job entry systems.

Global networks

Following multi-terminal systems in the late 60s. XX century The first type of networks was created - global computer networks (GCN). They connected supercomputers, which existed in single copies and stored unique data and software, with mainframe computers located at distances of up to many thousands of kilometers, through telephone networks and modems. This network technology has previously been tested in multi-terminal systems.

The first GCS in 1969 was ARPANET, which worked in the US Department of Defense and united different types of computers with different operating systems. They were equipped with additional modules to implement communication systems common to all computers on the network. It was on it that the foundations of network technologies that are still used today were developed.

The first example of the convergence of computer and telecommunications networks

GKS inherited communication lines from older and more global networks— telephone lines, because laying new long-distance lines was very expensive. Therefore, for many years they used analog telephone channels for transmission to this moment time for only one conversation. Digital data was transmitted over them at a very low speed (tens of kbit/s), and the capabilities were limited to the transfer of data files and email.

However, having inherited telephone lines communications, GKS did not take their basic technology, based on the principle of circuit switching, when each pair of subscribers was allocated a channel with a constant speed for the entire duration of the communication session. The GCS used new computer network technologies based on the principle of packet switching, in which data in the form of small portions of packets at a constant speed is issued into a non-switched network and received by their recipients on the network using address codes built into the packet headers.

Predecessors of local area networks

Appearance in the late 70s. XX century LSI led to the creation of minicomputers with low cost and rich functionality. They began to really compete with large computers.

Minicomputers of the PDP-11 family have gained wide popularity. They began to be installed in all, even very small production units to manage technical processes and individual technological installations, as well as in enterprise management departments to perform office tasks.

The concept of distributed throughout the enterprise emerged computer resources, although all the minicomputers still worked autonomously.

The emergence of LAN networks

By the mid-80s. XX century technologies for combining minicomputers into networks were introduced, based on switching data packets, as in the GCS.

They turned the construction of a single enterprise network, called a local (LAN) network, into an almost trivial task. To create it, you only need to buy network adapters for the selected LAN technology, for example, Ethernet, a standard cable system, install connectors (connectors) on its cables and connect the adapters to the minicomputer and to each other using these cables. Next, one of the operating systems intended for organizing a LAN network was installed on the computer server. After that, it began to work, and the subsequent connection of each new minicomputer did not cause any problems.

The inevitability of the Internet

If the advent of mini-computers made it possible to distribute computer resources evenly across the territories of enterprises, then the appearance in the early 90s. PC led to their gradual appearance, first in every workplace of any mental worker, and then in individual human dwellings.

The relative cheapness and high reliability of PCs first gave a powerful impetus to the development of LAN networks, and then led to the emergence of a global computer network - the Internet, which today covers all countries of the world.

The size of the Internet is growing by 7-10% every month. It represents the core that connects various local and global networks of enterprises and institutions around the world with each other.

If at the first stage data files and email messages were mainly transmitted via the Internet, today it mainly provides remote access to distributed information resources and electronic archives, to commercial and non-commercial information services in many countries. Its freely accessible archives contain information on almost all areas of knowledge and human activity - from new trends in science to weather forecasts.

Basic network technologies of LAN networks

Among them are the basic technologies on which the basis of any specific network can be built. Examples include such well-known LAN technologies as Ethernet (1980), Token Ring (1985) and FDDI (late 80s).

At the end of the 90s. Ethernet technology has become the leader in LAN network technology, combining its classic version with up to 10 Mbit/s, as well as Fast Ethernet (up to 100 Mbit/s) and Gigabit Ethernet (up to 1000 Mbit/s). All Ethernet technologies have similar operating principles that simplify their maintenance and the integration of LAN networks built on their basis.

During the same period, network functions that implement the above network functions began to be built into the kernels of almost all computer operating systems. information Technology. Even specialized communication operating systems like IOS from Cisco Systems have appeared.

How GCS technologies developed

GKS technologies on analog telephone channels, due to the high level of distortion in them, were distinguished by complex algorithms for monitoring and data recovery. An example of them is the X.25 technology developed in the early 70s. XX century More modern network technologies are frame relay, ISDN, ATM.

ISDN is an acronym meaning " digital network with integration of services”, allows for remote video conferencing. Remote access is ensured by installing ISDN adapters in PCs, which work many times faster than any modems. There is also special software that allows popular operating systems and browsers to work with ISDN. But the high cost of equipment and the need to lay special communication lines hinder the development of this technology.

WAN technologies have progressed along with telephone networks. After the advent of digital telephony, a special technology, Plesiochronous Digital Hierarchy (PDH), was developed, supporting speeds of up to 140 Mbit/s and used by enterprises to create their own networks.

New Synchronous Digital Hierarchy (SDH) technology in the late 80s. XX century expanded the capacity of digital telephone channels up to 10 Gbit/s, and Dense Wave Division Multiplexing (DWDM) technology - up to hundreds of Gbit/s and even up to several Tbit/s.

Internet technologies

Network ones are based on the use of hypertext language (or HTML language) - a special markup language that is an ordered set of attributes (tags) that are pre-implemented by website developers into each of their pages. Of course, in this case we are not talking about text or graphic documents (photos, pictures), which have already been “downloaded” by the user from the Internet, are in the memory of his PC and are viewed through text or images. We are talking about so-called web pages viewed through programs -browsers.

Developers of Internet sites create them in HTML language (now many tools and technologies have been created for this work, collectively called “website layout”) in the form of a set of web pages, and site owners place them on Internet servers on a rental basis from the owners of their memory servers (the so-called “hosting”). They work on the Internet around the clock, servicing the requests of its users to view the web pages loaded on them.

Browsers on user PCs, having received access through the server of their Internet provider to a specific server, the address of which is contained in the name of the requested Internet site, gain access to this site. Further, by analyzing the HTML tags of each page being viewed, browsers form its image on the monitor screen in the way it was intended by the site developer - with all the headings, font and background colors, various inserts in the form of photos, diagrams, pictures, etc. .

A computer network is an association of several computers to jointly solve information and computing problems.

The key concept of network technologies is a network resource, which can be understood as hardware and software components, participating in the process of sharing - in the process of network interaction. Access to network resources is provided by network services (network services)

The basic concepts of network technologies include such concepts as server, client, communication channel, protocol and many others. However, the concept of a network resource and a network service (service) are fundamental, since the need to organize work based on the sharing of computer resources, and therefore the creation of network resources and corresponding network services, is the root cause of the creation of computer networks themselves.

Highlight five types of network services: file, print, messages, application databases.

File service — implements centralized storage and sharing of files. This is one of the most important network services, it requires the presence of some network storage files (local network file server, ftp server, etc.), as well as the use of various security mechanisms (access control, file version control, information backup).

Print service — provides opportunities for centralized use of printers and other printing devices. This service accepts print jobs, manages the job queue, and organizes user interaction with network printers. Network printing technology is very convenient in a wide variety of computer networks, as it makes it possible to reduce the number of printers required, which ultimately allows you to reduce costs or use better equipment.

Messaging service — allows you to organize information exchange between users of a computer network. The messages in this case should be considered as text messages (Email, messages from network instant messengers), and media messages various systems voice and video communication.

Database service — is designed to organize centralized storage, search processing and ensure data protection of various information systems. Unlike easy storage and file sharing, the database service provides and management, which includes creating, modifying, deleting data, ensuring its integrity and protection.

Application Service — provides a method of operation in which the application is launched on the user's computer not from a local source, but from a computer network. Such applications may use server resources for data storage and computation. The advantage of using network applications is the ability to use them from anywhere connected to a computer network without the need to install the application on local computer, the ability for multiple users to collaborate, “transparent” update software, the ability to use commercial software on a subscription basis.

Application services are the newest and fastest growing type of network service. A good example office network applications can serve here Google online services Drive and Microsoft Office 365.

The history of the emergence of computer networks is directly related to the development of computer technology. The first powerful computers (the so-called Mainframes) occupied rooms and entire buildings. The procedure for preparing and processing data was very complex and time-consuming. Users prepared punch cards containing data and program commands and transmitted them to the computer center. Operators entered these cards into a computer, and users usually received printed results only the next day. This method of network interaction assumed completely centralized processing and storage.

Mainframe- a high-performance general-purpose computer with a significant amount of RAM and external memory, designed to perform intensive computing work. Typically, many users work with the mainframe, each of whom has only terminal devoid of its own computing power.

Terminal(from Latin terminalis - related to the end)

Computer terminal- input/output device, workplace on multi-user computers, monitor with keyboard. Examples of terminal devices: console, terminal server, thin client, terminal emulator, telnet.

Host(from the English host - host who receives guests) - any device that provides services in the “client-server” format in server mode over any interfaces and is uniquely defined on these interfaces. In a more particular case, a host can be understood as any computer, server connected to a local or global network.

Computer network (computer network, data network) - a communication system for computers and/or computer equipment (servers, routers and other equipment). To transmit information, various physical phenomena can be used, usually various types of electrical signals or electromagnetic radiation.

An interactive mode of operation would be more convenient and efficient for users, in which they can quickly manage the processing of their data from the terminal. But the interests of users were largely neglected in the early stages of the development of computing systems, because batch mode- this is the most effective mode of use computing power, since it allows you to perform more user tasks per unit of time than any other modes. Fortunately, the evolutionary processes cannot be stopped, and in the 60s the first interactive multi-terminal systems began to develop. Each user received a terminal at his disposal, with the help of which he could conduct a dialogue with the computer. And, although computing power was centralized, data input and output functions became distributed. This interaction model is often called "terminal-host" . The central computer must be running an operating system that supports this interaction, which is called centralized computing. Moreover, the terminals could be located not only on the territory of the computer center, but also be dispersed over a large territory of the enterprise. In fact, this was the prototype of the first local area networks (LAN). Although such a machine fully provides data storage and computing capabilities, connecting remote terminals to it is not network interaction, since the terminals, being, in fact, peripheral devices, provide only transformation of the form of information, but not its processing.

Figure 1. Multi terminal system

Local area network (LAN), (local area network, slang local area; English Local AreaNetwork, LAN ) - a computer network that usually covers a relatively small area or a small group of buildings (home, office, company, institute)

Computer (English computer - “calculator”),computer (electronic computer)- a computer for transmitting, storing and processing information.

The term “computer” and the abbreviation “EVM” (electronic computer), adopted in the USSR, are synonymous. However, after the appearance personal computers, The term "computer" was practically forced out of everyday use.

Personal computer, PC (English personal computer,PC ), personal computer a computer intended for personal use, the price, size and capabilities of which satisfy the needs of a large number of people. Created as a computing machine, the computer, however, is increasingly used as a tool for accessing computer networks. .

In 1969, the US Department of Defense decided that in case of war, America needed a reliable information transmission system. The Advanced Research Projects Agency (ARPA) proposed developing a computer network for this purpose. The development of such a network was entrusted to the University of California at Los Angeles, the Stanford Research Center, the University of Utah and the University of California at Santa Barbara. The first test of the technology occurred on October 29, 1969. The network consisted of two terminals, the first of which was located at the University of California, and the second, 600 km away, at Stanford University.

The computer network was called ARPANET; within the framework of the project, the network united four specified scientific institutions, all work was funded by the US Department of Defense. Then the ARPANET network began to actively grow and develop, scientists from different fields of science began to use it.

In the early 70s, a technological breakthrough occurred in the production of computer components - large integrated circuits (LSI) appeared. Their relatively low cost and high functionality have led to the creation of mini- computer (electronic computers), which became real competitors of mainframes. Mini-computer, or mini- computers (not to be confused with modern mini-computers), performed tasks of managing technological equipment, warehouses and other tasks at the enterprise department level. Thus, the concept of distributing computer resources throughout the enterprise emerged. However, all computers of one organization continued to work autonomously.

Figure 2. Autonomous use of several mini-computers in one enterprise

It was during this period, when users gained access to full-fledged computers, that the solution to combining individual computers to exchange data with other nearby computers was ripe. In each individual case this problem was solved in its own way. As a result, the first local computer networks appeared.

Since the creative process was spontaneous, and there was no single solution for connecting two or more computers, there was no question of any network standards.

Meanwhile, the first foreign organizations from Great Britain and Norway were connected to the ARPANET network in 1973, and the network became international. In parallel with ARPANET, other networks of universities and enterprises began to appear and develop.

In 1980, it was proposed to link the ARPANET and CSnet (Computer Science Research Network) together through a gateway using TCP/IP protocols so that all subsets of the CSnet networks would have access to a gateway on the ARPANET. This event led to agreement on the method of internetwork communication between a community of independent computer networks, can be considered the appearance Internet in its modern understanding.

Figure 3. Options for connecting a PC to the first LAN

In the mid-80s, the situation in local networks began to change. Standard technologies for connecting computers into a network have been established - Ethernet, Arcnet, Token Ring, Token Bus, a little later - FDDI. A powerful stimulus for their development was personal computers. These devices have become an ideal solution for creating a LAN. On the one hand, they had sufficient power to process individual tasks, and at the same time, they clearly needed to combine their computing power to solve complex problems.

All standard technologies local networks relied on the same switching principle, which was successfully tested and proved its advantages in transmitting data traffic in global computer networks - packet switching principle .

Internet (pronounced [internet]; English Internet, abbreviated from Interconnected Networks -interconnected networks; slang. no, no) - global telecommunications network of information and computing resources. Serves as a physical basis for World Wide Web Wide WEB) . Often referred to as World Wide Web, Global Network, or just Net.

Standard network technologies have made the task of building a local network almost trivial. To create a network, it was enough to purchase network adapters of the appropriate standard, for example Ethernet , standard cable, connect the adapters to the cable with standard connectors and install one of the popular network operating systems, for example Novell NetWare. After this, the network began to work, and the subsequent connection of each new computer did not cause any problems - naturally, if a network adapter of the same technology was installed on it.

Figure 4. Connecting several computers using a “common bus” scheme.

Network card , also known asnetwork card, network adapter, Ethernet adapter, NIC (English networkinterface controller) - a peripheral device that allows the computer to interact with other devices on the network.

Operating system, OS (English operatingsystem) - a basic set of computer programs that provide a user interface, control of computer hardware, work with files, input and output of data, and execution of application programs and utilities.

Network technology - this is an agreed set of standard protocols and software and hardware that implement them (for example, network adapters, drivers, cables and connectors), sufficient to build a computer network. The epithet “sufficient” emphasizes the fact that this set represents the minimum set of tools with which you can build a working network. Perhaps this network can be improved, for example, by allocating subnets in it, which will immediately require, in addition to standard Ethernet protocols, the use of the IP protocol, as well as special communication devices - routers. An improved network will likely be more reliable and faster, but at the expense of add-ons to the tools Ethernet technologies, which formed the basis of the network.

The term “network technology” is most often used in the narrow sense described above, but sometimes its expanded interpretation is also used as any set of tools and rules for building a network, for example, “end-to-end routing technology,” “secure channel technology,” “IP technology.” networks."

The protocols on which a network of a certain technology is built (in the narrow sense) were specifically developed for joint work, so the network developer does not require additional efforts to organize their interaction. Sometimes network technologies are called basic technologies, bearing in mind that the basis of any network is built on their basis. Examples of basic network technologies include, in addition to Ethernet, such well-known local network technologies as Token Ring and FDDI, or X.25 and frame relay technologies for territorial networks. To obtain a functional network in this case, it is enough to purchase software and hardware related to the same basic technology - network adapters with drivers, hubs, switches, cable system, etc. - and connect them in accordance with the requirements of the standard for this technology.

Creation of standard local network technologies

In the mid-80s, the situation in local networks began to change dramatically. Standard technologies for connecting computers into a network have been established - Ethernet, Arcnet, Token Ring. Personal computers served as a powerful stimulus for their development. These commodity products were ideal elements for building networks - on the one hand, they were powerful enough to run networking software, but on the other hand, they clearly needed to pool their computing power to solve complex problems, as well as share expensive peripheral devices and disk arrays. Therefore, personal computers began to predominate in local networks, not only as client computers, but also as data storage and processing centers, that is, network servers, displacing minicomputers and mainframes from these familiar roles.

Standard network technologies have turned the process of building a local network from an art into a routine task. To create a network, it was enough to purchase network adapters of the appropriate standard, for example Ethernet, a standard cable, connect the adapters to the cable with standard connectors and install one of the popular network operating systems on the computer, for example, NetWare. After this, the network began to work and connecting each new computer did not cause any problems - naturally, if a network adapter of the same technology was installed on it.

Local networks, in comparison with global networks, have introduced a lot of new things into the way users organize their work. Access to shared resources became much more convenient - the user could simply view lists of available resources, rather than remember their identifiers or names. After connecting to a remote resource, it was possible to work with it using commands already familiar to the user from working with local resources. The consequence and at the same time the driving force of this progress was the emergence of a huge number of non-professional users who did not need to learn special (and quite complex) commands for network work. And local network developers got the opportunity to implement all these conveniences as a result of the emergence of high-quality cable communication lines, on which even first-generation network adapters provided data transfer rates of up to 10 Mbit/s.

Of course, the developers of global networks could not even dream of such speeds - they had to use the communication channels that were available, since laying new cable systems for computer networks thousands of kilometers long would require colossal capital investments. And “at hand” there were only telephone communication channels, poorly suited for high-speed transmission of discrete data - a speed of 1200 bps was a good achievement for them. Therefore, economical use of communication channel bandwidth has often been the main criterion for the effectiveness of data transmission methods in global networks. Under these conditions, various procedures for transparent access to remote resources, standard for local networks, for global networks have long remained an unaffordable luxury.

Modern tendencies

Today, computer networks continue to develop, and quite quickly. The gap between local and global networks is constantly narrowing, largely due to the emergence of high-speed territorial communication channels that are not inferior in quality to local network cable systems. In global networks, resource access services appear that are as convenient and transparent as local network services. Similar examples are demonstrated in large numbers by the most popular global network - the Internet.

Local networks are also changing. Instead of a passive cable connecting computers, a variety of communication equipment appeared in them in large quantities - switches, routers, gateways. Thanks to this equipment, it became possible to build large corporate networks, numbering thousands of computers and having a complex structure. There has been a resurgence of interest in large computers, largely because, after the euphoria over the ease of working with personal computers subsided, it became clear that systems consisting of hundreds of servers were more difficult to maintain than several large computers. Therefore, in a new round of the evolutionary spiral, mainframes began to return to corporate computing systems, but as full-fledged network nodes supporting Ethernet or Token Ring, as well as the TCP/IP protocol stack, which became thanks to the Internet network standard de facto.

Another very important trend has emerged, affecting both local and global networks equally. They began to process information previously unusual for computer networks - voice, video images, drawings. This required changes to the operation of protocols, network operating systems and communications equipment. The difficulty of transmitting such multimedia information over a network is associated with its sensitivity to delays in the transmission of data packets - delays usually lead to distortion of such information at the end nodes of the network. Since traditional networking services such as file transfer or e-mail generate latency-insensitive traffic, and all network elements were designed with latency in mind, the advent of real-time traffic has created major problems.

Today, these problems are solved in various ways, including with the help of ATM technology specially designed for the transmission of various types of traffic. However, despite significant efforts being made in this direction, an acceptable solution to the problem is still far away, and much remains to be done in this area in order to achieve the cherished goal - the merging of technologies not only of local and global networks, but also the technologies of any information networks - computer, telephone, television, etc. Although today this idea seems like a utopia to many, serious experts believe that the prerequisites for such a synthesis are already exist, and their opinions differ only in assessing the approximate terms of such a merger - the terms are called from 10 to 25 years. Moreover, it is believed that the basis for unification will be the packet switching technology used today in computer networks, rather than the circuit switching technology used in telephony, which should probably increase interest in this type of network.

Modern network technologies

Plan

What is a local network?

Computer network hardware. Local area network topologies

Physical topologies of local area networks

Logical topologies of local area networks

Connectors and sockets

Coaxial cable

twisted pair

Transmitting information via fiber optic cables

Communication equipment

Equipment and technologies wireless networks

Technologies and protocols of local area networks

Addressing computers on the network and basic network protocols

Network facilities of MS Windows operating systems

Network Resource Management Concepts

Capabilities of the MS Windows family of operating systems for organizing work in a local network

Configuring network component settings

Configuring connection settings

Connecting a network printer

Connection network drive

What is a local network?

The problem of transferring information from one computer to another has existed since the advent of computers. To solve it, various approaches were used. The most common “courier” approach in the recent past was to copy information onto removable media (GMD, CD, etc.), transfer it to the destination and copy it again, but from the removable media to the recipient’s computer. Currently, such methods of moving information are giving way to network technologies. Those. computers are connected to each other in some way, and the user is able to transfer information to its destination without leaving the desk.

Totality computer devices, which have the ability to communicate with each other, are usually called a computer network. In most cases, there are two types of computer networks: local (LAN - LocalAreaNetwork) and global (WAN - Wide-AreaNetwork). In some classification options, a number of additional types are considered: urban, regional, etc., however, all these types (in essence) in most cases are variants of global networks of various scales. The most common option is to classify networks into local and global based on geography. Those. In this case, a local area network is understood as a collection of a finite number of computers located in a limited area (within one building or neighboring buildings), connected information channels, having high speed and reliability of data transmission and designed to solve a complex of interrelated problems.

Computer network hardware. Local area network topologies

All computers of subscribers (users) working within the local area network must be able to interact with each other, i.e. be connected with each other. The way such connections are organized significantly affects the characteristics of the local computer network and is called its topology (architecture, configuration). There are physical and logical topologies. The physical topology of a local area network refers to the physical placement of the computers that are part of the network and the way they are connected to each other by conductors. The logical topology determines the way information flows and very often does not coincide with the selected physical topology for connecting local area network subscribers.

Physical topologies of local area networks

There are four main physical topologies used in building local area networks.

The bus topology (Fig. 1) involves connecting all computers to one common conductor. At both ends of such a conductor there are special matching devices called terminators. The main advantages of this topology are low cost and ease of installation. Disadvantages include the difficulty of localizing the location of the fault and low reliability: damage to the cable anywhere leads to the cessation of information exchange between all computers on the network. Due to the nature of electrical signal propagation, even if two computers trying to exchange information are physically connected to each other, if there is no terminator at one end of such a “break” of the bus, communication between them will be impossible.

In a ring topology (Fig. 2), each network subscriber is connected to two nearby subscribers. The advantages and disadvantages are similar to those considered for the bus topology.

The star topology involves laying a separate cable for each computer in the network, connecting all network subscribers to a certain center. The center of the star can be a computer or a special connecting device called a hub (Fig. 3). The advantage of this topology is higher reliability. A break in any conductor “disconnects” only one subscriber. The bottleneck of this topology is the hub. If it breaks, the entire network is blocked. The disadvantage is the higher cost of the equipment (taking into account the increase in the total length of the conductors in comparison with previous topologies, as well as the cost of additional equipment - a hub).

In terms of reliability and speed of information exchange best characteristics has a fully connected topology (Fig. 4). In this case, network subscribers are provided with a separate communication channel with each of the other subscribers. However, in terms of cost, this topology is inferior to all other options.

Listed topologies are basic. Most local area networks created in various organizations have a more complex structure and are various combinations of the above topologies.

Logical topologies of local area networks

Logical topology determines the nature of information distribution over a computer network. When transmitting information from one network subscriber to another subscriber, this information is properly “formatted”. The transmitted data is formatted in standard fragments (packets, datagrams). In addition to the actual transmitted data (numbers, texts, pictures, etc.), the address (of the information receiver or both the receivers and the transmitter), control information (so that you can check whether the packet was received in full or only part of it) and a number of other things are added to the packet. information. Let's consider three main options for logical topologies of local computer networks.

The logical bus determines equal access to the network for all subscribers. In this case, the transmitter puts a packet of information into the network, and all other subscribers “hear” transmitted information analyze it. If the subscriber finds his address as part of the package, he “keeps” this information for himself, if the address turns out to be someone else’s, he ignores it. If, at the time of transmission of information by one subscriber, another subscriber “interjects” into the conversation, an overlap of packets occurs, called a collision. Collisions lead to “mixing” of packets and the inability to figure out “who said what.” Having detected a collision, the transmitting subscriber “falls silent” for a time interval of random duration, after which it repeats the attempt to transmit information. With a very large number of subscribers in the network, the probability of collisions increases sharply, and the network becomes inoperable.

The logical ring assumes that information goes full circle and comes to the source, i.e. to the point from which it was sent. In this case, each subscriber compares the “recipient” address with his own. If the addresses match, the information is copied to a buffer, the packet is marked as “reached the addressee” and is transmitted to the next subscriber. If the addresses do not match, the packet is transmitted without any marks. When a subscriber has received a package sent “with his own hand” and marked “accepted,” he does not transmit it further and another network subscriber can start working.

The logical star topology (and its version - tree) is focused on establishing a communication channel between the receiver and the transmitter using switches. Those. In the absence of a switch, it is impossible for even two network subscribers to communicate with each other. When transferring data from one subscriber to another, everyone else waits for the end of the transfer.

Connectors and sockets

Currently, several types of conductors are used in local area networks. Based on the physical nature of the transmitted signal, a distinction is made between electrical conductors and optical conductors. In addition, equipment can be used to organize local computer networks using wireless channels.

Coaxial cable

A coaxial cable (Fig. 5) is a conductor enclosed in a shielding braid. The conductor is protected from contact with the braid by a tubular insulator. Important characteristic cable systems in general and coaxial cable in particular is the characteristic resistance or impedance. In local area networks, a coaxial cable with a characteristic impedance of 50 Ohms is used and (much less often) in ARCnet networks a cable with a characteristic impedance of 93 Ohms is used. There are two types of coaxial cable - thick (outer diameter about 10 mm) and thin (outer diameter about 5 mm). With the same characteristic impedance value for a thick and thin coaxial cable various characteristics by the length of the cable segment and the number of supported network subscribers. On a thick coaxial cable maximum length segment 500 meters, maximum number of connection points 100. Thin coaxial cable has a maximum segment length of 185 meters, maximum number of connection points 30.