Telecommunications and computer networks. Lecture notes

by discipline "Computer networks and telecommunications"

INTRODUCTION... 65

2 CABLES AND INTERFACES... 10

3 DATA EXCHANGE ON THE NETWORK... 15

6 INTERNET SERVICES 40

8 WEB VIEWERS 54

INTRODUCTION 6

1 NETWORK CONCEPTS AND TERMS... 7

1.1 Basic concepts. 7

1.2 Classification of networks by scale. 7

1.3 Classification of networks based on the presence of a server. 7

1.3.1 Peer-to-peer networks. 7

1.3.2 Networks with a dedicated server. 8

1.4 Network selection. 9

2 CABLES AND INTERFACES... 10

2.1 Cable types. 10

2.1.1 Twisted pair cable – twisted pair 10

2.1.2 Coaxial cable. eleven

2.1.3 Fiber optic cable. 12

2.2 Wireless technologies. 12

2.2.1 Radio communication. 13

2.2.2 Microwave communications. 13

2.2.3 Infrared communication. 13

2.3 Cable parameters. 13

3 DATA EXCHANGE ON THE NETWORK... 15

3.1 General concepts. Protocol. Protocol stack. 15

3.2 ISO/OSI 16 model

3.3 Functions of ISO/OSI 18 model layers

3.4 Application interaction protocols and transport subsystem protocols. 21

3.5 Functional compliance of types of communication equipment with levels of the OSI 22 model

3.6 IEEE 802.24 Specification

3.7 According to the protocol stack. 25

4 NETWORK EQUIPMENT AND TOPOLOGIES.. 27

4.1 Network components. 27

4.1.1 Network cards. 27

4.1.2 Repeaters and amplifiers. 28

4.1.3 Concentrators. 29

4.1.4 Bridges. 29

4.1.5 Routers. thirty

4.1.6 Gateways. thirty

4.2 Types of network topology. 31

4.2.1 Tire. 31

4.2.2 Ring. 32

4.2.3 Star. 32

4.2.5 Mixed topologies. 33

5 GLOBAL INTERNET NETWORK.. 36

5.1 Theoretical basis Internet. 36

5.2 Working with Internet services. 37

6 INTERNET SERVICES 40

6.1 Terminal mode. 40

6.2 Electronic mail (E-Mail) 40

6.4 Teleconferencing service (Usenet) 41

6.5 World Wide Web (WWW) Service 43

6.6 Domain Name Service (DNS) 45

6.7 File Transfer Service (FTP) 48

6.8 Internet Relay Chat Service 49

6.9 ICQ service.. 49

7 CONNECTING TO THE INTERNET.. 51

7.1 Basic concepts. 51

7.2 Installing the modem. 52

7.3 Connecting to an Internet service provider's computer. 53

8 WEB VIEWERS 54

8.1 The concept of browsers and their functions. 54

8.2 Working with the program Internet Explorer 54

8.2.1 Opening and viewing Web pages. 56

8.2.3 Browser control techniques. 57

8.2.4 Working with multiple windows. 58

8.2.5 Setting browser properties. 58

8.3 Searching for information on the World Wide Web. 60

8.4 Receiving files from the Internet. 62

9 WORKING WITH ELECTRONIC MESSAGES... 64

9.1 Sending and receiving messages. 64

9.2 Working with Outlook Express. 65

9.2.1 Creation account. 65

9.2.2 Creating an email message. 66

9.2.3 Preparing responses to messages. 66

9.2.4 Reading teleconference messages. 67

9.3 Working with the address book. 67

INTRODUCTION

The material discussed in this lecture notes is not about a specific operating system or even a specific type of operating system. It examines operating systems (OS) from a very general perspective, and the described fundamental concepts and design principles are valid for most operating systems.

1 NETWORK CONCEPTS AND TERMS

1.1 Basic concepts

A network is a connection between two or more computers, allowing them to share resources.

1.2 Classification of networks by scale

The local network(Local Area Network) is a collection of networked computers located within a small physical region, such as a single building.

This is a set of computers and other connected devices that fit within the coverage area of ​​one physical network. Local networks are the basic building blocks for building internetworks and global networks.

Global networks(Wide Area Network) can connect networks around the world; third-party communications tools are typically used for internetworking.

WAN connections can be very expensive as communication costs increase with bandwidth. Thus, only a small number of WAN connections support the same bandwidth as regular LANs.

Regional networks(Metropolitan Area Network) use wide area network technologies to connect local networks in a specific geographic region, such as a city.

1.3 Classification of networks based on the presence of a server

1.3.1 Peer-to-peer networks

Computers in peer-to-peer networks can act as both clients and servers. Since all computers in this type of network have equal rights, peer-to-peer networks do not have centralized control over resource sharing. Any computer on this network can share its resources with any computer on the same network. Peer-to-peer relationships also mean that no one computer has higher access priority or greater responsibility for sharing resources.

Advantages of peer-to-peer networks:

– they are easy to install and configure;

– individual machines do not depend on a dedicated server;

– users are able to control their own resources;

– inexpensive type of networks to purchase and operate;

– no additional hardware or software is needed other than the operating system;

– there is no need to hire a network administrator;

– works well with a number of users not exceeding 10.

Disadvantages of peer-to-peer networks:

– applying network security to only one resource at a time;

– users must remember as many passwords as there are shared resources;

- must be produced backup separately on each computer to protect all shared data;

– when gaining access to a resource, a drop in performance is felt on the computer on which this resource is located;

– There is no centralized organizational scheme for searching and managing access to data.

1.3.2 Dedicated server networks

Microsoft prefers the term Server-based. A server is a machine (computer) whose main task is to respond to client requests. Servers are rarely managed by anyone directly - only to be installed, configured or maintained.

Advantages of networks with a dedicated server:

– they provide centralized management of user accounts, security and access, which simplifies network administration;

– more powerful equipment means more efficient access to network resources;

– users only need to remember one password to log into the network, which allows them to access all resources they are entitled to;

– such networks scale (grow) better with the increase in the number of clients.

Disadvantages of dedicated server networks:

– a server malfunction can make the network inoperable, at best – loss of network resources;

– such networks require qualified personnel to maintain complex specialized software;

– the cost of the network increases due to the need for specialized equipment and software.

1.4 Network selection

The choice of network depends on a number of circumstances:

– number of computers in the network (up to 10 – peer-to-peer networks);

– financial reasons;

– presence of centralized management, security;

– access to specialized servers;

– access to the global network.

2 CABLES AND INTERFACES

At the lowest level of network communications is the medium over which data is transmitted. In relation to data transmission, the term media (media, data transmission medium) can include both cable and wireless technologies.

2.1 Cable types

There are several different types of cables used in modern networks. Different network situations may require various types cables

2.1.1 Twisted pair cable

It is a network media used in many network topologies, including Ethernet, ARCNet, IBM Token Ring.

There are two types of twisted pair.

1. Unshielded twisted pair.

There are five categories of unshielded twisted pair cable. They are numbered in order of increasing quality from CAT1 to CAT5. Higher grade cables typically contain more pairs of conductors, and these conductors have more turns per unit length.

CAT1 – telephone cable, does not support digital data transmission.

CAT2 is a rarely used older type of unshielded twisted pair cable. It supports data transfer rates up to 4 Mbps.

CAT3, the minimum level of unshielded twisted pair cable required for today's digital networks, has a throughput of 10 Mbps.

CAT4 is an intermediate cable specification that supports data rates up to 16 Mbps.

CAT5 is the most efficient type of unshielded twisted pair cable, supporting data transfer rates up to 100 Mbps.

UTP cables connect each computer's network card to a network panel or network hub using an RJ-45 connector at each connection point.

An example of such a configuration is the 10Base-T Ethernet network standard, which is characterized by unshielded twisted pair cable (CAT3 to CAT5) and the use of an RJ-45 connector.

Flaws:

– sensitivity to interference from external electromagnetic sources;

– mutual signal overlap between adjacent wires;

– unshielded twisted pair is vulnerable to signal interception;

– large signal attenuation along the way (limited to 100 m).

2. Shielded twisted pair.

It has a similar design as the previous one and is subject to the same 100-meter limit. Typically contains four or more pairs of stranded copper insulated wires in the middle, along with electrically grounded braided copper mesh or aluminum foil, creating a shield from external electromagnetic influences.

Flaws:

– the cable is less flexible;

– requires electrical grounding.

2.1.2 Coaxial cable

This type of cable consists of a central copper conductor that is thicker than the wires in a twisted pair cable. The center conductor is covered with a layer of foamy plastic insulating material, which in turn is surrounded by a second conductor, usually woven copper mesh or aluminum foil. The external conductor is not used for data transmission, but acts as grounding.

Coaxial cable can transmit data at speeds up to 10 Mbps over a maximum distance of 185 m to 500 m.

The two main types of coaxial cable used in LANs are Thicknet and Thinnet.

Also known as RG-58 cable, it is the most used. It is the most flexible of all coaxial cable types and is approximately 6mm thick. It can be used to connect each computer to other computers in local network using a T-connector, British Naval Connector (BNC)-connector and 50-Ohm plugs (terminator terminators). Mainly used for 10Base-2 Ethernet networks.

This configuration supports data transfer rates of up to 10 Mbps over a maximum distance of 185 m between repeaters.

Is a thicker and more expensive coaxial cable. It is similar in design to the previous one, but less flexible. Used as the basis for 10Base-5 Ethernet networks. This cable is marked RG-8 or RG-11, approximately 12 mm in diameter. It is used as a linear bus. To connect to each network card, a special external transceiver AUI (Attachment unit interface) and a “vampire” (branch) are used that pierces the cable sheath to gain access to the wire.

It has a thick center conductor that provides reliable data transmission over distances of up to 500 m per cable segment. Often used to create connecting highways. Data transfer speed up to 10 Mbit/s.

2.1.3 Fiber optic cable

Provide excellent speed of information transfer over long distances. They are immune to electromagnetic noise and eavesdropping.

It consists of a central glass or plastic conductor surrounded by another layer of glass or plastic coating, and an outer protective sheath. Data is transmitted over the cable using a laser or LED transmitter that sends unidirectional light pulses through a central glass fiber. The glass coating helps keep the light focused in the inner conductor. At the other end of the conductor, the signal is received by a photodiode receiver, which converts the light signals into an electrical signal.

The data transfer speed for fiber optic cable reaches from 100 Mbit/s to 2 Gbit/s. Data can be reliably transmitted over distances of up to 2 km without a repeater.

Light pulses travel in only one direction, so you need to have two conductors: an incoming cable and an outgoing cable.

This cable is difficult to install and is the most expensive type of cable.

2.2 Wireless technologies

Methods wireless transmission data are a more convenient form. Wireless technologies vary in signal types, frequency, and transmission distance.

The three main types of wireless data transmission are: radio, microwave, and infrared.

2.2.1 Radio communications

Radio communication technologies send data at radio frequencies and have virtually no range limitations. Used to connect local networks over large geographical distances.

Flaws:

– radio transmission is expensive,

– subject to government regulation,

– extremely sensitive to electronic or atmospheric influences,

– is susceptible to interception and therefore requires encryption.

2.2.2 Microwave communications

Supports data transmission in the microwave range, uses high frequencies and is used both over short distances and in global communications.

Limitation: The transmitter and receiver must be within line of sight of each other.

Widely used in global information transmission using satellites and terrestrial satellite antennas.

2.2.3 Infrared communication

Operates at high frequencies approaching the frequencies of visible light. Can be used to establish two-way or broadcast data transmission over short distances. Typically LEDs are used to transmit infrared waves to the receiver.

These waves can be physically blocked and experience interference with bright light, so transmission is limited to short distances.

2.3 Cable parameters

When planning a network or expansion existing network Several issues regarding cabling need to be clearly considered: cost, distance, data speed, ease of installation, number of nodes supported.

A comparison of cable types by data transfer speed, cable cost, installation complexity, and maximum data transfer distance is presented in Table 2.1.

The number of nodes per segment and nodes in the network when building networks with different cable usage is presented in Table 2.2.

Table 2.1 – Comparative characteristics cables

Table 2.2 – Number of nodes depending on network type

3 DATA EXCHANGE ON THE NETWORK

3.1 General concepts. Protocol. Protocol stack.

The main goal that is pursued when connecting computers into a network is the ability to use the resources of each computer by all network users. In order to realize this feature, computers connected to the network must have the necessary means of interaction with other computers on the network.

The task of sharing network resources includes solving many problems - choosing a method for addressing computers and coordinating electrical signals when establishing electrical communication, ensuring reliable data transmission and processing error messages, generating sent and interpreting received messages, as well as many other equally important tasks.

The usual approach to solving a complex problem is to break it down into several subproblems. A certain module is assigned to solve each subtask. At the same time, the functions of each module and the rules for their interaction are clearly defined.

A special case of task decomposition is a multi-level representation, in which the entire set of modules that solve subtasks is divided into hierarchically ordered groups - levels. For each level, a set of query functions is defined, with which modules at a given level can be accessed by modules at a higher level to solve their problems.

This set of functions performed by a given layer for a higher layer, as well as the message formats exchanged between two neighboring layers during their interaction, is called an interface.

The rules for interaction between two machines can be described as a set of procedures for each level. Such formalized rules that determine the sequence and format of messages exchanged between network components lying at the same level, but in different nodes, are called protocols.

An agreed upon set of protocols at different levels, sufficient to organize internetworking, is called protocol stack.

When organizing interaction, two main types of protocols can be used. IN connection-oriented protocols(connection-oriented network service, CONS) before exchanging data, the sender and recipient must first establish a logical connection, that is, agree on the parameters of the exchange procedure that will be valid only within the framework of this connection. After completing the dialogue, they must terminate this connection. When a new connection is established, the negotiation procedure is carried out again.

The second group of protocols is connectionless protocols(connectionless network service, CLNS). Such protocols are also called datagram protocols. The sender simply transmits the message when it is ready.

3.2 ISO/OSI model

Just because a protocol is an agreement between two interacting entities, in this case two computers working on a network, does not mean that it is necessarily a standard. But in practice, when implementing networks, they tend to use standard protocols. These may be proprietary, national or international standards.

The International Standards Organization (ISO) has developed a model that clearly defines the different levels of interaction between systems, gives them standard names, and specifies what work each level should do. This model is called the Open System Interconnection (OSI) model or ISO/OSI model.

In the OSI model, communication is divided into seven layers or layers (Fig. 1). Each level deals with one specific aspect of interaction. Thus, the interaction problem is decomposed into 7 particular problems, each of which can be solved independently of the others. Each layer maintains interfaces with the layers above and below.

The OSI model describes only system communications, not end-user applications. Applications implement their own communication protocols by accessing system facilities. It should be borne in mind that the application can take over the functions of some of the upper layers of the OSI model, in which case, if necessary, internetworking it accesses directly the system tools that perform the functions of the remaining lower layers of the OSI model.

An end-user application can use system interaction tools not only to organize a dialogue with another application running on another machine, but also simply to receive the services of a particular network service.

So, let's say an application makes a request to an application layer, such as a file service. Based on this request software The application layer generates a standard format message, which contains service information (header) and, possibly, transmitted data. This message is then forwarded to the representative level.

The presentation layer adds its header to the message and passes the result down to the session layer, which in turn adds its header, and so on.

Finally, the message reaches the lowest, physical layer, which actually transmits it along the communication lines.

When a message arrives on another machine over the network, it moves up sequentially from level to level. Each level analyzes, processes and deletes the header of its level, performs functions corresponding to this level and passes the message to the higher level.

In addition to the term message, there are other names used by network specialists to designate a unit of data exchange. ISO standards for protocols of any level use the term “protocol data unit” - Protocol Data Unit (PDU). In addition, the names frame, packet, and datagram are often used.

3.3 Functions of ISO/OSI model layers

Physical level. This layer deals with the transmission of bits over physical channels, such as coaxial cable, twisted pair cable, or fiber optic cable. This level is related to the characteristics of physical data transmission media, such as bandwidth, noise immunity, characteristic impedance and others. At the same level, the characteristics of electrical signals are determined, such as requirements for pulse edges, voltage or current levels of the transmitted signal, type of coding, signal transmission speed. In addition, the types of connectors and the purpose of each contact are standardized here.

Physical layer functions are implemented in all devices connected to the network. On the computer side, the physical layer functions are performed by the network adapter or serial port.

Data link level. One of the tasks of the link layer is to check the availability of the transmission medium. Another task of the link layer is to implement error detection and correction mechanisms. To do this, at the data link layer, bits are grouped into sets called frames. The link layer ensures that each frame is transmitted correctly by placing a special sequence of bits at the beginning and end of each frame to mark it, and also calculates a checksum by summing all the bytes of the frame in a certain way and adding the checksum to the frame. When the frame arrives, the receiver again calculates the checksum of the received data and compares the result with the checksum from the frame. If they match, the frame is considered correct and accepted. If the checksums do not match, an error is recorded.

The link layer protocols used in local networks contain a certain structure of connections between computers and methods for addressing them. Although the data link layer provides frame delivery between any two nodes on a local network, it does this only in a network with a very specific connection topology, precisely the topology for which it was designed. Typical topologies supported by LAN link layer protocols include shared bus, ring, and star. Examples of link layer protocols are Ethernet, Token Ring, FDDI, 100VG-AnyLAN.

Network layer. This level serves to form a unified transport system that unites several networks with different principles for transmitting information between end nodes.

Network layer messages are usually called packets. When organizing packet delivery at the network level, the concept of “network number” is used. In this case, the recipient's address consists of the network number and the computer number on this network.

In order to transmit a message from a sender located on one network to a recipient located on another network, you need to make a number of transit transfers (hops) between networks, each time choosing the appropriate route. Thus, a route is a sequence of routers through which a packet passes.

The problem of choosing the best path is called routing and its solution is the main task of the network layer. This problem is complicated by the fact that the shortest path is not always the best. Often the criterion for choosing a route is the time of data transmission along this route; it depends on the capacity of communication channels and traffic intensity, which can change over time.

At the network level, two types of protocols are defined. The first type refers to the definition of rules for transmitting end node data packets from the node to the router and between routers. These are the protocols that are usually meant when people talk about network layer protocols. The network layer also includes another type of protocol, called routing information exchange protocols. Using these protocols, routers collect information about the topology of internetwork connections. Network layer protocols are implemented by operating system software modules, as well as router software and hardware.

Examples of network layer protocols are the TCP/IP stack IP Internetwork Protocol and the Novell IPX stack Internetwork Protocol.

Transport layer. On the way from the sender to the recipient, packets may be corrupted or lost. While some applications have their own error handling, there are others that prefer to deal with a reliable connection right away. The job of the transport layer is to ensure that applications or the upper layers of the stack - application and session - transfer data with the degree of reliability that they require. The OSI model defines five classes of service provided by the transport layer.

As a rule, all protocols, starting from the transport layer and above, are implemented by software of the end nodes of the network - components of their network operating systems. Examples of transport protocols include the TCP and UDP protocols of the TCP/IP stack and the SPX protocol of the Novell stack.

Session level. The session layer provides conversation management to record which party is currently active and also provides synchronization facilities. The latter allow you to insert checkpoints into long transfers so that in case of failure you can go back to the last checkpoint, instead of starting all over again. In practice, few applications use the session layer, and it is rarely implemented.

Presentation level. This layer provides assurance that information conveyed by the application layer will be understood by the application layer in another system. If necessary, the presentation layer converts data formats into some common presentation format, and at the reception, accordingly, performs the reverse conversion. In this way, application layers can overcome, for example, syntactic differences in data representation. At this level, data encryption and decryption can be performed, thanks to which the secrecy of data exchange is ensured for all application services at once. An example of a protocol that operates at the presentation layer is the Secure Socket Layer (SSL) protocol, which provides secure messaging for the application layer protocols of the TCP/IP stack.

Application layer. The application layer is really just a set of various protocols that enable network users to access shared resources such as files, printers, or hypertext Web pages, and to collaborate, such as through the email protocol. The unit of data that the application layer operates on is usually called a message.

There is a very wide variety of application layer protocols. Let us give as examples at least a few of the most common implementations of file services: NCP in the Novell NetWare operating system, SMB in Microsoft Windows NT, NFS, FTP and TFTP, which are part of the TCP/IP stack.

3.4 Application interaction protocols and transport subsystem protocols

Functions at all layers of the OSI model can be classified into one of two groups: either functions that depend on a specific technical implementation of the network, or functions that are oriented to work with applications.

The three lower levels - physical, channel and network - are network-dependent, that is, the protocols of these levels are closely related to the technical implementation of the network, to the communication equipment used.

The top three layers - session, presentation and application - are application-oriented and have little dependence on technical features building a network. The protocols at these layers are not affected by any changes in network topology, hardware replacement, or migration to another network technology.

The transport layer is the intermediate layer, it hides all the details of the functioning of the lower layers from the upper layers. This allows you to develop applications that are independent of the technical means directly involved in transporting messages.

Figure 2 shows the layers of the OSI model at which the various network elements operate.

A computer with a network OS installed on it interacts with another computer using protocols of all seven levels. Computers carry out this interaction through various communication devices: hubs, modems, bridges, switches, routers, multiplexers. Depending on the type, a communication device can operate either only at the physical layer (repeater), or at the physical and link level (bridge and switch), or at the physical, link and network layer, sometimes including the transport layer (router).

3.5 Functional correspondence of types of communication equipment to the levels of the OSI model

The best way The key to understanding the differences between network adapters, repeaters, bridges/switches, and routers is to look at how they work in terms of the OSI model. The relationship between the functions of these devices and the layers of the OSI model is shown in Figure 3.

A repeater, which regenerates signals, thereby allowing you to increase the length of the network, operates at the physical level.

The network adapter operates at the physical and data link layers. The physical layer includes that part of the functions of the network adapter that is associated with the reception and transmission of signals over the communication line, and gaining access to the shared transmission medium and recognizing the MAC address of the computer is already a function of the link layer.

Bridges do most of their work at the data link layer. For them, the network is represented by a set of MAC addresses of devices. They extract these addresses from headers added to packets at the data link layer and use them during packet processing to decide which port to send a particular packet to. Bridges do not have access to higher-level network address information. Therefore, they are limited in making decisions about possible paths or routes for packets to travel through the network.

Routers operate at the network layer of the OSI model. For routers, a network is a set of device network addresses and a set of network paths. Routers analyze all possible paths between any two network nodes and choose the shortest one. When choosing, other factors may be taken into account, for example, the state of intermediate nodes and communication lines, line capacity or the cost of data transmission.

In order for a router to perform the functions assigned to it, it must have access to more detailed information about the network than that available to the bridge. In addition to the network address, the network layer packet header contains data, for example, about the criteria that should be used when choosing a route, about the lifetime of the packet in the network, and about which upper-level protocol the packet belongs to.

By using additional information, a router can perform more packet operations than a bridge/switch. Therefore, the software required to operate the router is more complex.

Figure 3 shows another type of communication device - a gateway, which can operate at any level of the OSI model. A gateway is a device that performs protocol translation. A gateway is placed between communicating networks and serves as an intermediary, translating messages coming from one network into the format of another network. The gateway can be implemented either purely by software installed on a regular computer, or on the basis of a specialized computer. Translating one protocol stack into another is a complex intellectual task that requires the most complete information about the network, so the gateway uses the headers of all translated protocols.

3.6 IEEE 802 Specification

Around the same time that the OSI model was introduced, the IEEE 802 specification was published, which effectively extends the OSI networking model. This expansion occurs at the data link and physical layers, which determine how more than one computer can access a network without conflicting with other computers on the network.

This standard details these layers by breaking the data link layer into 2 sublayers:

– Logical Link Control (LLC) – logical link control sublevel. Manages connections between data channels and defines the use of logical interface points, called Services Access Points, which other computers can use to pass information to higher layers of the OSI model;

– Media Access Control (MAC) – device access control sublayer. Provides parallel access for several network adapters at the physical level, has direct interaction with the computer's network card and is responsible for ensuring error-free data transfer between computers on the network.

3.7 By protocol stack

A protocol suite (or protocol stack) is a combination of protocols that work together to provide network communication. These protocol suites are usually divided into three groups, corresponding to the OSI network model:

– network;

– transport;

– applied.

Network protocols provide the following services:

– addressing and routing of information;

– checking for errors;

– retransmission request;

– establishing rules of interaction in a specific network environment.

Popular network protocols:

– DDP (Delivery Datagram Protocol). The Apple data transfer protocol used in AppleTalk.

– IP (Internet Protocol). Part of the TCP/IP protocol suite that provides addressing and routing information.

– IPX (Internetwork Packet eXchange) and NWLink. A Novell NetWare networking protocol (and Microsoft's implementation of this protocol) used for routing and forwarding packets.

– NetBEUI. Developed jointly by IBM and Microsoft, this protocol provides transport services for NetBIOS.

Transport protocols are responsible for ensuring reliable transport of data between computers.

Popular transport protocols:

– ATP (AppleTalk Transaction Protocol) and NBP (Name Binding Protocol). AppleTalk session and transport protocols.

– NetBIOS/NetBEUI. The first one establishes a connection between computers, and the second one provides data transfer services for this connection.

– SPX (Sequenced Packet exchange) and NWLink. Novell's connection-oriented protocol used to provide data delivery (and Microsoft's implementation of this protocol).

– TCP (Transmission Control Protocol). Part of the TCP/IP protocol suite responsible for reliable data delivery.

Application protocols responsible for the interaction of applications.

Popular application protocols:

– AFP (AppleTalk File Protocol). Protocol remote control Macintosh files.

– FTP ( File Transfer Protocol – Data transfer protocol). Another member of the TCP/IP protocol suite, used to provide file transfer services.

– NCP (NetWare Core Protocol – NetWare Basic Protocol). Novell client shell and redirectors.

– SMTP (Simple Mail Transport Protocol). A member of the TCP/IP protocol suite responsible for transmitting electronic mail.

– SNMP (Simple Network Management Protocol). TCP/IP protocol used to manage and monitor network devices.

4 NETWORK EQUIPMENT AND TOPOLOGIES

4.1 Network components

There are many network devices that can be used to create, segment, and enhance a network.

4.1.1 Network cards

Network adapter(Network Interface Card, NIC) is a computer peripheral device that directly interacts with a data transmission medium that directly or through other communications equipment connects it with other computers. This device solves the problem of reliable exchange of binary data, represented by corresponding electromagnetic signals, over external communication lines. Like any computer controller, the network adapter operates under the control of the operating system driver.

Most modern standards for local networks assume that a special communication device (hub, bridge, switch or router) is installed between the network adapters of interacting computers, which takes on some functions for controlling the data flow.

A network adapter typically performs the following functions:

– Formatting of transmitted information in the form of a frame of a certain format. The frame includes several service fields, including the address of the destination computer and the frame checksum.

– Gaining access to the data transmission medium. Local networks mainly use communication channels shared between a group of computers (common bus, ring), access to which is provided using a special algorithm (the most often used is the random access method or the method of passing an access token along the ring).

– Encoding a sequence of frame bits using a sequence of electrical signals when transmitting data and decoding when receiving them. Coding must ensure the transmission of original information over communication lines with a certain bandwidth and a certain level of interference so that the receiving side can recognize the sent information with a high degree of probability.

– Converting information from parallel to serial form and vice versa. This operation is due to the fact that in computer networks information is transmitted in serial form, bit by bit, and not byte by byte, as inside a computer.

– Synchronization of bits, bytes and frames. For stable reception of transmitted information, it is necessary to maintain constant synchronization of the receiver and transmitter of information.

Network adapters differ in the type and capacity of the internal data bus used in the computer - ISA, EISA, PCI, MCA.

Network adapters also differ in the type of network technology adopted on the network - Ethernet, Token Ring, FDDI, etc. As a rule, a specific network adapter model operates on a specific network technology (for example, Ethernet).

Due to the fact that each technology now has the ability to use different transmission media, a network adapter can support both one and several media at the same time. In the case when the network adapter supports only one data transmission medium, but it is necessary to use another, transceivers and converters are used.

Transceiver(transceiver, transmitter+receiver) is part of the network adapter, its terminal device that connects to the cable. In Ethernet versions it turned out to be convenient to release network adapters with an AUI port to which a transceiver can be connected for the required environment.

Instead of selecting a suitable transceiver, you can use converter, which can match the output of a transceiver designed for one medium to another (for example, a twisted-pair output is converted to a coaxial cable output).

4.1.2 Repeaters and amplifiers

As mentioned earlier, the signal weakens as it moves across the network. To prevent this attenuation, repeaters and/or amplifiers can be used to amplify the signal passing through them.

Repeaters are used in digital signal networks to combat signal attenuation (weakening). When the repeater receives a weakened signal, it cleans the signal, amplifies it, and sends it to the next segment.

Amplifiers, although they have a similar purpose, are used to increase transmission range in networks using an analog signal. This is called broadband transmission. The carrier is divided into several channels so that different frequencies can be transmitted in parallel.

Typically, the network architecture determines the maximum number of repeaters that can be installed on a single network. The reason for this is a phenomenon known as propagation delay. The period required for each repeater to clean up and amplify the signal, multiplied by the number of repeaters, can result in noticeable delays in the transmission of data across the network.

4.1.3 Hubs

A hub (HUB) is a network device that operates at the physical layer of the OSI network model, serving as the central connection point and link in a star network configuration.

There are three main types of hubs:

– passive (passive);

– active (active);

– intellectual (intelligent).

Passive hubs require no power and act as a physical connection point without adding anything to the signal passing through).

Active ones require energy, which is used to restore and strengthen the signal.

Smart hubs can provide services such as packet switching and traffic routing.

4.1.4 Bridges

A bridge is a device used to connect network segments. Bridges can be considered an improvement on repeaters because they reduce network load: bridges read the address network card(MAC address) of the receiving computer from each incoming data packet and look at special tables to determine what to do with the packet.

The bridge operates at the data link layer of the OSI network model.

A bridge functions as a repeater, it receives data from any segment, but it is more discriminating than a repeater. If the recipient is on the same physical segment as the bridge, then the bridge knows that the packet is no longer needed. If the recipient is on a different segment, the bridge knows to forward the packet.

This processing reduces network load because the segment will not receive messages that do not belong to it.

Bridges can connect segments that use different types of media (10BaseT, 10Base2), as well as with different media access schemes (Ethernet, Token Ring).

4.1.5 Routers

A router is a network communication device that operates at the network layer of the network model and can connect two or more network segments (or subnets).

It functions like a bridge, but to filter traffic it uses not the address of the computer's network card, but rather the network address information carried in the network layer portion of the packet.

After receiving this information, the router uses the routing table to determine where to route the packet.

There are two types of routing devices: static and dynamic. The former use a static routing table, which must be created and updated by the network administrator. The second ones create and update their tables themselves.

Routers can reduce network congestion, increase throughput, and improve the reliability of data delivery.

A router can be either a special electronic device or a specialized computer connected to several network segments using several network cards.

It can connect multiple small subnets using different protocols, as long as the protocols used support routing. Routed protocols have the ability to redirect data packets to other network segments (TCP/IP, IPX/SPX). Non-routable protocol – NetBEUI. It cannot operate outside of its own subnet.

4.1.6 Gateways

A gateway is a method of communicating between two or more network segments. Allows disparate systems to communicate on the network (Intel and Macintosh).

Another function of gateways is protocol conversion. The gateway can receive IPX/SPX directed to a TCP/IP client on the remote segment. The gateway converts the source protocol to the desired destination protocol.

The gateway operates at the transport layer of the network model.

4.2 Types of network topology

Network topology refers to a description of its physical location, that is, how computers are connected to each other on the network and through which devices they are included in the physical topology.

There are four main topologies:

– Bus (bus);

– Ring (ring);

– Star (star);

– Mesh (cell).

The physical bus topology, also called a linear bus, consists of a single cable to which all computers in the segment are connected (Fig. 4.1).

Messages are sent over the line to all connected stations, regardless of who the recipient is. Each computer examines every packet on the wire to determine the recipient of the packet. If the packet is intended for another station, the computer rejects it. If the package is intended this computer, then it will receive and process it.

Figure 4.1 – Bus topology

The main bus cable, known as the backbone, has terminations (terminators) at both ends to prevent signal reflections. Typically, bus topology networks use two types of media: thick and thin Ethernet.

Flaws:

– it is difficult to isolate problems with a station or other network component;

– faults in the backbone cable can lead to failure of the entire network.

4.2.2 Ring

Ring topology is used primarily in Token Ring and FDDI (fiber optic) networks.

In a physical ring topology, the data lines actually form a logical ring to which all computers on the network are connected (Figure 4.2).

Figure 4.2 – Ring topology

Access to the media in the ring is carried out through tokens, which are sent in a circle from station to station, giving them the opportunity to forward the packet if necessary. A computer can only send data when it owns the token.

Since each computer in this topology is part of a ring, it has the ability to forward any data packets it receives that are addressed to another station.

Flaws:

– problems at one station can lead to a failure of the entire network;

– when reconfiguring any part of the network, it is necessary to temporarily disconnect the entire network.

4.2.3 Star

In a Star topology, all computers on the network are connected to each other using a central hub (Fig. 4.3).

All data that the station sends is sent directly to the hub, which forwards the packet towards the recipient.

In this topology, only one computer can send data at a time. If two or more computers try to send data at the same time, they will all fail and be forced to wait a random amount of time to try again.

These networks scale better than other networks. Problems at one station do not bring down the entire network. Having a central hub makes it easy to add a new computer.

Flaws:

– requires more cable than other topologies;

– failure of the hub will disable the entire network segment.

Figure 4.3 – Star topology

The Mesh (cell) topology connects all computers in pairs (Fig. 4.4).

Figure 4.4 – Cell topology

Mesh networks use significantly more cable than other topologies. These networks are much more difficult to install. But these networks are fault-tolerant (able to operate in the presence of damage).

4.2.5 Mixed topologies

In practice, there are many combinations of major network topologies. Let's look at the main ones.

Star Bus

The mixed Star Bus topology (star on a bus) combines the Bus and Star topologies (Fig. 4.5).

The Star Ring topology is also known as a Star-wired Ring because the hub itself is designed as a ring.

This network is identical to a star topology, but the hub is actually wired together as a logical ring.

Just like a physical ring, this network sends tokens to determine the order in which computers transmit data.

Figure 4.5 – Star-on-bus topology

Hybrid Mesh

Since implementing a true mesh topology on large networks can be expensive, a Hybrid Mesh topology network can provide some of the significant benefits of a true mesh network.

Mainly used to connect servers storing critical data (Fig. 4.6).

Figure 4.6 – “Hybrid cell” topology

5 GLOBAL INTERNET

5.1 Theoretical foundations of the Internet

Early experiments on transmitting and receiving information using computers began in the 50s and were of a laboratory nature. Only in the late 60s, with funds from the Advanced Development Agency of the US Department of Defense, was it created national network. She got the name ARPANET. This network connected several major scientific, research and educational centers. Its main task was to coordinate groups of teams working on common scientific and technical projects, and its main purpose was the exchange of files with scientific and design documentation by e-mail.

The ARPANET went live in 1969. The few nodes included in it at that time were connected by dedicated lines. Reception and transmission of information was provided by programs running on host computers. The network gradually expanded by connecting new nodes, and by the early 80s, based on the largest nodes, their own regional networks were created, recreating the general ARPANET architecture at a lower level (on a regional or local scale).

For real the birth of the Internet It is generally accepted that the year is 1983. This year has seen revolutionary changes in computer communications software. The birthday of the Internet in the modern sense of the word was the date of standardization of the TCP/IP communication protocol, which underlies the World Wide Web to this day.

TCP/IP is not one network protocol, but several protocols lying at different levels of the OSI network model (this is the so-called protocol stack). Of these, TCP is a transport layer protocol. It controls how information is transferred. IP address protocol. It belongs to the network layer and determines where transmission occurs.

Topic 9. Telecommunications

Lecture outline

1. Telecommunications and computer networks

2. Characteristics of local and global networks

3. System software

4. OSI model and information exchange protocols

5. Data transmission media, modems

6. Tele capabilities information systems

7. Possibilities of the World Wide Web

8. Prospects for creating an information highway

Telecommunications and computer networks

Communication is the transfer of information between people, carried out using various means (speech, symbolic systems, communication systems). As communication developed, telecommunications appeared.

Telecommunications is the transmission of information over a distance using technical means (telephone, telegraph, radio, television, etc.).

Telecommunications are an integral part of the country's industrial and social infrastructure and are designed to meet the needs of individuals and legal entities, government bodies for telecommunications services. Thanks to the emergence and development of data networks, a new highly efficient way of interaction between people has emerged - computer networks. The main purpose of computer networks is to provide distributed data processing and increase the reliability of information and management solutions.

A computer network is a collection of computers and various devices that provide information exchange between computers on the network without the use of any intermediate storage media.

In this case, there is a term - network node. A network node is a device connected to other devices as part of a computer network. Nodes can be computers or special network devices such as a router, switch or hub. A network segment is a part of the network limited by its nodes.

A computer on a computer network is also called a “workstation.” Computers on a network are divided into workstations and servers. At workstations, users solve application problems (work in databases, create documents, make calculations). The server serves the network and provides its own resources to everyone network nodes including workstations.

Computer networks are used in various fields, affect almost all areas of human activity and are an effective tool for communication between enterprises, organizations and consumers.

The network provides faster access to various sources of information. Using the network reduces resource redundancy. By connecting several computers together, you can get a number of advantages:

· expand the total amount of available information;

· share one resource with all computers (common database, network printer, etc.);

· simplifies the procedure for transferring data from computer to computer.

Naturally, the total amount of information accumulated on computers connected to a network, compared to one computer, is incomparably greater. As a result, the network provides a new level of employee productivity and effective communication of the company with manufacturers and customers.

Another purpose of a computer network is to ensure the efficient provision of various computer services to network users by organizing their access to resources distributed in this network.

In addition, the attractive side of networks is the availability of e-mail and workday planning programs. Thanks to them, managers of large enterprises can quickly and effectively interact with a large staff of their employees or business partners, and planning and adjusting the activities of the entire company is carried out with much less effort than without networks.

Computer networks as a means of realizing practical needs find the most unexpected applications, for example: selling air and railway tickets; access to information from reference systems, computer databases and data banks; ordering and purchasing consumer goods; payment of utility costs; exchange of information between the teacher’s workplace and students’ workplaces (distance learning) and much more.

Thanks to the combination of database technologies and computer telecommunications, it has become possible to use so-called distributed databases. Huge amounts of information accumulated by humanity are distributed across various regions, countries, cities, where they are stored in libraries, archives, and information centers. Typically, all large libraries, museums, archives and other similar organizations have their own computer databases that contain the information stored in these institutions.

Computer networks allow access to any database that is connected to the network. This relieves network users from the need to maintain a giant library and makes it possible to significantly increase the efficiency of searching for the necessary information. If a person is a user of a computer network, then he can make a request to the appropriate databases, receive an electronic copy of the necessary book, article, archival material over the network, see what paintings and other exhibits are in a given museum, etc.

Thus, the creation of a unified telecommunications network should become the main direction of our state and be guided by the following principles (principles taken from the Law of Ukraine “On Communications” dated February 20, 2009):

1. consumer access to publicly available telecommunications services that
   they need to satisfy their own needs, participate in political,
   economic and social life;
2. interaction and interconnectedness of telecommunication networks to ensure
   communication capabilities between consumers of all networks;
3. ensuring the sustainability of telecommunication networks and managing these networks with
   taking into account their technological features on the basis of uniform standards, norms and rules;
4. state support for the development of domestic production of technical
   telecommunications means;

5. encouraging competition in the interests of consumers of telecommunication services;

6. increasing the volume of telecommunications services, their list and the creation of new jobs;

7. implementation of world achievements in the field of telecommunications, attraction and use of domestic and foreign material and financial resources, latest technologies, management experience;

8. promoting the expansion of international cooperation in the field of telecommunications and the development of the global telecommunications network;

9. ensuring consumer access to information on the procedure for obtaining and the quality of telecommunications services;

10. efficiency, transparency of regulation in the field of telecommunications;

11. creation of favorable conditions for activity in the field of telecommunications, taking into account the characteristics of technology and the telecommunications market.

Study materials for full-time students

5. Sample of completing an individual assignment (abstract) - Download

7. Sample of the created website - Download

8. Sample of the created web page - Download

9. Application for color selection - "Color" - Download

11. Text for creating a web page and website yourself - Download

12. Drawings for creating a web page and website yourself - Download

13. EBook: Technology for preparing abstracts and tests - Download

Educational materials for students of distance and correspondence courses

4. Sample test paper for students of distance and correspondence courses in the KST course: Kontrol_rabota - Download

Computing or computer networks

Basic concepts of the discipline "Computer networks and telecommunications"

The goal of teaching students the basics of computer networks and telecommunications is to provide knowledge of the theoretical and practical fundamentals in the organization and functioning of computer networks and telecommunications, the ability to use distributed data, application programs and network resources in professional activities.

Currently, personal computers are practically not used offline; they are usually combined into computer or computer networks.

Computer network is a collection of computers and telecommunications equipment that ensures the information exchange of computers on a network. The main purpose of computer networks is to provide access to distributed resources.

Telecommunications(Greek tele - into the distance, far away and lat. communicatio - communication) is the transmission and reception of any information (sound, image, data, text) over a distance via various electromagnetic systems (cable and fiber optic channels, radio channels and other wired and wireless channels communications).

Telecommunications network is a system of technical means through which telecommunications are carried out.

Telecommunication networks include:

1. Computer networks (for data transmission).
2. Telephone networks (transmission of voice information).
3. Radio networks (transmission of voice information - broadcasting services).
4. Television networks (voice and image transmission - broadcast services).

The subject of the discipline is theoretical and practical foundations in the field of computer networks and telecommunications.

Training program The course with a volume of 198 academic hours is divided into two content (educational) modules of 2.0 and 3.5 credits (ECTS credit volume is 36 academic hours) and consists of classroom lessons and independent work of students.

The objective of the discipline Computer Networks and Telecommunications:

• formation of knowledge of theoretical and practical foundations in the use of computer networks;
• teach how to connect a PC to networks and work in them;
• teach how to use hardware, software and information resources of networks;
• teach how to work with network application programs.

As a result of studying the discipline, students must:
KNOW:

• technologies and principles of building computer networks;
• principles of operation and interaction of hardware and software computer equipment;
• ways to configure Microsoft Windows OS to work in networks;
• network applications;
• application programs for creating Web sites and Web pages;
• Ukrainian and international search tools on the Internet;
• main business opportunities on the Internet.

BE ABLE TO:

• use computer systems in professional activities;
• connect PCs to networks and work in them;
• work with network application programs;
• create and design Web pages and Web sites.

BE AWARE OF:

• with the main trends in the development of methods and technologies of computer networks;
• with mechanisms for data transmission via communication channels;
• with possible LAN resources;
• with Internet service.

Used Books:

1. Computer networks and telecommunications: primary sourcebook / V. A. Tkachenko, O. V. Kasilov, V. A. Ryabik. – Kharkiv: NTU “KhPI”, 2011. – 224 p.
2. Broido V.L. Computing systems, networks and telecommunications: Textbook for universities. 2nd ed. - St. Petersburg: Peter, 2006 - 703 p.
3. Computer networks. Principles, technologies, protocols: Textbook for universities. 4th ed. / V.G. Olifer, N.A. Olifer – St. Petersburg. Peter, 2010. – 944 p.
4. Moore M. et al. Telecommunications. Beginner's Guide. / Authors: Moore M., Pritsk T., Riggs K., Southwick P. - St. Petersburg: BHV - Petersburg, 2005. - 624 p.
5. Denisova A., Vikharev I., Belov A., Naumov G. Internet. Self-instruction manual. 2nd ed. – St. Petersburg. Peter. 2004.– 368 p.
6. Hester N. Frontpage 2002 for Windows: Trans. From English - M.: DMK Press, 2002. – 448 p.

Computer networks and telecommunications

Domain Name System DNS

The correspondence between domain names and IP addresses can be established either by local host tools or by means of a centralized service. In the early days of the Internet, a text file with the known name hosts was manually created on each host. This file consisted of a number of lines, each of which contained one “IP address - domain name” pair, for example 102.54.94.97 - rhino.acme.com.

As the Internet grew, hosts files also grew, and creating a scalable name resolution solution became a necessity.

This solution was a special service - the Domain Name System (DNS). DNS is a centralized service based on a distributed domain name-IP address mapping database. The DNS service operates using a client-server protocol. It defines DNS servers and DNS clients. DNS servers maintain a distributed mapping database, and DNS clients contact servers with requests to resolve a domain name to an IP address.

The DNS service uses text files almost the same format as the hosts file, and the administrator also prepares these files manually. However, DNS relies on a hierarchy of domains, and each DNS server stores only part of the network's names, rather than all of the names, as happens when using hosts files. As the number of nodes in the network grows, the problem of scaling is solved by creating new domains and subdomains of names and adding new servers to the DNS service.

Each name domain has its own DNS server. This server can store domain name-IP address mappings for the entire domain, including all its subdomains. However, this solution turns out to be poorly scalable, since when adding new subdomains, the load on this server may exceed its capabilities. More often, the domain server only stores names that end at the next lower level in the hierarchy than the domain name. (Similar to a file system directory, which contains records about files and subdirectories directly included in it.) It is with this organization of the DNS service that the name resolution load is distributed more or less evenly among all DNS servers on the network. For example, in the first case, the DNS server of the mmt.ru domain will store mappings for all names ending in mmt.ru: wwwl.zil.mmt.ru, ftp.zil.mmt.ru, mail.mmt.ru, etc. In the second case, this server stores only mappings of names like mail.mmt.ru, www.mmt.ru, and all other mappings should be stored on the DNS server of the zil subdomain.

Each DNS server, in addition to the name mapping table, contains links to the DNS servers of its subdomains. These links link individual DNS servers into a single DNS service. Links are the IP addresses of the corresponding servers. To service the root domain, several DNS servers that duplicate each other are allocated, the IP addresses of which are widely known (they can be found, for example, in InterNIC).

The procedure for resolving a DNS name is in many ways similar to the procedure for the file system searching for a file address by its symbolic name. Indeed, in both cases, the compound name reflects the hierarchical structure of the organization of the corresponding directories - file directories or DNS tables. Here the domain and domain DNS server are analogous to a file system directory. Domain names, like symbolic file names, are characterized by naming independence from physical location.

The procedure for searching for a file address by symbolic name consists of sequentially viewing directories, starting with the root. In this case, the cache and the current directory are first checked. To determine an IP address from a domain name, you also need to view all DNS servers that serve the chain of subdomains included in the host name, starting with the root domain. The significant difference is that file system is located on a single computer, and the DNS service is distributed in nature.

There are two main DNS name resolution schemes. In the first option, the work of finding an IP address is coordinated by the DNS client:

The DNS client contacts the root DNS server with the fully qualified domain name;

The DNS server responds with the address of the next DNS server serving the top-level domain specified in the high part of the requested name;

The DNS client makes a request to the next DNS server, which sends it to the DNS server of the desired subdomain, and so on, until a DNS server is found that stores the mapping of the requested name to the IP address. This server gives the final response to the client. This interaction pattern is called non-recursive or iterative, when the client itself iteratively performs a sequence of requests to different name servers. Since this scheme loads the client with quite complex work, it is rarely used. The second option implements a recursive procedure:

The DNS client queries the local DNS server, that is, the server that services the subdomain to which the client name belongs;

If the local DNS server knows the answer, it immediately returns it to the client; this may correspond to the case where the requested name is in the same subdomain as the client's name, and may also correspond to the case where the server has already learned this match for another client and stored it in its cache;

If the local server does not know the answer, then it makes iterative requests to the root server, etc., in exactly the same way as the client did in the first option; Having received the answer, it transmits it to the client, which all this time was simply waiting for it from its local DNS server.

In this scheme, the client delegates work to its server, hence the scheme is called indirect or recursive. Almost all DNS clients use a recursive procedure.

TCP/IP protocol stack.

The TCP/IP stack, also called the DoD stack and the Internet stack, is one of the most popular and promising communication protocol stacks. If it is currently distributed mainly in networks with UNIX OS, then its implementation in latest versions network operating systems for personal computers(Windows NT, NetWare) is a good prerequisite for rapid growth in the number of TCP/IP stack installations.

The stack was developed at the initiative of the US Department of Defense (DoD) more than 20 years ago to connect the experimental ARPAnet network with other satellite networks as a set of common protocols for heterogeneous computing environments. The ARPA network supported developers and researchers in military fields. In the ARPA network, communication between two computers was carried out using Internet protocol Protocol (IP), which to this day is one of the main ones in the TCP/IP stack and appears in the name of the stack.

Berkeley University made a major contribution to the development of the TCP/IP stack by implementing stack protocols in its version of the UNIX OS. The widespread adoption of the UNIX operating system also led to the widespread adoption of IP and other stack protocols. Worldwide works on the same stack information network Internet, whose division, the Internet Engineering Task Force (IETF), is a major contributor to the improvement of stack standards published in the form of RFC specifications.

Since the TCP/IP stack was developed before the advent of the ISO/OSI open systems interconnection model, although it also has a multi-level structure, the correspondence of the TCP/IP stack levels to the levels of the OSI model is rather conditional.

The lowest (level IV) - the level of network interfaces - corresponds to the physical and data link layers of the OSI model. This level in the TCP/IP protocols is not regulated, but supports all popular standards of the physical and data link layer: for local channels these are Ethernet, Token Ring, FDDI, for global channels - their own protocols for operating on analog dial-up and leased lines SLIP/PPP, which establish point-to-point connections via WAN serial links, and WAN protocols X.25 and ISDN. A special specification has also been developed that defines the use of ATM technology as a data link layer transport.

The next layer (layer III) is the internetworking layer, which deals with the transmission of datagrams using various local networks, X.25 area networks, ad hoc lines, etc. As the main network layer protocol (in terms of the OSI model) in the stack The IP protocol is used, which was originally designed as a protocol for transmitting packets in composite networks consisting of a large number of local networks connected by both local and global connections. Therefore, the IP protocol works well in networks with complex topologies, rationally using the presence of subsystems in them and economically using the bandwidth of low-speed communication lines. The IP protocol is a datagram protocol.

The internetworking layer also includes all protocols associated with the compilation and modification of routing tables, such as the routing information collection protocols RIP (Routing Internet Protocol) and OSPF (Open Shortest Path First), as well as the Internet Control Message Protocol (ICMP). ). The latter protocol is designed to exchange information about errors between the router and the gateway, the source system and the destination system, that is, to organize feedback. Using special ICMP packets, it is reported that it is impossible to deliver a packet, that the lifetime or duration of assembling a packet from fragments has been exceeded, anomalous parameter values, a change in the forwarding route and type of service, the state of the system, etc.

The next level (level II) is called basic. The Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) operate at this layer. The TCP protocol provides a stable virtual connection between remote application processes. The UDP protocol ensures the transmission of application packets using the datagram method, that is, without establishing a virtual connection, and therefore requires less overhead than TCP.

The top level (level I) is called application. Over many years of use in the networks of various countries and organizations, the TCP/IP stack has accumulated a large number of protocols and application level services. These include such widely used protocols as the FTP file copy protocol, the telnet terminal emulation protocol, and the SMTP mail protocol used in e-mail the Internet and its Russian branch RELCOM, hypertext services for accessing remote information, such as WWW and many others. Let's take a closer look at some of them that are most closely related to the topics of this course.

The SNMP (Simple Network Management Protocol) protocol is used to organize network management. The management problem is divided here into two problems. The first task is related to the transfer of information. Control information transfer protocols determine the procedure for interaction between the server and the client program running on the administrator's host. They define the message formats that are exchanged between clients and servers, as well as the formats for names and addresses. The second challenge is related to controlled data. The standards regulate what data should be stored and accumulated in gateways, the names of this data, and the syntax of these names. The SNMP standard defines a specification for a network management information database. This specification, known as the Management Information Base (MIB), defines the data elements that a host or gateway must store and the permissible operations on them.

The FTP (File Transfer Protocol) implements remote access to the file. In order to ensure reliable transfer, FTP uses the connection-oriented protocol - TCP - as its transport. In addition to file transfer protocol, FTP offers other services. This gives the user the opportunity to interact interactively with a remote machine, for example, he can print the contents of its directories; FTP allows the user to specify the type and format of the data to be stored. Finally, FTP authenticates users. Before accessing the file, protocol requires users to provide their username and password.

In the TCP/IP stack, FTP offers the most comprehensive set of file services, but is also the most complex to program. Applications that do not require all the capabilities of FTP can use another, more cost-effective protocol - the simplest file transfer protocol TFTP (Trivial File Transfer Protocol). This protocol only implements file transfer, and the transport used is a simpler than TCP, connectionless protocol - UDP.

The telnet protocol provides a stream of bytes between processes and between a process and a terminal. Most often, this protocol is used to emulate a remote computer terminal.

BGP protocol

The general scheme of how BGP works is as follows. BGP routers of neighboring systems that decide to exchange routing information establish connections with each other using the BGP protocol and become BGP neighbors (BGP peers).

Next, BGP uses an approach called path vector, which is a development of the distance vector approach. BGP neighbors send (announce, advertise) path vectors to each other. The path vector, unlike the distance vector, contains not just the network address and distance to it, but the network address and a list of attributes (path attributes) that describe various characteristics of the route from the sending router to the specified network. In the following, for brevity, we will call a set of data consisting of a network address and attributes of the path to this network a route to this network.

BGP Implementation

A pair of BGP neighbors establishes a connection with each other using the TCP protocol, port 179. Neighbors belonging to different ASs must be directly accessible to each other; for neighbors from the same AS there is no such restriction, since the internal routing protocol will ensure the availability of all necessary routes between nodes of one autonomous system.

The flow of information exchanged between BGP neighbors over TCP consists of a sequence of BGP messages. The maximum message length is 4096 octets, the minimum is 19. There are 4 types of messages.

BGP Message Types

• OPEN - sent after a TCP connection is established. The response to OPEN is a KEEPALIVE message if the other party agrees to become a BGP neighbor; otherwise, a NOTIFICATION message is sent with a code explaining the reason for the refusal, and the connection is terminated.
• KEEPALIVE - the message is intended to confirm consent to establish neighbor relations, as well as to monitor the activity of an open connection: for this, BGP neighbors exchange KEEPALIVE messages at certain intervals.
• UPDATE - message is intended for announcing and withdrawing routes. After a connection is established, UPDATE messages forward all the routes that the router wants to advertise to its neighbor (full update), after which only data about added or deleted routes as they become available is forwarded (partial update).
• NOTIFICATION - This type of message is used to inform the neighbor about the reason for closing the connection. After this message is sent, the BGP connection is closed.

BGP Message Format

A BGP message consists of a header and a body. The header is 19 octets long and consists of the following fields:

· marker: in the OPEN message always, and when working without authentication - in other messages, filled with units. Otherwise contains authentication information. A related function of the marker is to increase the reliability of highlighting the message boundary in the data stream.

· message length in octets, including header.

IGRP protocol

Interior Gateway Routing Protocol (IGRP) is a routing protocol developed in the mid-1980s. by Cisco Systems, Inc. The primary goal was to provide a survivable protocol for routing within an autonomous system (AS) that has an arbitrarily complex topology and includes media with varying bandwidth and latency characteristics.

IGRP is a distance vector Interior Router Protocol (IGP). Distance vector routing protocols require each router to send all or part of its routing table at regular intervals to all neighboring routers in route update messages. As routing information propagates through the network, routers can calculate distances to all nodes on the interconnected network.

IGRP uses a combination (vector) of metrics. Internetwork delay, bandwidth, reliability and load - all these indicators are taken into account as coefficients when making a routing decision. Network administrators can set weighting factors for each of these metrics. IGRP provides a wide range of values ​​for its indicators.

To provide additional flexibility, IGRP allows multipath routing. Duplicate lines of the same bandwidth can carry a separate stream of traffic in a round-robin fashion, with automatic switching to the second line if the first line fails.

Package format

The first field of the IGRP packet contains the version number.

Operation code field (opcode). This field indicates the packet type. An opcode of 1 indicates an update packet (contains a header immediately followed by routing table data records); equal 2-packet request (used by the source to query the routing table from another router.

Edition field. This release number value is used to allow routers to avoid processing updates that contain information they have already seen.

The next three fields indicate the number of subnets, the number of main networks, and the number of external networks in the update package.

Checksum field. Calculating a checksum allows the receiving router to verify the validity of the incoming packet.

Stability characteristics

IGRP has a number of features designed to enhance its stability. These include:

Temporary change holds are used to prevent regular correction messages from illegally reinstating a route that may have been corrupted. The change retention period is usually calculated to be longer than the period of time required to adjust the entire network to accommodate any routing change.

Split Horizons The concept of split horizons stems from the fact that it is never useful to send information about a route back in the direction it came from. The split-horizon rule helps prevent route loops.

Route cancellation adjustments are designed to combat larger route loops. Increasing routing metric values ​​typically indicate routing loops. In this case, cancellation adjustments are sent to remove this route and place it in a holding state.

IGRP provides a number of timers and variables containing time intervals. This includes

• adjustment timer (defines how often route adjustment messages should be sent),
• dead route timer, determines how long the router should wait if there are no messages about updating a particular route before declaring that route dead
• change retention period
• sleep timer. specifies how much time must pass before a router must be excluded from the routing table.

Network layer protocols are typically implemented in the form software modules and are executed on end computers called hosts, as well as on intermediate nodes - routers, called gateways. The functions of routers can be performed by both specialized devices and universal ones.

Internetworking concept

The main idea of ​​introducing the network layer is as follows. A network is generally considered as a collection of several networks and is called a composite network or internetwork (internetwork or internet). The networks that are part of a composite network are called subnets (subnet), constituent networks or simply networks (Fig. 5.1). Subnets are connected to each other by routers. Components of a composite network can be both local and global networks. The internal structure of each network is not shown in the figure because it is not relevant when considering the network protocol. All nodes within the same subnet interact using the same technology. Thus, the composite network shown in the figure includes several networks different technologies: local networks Ethernet, Fast Ethernet, Token Ring, FDDI and global frame relay networks, X.25, ISDN. Each of these technologies is sufficient to organize the interaction of all nodes in its subnet, but is not capable of building an information connection between randomly selected nodes belonging to different subnets, for example, between node A and node B in Fig. 5.1. Consequently, to organize interaction between any arbitrary pair of nodes of this “large” composite network, additional means are required. Such tools are provided by the network layer.

The network layer acts as a coordinator, organizing the work of all subnets that lie along the path of a packet through the composite network. To move data within subnets, the network layer turns to the technologies used in those subnets.

Although many local network technologies (Ethernet, Token Ring, FDDI, Fast Ethernet, etc.) use the same node addressing system based on MAC addresses, there are many technologies (X.25, ATM, frame relay), in which other addressing schemes are used. Addresses assigned to nodes in accordance with subnet technologies are called local. In order for the network layer to fulfill its task, it needs its own addressing system, independent of the methods of addressing nodes in individual subnets, which would allow the network layer to universally and unambiguously identify any node in a composite network.

A natural way to form a network address is to uniquely number all subnets of a composite network and number all nodes within each subnet. Thus, a network address is a pair: a network (subnet) number and a host number.

The node number can be either the local address of this node (this scheme is adopted in the IPX/SPX stack), or some number, unrelated to local technology, that uniquely identifies a node within a given subnet. In the first case, the network address becomes dependent on local technologies, which limits its use. For example, IPX/SPX network addresses are designed to work in composite networks that connect networks that use only MAC addresses or addresses of a similar format. The second approach is more universal; it is typical for the TCP/IP stack. In both cases, each node of the composite network has, along with its local address, another one - a universal network address.

Data that arrives at the network layer and needs to be transmitted through the composite network is provided with a network layer header. The data together with the header form a packet. The network layer packet header has a unified format that is independent of the link layer frame formats of those networks that may be part of the internetwork, and carries, along with other service information, data about the number of the network to which this packet is intended. The network layer determines the route and moves the packet between subnets.

When a packet is transmitted from one subnet to another, the network layer packet encapsulated in the arriving link frame of the first subnet is stripped of the headers of that frame and surrounded by the headers of the link layer frame of the next subnet. The information on the basis of which this replacement is made is the service fields of the network layer packet. The destination address field of the new frame specifies the local address of the next router.

Ethernet hubs

In Ethernet technology, devices that combine several physical segments of a coaxial cable into a single shared medium have been used for a long time and are called “repeaters” because of their main function - repeating on all their ports the signals received at the input of one of the ports. In coaxial cable networks, two-port repeaters connecting only two cable segments were common, so the term hub was not usually applied to them.

With the advent of the lOBase-T specification for twisted pair cables, the repeater became an integral part of the Ethernet network, since without it, communication could only be organized between two network nodes. Multiport Ethernet repeaters on twisted pair began to be called concentrators or hubs, since connections between a large number of network nodes were actually concentrated in one device. An Ethernet hub typically has between 8 and 72 ports, with the majority of the ports being dedicated to connecting twisted pair cables. In Fig. Figure 2 shows a typical Ethernet hub designed to form small segments of shared media. It has 16 lOBase-T ports with RJ-45 connectors, as well as one AUI port for connecting an external transceiver.

Typically a transceiver operating on coaxial or fiber optics is connected to this port. Using this transceiver, the hub is connected to a backbone cable connecting several hubs to each other, or thus connecting a station more than 100 m away from the hub.

Rice. 15. Ethernet hub.

To connect lOBase-T technology hubs to each other in a hierarchical system, a coaxial or fiber optic cable is not required; you can use the same ports as for connecting end stations, subject to one circumstance. The fact is that the regular RJ-45 port intended for connecting a network adapter, called MDI-X (crossed MDI), has an inverted connector pinout so that the network adapter can be connected to a hub using a standard connecting cable that does not cross over the pins.

If you connect hubs via a standard MDI-X port, you must use a non-standard cable with crossover pairs. Therefore, some manufacturers provide the hub with a dedicated MDI port that does not cross pairs. Thus, two hubs can be connected using a regular uncrossed cable if this is done through the MDI-X port of one hub and the MDI port of the second. More often than not, one hub port can act as both an MDI-X port and an MDI port, depending on the position of the pushbutton switch.

A multiport Ethernet repeater hub can be considered differently when using the 4-hub rule. In most models, all ports are connected to a single repeater unit, and when a signal passes between two repeater ports, the repeater unit introduces only one delay. Therefore, such a hub should be considered one repeater with the restrictions imposed by the 4-hub rule. But there are other repeater models, in which several ports have their own repeat unit.

In this case, each repetition block must be considered a separate repeater and counted separately in the 4-hub rule.
Some differences may be demonstrated by concentrator models operating on single-mode fiber optic cable. The range of a cable segment supported by an FDDI hub on such a cable can vary significantly depending on the power of the laser emitter - from 10 to 40 km.

However, if the existing differences in the performance of the main function of the hubs are not so great, then they are far exceeded by the dispersion in the capabilities of the hubs to implement additional functions. Disabling ports.

Very useful when operating a network is the ability of a hub to disable incorrectly functioning ports, thereby isolating the rest of the network from problems that arise in the node. This feature is called autopartitioning. For the FDDI concentrator, this function is the main one for many error situations, as it is defined in the protocol. At the same time, for an Ethernet hub or Token Ring, the auto-segmentation function for many situations is optional, since the standard does not describe the hub's response to this situation. The main reason for port shutdown in the Ethernet and Fast Ethernet standards is the failure to respond to the link test pulse sequence sent to all ports every 16 ms. In this case, the faulty port is placed in the "disabled" state, but link test pulses will continue to be sent to the port so that when the device is restored, work with it will continue automatically.

Let's look at situations in which Ethernet and Fast Ethernet hubs perform port shutdown:

o Errors at the frame level. If the intensity of frames with errors passing through the port exceeds a specified threshold, the port is turned off, and then, if there are no errors for a specified time, it is turned on again. Such errors can be: incorrect checksum, incorrect frame length (more than 1518 bytes or less than 64 bytes), unformatted frame header.
o Multiple collisions. If the hub detects that the source of the collision was the same port 60 times in a row, then the port is disabled. After some time, the port will be enabled again.

o Lengthy transmission (jabber). Like a network adapter, a hub controls the time it takes for one frame to pass through a port. If this time exceeds the transmission time of a frame of maximum length by 3 times, the port is disabled.

Support for redundant connections

Since the use of redundant links in hubs is defined only in the FDDI standard, for other standards hub developers support this function using their proprietary solutions. For example, Ethernet/Fast Ethernet hubs can only form hierarchical links without loops. Therefore, backup links should always connect disabled ports so as not to disrupt the logic of the network.

Typically, when configuring a hub, the administrator must determine which ports are primary and which in relation to them are backup (Figure 16). If for any reason a port goes down (the auto-segmentation mechanism is triggered), the hub makes its backup port active.

Rice. 16.

Rice. 16. Redundant connections between Ethernet hubs.

When considering some models of hubs, the question arises - why does this model have such a large number of ports, for example 192 or 240? Does it make sense to split a 10 or 16 Mbps environment between so many stations? Perhaps ten to fifteen years ago the answer in some cases might have been positive, for example, for those networks in which computers used the network only to send small email messages or to rewrite a small text file.

Today there are very few such networks left, and even 5 computers can completely load an Ethernet or Token Ring segment, and in some cases, a Fast Ethernet segment. Why then do you need a hub with a large number of ports if they are practically impossible to use due to limitations on bandwidth per station? The answer is that such hubs have multiple, unconnected internal buses that are designed to create multiple shared media.

For example, the hub shown in Fig. 17, has three internal Ethernet buses. If, for example, such a hub has 72 ports, then each of these ports can be associated with any of the three internal buses. In the figure, the first two computers are connected to Ethernet bus 3, and the third and fourth computers are connected to Ethernet bus 1. The first two computers form one shared segment, and the third and fourth computers form another shared segment.

Rice. 17. Multi-segment hub.

Computers connected to different segments cannot communicate with each other through a hub, since the buses inside the hub are not connected in any way. Multi-segment hubs are needed to create shared segments, the composition of which can easily change. Most multi-segment hubs, such as Nortel Networks' System 5000 or 3Com's PortSwitch Hub, allow the operation of connecting a port to one of the internal buses to be performed purely programmatically, for example, using local configuration via the console port.

As a result, the network administrator can connect user computers to any ports on the hub, and then use the hub configuration program to manage the composition of each segment. If segment 1 becomes overloaded tomorrow, its computers can be distributed among the remaining segments of the hub.

The ability of a multi-segment hub to programmatically change port connections to internal buses is called configuration switching.
ATTENTION
Configuration switching has nothing in common with frame switching, which is performed by bridges and switches. Multi-segment hubs are a programmable foundation large networks. To connect segments to each other, another type of device is needed - bridges/switches or routers. Such a gateway device must connect to multiple ports on a multi-segment hub connected to different internal buses, and transfer frames or packets between segments in the same way as if they were formed by separate hub devices.

For large networks, a multi-segment hub plays the role of an intelligent cross-connect cabinet that makes a new connection not by mechanically moving the cable plug to a new port, but by software changing the internal configuration of the device. Managing the hub using the SNMP protocol.

As you can see from the descriptions of additional features, many of them require configuration of the hub. This configuration can be done locally via the RS-232C interface, which is available on any hub that has a control unit. In addition to configuring a large network, the function of monitoring the status of the hub is very useful: is it operational, and what state are its ports in?

Computer networks and telecommunications of the 21st century

Introduction

2.1 Types of LAN architectures

2.3 Access methods in computer networks

3. Local networks for scientific purposes

4. Telecommunications

List of used literature

Introduction

A computer network is an association of several computers for the joint solution of information, computing, educational and other problems.

One of the first to arise during the development computer technology tasks that required the creation of a network of at least two computers - providing many times greater reliability than one machine could provide at that time when managing a critical process in real time. Thus, when launching a spacecraft, the required rate of reaction to external events exceeds human capabilities, and failure of the control computer threatens with irreparable consequences. IN the simplest scheme the work of this computer is duplicated by a second one of the same type, and if the active machine fails, the contents of its processor and RAM are very quickly transferred to the second one, which takes over control (in real systems everything, of course, is much more complicated).

Computer networks have given rise to significantly new information processing technologies - network technologies. In the simplest case, network technologies allow the sharing of resources - large-capacity storage devices, printing devices, Internet access, databases and data banks. The most modern and promising approaches to networks involve the use of collective division of labor in working together with information - development of various documents and projects, management of an institution or enterprise, etc.

Computer networks and network information processing technologies have become the basis for building modern information systems. The computer should now be considered not as a separate processing device, but as a “window” into computer networks, a means of communication with network resources and other network users.

1. Computer network hardware

Local networks (LAN computers) unite a relatively small number of computers (usually from 10 to 100, although sometimes much more are found) within one room (educational computer class), building or institution (for example, a university). The traditional name - local area network (LAN) - is rather a tribute to those times when networks were mainly used to solve computing problems; today, in 99% of cases, we are talking exclusively about the exchange of information in the form of texts, graphic and video images, and numerical arrays. The usefulness of drugs is explained by the fact that from 60% to 90% of the information an institution needs circulates within it, without needing to go outside.

The creation of automated enterprise management systems (ACS) had a great influence on the development of drugs. ACS include several automated workstations (AWS), measuring systems, and control points. Another important field of activity in which LS has proven its effectiveness is the creation of educational computer technology classes (ECT).

Thanks to the relatively short lengths of communication lines (usually no more than 300 meters), information can be transmitted digitally over the LAN at a high transmission speed. At long distances, this transmission method is unacceptable due to the inevitable attenuation of high-frequency signals; in these cases, it is necessary to resort to additional technical (digital-to-analog conversions) and software (error correction protocols, etc.) solutions.

A characteristic feature of the LAN is the presence of a high-speed communication channel connecting all subscribers for transmitting information in digital form. There are wired and wireless channels. Each of them is characterized by certain values ​​of parameters that are essential from the point of view of drug organization:

1. data transfer speed;

2. maximum line length;

3. noise immunity;

4. mechanical strength;

5. convenience and ease of installation;

6. cost.

Currently, there are four types of network cables commonly used:

1. coaxial cable;

2. unprotected twisted pair;

3. protected twisted pair;

4. fiber optic cable.

The first three types of cables transmit an electrical signal through copper conductors. Fiber optic cables transmit light along glass fibers.

Most networks allow several cabling options.

Coaxial cables consist of two conductors surrounded by insulating layers. The first layer of insulation surrounds the central copper wire. This layer is braided from the outside with an external shielding conductor. The most common coaxial cables are thick and thin "Ethernet" cables. This design provides good noise immunity and low signal attenuation over distances.

There are thick (about 10 mm in diameter) and thin (about 4 mm) coaxial cables. Having advantages in noise immunity, strength, and length, a thick coaxial cable is more expensive and more difficult to install (it is more difficult to pull through cable channels) than a thin one. Until recently, thin coaxial cable represented a reasonable compromise between the basic parameters of LAN communication lines and was most often used for organizing large LANs of enterprises and institutions. However, thicker, more expensive cables provide better data transmission over longer distances and are less susceptible to electromagnetic interference.

Twisted pairs are two wires twisted together with six turns per inch to provide EMI protection and electrical resistance matching. Another name commonly used for such wire is "IBM Type-3". In the USA, such cables are laid during the construction of buildings to provide telephone communications. However, using telephone wire, especially when it is already placed in a building, can create big problems. First, unprotected twisted pairs are susceptible to electromagnetic interference, such as electrical noise generated by fluorescent lights and moving elevators. Interference can also be caused by signals transmitted in a closed loop in telephone lines running along a local network cable. Additionally, poor quality twisted pairs may have a variable number of turns per inch, which skews the calculated electrical resistance.

It is also important to note that telephone wires are not always laid in a straight line. A cable connecting two adjacent rooms can actually travel halfway around the building. Underestimating the cable length in this case may result in it actually exceeding the maximum permissible length.

Protected twisted pairs are similar to unprotected twisted pairs, except that they use thicker wires and are protected from external influences by the neck of the insulator. The most common type of such cable used in local networks, "IBM Type-1" is a secure cable with two twisted pairs of continuous wire. In new buildings, Type-2 cable may be a better option because it includes, in addition to the data line, four unprotected pairs of continuous wire for carrying telephone conversations. Thus, “type-2” allows you to use one cable to transmit both telephone conversations and data over a local network.

Protection and careful TPI make rugged twisted pair cable a reliable cabling alternative. However, this reliability comes at a cost.

Fiber optic cables transmit data in the form of light pulses to glass “wires.” Most LAN systems today support fiber optic cabling. Fiber optic cable has significant advantages over any copper cable option. Fiber optic cables provide the highest transmission speeds; they are more reliable because they are not subject to loss of information packets due to electromagnetic interference. Optical cable is very thin and flexible, making it easier to transport than heavier copper cable. However, the most important thing is that only optical cable has sufficient bandwidth, which will be required for faster networks in the future.

The price of fiber optic cable is still significantly higher than copper. Compared to copper cable, installing an optical cable is more labor-intensive; its ends must be carefully polished and aligned to ensure a reliable connection. However, now there is a transition to fiber optic lines, which are absolutely immune to interference and have no competition in terms of throughput. The cost of such lines is steadily decreasing, and the technological difficulties of joining optical fibers are being successfully overcome.

Wireless connection on radio waves can be used to organize networks within large premises such as hangars or pavilions, where the use of conventional communication lines is difficult or impractical. In addition, wireless lines can connect remote segments of local networks at distances of 3 - 5 km (with a wave channel antenna) and 25 km (with a directional parabolic antenna) subject to direct visibility. Organizations wireless network significantly more expensive than usual.

To organize educational LANs, twisted pair is most often used, as it is the cheapest, since the requirements for data transfer speed and line length are not critical.

Network adapters (or network cards, as they are sometimes called) are required to connect computers using LAN links. The most famous are: adapters of the following three types:

1. ArcNet; 2. Token Ring; 3. Ethernet.

2. LAN configuration and organization of information exchange

2.1 Types of LAN architectures

In the simplest networks with a small number of computers, they can be completely equal; the network in this case ensures the transfer of data from any computer to any other for collective work on information. Such a network is called peer-to-peer.

However, in large networks with a large number of computers, it turns out to be advisable to allocate one (or several) powerful computers to serve network needs (data storage and transmission, printing to a network printer). Such dedicated computers are called servers; they run a network operating system. A high-performance computer with large RAM and a high-capacity hard drive (or even several hard drives) is usually used as a server. The keyboard and display are not required for the network server, since they are used very rarely (to configure the network OS).

All other computers are called workstations. Workstations may not have hard drives or even disk drives at all. Such workstations are called diskless. The initial loading of the OS onto diskless workstations occurs over a local network using RAM chips specially installed on the network adapters of workstations that store the boot program.

Depending on the purpose and technical solutions, LANs can have different configurations (or, as they also say, architecture, or topology).

In a ring LAN, information is transmitted over a closed channel. Each subscriber is directly connected to its two closest neighbors, although in principle it is capable of contacting any subscriber on the network.

In a star-shaped (radial) LAN, in the center there is a central control computer that sequentially communicates with subscribers and connects them with each other.

In a bus configuration, computers are connected to a common channel (bus), through which they can exchange messages.

In a tree view, there is a “main” computer, to which the computers of the next level are subordinate, etc.

In addition, configurations without a distinct nature of connections are possible; the limit is a mesh configuration, where every computer on the network is directly connected to every other computer.

In large LANs of enterprises and institutions, a bus (neck) topology is most often used, corresponding to the architecture of many administrative buildings with long corridors and employee offices along them. For training purposes in CUVT, ring and star-shaped drugs are most often used.

In any physical configuration, support for access from one computer to another, the presence or absence of a dedicated computer (as part of KUVT it is called “teacher”, and the rest - “students”), is performed by the network program operating system, which is an add-on to the OS of individual computers. It is quite typical for modern highly developed personal computer operating systems to have network capabilities (for example, OS/2, WINDOWS 95-98).

2.2 Network communication components

The process of data transmission over a network is determined by six components:

1. source computer;

2. protocol block;

3. transmitter;

4. physical cable network;

5. receiver;

6. destination computer.

The source computer can be a workstation, a file server, a gateway, or any computer connected to the network. The protocol block consists of a chipset and driver software for the network interface card. The protocol block is responsible for the logic of network transmission. The transmitter sends an electrical signal through a physical circuit diagram. The receiver recognizes and receives the signal transmitted over the network and routes it to be converted into a protocol block. The data transfer cycle begins with the source computer transmitting the original data to the protocol block. The protocol block organizes the data into a transmission packet containing the corresponding request to the serving devices, information on processing the request (including, if necessary, the recipient's address), and the original data to be transmitted. The packet is then sent to the transmitter to be converted into a network signal. The package is distributed across network cable until it reaches the receiver, where it is recoded into data. Here, control passes to the protocol block, which checks the data for errors, transmits a “receipt” of packet receipt to the source, reformats the packets and transmits them to the destination computer.