The combination of the components discussed above into a network can be done in various ways and means. Based on the composition of their components, methods of their connection, scope of use and other characteristics, networks can be divided into classes in such a way that the belonging of the described network to a particular class can sufficiently fully characterize the properties and quality parameters of the network.
However, this kind of classification of networks is rather arbitrary. The most widespread today is the division of computer networks based on territorial location.
Based on this feature, networks are divided into three main classes:
LAN – local area networks;
MAN – Metropolitan Area Networks.
WAN – global networks (Wide Area Networks);
Local network (LAN) is a communications system that supports, within a building or some other limited area, one or more high-speed digital information transmission channels made available to connected devices for short-term exclusive use. The areas covered by the drug may vary significantly.
The length of communication lines for some networks can be no more than 1000 m, while other networks are able to serve an entire city. The serviced areas can be factories, ships, airplanes, as well as institutions, universities, and colleges. As a rule, coaxial cables are used as a transmission medium, although networks on twisted pair and optical fiber are becoming increasingly widespread, and recently the technology of wireless local networks has also been rapidly developing, which uses one of three types of radiation: broadband radio signals, low-power radiation ultrahigh frequencies (microwave radiation) and infrared rays.
Short distances between network nodes, the transmission medium used and the associated low probability of errors in the transmitted data make it possible to maintain high exchange rates - from 1 Mbit/s to 100 Mbit/s (at present there are already industrial designs of LANs with speeds of the order of 1 Gbit/s ).
City networks, as a rule, cover a group of buildings and are implemented on fiber optic or broadband cables. According to their characteristics, they are intermediate between local and global networks. Recently, in connection with the laying of high-speed and reliable fiber optic cables in urban and intercity areas, and new promising network protocols, for example, ATM (Asynchronous Transfer Mode), which in the future can be used both in local and global networks.
Global networks, unlike local ones, as a rule, cover much larger territories and even most regions of the globe (an example is the Internet). Currently, analogue or digital wire channels are used as a transmission medium in global networks, as well as satellite channels communications (usually for communication between continents). Limitations on transmission speed (up to 28.8 Kbit/s on analogue channels and up to 64 Kbit/s on user sections digital channels) and the relatively low reliability of analog channels, requiring the use of error detection and correction tools at the lower levels of protocols, significantly reduce the speed of data exchange in global networks compared to local ones.
There are other classification features of computer networks.
By area of operation networks are divided into:
Banking networks,
Networks of scientific institutions,
University Networks;
According to the form of functioning can be distinguished:
Commercial networks;
Free networks,
Corporate networks
Public networks;
By the nature of the functions being implemented networks are divided into:
Computational, designed to solve control problems based on computational processing of initial information;
Informational, designed to obtain reference data at the request of users; mixed, in which computational and information functions are implemented.
By control method computer networks are divided into:
Networks with decentralized control;
Centralized management;
Mixed control.
In the first case, each computer included in the network includes a complete set software to coordinate ongoing network operations. Networks of this type are complex and quite expensive, since the operating systems of individual computers are developed with a focus on collective access to the common memory field of the network.
In mixed networks, tasks that have the highest priority and, as a rule, are associated with the processing of large volumes of information, are solved under centralized control.
By software compatibility there are networks:
Homogeneous;
Homogeneous (consisting of software-compatible computers)
Heterogeneous or heterogeneous (if the computers on the network are software incompatible).
Local networks
There are two approaches to building local networks and, accordingly, two types: client/server networks and peer-to-peer networks.
Client/server networks
Client/server networks use a dedicated computer (server) that hosts shared files and provides printing services to many users (Figure 1).
Rice. 1.Client/server networks
Server –a computer connected to a network and providing its users with certain services.
Servers can perform data storage, database management, remote job processing, job printing, and a number of other functions that network users may need. The server is the source of network resources. There can be quite a few servers on the network, and each of them can serve its own group of users or manage certain databases.
Workstation– a personal computer connected to a network through which the user gains access to its resources. A network workstation operates in both network and local modes. It is equipped with its own operating system (MSDOS, Windows, etc.) and provides the user with all the necessary tools for solving applied problems. Workstations connected to the server are called clients. Can be used as clients powerful computers for resource-intensive processing spreadsheets, and low-power PCs for simple word processing. In contrast, powerful computers are usually installed as servers. Due to the need to ensure simultaneous processing of requests from a large number of clients and good protection network data from unauthorized access, the server must run a specialized operating system.
Examples: Novell Net Ware, Windows NT Server, IBM OS/2 Lan Server, Banyan Vines.
Peer-to-peer networks
Peer-to-peer networks do not use dedicated servers (Figure 2). At the same time as serving the user, a computer in a peer-to-peer network can take on the functions of a server, performing print jobs and responding to file requests from other workstations on the network. Of course, if the computer does not provide its disk space or your printer, then it is only a client in relation to other workstations that perform server functions. Windows 95 has built-in capabilities for building a peer-to-peer network. If you need to connect to other peer-to-peer networks, Windows 95 supports the following networks:
Net Ware Lite
Artisoft LANtastic.
Rice. 2.Location of computers in peer-to-peer networks.
Network topology
Under topology is understood as a description of the properties of a network inherent in all its homomorphic transformations, i.e. such changes appearance network, the distances between its elements, their relative positions, at which the relationship between these elements does not change.
The topology of a computer network is largely determined by the way computers are connected to each other. Topology largely determines many important properties of a network, such as reliability (survivability), performance, etc. There are different approaches to classifying network topologies. According to one of them, local network configurations are divided into two main classes: broadcast And sequential.
In broadcast configurations, each PC (receiver/transmitter) physical signals) transmits signals that can be perceived by other PCs. Such configurations include “common bus”, “tree”, “star with a passive center” topologies. A star network can be thought of as a type of "tree" that has a root with a branch to each connected device.
In sequential configurations, each physical sublayer transmits information to only one PC. Examples of sequential configurations are: random (random connection of computers), hierarchical, ring, chain, smart star, snowflake, and
other.
The most optimal from the point of view of reliability (the ability of the network to function in the event of failure of individual nodes or communication channels) is mesh network, i.e. a network in which each network node is connected to all other nodes, however, with a large number of nodes, such a network requires a large number of communication channels and is difficult to implement due to technical difficulties and high cost. Therefore, almost all networks are incompletely connected.
Although for a given number of nodes in a partial network there may be a large number of options for connecting network nodes, in practice the three most widely used (basic) LAN topologies are usually used:
1. common bus;
2. ring;
3. star.
Bus topology (Fig. 3), when all network nodes are connected to one open channel, usually called a bus.
Fig 3.Bus topology.
In this case, one of the machines serves as a system serving device, providing centralized access to shared files and databases, printing devices and other computing resources.
Networks of this type have gained great popularity due to their low cost, high flexibility and data transfer speed, and ease of network expansion (connecting new subscribers to the network does not affect its basic characteristics). Disadvantages of the bus topology include the need to use rather complex protocols and vulnerability to physical damage to the cable.
Ring topology (Fig. 4), when all network nodes are connected to one closed ring channel .
Fig 4.Ring topology.
This network structure is characterized by the fact that information along the ring can be transmitted only in one direction and all connected PCs can participate in its reception and transmission. In this case, the recipient subscriber must mark the received information with a special marker, otherwise “lost” data may appear that interferes with the normal operation of the network.
As a daisy-chain configuration, the ring is particularly vulnerable to failure: failure of any cable segment results in loss of service to all users. LAN developers have put a lot of effort into dealing with this problem. Protection against damage or failure is provided either by closing the ring to the reverse (redundant) path, or by switching to a spare ring. In both cases, the general ring topology is maintained.
Star topology (Fig. 5), when all network nodes connect to one central node called a host ( host) or hub ( hub).
Fig 5.Star topology.
The configuration can be thought of as a further development of a rooted tree structure with a branch to each connected device. At the center of the network is usually a switching device that ensures the viability of the system. LANs of this configuration are most often used in automated institutional control systems that use a central database. Star LANs are generally less reliable than bus or hierarchical networks, but this problem can be solved by duplicating the equipment at the central node. Disadvantages also include significant cable consumption (sometimes several times higher than the consumption in similar LAN capabilities with a common bus or hierarchical).
Networks can also have a mixed topology ( hybrid) when individual parts of the network have different topologies. An example is a local FDDI network, in which the main (backbone) nodes are connected to a ring channel, and the remaining nodes are connected to them via a hierarchical topology.
Connecting computers and devices into a network can be done in various ways and means. Based on the composition of their components, methods of their connection, scope of use and other characteristics, networks can be divided into classes in such a way that the belonging of the described network to a particular class can sufficiently fully characterize the properties and quality parameters of the network.
However, this kind of classification of networks is rather arbitrary. The most widespread division of computer networks today is based on territorial location. Based on this feature, networks are divided into three main classes:
LAN (Local Area Networks) – local networks;
MAN (Metropolitan Area Networks) – regional (city or corporate) networks;
WAN (Wide Area Networks) – global networks.
A local area network (LAN) is a communications system that supports, within a building or some other limited area, one or more high-speed channels for transmitting digital information, provided to connected devices for short-term exclusive use. The areas covered by the drug may vary significantly.
The length of communication lines for some networks can be no more than 1000 m, while other networks are able to serve an entire city. The serviced areas can be factories, ships, airplanes, as well as institutions, universities, and colleges. As a rule, coaxial cables are used as a transmission medium, although networks on twisted pair and optical fiber are becoming increasingly widespread, and recently the technology of wireless local networks has also been rapidly developing, which uses one of three types of radiation: broadband radio signals, low-power radiation ultrahigh frequencies (microwave radiation) and infrared rays.
The short distances between network nodes, the transmission medium used and the associated low probability of errors in the transmitted data make it possible to maintain high exchange rates - from 1 Mbit/s to 100 Mbit/s (at present there are already industrial designs of LANs with speeds of about 1 Gbit /With).
Regional networks, as a rule, cover a group of buildings and are implemented on fiber optic or broadband cables. According to their characteristics, they are intermediate between local and global networks.
Global networks, unlike local ones, as a rule, cover much larger territories and even most regions of the globe (an example is the Internet). Currently, analogue or digital wire channels, as well as satellite communication channels (usually for communication between continents), are used as transmission media in global networks. Limitations on transmission speed and the relatively low reliability of analog channels, requiring the use of error detection and correction tools at the lower levels of protocols, significantly reduce the speed of data exchange in global networks compared to local ones.
There are other classification features of computer networks. So, for example:
– according to the area of operation, the networks can be divided into banking research institutions and universities;
– according to the form of functioning, we can distinguish between commercial and free networks, corporate and general use;
– according to the nature of the implemented functions, networks are divided into computational ones (intended to solve control problems based on computational processing of initial information); informational (intended to obtain reference data at the request of users); mixed (they implement computational and information functions);
– according to the control method, computer networks are divided into networks with decentralized, centralized and mixed control. In the first case, each computer that is part of the network includes a full set of software tools for coordinating network operations. Networks of this type are complex and quite expensive, since the operating systems of individual computers are developed with a focus on collective access to the common memory field of the network. In mixed networks, tasks that have the highest priority and, as a rule, are associated with the processing of large volumes of information, are solved under centralized control.
Local networks
A local network is created, as a rule, to share computer resources or data (usually within the same organization). From a technical point of view, a local network is a collection of computers and communication channels that unite computers into a structure with a specific configuration, as well as network software that controls the operation of the network. The method of connecting computers into a local network is called topology.
Topology largely determines many important properties of a network, such as reliability (survivability), performance, etc. There are different approaches to classifying network topologies. Based on performance, they are divided into two main classes: broadcast and serial.
In broadcast configurations, each computer transmits signals that can be received by other computers. Such configurations include “common bus”, “tree”, “star with a passive center” topologies. A star network can be thought of as a type of "tree" that has a root with a branch to each connected device.
In sequential configurations, each physical sublayer transmits information to only one PC. Examples of sequential configurations are: random (random connection of computers), hierarchical, “ring”, “chain”, “star with an intellectual center”, “snowflake” and others.
Bus topology
Figure 10.2. Local network bus topology
With such a connection, exchange can be carried out between any computers on the network, regardless of the others. If the connection of one computer to the common bus is damaged, this computer is disconnected from the network, but the entire network is operational. In this sense, the network is quite stable, but if a bus is damaged, the entire network fails.
Ring topology
Figure 10.3. Ring LAN topology
This connection also transfers data serially from computer to computer, but compared to a simple serial connection, data can be transferred in two directions, which makes it more resilient to network problems. One break does not disable the network, but two breaks make the network inoperable. The ring network is widely used, mainly due to the high data transfer speed. Ring networks the fastest.
Star topology
Figure 10.4. Star-shaped local network topology
When connected by a star, the network is very resistant to damage. If one of the connections is damaged, only one computer is disconnected from the network. In addition, this connection scheme allows the creation of complex branched networks. Devices that allow the organization of complex network structures are called hubs and switches.
BELARUSIAN NATIONAL TECHNICAL UNIVERSITY
INTERNATIONAL INSTITUTE OF DISTANCE EDUCATION
TEST
IN THE ACADEMIC DISCIPLINE: Computer networks
Types of computer networks
Computer networks can be classified according to various criteria.
I. According to management principles:
1. Peer-to-peer - without a dedicated server. In which control functions are alternately transferred from one workstation to another;
2. Multi-peer is a network that includes one or more dedicated servers. The remaining computers of such a network (workstations) act as clients.
II. By connection method:
1. "Direct connection"- two personal computers are connected by a piece of cable. This allows one computer (master) to access the resources of the other (slave);
2. "Common bus" - connecting computers to one cable;
3. "Star" - connection through a central node;
4. "Ring" - serial connection PC in two directions.
III. By territory coverage:
1. Local network(a network in which computers are located at a distance of up to a kilometer and are usually connected using high-speed communication lines.) - 0.1 - 1.0 km; LAN nodes are located within the same room, floor, or building.
2. Corporate network(within the limits of one organization, company, plant). The number of nodes in a FAC can reach several hundred. At the same time, the corporate network usually includes not only personal computers, but also powerful computers, as well as various technological equipment (robots, assembly lines, etc.).
A corporate network makes it easier to manage an enterprise and manage a technological process, and to establish clear control over information and production resources.
3. Global network(a network whose elements are located at a considerable distance from each other) - up to 1000 km.
Both specially laid (for example, transatlantic fiber optic cable) and existing communication lines (for example, telephone networks). The number of nodes in a WAN can reach tens of millions. The global network includes separate local and corporate networks.
4. World Wide Web- unification of global networks (Internet).
COMPUTER NETWORK TOPOLOGY
Network topology is the geometric shape and physical arrangement of computers in relation to each other. Network topology allows you to compare and classify different networks. There are three main types of topology:
1) Star;
2) Ring;
BUS TOPOLOGY
This topology uses one transmission channel per base coaxial cable, called a "bus". All network computers are connected directly to the bus. At the ends of the bus cable, special plugs are installed - “terminators”. They are necessary in order to extinguish the signal after passing through the bus. The disadvantages of the "Bus" topology include the following:
Data transmitted over the cable is available to all connected computers;
If the “bus” is damaged, the entire network stops functioning.
RING TOPOLOGY
The ring topology is characterized by the absence of connection endpoints; the network is closed, forming an unbroken ring through which data is transmitted. This topology implies the following transmission mechanism: data is transmitted sequentially from one computer to another until it reaches the recipient computer. The disadvantages of the “ring” topology are the same as those of the “bus” topology:
Public availability of data;
Instability to cable system damage.
STAR TOPOLOGY
In a star network, all computers are connected to a special device called a network hub, which performs data distribution functions. There are no direct connections between two computers on the network. Thanks to this, it is possible to solve the problem of public data availability, and also increases the resistance to damage to the cable system. However, network functionality depends on the status of the network hub.
Carrier access methods in computer networks
IN various networks There are various procedures for exchanging data between workstations.
The International Institute of Electrical and Electronics Engineers (IEEE) has developed standards (IEEE802.3, IEEE802.4 and IEEE802.5) that describe methods for accessing network data channels.
The most widespread are specific implementations of access methods: Ethernet, ArcNet and Token Ring. These implementations are based on the IEEE802.3, IEEE802.4 and IEEE802.5 standards, respectively.
Ethernet access method
This access method, developed by Xerox in 1975, is the most popular. It provides high speed data transmission and reliability.
For this method access uses a "common bus" topology. Therefore, a message sent by one workstation, is received simultaneously by all other stations connected to the common bus. But the message is intended for only one station (it includes the address of the destination station and the address of the sender). The station to which the message is intended receives it, the others ignore it.
The Ethernet access method is a multiple access method that listens to the carrier and resolves conflicts called collisions (CSMA/CD -Carter Sense Multiple Access with Collision Detection).
Before transmission begins, the workstation determines whether the channel is free or busy. If the channel is free, the station begins transmitting.
Ethernet does not exclude the possibility of simultaneous transmission of messages by two or more stations. The equipment automatically recognizes such conflicts. After detecting a conflict, stations delay transmission for some time. This time is short and different for each station. After a delay, transmission resumes.
In reality, conflicts lead to a decrease in network speed only if several tens or hundreds of stations are operating.
ArcNet access method
This method was developed by Datapoint Corp. It, too, has become widespread, largely due to the fact that ArcNet hardware is cheaper than Ethernet or Token-Ring hardware.
ArcNet is used in local networks with star topology. One of the computers creates a special token (message special type), which is sequentially transmitted from one computer to another.
If a station wishes to send a message to another station, it must wait for the token and append a message to it, complete with the source and destination addresses. When the packet reaches the destination station, the message will be “unhooked” from the token and transmitted to the station.
Token-Ring access method
The Token-Ring access method was developed by IBM and is designed for a ring network topology.
This method is similar to ArcNet, as it also uses a token passed from one station to another. Unlike ArcNet, the Token-Ring access method allows you to assign different priorities to different workstations.
Data transmission media, their characteristics
Coaxial cable
Coaxial cable was the first type of cable used to connect computers to a network. This type of cable consists of a central copper conductor covered with a plastic insulating material, which in turn is surrounded by copper mesh and/or aluminum foil. This outer conductor provides grounding and protects the center conductor from external electromagnetic interference. When laying networks, two types of cable are used - “Thick coaxial cable” (Thicknet) and “Thin coaxial cable” (Thinnet). Networks based on coaxial cable provide transmission speeds of up to 10 Mbit/s. Maximum length segment ranges from 185 to 500 m depending on the cable type.
"Twisted Pair"
Twisted pair cable is one of the most common types of cable today. It consists of several pairs of copper wires covered with a plastic sheath. The wires that make up each pair are twisted around each other, which provides protection against mutual interference. Cables of this type are divided into two classes - “Shielded twisted pair” and “Unshielded twisted pair”. The difference between these classes is that shielded twisted pair cable is more protected from external electromagnetic interference due to the presence of an additional shield of copper mesh and/or aluminum foil surrounding the cable wires. Twisted pair networks, depending on the cable category, provide transmission speeds from 10 Mbit/s – 1 Gbit/s. The cable segment length cannot exceed 100 m (up to 100 Mbps) or 30 m (1 Gbps).
Fiber optic cable
Fiber optic cables are the most advanced cable technology, providing high speed data transmission over long distances, resistant to interference and eavesdropping. A fiber optic cable consists of a central glass or plastic conductor surrounded by a layer of glass or plastic coating and an outer protective sheath. Data transmission is carried out using a laser or LED transmitter that sends unidirectional light pulses through a central conductor. The signal at the other end is received by a photodiode receiver, which converts light pulses into electrical signals that can be processed by a computer. Transmission speeds for fiber optic networks range from 100 Mbit/s to 2 Gbit/s. The segment length limit is 2 km.
Modern networks can be classified according to various criteria:
By remoteness of computers:
Local LAN (Local Area Network) is a network within an enterprise, institution, or one organization. Computers are located at a distance of up to several kilometers and are usually connected using high-speed communication lines.
Regional MAN (Metropolitan Area Network) - unites users of a region, city, and small countries. Telephone lines are used as communication channels. The distance between network nodes ranges from 10 to 1000 km.
Global WAN (Wide Area Network) - includes other global networks, local networks, as well as computers separately connected to it.
By purpose and list of services provided:
- General use of files and printers - with the help of a special computer (file server, printer server) user access to files and printers is organized.
General use of databases - using a special computer (database server), user access to the database is organized.
Application of Internet technologies - e-mail, World Wide Web, teleconferences, video conferences, file transfer over the Internet.
By way of organizing interaction:
- Peer-to-peer networks - all computers in a peer-to-peer network have equal rights, and any network user can access data stored on any computer. The main advantage of peer-to-peer networks is the ease of installation and operation. Main disadvantage is that in peer-to-peer networks it is difficult to resolve information security issues. Therefore, this method of organizing a network is used for networks with a small number of computers and where the issue of data protection is not fundamental.
- Dedicated server networks ( hierarchical networks) - when installing a network, one or more servers- computers that manage data exchange over the network and resource distribution. Any computer that has access to the server's services is called network client or workstation. The server itself can only be a client of a server at a higher hierarchy level. The hierarchical network model is the most preferable, as it allows you to create the most stable network structure and more rationally distribute resources. Another advantage of a hierarchical network is a higher level of data protection.
The disadvantages of a hierarchical network, compared to peer-to-peer networks, include:
The need for an additional OS for the server.
Greater complexity of network installation and upgrades.
The need for highlighting separate computer as a server
Based on server usage technology:
File-server architecture networks use a file server on which most programs and data are stored. At the user's request, they are sent to him necessary program and data. Information processing is performed at the workstation.
Networks with a client-server architecture - data is exchanged between the client application and the server application. Data is stored and processed on a powerful server, which also controls access to resources and data. The workstation receives only the results of the query.
By information transfer speed computer networks divided into low, medium and high speed:
Low-speed networks - up to 10 Mbit/s;
Medium-speed networks - up to 100 Mbit/s;
High-speed networks - over 100 Mbit/s.
Based on the type of transmission medium, networks are divided into:
Wired (coaxial cable, twisted pair, fiber optic);
Wireless with information transmission via radio channels or in the infrared range.
By topology (how computers are connected to each other):
Common bus;
Network topology
Network topology refers to the physical or electrical configuration of the network's cabling and connections.
Several specialized terms are used in network topology:
Network node - a computer or network switching device;
A network branch is a path connecting two adjacent nodes;
A terminal node is a node located at the end of only one branch;
Intermediate node - a node located at the ends of more than one branch;
Adjacent nodes are nodes connected by at least one path that does not contain any other nodes.
Any computer network can be considered as a collection of nodes. The configuration of physical connections is determined by the electrical connections between computers and may differ from the configuration of logical connections between network nodes. Logical connections represent data transmission routes between network nodes, formed by appropriately configuring the equipment.
There are three main types of physical local area network topology:
Ring topology provides for the connection of network nodes in a closed curve, i.e. transmission medium cable. In such a network, each node is connected to two and only two branches. Information along the ring is transmitted from node to node, usually in one direction. Each intermediate node between the transmitter and the receiver relays the sent message.
The receiving node recognizes and receives only messages addressed to it. In a network with a ring topology, it is necessary to take special measures so that in the event of a failure or disconnection of any station, the communication channel between the remaining stations is not interrupted. The advantage of this topology is ease of management, the disadvantage is the possibility of failure of the entire network if there is a failure in the channel between two nodes.
Bus topology one of the simplest, implemented using a cable to which all computers are connected. All signals transmitted by any computer on the network travel along the bus in both directions to all other computers.
Star topology uses a separate cable for each computer, running from a central device called hub or hub. A hub broadcasts signals received on any one of its ports to all other ports, causing signals sent by one node to reach the rest of the computers. In such a network there is only one intermediate node. A star-based network is more resilient to damage than a bus-based network, since cable damage directly affects only the computer to which it is connected, and not the entire network.
While small networks typically have a typical star, ring, or bus topology, large networks typically have random connections between computers. In such networks, individual subnetworks can be identified at random, having a standard topology, which is why they are called networks with mixed topology. The choice of a particular topology is determined by the area of application of the network, the geographical location of its nodes and the size of the network as a whole.
Open systems interconnection model. The main task solved when creating computer networks is to ensure compatibility of equipment in terms of electrical and mechanical characteristics and ensuring compatibility of information support (programs and data) in terms of coding system and data format. The solution to this problem belongs to the field of standardization. One example of solving this problem is the so-called open systems interconnection model OSI (Model of Open System Interconnections).
According to the OSI model, the architecture of computer networks should be considered at different levels (the total number of levels is up to seven). The highest level is applied. At this level the user interacts with computing system. The lowest level is physical. It ensures the exchange of signals between devices. Data exchange in communication systems occurs by moving it from the upper level to the lower one, then transporting it and, finally, playing it back on the client's computer as a result of moving from the lower level to the upper one.
Let's consider how in the OSI model data exchange occurs between users located on different continents.
1. At the application level, using special applications, the user creates a document (message, drawing, etc.).
2. At the presentation level, the operating system of his computer records where the created data is located (in RAM, in a file on the hard drive, etc.), and provides interaction with the next level.
3. At the session level, the user's computer interacts with a local or global network. Protocols at this level check the user's rights to “go on the air” and transmit the document to the transport layer protocols.
4. At the transport layer, the document is converted into the form in which data is supposed to be transmitted on the network being used. For example, it can be cut into small standard size bags.
5. The network layer determines the route for data movement in the network. So, for example, if at the transport level the data was “cut” into packets, then at the network level each packet must receive an address to which it should be delivered regardless of other packets.
6. The connection layer (Link layer) is necessary in order to modulate the signals circulating at the physical layer in accordance with the data received from the network layer. For example, in a computer these functions are performed network card or modem.
The actual data transfer occurs at the physical layer. There are no documents, no packets, not even bytes - only bits, that is, the elementary units of data representation. Restoring a document from them will occur gradually, when moving from the lower to the upper level on the client’s computer.
The physical layer facilities lie outside the computer. In local networks, this is the equipment of the network itself. For remote communications using telephone modems, these lines telephone communication, switching equipment for telephone exchanges, etc.
On the computer of the information recipient, the reverse process of converting data from bit signals to a document occurs.
The different protocol layers of the server and client do not communicate with each other directly, but they communicate through physical layer. Gradually moving from the upper level to the lower one, the data is continuously transformed, “overgrown” with additional data, which is analyzed by the protocols of the corresponding levels on the adjacent side. This creates an effect virtual interactions between levels.
In order for different computers on a network to communicate with each other, they must “speak” the same language, that is, use the same protocol. A protocol is a “language” used to communicate over a network.
There are many protocols, each performing different tasks. Different protocols are used at different layers of the OSI model.
Ethernet is a Connection Layer protocol used by most modern local area networks. The Ethernet protocol provides a unified interface to the network transmission medium, which allows operating system use several Network layer protocols simultaneously to receive and transmit data. Token Ring is an alternative to the “classic” Ethernet protocol at the Connection Level.
To be able to transmit information over network communication channels, it is necessary to install a messaging (packet) protocol. There are several such protocols. The most widely used are: NetBEUI , IPX/SPX , TCP/IP . Protocols NETBEUI And IPX/SPX- used in local networks. Protocols TCP/IP are the basic protocols of the global Internet.
Network equipment
The main components of the network are workstations, servers, transmission media (cables) And network equipment.
Workstations are called network computers on which network users implement applied tasks.
Network servers- these are hardware and software systems that perform the functions of managing the distribution of public network resources. A server can be any computer connected to a network that contains resources used by other devices on the network. Quite powerful computers are used as server hardware.
The following types are distinguished network equipment:
Network cables (coaxial, consisting of two concentric conductors insulated from each other, the outer one of which has the appearance of a tube; cables on twisted pairs, formed by two wires intertwined with each other; fiber optic etc.).
Network cards (Network interface adapters)- these are controllers connected to motherboard computers designed to transmit signals to the network and receive signals from the network. A network cable is connected to the adapter connectors.
Hubs (Hub) - This central devices a cable system or physical star network that, when it receives a packet on one of its ports, forwards it to all the others. A hub with a set of different types of ports allows you to combine network segments with different cable systems. You can connect either a separate network node or another hub or cable segment to the hub port.
The following devices are used to connect local networks to each other:
Bridges- network devices that connect two separate segments limited by their physical length. Bridges also amplify and convert signals for other types of cable. This allows you to expand maximum size networks.
Bridges transfer data between networks in packet form without making any changes to them. The figure below shows three local networks connected by two bridges. In addition, bridges can filter packets, protecting the entire network from local data flows and allowing only data that is intended for other network segments to pass through.
Gateways (Gateway) - hardware and software systems connecting heterogeneous networks or network devices. Gateways allow you to solve problems of differences in protocols or addressing systems. A gateway, unlike a bridge, is used in cases where the connected networks have different network protocols. A message from one network arriving at the gateway is converted into another message that meets the requirements of the next network.
Routers (Router) - standard devices networks that operate at the network layer and allow packets to be forwarded and routed from one network to another. It allows, for example, large messages to be split into smaller pieces, thereby ensuring the interaction of local networks with different packet sizes. A router can forward packets to a specific address (bridges can only filter out unnecessary packets), choose the best path for the packet to pass through.
Firewalls (firewall, firewalls ) - this is a software and/or hardware barrier between two networks, allowing the establishment of only authorized internet connections, implementing control over information entering and exiting the local network, and ensuring protection of the local network by filtering information.
Most firewalls are built on classical access control models, according to which a subject (user, program, process or network packet) is allowed or denied access to any object (file or network node) upon presentation of some unique element, inherent only to this subject. In most cases, this element is a password. For a network packet, such an element is the addresses or flags found in the packet header, as well as some other parameters.
Communication network– a system of nodes and connections between them. The nodes carry out the functions of creating, transforming, storing and consuming a communication product. Connections (transmission channels, communication lines) serve to transfer the product between nodes. Depending on the type of product, material, energy, and information networks are distinguished. Examples of physical networks: road and railway communications; water and gas supply.
Information network– a communication network in which the product of communication is information. Examples: telephone networks, television, radio broadcasting.
Computing, or computer network– an information network whose nodes are computers and other computing equipment. In addition to special network equipment, you also need network software. Thanks to the interaction of computers on a network, a number of new possibilities become available.
First - sharing hardware and software resources. Yes, when public access to an expensive peripheral device (printer, plotter, scanner, fax, etc.), the costs for each individual user are reduced. Network versions of application software are used in the same way.
Second - sharing to data resources. With centralized storage of information, the processes of ensuring its integrity are significantly simplified, as well as backup, which ensures high reliability. Having alternate copies on two machines at the same time allows you to continue working if one of them is unavailable.
Third, accelerating data transfer and providing new forms of interaction between users in one team when working on a common project.
Fourth - use common funds communications between various application systems (communication services, data transmission, video, speech, etc.).
One of the important classification features of networks is their size. The size of the network influences the choice of equipment used and transmission technologies used.
Local area network(LAN, or LAN - Local Area Network) unites nearby computers within a limited area, room, building. Distinctive Features LAN – minimum time latency and low error rate. LANs can be elements of larger-scale entities: campus, or corporate network(CAN - Campus Area Network), uniting local networks of nearby buildings; municipal network, or city-scale network (MAN - Metropolitan Area Network); regional or wide-area network (WAN - Wide Area Network), covering a large area; wide area network(WAN, or GAN - Global Area Network), which has the size of a country and a continent.
Based on the management method, networks are divided into peer-to-peer and with dedicated server(centralized control). In peer-to-peer networks, all nodes have equal rights - each node can act as both a client and a server. Under client refers to a hardware and software object that requests some services. And under server– a combination of hardware and software that provides these services. A computer connected to a local network, depending on the tasks solved on it, is called a workstation or server.
Peer-to-peer LANs are quite easy to maintain, but cannot provide adequate information protection if the network size is large. The costs of organizing peer-to-peer computer networks are relatively small. However, as the number of workstations increases, the efficiency of network use decreases sharply. Therefore, peer-to-peer LANs are used only for small workgroups - no more than 20 computers.
A dedicated server implements network management (administration) functions in accordance with specified policies - sets of rules for dividing and limiting the rights of network participants. LANs with a dedicated server have good means ensuring data security are capable of supporting thousands of users, but require constant qualified maintenance by a system administrator.
Depending on the data transmission technology used, there are different broadcast networks and networks with transmission from node to node. Broadcast transmission is used mainly in small networks, and in large networks it is used for transmission from node to node.
In broadcast networks, all nodes on the network share single channel communications. Messages sent by one computer, called packets, are received by all other machines. Each packet contains the address of the message recipient. If the packet is addressed to another computer, it is ignored. Thus, after verifying the address, the recipient processes only those packets that are intended for it.
Networks with transmission from node to node consist of pairwise connected machines. In such a network, to get to its destination, a packet passes through a number of intermediate machines. However, there are often alternative paths from the source to the recipient.
The method of connecting computers together in a network is called topology. There are three most common topologies used in LANs. These are the so-called tire, ring And star-shaped structures.
In the case of a bus (linear) structure, all computers are connected in a chain using one common coaxial cable. If at least one of the sections of the network with a bus structure is damaged, the entire network as a whole becomes inoperable. The fact is that then there is a break in the only physical channel necessary for the movement of the signal.
The ring structure is used mainly in Token Ring networks and differs from the bus structure in that all computers are connected in pairs to each other, forming a closed loop. Also, if one of the network segments malfunctions, the entire network goes down.
In a star network, the central node to which all others connect is hub(Hub – “hub”). Its main function is to ensure communication between computers on the network. This structure preferable, since if one of the workstations or the cable connecting it to the hub fails, all the others remain operational.
When building networks, cellular ( fully connected) a topology in which each node is connected to all other individual links. The costs of creating redundant channels are offset by high reliability - there are almost always several paths for signals to pass from sender to recipient, so if some channels are disconnected, signals can be transmitted through others.
The following are distinguished: switching methods data in information networks: circuit switching, packet switching And message switching.
When switching circuits, the entire connection path is first established - from the sender to the recipient. This path consists of several sections connected by switches and/or multiplexers. All data is transmitted along the established route. Once the transfer is complete, the connection is terminated. Example – telephone conversation: The channel is busy during the entire call, even if the callers are silent. The transmission speed over such a channel is limited to the area with the lowest bandwidth.
With the second method, messages are divided into packets of a fixed length, which can be delivered across the network via independent routes, ensuring uniform network load. In this case, packets of different messages can be transmitted over one channel. As an example, let's give an analogy: during rush hour, a group of students gets from the hostel to the university using different transport, each in their own way.
Message switching is similar to packet switching, but at a higher level (message switching nodes can be connected by either a circuit-switched network or a packet-switched network). The main difference is that the size of the data block is determined not by technological limitations, but by the content of the information in the message. It could be text document, email, file. Example - a group of tourists follows a route, and at each point the composition of the group is checked. This scheme is used to transmit messages that do not require an immediate response, such as email messages.
15.3 OSI/ISO network model
The functioning of network equipment is impossible without interconnected standards. Harmonization of standards is achieved both through consistent technical solutions and through grouping of standards. Each specific network has its own basic set of protocols - the “language” of data transmission. Protocol– formalized rules for the interaction of several computers, which can be described as a set of procedures that determine the sequence and format of messages exchanged between network components located at the same level, but in different nodes.
The International Organization for Standardization (ISO) has proposed model architecture computer network OSI(Open System Interconnection - communication open networks). This model, which most users try to adhere to, divides communication functions on the network into seven levels. Data exchange occurs by moving it on the sender's computer from the upper level to the lower one, then transporting it over the communication channel and converting it back on the recipient's computer from the lower level to the upper one.
The highest level is application layer(Application Layer) is the interface between application programs and OSI model processes.
The Presentation Layer defines the format for data exchange and serves for encryption, compression and code conversion of data.
Session layer(Session Layer) performs the functions of coordinating communications between workstations. The layer provides the creation of a communication session, control of the transmission and reception of message packets, and termination of the session.
The Transport Layer divides or assembles messages into packets when more than one packet is in the process of transmission or reception, as well as controls the order in which message components pass through. In addition, at this level, through gateways, the network layers of various incompatible networks are negotiated. Guarantees delivery of packets without errors, in the same sequence, without losses and duplication with confirmation of receipt.
The Network Layer provides the translation of logical address names into physical ones. Based on specific network conditions and the priority of the service, routing is carried out, that is, the choice of transmission route for a data packet in the network, and control of the data flow in the network (data buffering, error control when establishing a connection).
The Data Link layer defines the rules for using the physical layer by network nodes. This layer is divided into two sublayers: Media Access Control, related to network access and management, and Logical Link Control, related to transmission and reception. user messages. It is at the Data Link level that data transmission is ensured in frames, which are blocks of data containing additional control information. Error correction is performed automatically by resending the frame. In addition, at this level the correct sequence of transmitted and received frames is ensured.
Lowest – physical layer(Physical Layer) defines the physical, mechanical and electrical characteristics of communication lines. At this level, the transformation of data coming from link layer, into signals that are then transmitted over communication lines. In local networks, this conversion is carried out using network adapters, in global networks modems are used for this purpose.
Each level actually interacts only with neighboring levels (upper and lower), and virtually only with a similar level at the end of the line. Real interaction is the direct transfer of information in which the data remains unchanged. Virtual interaction is indirect interaction and data transfer, and the data can be modified during the transfer process.
Physical connection actually takes place only at the lowest level. Horizontal connections between all other levels are virtual; in reality they are carried out by transferring and converting information first downwards, sequentially to the lowest level, where the real transfer occurs, and then at the other end - reverse transmission upwards sequentially to the appropriate level.
