Combining the components discussed above into a network can be done in various ways and means. According to the composition of their components, methods of their connection, sphere of use and other features, networks can be divided into classes so that the belonging of the described network to one or another class can sufficiently characterize the properties and quality parameters of the network.
However, this kind of network classification is rather arbitrary. The most widespread today is the division of computer networks on the basis of territorial distribution.
On this basis, networks are divided into three main classes:
LAN - Local Area Networks;
MAN stands for Metropolitan Area Networks.
WAN - Wide Area Networks;
Local area network (LAN) Is a communication system that supports, within a building or some other limited area, one or more high-speed digital information transmission channels provided to connected devices for short-term exclusive use. The areas covered by a drug can vary significantly.
The length of communication lines for some networks can be no more than 1000 m, while other LANs are able to serve the whole city. Served territories can be both factories, ships, aircraft, and institutions, universities, colleges. As a transmission medium, coaxial cables are usually used, although networks on twisted pair and fiber optics are becoming more widespread, and recently wireless LAN technology is also rapidly developing, which uses one of three types of radiation: broadband radio signals, low-power radiation ultra-high frequencies (microwave radiation) and infrared rays.
Small distances between network nodes, the used transmission medium and the associated low probability of errors in the transmitted data allow maintaining high exchange rates - from 1 Mbit / s to 100 Mbit / s (currently there are already industrial models of LANs with speeds of the order of 1 Gbit /with).
City networks usually cover a group of buildings and are implemented on fiber optic or broadband cables. By 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 sections, 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, they cover much larger territories and even most regions of the world (an example is the Internet). At present, analog or digital wire channels, as well as satellite communication channels (usually for communication between continents), are used as the transmission medium in global networks. Limitations on the transmission rate (up to 28.8 Kbit / s on analog channels and up to 64 Kbit / s on the user sections of digital channels) and the relatively low reliability of analog channels, requiring the use of error detection and correction tools at the lower protocol levels significantly reduce the exchange rate data in global networks compared to local ones.
There are other classifications of computer networks.
By area of operation networks are divided into:
Banking networks,
Scientific networks,
University networks;
By the form of functioning can be distinguished:
Commercial networks;
Free networks,
Corporate networks
Public networks;
By the nature of the functions implemented networks are divided into:
Computing, 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:
Decentralized networks;
Centralized management;
Mixed control.
In the first case, each computer included in the network includes a complete set of software tools for coordinating the network operations performed. 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 the conditions of mixed networks under centralized control, the solution of tasks with the highest priority and, as a rule, associated with the processing of large amounts of information is carried out.
By software compatibility there are networks:
Uniform;
Homogeneous (consisting of software compatible computers)
Heterogeneous or heterogeneous (if the computers included in 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
In client / server networks, a dedicated computer (server) is used, on which public files are concentrated and which provides a print service for many users (Fig. 1).
Rice. 1.Client / server networks
Server -a computer connected to a network and providing its users with certain services.
Servers can handle data storage, database management, remote job processing, job printing, and a variety of other functions that may be required by network users. Server is the source of network resources. There can be quite a lot of servers on the network, and each of them can serve its own group of users or manage certain databases.
Work station- a personal computer connected to the 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 that connect to the server are called clients. The clients can be used as powerful computers for resource-intensive processing of spreadsheets, and low-power PCs for simple word processing. In contrast, powerful computers are usually installed as servers. Due to the need to ensure the simultaneous processing of requests from a large number of clients and good protection of network data from unauthorized access, the server must operate under a specialized operating system.
Examples: Novell Net Ware, Windows NT Server, IBM OS / 2 Lan Server, Banyan Vines.
Peer-to-peer networks
Dedicated servers are not used in peer-to-peer networks (Fig. 2). While serving the user, a peer-to-peer computer can take over the server functions, performing print jobs and responding to file requests from other workstations on the network. Of course, if a computer does not share its disk space or its printer, then it is only a client in relation to other workstations that perform the functions of a server. Windows 95 has built-in peer-to-peer networking capabilities. 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 we understand the description of the properties of a network inherent in all its homomorphic transformations, i.e. such changes in the appearance of the network, the distances between its elements, their relative position, at which the ratio of these elements to each other does not change.
The topology of a computer network is largely determined by the way computers connect 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, LAN configurations are divided into two main classes: broadcast and consecutive.
In broadcast configurations, each PC (physical transceiver) transmits signals that can be picked up by the rest of the PCs. These configurations include common bus, tree, and star passive center topologies. A star with a passive center can be thought of as a kind of tree, rooted with a branch to each connected device.
In sequential configurations, each physical sublayer transmits information to only one PC. Examples of sequential configurations are: arbitrary (arbitrary connection of computers), hierarchical, "ring", "chain", "star with a smart center", "snowflake" and
other.
The most optimal from the point of view of reliability (the possibility of the network functioning in the event of failure of individual nodes or communication channels) is mesh network, i.e. a network in which each node of the network 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 non-fully connected network, there can be a large number of options for connecting network nodes, in practice, three most widespread (basic) LAN topologies are usually used:
1. common bus;
2. ring;
3.star.
Bus topology (fig. 3) when all nodes on a network are connected to one open channel, commonly called a bus.
Fig 3.Bus topology.
In this case, one of the machines serves as a system service device providing centralized access to shared files and databases, printers and other computing resources.
Networks of this type have gained great popularity due to their low cost, high flexibility and data transfer speed, ease of network expansion (the connection of new subscribers to the network does not affect its basic characteristics). The disadvantages of 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 on 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 interfere with the normal operation of the network.
As a sequential configuration, the ring is especially vulnerable to failures: the failure of a segment of the cable leads to the termination of service for all users. LAN developers have gone to great lengths to deal 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 overall ring topology is preserved.
Star topology (Figure 5) when all nodes on the network connect to one central node called host ( host) or hub ( hub).
Fig 5.Topology "Star".
The configuration can be seen as a further development of the tree-rooted structure, with a branch to each connected device. At the center of the network, a switching device is usually located to ensure the viability of the system. LANs of this configuration are most often used in automated office control systems using a central database. Star-shaped LANs are generally less reliable than shared bus or hierarchical networks, but duplication of central hub hardware solves this problem. The disadvantages can also be attributed to the significant consumption of the cable (sometimes several times higher than the consumption in LANs with similar capabilities with a common bus or hierarchical).
Networks can also be of mixed topology ( hybrid) when individual parts of the network have different topologies. An example is the local FDDI network, in which the main (backbone) nodes are connected to the ring channel, and the remaining nodes are connected to them in a hierarchical topology.
Combining computers and devices into a network can be done in various ways and means. According to the composition of its components, the way they are connected, the scope of use and other features, the networks can be divided into classes so that the belonging of the described network to one or another class could sufficiently characterize the properties and quality parameters of the network.
However, this kind of network classification is rather arbitrary. The most widespread today is the division of computer networks on the basis of territorial distribution. On this basis, networks are divided into three main classes:
LAN (Local Area Networks) - local area networks;
MAN (Metropolitan Area Networks) - regional (city or corporate) networks;
WAN (Wide Area Networks) - global networks.
A local area network (LAN) is a communication system that supports, within a building or some other limited area, one or more high-speed digital information transmission channels provided to connected devices for short-term exclusive use. The areas covered by a drug can vary significantly.
The length of communication lines for some networks can be no more than 1000 m, while other LANs are able to serve the whole city. Served territories can be both factories, ships, aircraft, and institutions, universities, colleges. As a transmission medium, coaxial cables are usually used, although networks on twisted pair and fiber optics are becoming more widespread, and recently wireless LAN technology is also rapidly developing, which uses one of three types of radiation: broadband radio signals, low-power radiation ultra-high frequencies (microwave radiation) and infrared rays.
Small distances between network nodes, the used transmission medium and the associated low probability of errors in the transmitted data allow maintaining high exchange rates - from 1 Mbit / s to 100 Mbit / s (currently there are already industrial models of LANs with speeds of the order of 1 Gbit /with).
Regional networks, as a rule, cover a group of buildings and are implemented on fiber optic or broadband cables. By 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). At present, analog or digital wire channels, as well as satellite communication channels (usually for communication between continents), are used as the transmission medium in global networks. Limitations on the transmission rate and the relatively low reliability of analog channels, requiring the use of error detection and correction tools at the lower levels of the protocols, significantly reduce the data exchange rate in global networks as compared to local ones.
There are other classifications of computer networks. For example:
- according to the sphere of functioning, the networks can be divided into banking scientific institutions, university;
- according to the form of functioning, commercial and free networks, corporate and public ones, can be distinguished;
- by the nature of the implemented functions, the networks are subdivided into computational ones (designed to solve control problems based on computational processing of initial information); informational (designed to obtain reference data at the request of users); mixed (computational and informational functions are implemented in them);
- according to the method of management, computer networks are divided into networks with decentralized, centralized and mixed control. In the first case, each computer included in the network includes a complete set of software tools for coordinating the network operations performed. 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 the conditions of mixed networks under centralized control, the solution of tasks with the highest priority and, as a rule, associated with the processing of large amounts of information is carried out.
Local networks
A local network is created, as a rule, for the joint use of computer resources or data (usually in one organization). From a technical point of view, a local network is a collection of computers and communication channels that combine computers into a structure with a specific configuration, as well as network software that controls the operation of the network. The way computers are connected to a local network is called a topology.
Topology largely determines many important properties of a network, such as reliability (survivability), performance, etc. There are different approaches to classifying network topologies. In terms of performance, they are divided into two main classes: broadcast and sequential.
In broadcast configurations, each computer transmits signals that can be picked up by other computers. These configurations include common bus, tree, and star passive center topologies. A star with a passive center can be thought of as a kind of tree, rooted with a branch to each connected device.
In sequential configurations, each physical sublayer transmits information to only one PC. Examples of sequential configurations are: arbitrary (arbitrary connection of computers), hierarchical, "ring", "chain", "star with an intelligent center", "snowflake" and others.
Bus topology
Figure 10.2. Bus topology of a local area network
With such a connection, the exchange can be carried out between any computers on the network, independently of the others. If the communication of one computer with a common bus is damaged, this computer is disconnected from the network, but the whole network is working. In this sense, the network is quite stable, but if the bus is damaged, then the entire network is out of order.
Ring topology
Figure 10.3. LAN ring 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 increases resilience to network problems. One break does not disable the network, but two breaks make the network unusable. The ring network is quite widely used, mainly due to the high data transfer rate. Ring networks are the fastest.
Star topology
Figure 10.4. Star topology LAN
When connected in 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 you to organize complex network structures are called hubs and switches.
BELARUSIAN NATIONAL TECHNICAL UNIVERSITY
INTERNATIONAL INSTITUTE OF DISTANCE EDUCATION
TEST
ACADEMIC DISCIPLINE: Computer networks
Types of computer networks
Computer networks can be classified according to various criteria.
I... By management principles:
1. Peer-to-peer - do not have a dedicated server. In which control functions are alternately transferred from one workstation to another;
2. Multi-rank is a network that includes one or more dedicated servers. The rest of the computers on this network (workstations) act as clients.
II... By connection method:
1. "Direct connection"- two personal computers are connected with 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 the central node;
4. "Ring"- serial connection of the PC in two directions.
III... By area coverage:
1. The 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 one room, floor, building.
2... Corporate network(within are within the same organization, firm, plant). The number of nodes in a KVS can reach several hundred. Moreover, a corporate network usually includes not only personal computers, but also powerful computers, as well as various technological equipment (robots, assembly lines, etc.).
The corporate network makes it possible to facilitate enterprise management and technological process management, establish clear control over information and production resources.
3. Global network(a network, the elements of which are remote from each other at a considerable distance) - up to 1000 km.
As communication lines in global networks, both specially laid (for example, transatlantic fiber-optic cable) and existing communication lines (for example, telephone networks) are used. The number of nodes in the hot water supply can reach tens of millions. The global network includes separate local and corporate networks.
4. Worldwide network- integration of global networks (Internet).
TOPOLOGY OF COMPUTER NETWORKS
Network topology is the geometric shape and physical location 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;
TIRE TOPOLOGY
This topology uses a single coaxial transmission channel called a "bus". All networked 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;
In the event of a bus failure, the entire network ceases to function.
TOPOLOGY "RING"
Ring topology is characterized by the absence of connection endpoints; the network is closed, forming an unbreakable ring through which data is transmitted. This topology implies the following transmission mechanism: data is transferred sequentially from one computer to another until it reaches the destination computer. The disadvantages of the ring topology are the same as for the bus topology:
Public availability of data;
Immunity to damage to the cable system.
TOPOLOGY "STAR"
In a star network, all computers are connected to a special device called a network hub or hub that performs the functions of distributing data. There are no direct connections between two computers on the network. This makes it possible to solve the problem of public data availability, and also increases the resistance to damage to the cabling system. However, the functionality of the network depends on the state of the network hub.
Carrier access methods in computer networks
Different networks have different 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 links.
The most widespread are specific implementations of access methods: Ethernet, ArcNet, and Token Ring. These implementations are based on IEEE802.3, IEEE802.4, and IEEE802.5, respectively.
Ethernet access method
This access method, developed by Xerox in 1975, is the most popular. It provides high data transfer speed and reliability.
This access method uses a shared 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 only for one station (it includes the destination station address and the sender's address). The station to which the message is intended receives it, the others ignore it.
The Ethernet access method is a Carter Sense Multiple Access with Collision Detection (CSMA / CD) method.
Before starting the transmission, the workstation determines whether the channel is free or busy. If the channel is free, the station starts transmitting.
Ethernet does not exclude the possibility of simultaneous transmission of messages by two or more stations. The hardware will automatically recognize such conflicts. After detecting a collision, the stations delay the transmission for some time. This time is short and for each station its own. After the delay, transmission resumes.
In reality, conflicts lead to a decrease in network performance only if several tens or hundreds of stations are working.
ArcNet access method
This method was developed by Datapoint Corp. It has also become widespread, mainly due to the fact that ArcNet equipment is cheaper than Ethernet or Token-Ring equipment.
ArcNet is used in star topology LANs. One of the computers creates a special token (a message of a special type), which is sequentially transmitted from one computer to another.
If a station wishes to transmit a message to another station, it must wait for the marker and add a message to it, supplemented by the sender and destination addresses. When the packet arrives at the destination station, the message will be "unhooked" from the marker and forwarded 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 in that it also uses a token passed from one station to another. Unlike ArcNet, the Token-Ring access method has the ability 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 plastic insulating material, which in turn is surrounded by a copper mesh and / or aluminum foil. This outer conductor provides grounding and protects the center conductor from external electromagnetic interference. There are two types of cable used for networking - Thicknet and Thinnet. Coaxial cable networks provide transmission speeds of up to 10 Mbps. The maximum segment length ranges from 185 to 500 m, depending on the cable type.
"Twisted pair"
Twisted pair cable is one of the most common cable types 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 the shielded twisted pair 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 at speeds from 10 Mbit / s - 1 Gbit / s. The length of a cable segment 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 covering and an outer sheath. Data transmission is carried out using a laser or LED transmitter that sends unidirectional light pulses through the center conductor. The signal at the other end is received by a photodiode receiver, which converts the light pulses into electrical signals that can be processed by a computer. The transmission speed for fiber optic networks ranges from 100 Mbps to 2 Gbps. The segment length is limited to 2 km.
Modern networks can be classified according to various criteria:
By the remoteness of computers:
Local LAN (Local Area Network) - a network within an enterprise, institution, one organization. Computers are located up to several kilometers away and are usually connected using high-speed communication lines.
Regional MAN (Metropolitan Area Network) - unite users of the region, city, small countries. Telephone lines are used as communication channels. The distance between the nodes of the network is from 10 to 1000 km.
Global WAN (Wide Area Network) - includes other global networks, local area networks, as well as separately connected computers.
By appointment and the list of services provided:
- General use of files and printers - using a special computer (file-server, printer-server), user access to files and printers is organized.
General use of databases - with the help of a special computer (database server), user access to the database is organized.
Application of Internet technologies - e-mail, the World Wide Web, teleconferences, video conferencing, file transfer over the Internet.
By the way of organizing interaction:
- Peer-to-peer networks - all computers in a peer-to-peer network are equal, and any user on the network can access data stored on any computer. The main advantage of peer-to-peer networks is ease of installation and operation. The main disadvantage is that in the conditions of peer-to-peer networks, it is difficult to solve 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 critical.
- Dedicated Server Networks ( hierarchical networks) - when installing a network, one or more servers- computers that control the exchange of data over the network and the allocation of resources. Any computer that has access to the server's services is called client of the network or workstation... The server itself can only be a client of a server at a higher level of the hierarchy. The hierarchical network model is the most preferable, as it allows you to create the most stable network structure and more rationally allocate resources. Also, the 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.
Higher complexity of network installation and modernization.
The need to allocate a separate computer as a server
Server technology:
File Server Networks — Uses a file server where most programs and data are stored. At the request of the user, the necessary program and data are sent to him. Information processing is performed at the workstation.
Client-server networks - communication takes place between the client application and the server application. Data storage and processing is performed on a powerful server, which also controls access to resources and data. The workstation only receives the query results.
By speed of information transfer computer networks are divided into low, medium and high speed:
Low-speed networks - up to 10 Mbps;
Medium-speed networks - up to 100 Mbit / s;
High-speed networks - over 100 Mbps.
By the type of transmission medium, networks are divided into:
Wired (coaxial, twisted pair, fiber optic);
Wireless with transmission of information 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 cabling system and network connections.
Several specialized terms are used in network topology:
Network node - a computer or a network switching device;
A branch of a network is a path connecting two adjacent nodes;
End node - a node located at the end of only one branch;
An intermediate knot is a knot located at the ends of more than one branch;
Adjacent nodes are nodes that are connected by at least one path that does not contain any other nodes.
Any computer network can be viewed as a collection of nodes. The configuration of physical links is determined by the electrical connections between computers and may differ from the configuration of logical links between network nodes. Logical links are data transmission routes between network nodes, formed by appropriate configuration of the equipment.
There are three main types of physical LAN topology:
Ring topology provides for the connection of network nodes with a closed curve, i.e. transmission medium cable. In such a network, two and only two branches are connected to each node. Information on 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, special measures must be taken so that in the event of failure or shutdown of a station, the communication channel between the other stations is not interrupted. The advantage of this topology is simplicity of management, the disadvantage is the possibility of failure of the entire network in case of a failure in the link between two nodes.
Bus topology one of the simplest, is implemented using a cable to which all computers are connected. All signals transmitted by any computer to the network go along the bus in both directions to all other computers.
Star topology uses a separate cable for each computer, routed from a central device called hub or hub. A hub broadcasts signals from any of its ports to all other ports, with the result that signals sent by one node reach the rest of the computers. In such a network, there is only one intermediate node. A star-based network is more resistant 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 tend to have a typical topology - star, ring, or shared bus, large networks tend to have arbitrary connections between computers. In such networks, individual subnets can be selected arbitrarily, having a typical topology, therefore they are called mixed topology networks... The choice of a particular topology is determined by the scope of the network, the geographic location of its nodes and the dimension of the network as a whole.
Open Systems Interconnection Model. The main task solved when creating computer networks is to ensure the compatibility of equipment in terms of electrical and mechanical characteristics and to ensure the compatibility of information support (programs and data) in terms of the 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 topmost level is applied. At this level, the user interacts with the computing system. The lowest level is physical. It provides signal exchange between devices. The exchange of data in communication systems occurs by moving them from the upper level to the lower, then transporting and, finally, replaying them on the client's computer as a result of moving from the lower level to the upper one.
Let us consider how the OSI model exchanges data between users located on different continents.
1. At the application level, using special applications, the user creates a document (message, picture, 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 a hard disk, etc.), and provides interaction with the next level.
3. At the session level, the user's computer interacts with the local or global network. The protocols of this layer check the user's rights to "go on the air" and transfer the document to the protocols of the transport layer.
4. At the transport level, the document is transformed into the form in which it is supposed to transfer data in the used network. For example, it can be cut into small standard sized bags.
5. The network layer determines the route of data movement in the network. So, for example, if at the transport layer the data were "sliced" 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 by a network card or modem.
The real data transfer takes place at the physical level. There are no documents, no packets, not even bytes - only bits, that is, elementary units of data representation. The recovery of the document from them will occur gradually, when moving from the lower to the upper level on the client's computer.
The physical layer means lie outside the computer. In local area networks, this is the equipment of the network itself. For remote communication using telephone modems, these are telephone lines, switching equipment of telephone exchanges, etc.
On the computer of the recipient of information, the reverse process of converting data from bit signals to a document takes place.
The different protocol layers of the server and client do not interact with each other directly, but they interact through the physical layer. Gradually moving from the upper level to the lower one, the data is continuously transformed, "overgrown" with additional data, which are analyzed by the protocols of the corresponding levels on the adjacent side. This creates the effect virtual interaction of levels with each other.
In order for different computers on the network to communicate with each other, they must "speak" the same language, that is, use the same protocol. A protocol is the “language” used to communicate over a network.
There are many protocols, each of which performs different tasks. Different protocols are used at different layers of the OSI model.
Ethernet is a Link-Layer protocol used by most modern LANs. The Ethernet protocol provides a unified interface to the network transmission medium that allows the operating system to use multiple Network Layer protocols simultaneously to receive and transmit data. Token Ring is an alternative to the "classic" Ethernet protocol at the Link Layer.
To be able to transfer information over network communication channels, it is necessary to install a message exchange protocol (packets). There are several such protocols. The most commonly 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 hardware
The main components of the network are workstations, servers, transmission media (cables) and network hardware.
Workstations network computers on which network users implement application tasks are called.
Network servers are hardware and software systems that perform the functions of managing the distribution of shared network resources. A server can be any computer connected to the network that contains resources used by other devices on the network. Rather powerful computers are used as the server hardware.
There are the following types network equipment:
Network cables (coaxial, consisting of two concentric conductors isolated from each other, of which the outer looks of a tube; cables on twisted pairs formed by two wires intertwined with each other; fiber optic and etc.).
Network Cards (Network Interface Adapters)- These are controllers that are connected to the computer's motherboard, designed to transmit signals to the network and receive signals from the network. A network cable is connected to the connectors of the adapters.
Concentrators (Hub) - these are the central devices of the cable system or network of physical topology "star", which, when receiving 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. A hub port can be used to connect either a separate network node or another hub or cable segment.
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 cable types. This allows you to expand the maximum network size.
Bridges transfer data between networks in batches without making any changes to them. The figure below shows three LANs connected by two bridges. In addition, bridges can filter packages, protecting the entire network from local data streams and letting out only those data that are intended for other network segments.
Gateways (Gateway) - software and hardware systems connecting heterogeneous networks or network devices. Gateways allow you to solve the problem 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 arriving at the gateway from one network is converted into another message that meets the requirements of the next network.
Routers (Router) - standard network devices operating at the network level and allowing to forward and route packets from one network to another. It allows, for example, to split large messages into smaller chunks, thereby ensuring the interaction of local networks with different packet sizes. The router can forward packets to a specific address (bridges can only filter out unnecessary packets), choose the best path for the packet to pass.
Firewalls (firewall, firewalls ) - this is a software and / or hardware barrier between two networks, which allows only authorized interconnections to be established, realizing control over information entering and leaving the local network, and ensuring the protection of the local network by filtering information.
Most firewalls are built on classical models of access control, according to which a subject (user, program, process or network packet) is allowed or denied access to an 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 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) are used to transfer the product between nodes. Depending on the type of product, material, energy, information networks are distinguished. Examples of material networks: road and rail links; water and gas supply.
Information network- a communication network in which the product of communications is information. Examples: telephone networks, television, radio broadcasting.
Computing, or computer network- an information network, the nodes of which are computers and other computing equipment. In addition to dedicated networking hardware, networking software is also required. Thanks to the interaction of computers on the network, a number of new possibilities become available.
The first is the sharing of hardware and software resources. So, with shared access to an expensive peripheral device (printer, plotter, scanner, fax, etc.), the costs for each individual user are reduced. The network versions of the application software are used in the same way.
The second is sharing data resources. With centralized storage of information, the processes of ensuring its integrity, as well as backup, are greatly simplified, 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.
The third is the acceleration of data transfer and the provision of new forms of user interaction in one team when working on a common project.
Fourth, the use of common means of communication between different application systems (communication services, data transmission, video, voice, etc.).
One of the important classification features of networks is their size. The size of the network affects the choice of equipment used and the transmission technologies used.
Local computing network(LAN, or LAN - Local Area Network) unites closely spaced computers within a limited area, room, building. Distinctive features of a LAN are minimal latency and low error rate. LANs can be elements of larger-scale formations: a campus or corporate network (CAN - Campus Area Network), which unites local networks of closely located buildings; municipal network, or city-scale network (MAN - Metropolitan Area Network); regional, or wide-scale network (WAN - Wide Area Network), covering a significant territory; global area network(WAN, or GAN - Global Area Network), having the size of the country and continent.
According to the method of management, networks are divided into peer-to-peer and with dedicated server(centralized management). In peer-to-peer networks, all nodes are equal - each node can act as both a client and a server. Under client refers to a hardware and software entity that requests some services. And under server- the 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 (workstation) or a server (server).
Peer-to-peer LANs are fairly easy to maintain, but they cannot provide adequate information protection for a large network. The cost of organizing peer-to-peer computer networks is relatively small. However, with an increase in the number of workstations, the efficiency of network utilization decreases dramatically. Therefore, peer-to-peer LANs are used only for small workgroups - no more than 20 computers.
A dedicated server implements the functions of network management (administration) in accordance with the specified policies - sets of rules for dividing and limiting the rights of network participants. Dedicated Server LANs have good data security, are capable of supporting thousands of users, but require constant qualified maintenance by a system administrator.
Depending on the used data transmission technology, a distinction is made between broadcast networks and networks with transfer from node to node... Broadcast transmission is used mainly in small networks, and in large networks, transmission from node to node.
In broadcast networks, all nodes on the network share a single communication channel. Messages sent by one computer, called packets, are received by all other computers. Each packet contains the address of the recipient of the message. If the packet is addressed to another computer, then it is ignored. Thus, after verifying the address, the recipient processes only those packets that are intended for him.
Node-to-node networks are made up of machines connected in pairs. In such a network, in order to reach its destination, a packet passes through a series of intermediate machines. However, there are often alternative paths from source to destination.
The method of connecting computers to each other in a network is called topology... There are three most common topologies used in a LAN. These are the so-called tire, annular and star-shaped structures.
In the case of a bus (linear) structure, all computers are chained together using one common coaxial cable. If, however, at least one of the sections of the network with a bus structure is disrupted, the entire network as a whole becomes inoperative. 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, in the event of a malfunction of one of the network segments, the entire network fails.
In a star network, the central hub to which everyone else connects is hub(Hub - "hub"). Its main function is to provide communication between computers on the network. This structure is preferable, because in the event of failure of one of the workstations or the cable connecting it to the hub, all the others remain operational.
When building networks, mesh ( fully connected a) topology in which each node connects to all other separate links. The costs of creating redundant channels are compensated for by high reliability - there are almost always several paths for signals to pass from the sender to the receiver, so when some channels are disconnected, signals can be transmitted over others.
Distinguish the following switching methods data in information networks: circuit switching, packet switching and message switching.
Circuit switching first establishes the entire path of the connection - from sender to receiver. This path consists of several sections connected by switches and / or multiplexers. All data is transmitted along the established route. When the transfer is complete, the connection is terminated. An example is a telephone conversation: the channel is busy during the entire conversation, even if the subscribers are silent. The transmission speed over such a channel is limited to the section with the lowest bandwidth.
In the second method, messages are split into packets of fixed length, which can be delivered over the network by independent routes, providing an even load of the network. In this case, packets of different messages can be transmitted over one channel. As an example, let us give an analogy: at rush hour a group of students travels from the hostel to the university by different transport, each in its own way.
Message switching resembles packet switching, but at a higher level (in this case, message switching nodes can be connected both by a circuit-switched network and a packet-switched network). The main difference is that the size of the data block is determined not by technological constraints, but by the content of the information in the message. It can be a text document, email, file. Example - a group of tourists follows the route, at each point the composition of the group is checked. This pattern is used to transmit messages that do not require an immediate response, such as e-mail messages.
15.3 OSI / ISO Network Model
The functioning of network equipment is impossible without interrelated 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 by network components that are at the same level, but in different nodes.
International Organization for Standardization ISO (International Standards Organization) proposed model computer network architecture OSI(Open System Interconnection - communication of open networks). This model, which most users try to adhere to, divides the communication functions of the network into seven levels... Data exchange occurs by moving them on the sender's computer from the upper level to the lower level, then transporting it through the communication channel and reverse transformation on the recipient's computer from the lower level to the upper level.
The highest level is application layer(Application Layer - application) is the interface between applications and processes of the OSI model.
The presentation layer (Presentation Layer) defines the format for data exchange, is used for encryption, compression and code transformation of data.
The Session Layer performs the function of coordinating communication between workstations. The layer provides the creation of a communication session, control of the transmission and reception of message packets, and the termination of the session.
The Transport Layer carries out the division or assembly of messages into packets in the case when more than one packet is in the process of transmission or reception, as well as control of the order of passage of the message components. In addition, at this level, the network layers of the various incompatible networks are negotiated through the gateways. It guarantees the delivery of packets without errors, in the same sequence, without loss and duplication with acknowledgment.
The Network Layer translates logical address names into physical ones. Based on the specific network conditions, the priority of the service, routing is carried out, that is, the choice of the route for transmitting the data packet in the network, and the control of the data flow in the network (data buffering, control of errors when establishing a connection).
The Data Link defines the rules for using the physical layer by network nodes. This layer is subdivided into two sub-layers: Media Access Control, associated with network access and management, and Logical Link Control, associated with the transmission and reception of user messages. It is at the Data Link level that data transmission is ensured by frames, which are data blocks containing additional control information. Error correction is done automatically by re-sending the frame. In addition, the correct sequence of transmitted and received frames is ensured at this level.
The lowest is physical layer(Physical Layer) defines the physical, mechanical and electrical characteristics of communication lines. At this level, the data coming from the data link layer is converted into signals, which are then transmitted over the 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), 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 an indirect interaction and data transfer, and the data can be modified during the transfer.
Physical connection really only takes place at the lowest level. Horizontal connections between all other levels are virtual, in reality they are carried out by transferring and transforming information first down, sequentially to the lowest level, where the real transfer takes place, and then at the other end - backward transfer up sequentially to the corresponding level.
