Connecting computers and devices into a network can be done different ways and means. Based on the composition of their components, the methods of their connection, the 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 using twisted pair and optical fiber are becoming increasingly widespread, and in Lately Wireless local area network technology is also rapidly developing, using one of three types of radiation: broadband radio signals, low-power microwave radiation (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 support high speeds exchange - from 1 Mbit/s to 100 Mbit/s (currently there are already industrial designs of LANs with speeds of about 1 Gbit/s).
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 Internet network). 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. For example:
– according to the area of operation, the networks can be divided into banking research institutions and universities;
– based on the form of operation, one can distinguish between commercial and free networks, corporate and public;
– 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 included in the network includes a complete set software to coordinate ongoing network operations. Networks of this type are complex and quite expensive, since OS individual computers are developed with a focus on shared access to general field network memory. 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 usually created for sharing computer resources or data (usually in 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 a network software, managing 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-type 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. Bus topology local network
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 are 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.
LAN classification
Local networks can be classified according to:
- management level;
- purpose;
- uniformity;
- administrative relations between computers;
- topology;
- architecture.
The following LANs are distinguished by management level: :
- Workgroup LANs, which consist of several PCs running the same operating system. In such a LAN, as a rule, there are several dedicated servers: a file server, a print server;
- LAN of structural units (departments). These LANs contain several dozen PCs and servers such as: file server, print server, database server;
- LAN of enterprises (firms). These LANs can contain over 100 computers and servers such as: file server, print server, database server, mail server and other servers.
Based on their purpose, networks are divided into :
- computer networks designed for computational work;
- information and computer networks, which are intended both for conducting settlement work and for providing information resources;
- information-advising, which, based on data processing, generate information to support decision-making;
- information-control networks, which are designed to manage objects based on information processing.
The types of computers used can be distinguished:
- homogeneous networks that contain the same type of computers and system software;
- heterogeneous networks that contain different types of computers and system software.
According to the administrative relationships between computers, we can distinguish:
- LAN with centralized management (with dedicated servers);
- LANs without centralized control (decentralized) or peer-to-peer (single-level) networks.
In local networks with centralized management, the server ensures interactions between workstations, performs the functions of storing public data, organizes access to this data and transmits the data to the client. The client processes the received data and provides the processing results to the user. It should be noted that data processing can also be carried out on the server.
Local networks with centralized management, in which the server is intended only for storing and issuing information to clients upon requests, are called networks with a dedicated file server. Systems in which the server, in addition to storing information, also processes information are called “client-server” systems.
It should be noted that in server local networks, only server resources are directly available to the client. But workstations that are part of a centrally managed LAN can simultaneously organize a peer-to-peer local network with all its capabilities among themselves.
Software, work manager A centrally controlled LAN consists of two parts:
- network operating system installed on the server;
- software on a workstation, which is a set of programs running under the operating system that is installed on the workstation. At the same time, different operating systems can be installed on different workstations on the same network.
In large hierarchical local networks, UNIX and LINUX are used as network operating systems, which are more reliable. For medium-sized local networks, the most popular network OS is Windows 2008 Server.
Depending on how the server is used in hierarchical networks, the following types of servers are distinguished:
- File server. In this case, the server contains shared files and/or shared programs.
- Database server. The server hosts a network database.
- Print server. A fairly powerful printer is connected to the computer, on which information can be printed from several workstations at once.
- Mail server . The server stores information sent and received both over a local network.
Advantages:
- higher data processing speed;
- has a reliable system for protecting information and ensuring secrecy;
- easier to manage compared to peer-to-peer networks.
Flaws:
- the network is more expensive due to a dedicated server;
- less flexible compared to a peer-to-peer network.
All computers on the local network are connected by communication lines. Geometric location of communication lines relative to network nodes and physical connection nodes to the network is called physical topology. Depending on the topology, networks are distinguished: bus, ring, star, hierarchical and arbitrary structures.
There are physical and logical topologies. Logical and physical network topologies are independent of each other. Physical topology is the geometry of the network, and logical topology determines the directions of data flows between network nodes and methods of data transmission.
All existing configurations can be divided into two main classes: broadcast and serial.
In the case of a broadcast LAN configuration, signals transmitted by one device connecting to the physical medium are perceived by all others. In a broadcast LAN, only one station can operate at any time. All workstations can communicate directly with any workstation on the network.
To build a broadcast configuration, it is necessary to use relatively powerful receivers and transmitters. Consequently, there is a need to limit the length of cable segments and the number of connections. If limits are exceeded, an analog amplifier or digital repeater is used. In addition, the means of connection to the physical medium are selected such that they do not cause significant signal attenuation.
The main types of broadcast topologies "bus", "tree" and "star" are shown in the diagrams (Figure 3).
Figure 3 - Types of broadcast topologies:
a) “tire”; b) “tree”; c) "star"
In the case of a sequential LAN configuration, each device connecting to the physical medium transmits information to only one device. At the same time, the requirements for transmitters and receivers are reduced, since all stations actively participate in the transmission.
The main types of sequential topologies: “ring”, “chain”, “snowflake” and “mesh” are shown in the diagram (Figure 4).
Figure 4 - Types of sequential topologies “ring”, “chain”, “snowflake” and “mesh”
Consider the following physical topologies:
- physical “bus” (bus);
- physical “star” (star);
- physical “ring” (ring);
Bus topology
- easy to connect a new PC;
- there is the possibility of centralized management;
- The network is resistant to failures of individual PCs and to interruptions in the connection of individual PCs.
Disadvantages of star topology networks:
- hub failure affects the operation of the entire network;
- high cable consumption;
Ring topology
In a network with a ring topology, all nodes are connected by communication channels into a continuous ring (not necessarily a circle) through which data is transmitted. The output of one PC is connected to the input of another PC. Having started the movement from one point, the data ultimately ends up at its beginning. Data in a ring always moves in the same direction.
The receiving workstation recognizes and receives only the message addressed to it. A network with a physical ring topology uses token access, which grants a station the right to use the ring in a specific order. The logical topology of this network is a logical ring.
This network is very easy to create and configure. The main disadvantage of ring topology networks is that damage to the communication line in one place or PC failure leads to the inoperability of the entire network.
As a rule, the “ring” topology is not used in its pure form due to its unreliability, so in practice various modifications of the ring topology are used.
In general, the IT infrastructure of various enterprises can be distinguished by:
- scale;
- composition of components;
- equipment level, etc.
Based on this, certain types of IT infrastructures can be presented in the form of basic configurations, which are shown in Figures 5, 6 and 7.
Figure 5 - Small local network.
Small local network. Typically consists of 1-3 servers, network switches, 5-30 workstations.
Figure 6 - Local network and telephone network with MiniPBX.
Local network and telephone network with MiniPBX. Includes all the components of a “small local network” with the addition of an internal MiniPBX for switching phones within the office
Figure 7 - Local network and digital telephone network at several sites.
Local network and digital telephone network at several sites, combined into a virtual private network. The organization's local network is used for IP telephony. It is possible to combine digital telephone networks divisions of the organization via the Internet using virtual private networks.
The level of training of specialists servicing the IT infrastructure of enterprises must be very high, requiring responsibility for work on which the functioning and security of corporate computer networks will depend.
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.
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 the order of 1 Gbit /With).
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 in both 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 (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:
Networks of scientific institutions,
University Networks;
According to the form of functioning can be distinguished:
Commercial 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 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.
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.
Work station – 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.), providing the user with all necessary tools for solving applied problems. Workstations connected to the server are called clients. Can be used as clients powerful computers for resource-intensive spreadsheet processing, 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 of 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 a computer does not share its disk space or its 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-type 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 mesh 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 service 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 further development rooted tree structures with branches 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 using central base data. 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 can also include significant cable consumption (sometimes several times higher than the consumption in LANs with a common bus or hierarchical ones with similar capabilities).
Networks can also have a mixed topology ( hybrid) when individual parts of the network have different topologies. An example would be 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.
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 - Email, World Wide Web, teleconferencing, video conferencing, 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. The 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 that it is more high level 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 to allocate a 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 (on 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. Configuration 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 has two and only two branches attached to it. 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 ensure compatibility information support(programs and data) by 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 top level to the lower one, then transport and, finally, reverse playback 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 network level each packet must be given an address to which it should be delivered independently 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 LAN 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 telephone exchanges and so on.
On the computer of the recipient of the information, reverse process converting data from bit signals to document.
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, 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 exchange data when operating on 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 the operating system to 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 via network channels communication, it is necessary to establish a protocol for exchanging messages (packets). 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 base protocols global network Internet.
network hardware
The main components of the network are workstations, servers, transmission media (cables) And network hardware.
Workstations are called network computers on which network users implement applied tasks.
Network servers- these are hardware and software systems that perform distribution management functions network resources public access. 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 isolated 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 and 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) are the central devices of a cable system or a star physical topology network, 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. 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 the maximum network size.
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 level and allow packets to be forwarded and routed from one network to another. It allows, for example, large messages to be split into smaller portions, thereby ensuring the interaction of local networks with different sizes package. The router can forward packets to a specific address (bridges can only filter out unnecessary packets), select the best way to pass the package.
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.
Depending on the scalability of the network, it will depend on how to this enterprise, network management will occur. There are several control methods. Local computer networks are divided into two subgroups based on the management method: peer-to-peer and hierarchical (multi-level) networks.
Peer-to-peer networks
In a peer-to-peer network, all computers have equal rights: there is no hierarchy among computers and there is no dedicated server. Typically, each computer functions as both a client and a server; in other words, there is no single computer responsible for administering the entire network. All users independently decide what data on their computer to make publicly available over the network.
Peer-to-peer networks are also called workgroups. A work group is a small team, so peer-to-peer networks most often have no more than 30 computers. Peer-to-peer networks are relatively simple.
Because each computer is both a client and a server, there is no need for a powerful central server or other components required for more complex networks.
Peer-to-peer networks are usually cheaper than server-based networks, but require more powerful (and more expensive) computers. In a peer-to-peer network, the performance and security requirements for network software are generally lower than in networks with a dedicated server.
Figure 5. Peer-to-peer network
Hierarchical networks
Hierarchical networks have one or more servers that store information shared by different users. In order to increase the reliability of information storage, two disks operating in parallel and duplicating each other can be installed on the server,
Moreover, if one of them fails, the other is automatically switched on. Depending on how the server is used in hierarchical networks, the following types of servers are distinguished:
File server. In this case, the server contains shared files and/or shared programs. One example of using a file server is hosting MS Office programs on it. In this case, only a small (client) part of these programs is located on workstations, requiring insignificant resources. Programs that allow this mode of operation are called programs with the ability to be installed on a network.
Database server. In this case, the server hosts a database (for example, Consultant Plus, Garant, Bank Client Accounts, etc.). The database on the server can be replenished from various workstations and/or provide information upon requests from the workstation.
Clients of the Hierarchical Network can use operating systems: Windows XP, Windows Vista 7, servers require special server versions of operating systems.
Figure 6. Hierarchical network
In our service center a hierarchical network will be used. For our case this is the most suitable option. To prevent our network from turning into an information dump, and also to increase the reliability of information storage, it is necessary to have several servers. In this case, a file server, an Internet server and a database server. The server will host MS Office, 1C and other programs, and the workstations will contain only a small (client) part of these programs, requiring little resources. It is also necessary for each user to allocate his rights on the local network.
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