At physical connection of two or more computers the computer network is formed. Generally, for creation of computer networks the special hardware – the network equipment and the special software – network software is necessary.
Set of the computers connected through communication channels and the peripherals providing users with means of exchanging with information and collective use of resources of a network is called as a computer network: hardware, program and information.
Main components of a network:
• Servers – the computers being sources of resources of a network.
• Local computers users (workstations) – the computers connected to a network, through which users get access to its resources.
• Communication channels – the lines connecting computers among themselves.
Record and information transfer on communication channels has the following advantages:
1. Simplifies process of formation and information control
2. The principle of single registration of information in the primary document and the machine carrier is observed
3. High reliability of information is provided
The remote data transmission based on use of communication channels represents data transmission in the form of electric signals, which can be continuous in time and discrete, i.e. have continuous character in time. Cable and telephone communication channels are most widely used. The electric signals transferred on a cable communication channel, are discrete, and by phone – continuous.
Depending on the directions on which information is sent, distinguish communication channels:
• Simplex (transfer goes only in one direction)
• Half-duplex (during every moment of time it is made either transfer, or information reception)
• Duplex (transfer and reception of information are carried out at the same time in two directions)
Purpose of all types of computer networks is defined by two functions:
1. Ensuring sharing of equipment rooms and program resources of a network;
2. Ensuring joint access to resources of data.
The protocol is a set of rules (agreements) according to which there is an information transfer through a computer network.
Network topology. The topology of a network represents the logic scheme (configuration) of connections of computers (knots) of a network by means of communication channels. In local networks one of the following topology is, as a rule, used: tire, ring or star-shaped. Other topology is derivative of the listed topology.
The network of tire topology uses one communication channel (tire) uniting all computers of a network. The data transferred by one knot of a network, arrive in all knots, but only one of them for which these data are intended, distinguishes and accepts them.
The network of ring topology has the closed ring as a communication channel. Such topology is used, in particular, by the Token Ring networks. The message is consistently transferred on a ring from knot to knot in one direction. Need of relaying of data leads to decrease in reliability as malfunction in any of knots of a network does it disabled.
The network of star-shaped topology has the central hub (CH) – the file server uniting all computers. Operability of a network completely depends on its reliability. All data circulating between computers pass through CH on separate communication channels. A characteristic example of this topology – Arcnet.
On territorial prevalence of a network can be local, global, and regional.
Local network (LAN – Local Area Network) – a network within the enterprise, the establishments, one organization.
Regional network (MAN – Metropolitan Area Network) – a network in city boundaries or area.
Global network (WAN – Wide Area Network) – a network in the territory of the state or group of the states.
Classification of networks:
1. Till speed of information transfer computer networks share on: low-high-speed networks – to 10 Mbps; medium-rate networks – to 100 Mbps; high-speed networks – over 100 Mbps.
2. As the environment of transfer of a network are divided on: wire (on a coaxial cable, on twisted pair, fiber-optical); wireless with information transfer on radio channels or in an infra-red range.
3. On a way of the organization of interaction of computers of a network divide on peer and with the allocated server (hierarchical networks).
Network security – differentiation of powers of access to files and resources of the computer network.
Network safety is provided with an assignment way to each user having access to a network, an identification code and the password checked at an entrance in a network or at access to resources. Network managers are engaged in control of net work safety.
Classification of viruses
Viruses can be divided into classes on the following signs:
• On virus habitat (network, file, loading)
• On a way of infection of habitat (resident, non-resident)
• By destructive possibilities (harmless, harmless, dangerous, very dangerous).
• On features of algorithm of a virus («partner viruses», «viruses worms», parasitic», «student's», a stealth viruses, polymorph-viruses, macro viruses)
Types of antiviruses: detectors (find concrete viruses on a signature and signal about their presence), auditors (find presence of viruses, periodically comparing current state of programs, catalogs and system areas to an initial condition), filters (small resident programs which reveal suspicious actions at operation of the computer), doctors (reveal and delete viruses from random access memory and treat the infected files on disks), vaccines and immunization (there silent programs preventing in fiction of files).
Web as architecture example «client server»
The client is any Internet the browser that is the browser the Internet of pages. Our home or office computers are client computers.
The server is a program which costs on the remote computers servers which have been constantly connected to the Internet. On these computers servers’ sites and all other multimedia contents, such as music and cinema and so on are stored a web.
The architecture the client – the server is an interaction between the client and the server. The scheme is simple:
1. The user in the client browser enters the server address, and pressing Enter gives team to the browser on connection with the Internet and sending to the server of the client standardized inquiry.
2. The server accepts inquiry and after its interpretation finds the necessary file and sends it in the form of information blocks. If the isn't, the server sends the standardized error message.
3. The client that is «our» browser receives the server answer and after its interpretation deduces it on the monitor, or gives out an error message then breaks off connection.
The client makes sessions of single-user connection, unlike the server which works with huge mass of inquiries at once.
It is necessary to distinguish technology the client server in a broad sense which can be used in any computer systems from actually architecture the client server with reference to information appendices in general and to automate control systems for the enterprise especially.
According to the on-line dictionary of computer terms, the client server is a type of the distributed system in which there is a server which is carrying out inquiries of the client, and the server and the client communicate among themselves with use of this or that protocol.
The client is understood as the program using resources, and as the server (on English – the servant) the program serving inquiries of clients on receiving resources of a certain look. So wide definition includes practically any program technology in which participates more than one program functions between which are distributed asymmetrically. Respectively, speak about KS technology used with operating systems, local and global networks etc.
Such wide definition gives rise to some confusion. So, the file - the server system too uses technology the client server, however from the point of view of architecture of applied programs is important what sort the server provides resources to clients.
The concept of architecture the client server in enterprise management systems is connected with division of any applied program into three main components or a layer. These three components are:
• Component of representation (visualization) of data;