3.6. COMPUTER NETWORKS

The concept of network computers practically dating the advent of minicomputers and microcomputers and the need for their interconnection. Sixties of the twentieth century was dominated by the large computers (mainframes), for which users were tied 'dumb' terminal, a device that had a monitor to display the image that he was sent by the keyboard and the computer that sent data arising mainly as a result of fulfilling some forms presented to the user, and a connection with him realized via telecommunication lines and modems on the principle of discrete time scheduling terminal access-user computer (time sharing).

Such a concept has been slow and seventies approach to using mini computers, computers smaller than 'mainframe', which led to a shorter special cables inside the building were connected terminals. So, the same principle but without telecommunication lines and modems, and therefore much faster and safer communication. So the companies have separate departments mini-computers that are interconnected been separated and each served its user group and enabled sharing (share) resources shared printers, plotters and the like. But the exchange of data between themselves mini-computers have become a problem, and it started making the client software to connect them.

The real expansion of the development and use of network begins appearance PC computers (IBM). Mini computers have not had such a rich program support is what is offered for the PC, and soon the presence of PC in the office is becoming commonplace. Results of the transferred between the PC and diskettes and soon appeared serious problem of MAINTAINING the INTEGRITY of data on a multitude of dispersed PC computers. LAN (Local Area Network) solves this problem by offering interconnectedness among network participants and SHARING of common software and hardware resources, mostly on the level within one institution or company. More connected network composed of an institution or company (e.g. electric power) constitute a widespread group of participants, called the WAN (Wide Area Network), a network of state and interstate character, to which nodes are linked to the previous example CARNet (Croatian Academic and Research Network) . All together the interconnected worldwide constitute INTERNET (INTERnational NETwork), a global system of computer connectivity through computer networks and providing various services.

In support of the program operating system of personal computers are often incorporated into and support the work of the network (Windows 9X, Windows XP, Mac OS X ...). PC operating systems do not necessarily require a network connection to the server and the individual work, but only if you want special services like web or e-mail. Indeed in the P2P (Peer-to-Peer) communication PCs communicate directly through the network without the mediation server.

Maintaining data integrity, their exchange, resource sharing and mutual communication (e-mail) are the basic features of the network. Resources that are commonly shared by the disk space (folders) and printers. Slightly better printers are designed as network devices, which simplifies the process of access, but also creates new security issues regarding the abuse.

A computer network is a collection of interconnected individual systems using a common telecommunication subsystems, according to the scheme in the following Figure:

Figure 3.6.1 Principle of operation of computer network.

Computers and terminals represent one group - subsystem, and communication between them allows telecommunications subsystem according to exact rules, called PROTOCOLS.

Limit connection between the computer and telecommunications subsystem subsystem called INTERFACES, connections marked lines like ' ——I ' in Figure 3.6.1. Therefore, protocols define the interaction between the subsystems, and the interface is possible. Both subsystems together form a computer network.

The network aims to:

shared use of software implementedn as commo applications for all users or applications that distributed processing and the results combined integrating access to network resources (servers, NAS, printers, plotters ...) access to common databases electronic communications and mail (send data and mail) management and administration of data connect different platforms and operatig systems (DOS, Windows, UNIX ...) increase the efficiency of operations acquiring new knowledge

Concentrated architecture

The simplest configuration (architecture) computer network is a HOST architecture, when one powerful computer (mainframe) 'attacking' users through terminals. The terminal is a device consisting of a monitor and keyboard without their own systems (resources) of data storage and printing support.

Figure 3.6.2 Host computer network architecture.

Properties of this architecture are:

In the event of a host, no user can no longer work or communicate. In essence, this is not network communication but they are realized when the two or more host's bound together, but nothing does not change significantly over the user-computer. PC with software designed for this purpose, can function as a terminal. Typical such a connection when the PC is connected to the B.B.S. information and communication system, which still allows the transfer of data (copying).

Distributed architecture

The opposite of the host computer network architecture is a CLIENT-SERVER architecture based on connecting multiple smaller computers with a server which gives the user help in their work, but the user is working on his computer (workstation) most of the time disconnected from the server. One of the first widely accepted concept of an operating system on Windows NT Server and Windows 95 on a user's computer, WORKING STATIONS associated with the server.

Figure 3.6.3 Client-server architecture of computer network.

The scheme is similar to the previous but has a completely different way of functioning. Workstation with its own software, either from its disk, or downloaded from the server in memory, independently handles data processing and returns them to the server where it is stored and available to all network users (file server). Individual PC peripherals (like a printer or CD-ROM) can be declared as a common-divisible (SHARE) network resources. Monitor print jobs to the same server or a separate server to monitor all printing stations at the printers as a laser printer, a color printer or plotter (print server), which are connected to it, or whether the printer is one of the stations serve the entire network as part of the network resources under the control of the print server. The server can support the execution of applications (application server), or to assume the tasks of processing databases (database server). All listed server activity may or may not performed an one computer.

If shutting down the server, workstation, if it has its own permanent memory resources (disk) alone will still work, but can not use shared network resources.

Properties of a client-server architecture are:

distributed computing power and users software and data cheaper network resources possible independent user experience disburdening computer server and of interactive work possible to use multiple servers with separate tasks shared distibuted peripherals and distribution access peripherals by the server greater overall flexibility and upgrade of system if necessary

Very important feature of today's network of software and connectivity stations with different operating systems for the server, which in this context includes adequate program support, and the possibility that the inter-relationship between the server installed with your host computer terminals and allows teachers to work through the host terminal associated with it, the connection to other host servers as well as assistance serving smaller groups. Such a system offers various possibilities of use. The most important representative of a client-server network OS Novell NetWare and Windows NT family of operating systems. Increasingly, the role of the server in the network take over computers with Linux operating system, and modern technology allows you to connect several of them in one common entity, which are very powerful computational resources (CLUSTER - a cluster of computers) - a super-computer.

A possible way of using a client-server network architecture is to connect computers in a peer-to-peer network (same-to-same) in which all computers are equal and each computer can be a server and workstation, one equal configurations based on the extension of OS computers OS network, which can not satisfy the complicated demands respect. It is suitable for several computers in a network which is not much traffic. OS known network of this kind was LANTASTIC that enabled client-server and P2P configuration. Configurations with different types of OS on computers basically not supported on this type of network. OS/2, Windows 9X, Windows XP, Mac OS and Linux operating systems include support for this type of networking. Therefore, the dedicated type of this network systems have become history.

It seems that history repeats itself in some way. Instead of 'mainframe' computers are now available large computer centers of power (DATACENTER) in which computers are interconnected with regard to association of processor, memory and similar resources and hardware device that enables such VIRTUAL MACHINES as a way of utilization of these resources. Architecture is basically on a client-server concept, but it is a very powerful server / servers. And if such a set of computers connected server (CLUSTER), the user is connected through the terminal of software in order to use in terms of programming offered resources concept resembles the 'mainframe'.