Purpose and a brief description of main components of computer networks.

Computer network called a set of interconnected and distributed computers over a certain territory.

computer network– a computing complex that includes a geographically distributed system of computers and their terminals combined into a single system.

According to the degree of geographical distribution, computer networks are divided into local, city, corporate, global, etc.

The computer network consists of three components:

Data transmission networks, including data transmission channels and switching facilities;

Computers, networked data transmission;

Network software.

computer network- this is a complex complex interconnected software and hardware components:

computers(host computers, network computers, workstations, servers) located in network nodes;

network operating system and application software, managing computers;

communication equipment– equipment and data transmission channels with accompanying peripheral devices; interface boards and devices (network cards, modems); routers and switching devices.

Software and hardware components of a computer network

Computer network- a spatially distributed system of software and hardware components connected by computer communication lines.

Among the hardware Computers and communications equipment can be distinguished. Software components consist of operating systems and network applications.

Currently, computers are used on the network various types and classes with different characteristics. This is the basis of any computer network. Computers and their characteristics determine the capabilities of a computer network. But in Lately and communications equipment (cable systems, repeaters, bridges, routers, etc.) began to play an equally important role. Some of these devices, given their complexity, cost and other characteristics, can be called computers that solve very specific tasks to ensure the operability of networks.

For efficient work networks are used special network operating systems (network OS), which, unlike personal operating systems, are designed to solve special problems of managing the operation of a network of computers. Network operating systems are installed on specially dedicated computers.

Network Applications- These are application software systems that expand the capabilities of network operating systems. Among them we can highlight mailers, group work systems, network databases, etc.

As network operating systems evolve, some network application functions become regular operating system functions.

All devices connected to the network can be divided into three functional groups:

1) workstations;

2) network servers;

3) communication nodes.

1) Workstation A workstation is a personal computer connected to a network on which a network user performs his or her work. Each workstation processes its own local files and uses its own operating system. But at the same time, network resources are available to the user.

There are three types of workstations:

Workstation with local disk,

Diskless workstation,

Remote workstation.

On a workstation with a disk (hard or floppy) operating system loaded from this local disk. For a diskless station, the operating system is loaded from the file server disk. This capability is provided by a special chip installed on the network adapter of the diskless station.

A remote workstation is a station that connects to a local network via telecommunications channels (for example, using a telephone network).

2) Network server, a network server is a computer connected to a network and providing network users with certain services, such as data storage common use, printing jobs, processing a request to a DBMS, remote processing of jobs, etc.

Based on the functions performed, we can distinguish the following groups servers.

File server, file server - a computer that stores data of network users and provides user access to this data. Typically, this computer has a large amount of disk space. The file server allows users to access shared data simultaneously.

The file server performs the following functions:

Data storage;

Data archiving;

Data transfer.

Database server - a computer that performs the functions of storing, processing and managing database files (DB).

The database server performs the following functions:

Storing databases, maintaining their integrity, completeness, and relevance;

Receiving and processing requests to databases, as well as sending processing results to the workstation;

Coordination of data changes made by different users;

Support distributed databases data, interaction with other database servers located in another location.

Application server, application server - a computer that is used to run user applications.

Communication server, communications server - a device or computer that provides local network users with transparent access to their serial ports input/output.

Using a communications server, you can create a shared modem by connecting it to one of the server's ports. The user, having connected to the communication server, can work with such a modem in the same way as if the modem was connected directly to the workstation.

An access server is a dedicated computer that allows remote processing of tasks. Programs initiated from a remote workstation are executed on this server.

Commands entered by the user from the keyboard are received from the remote workstation, and the results of the task are returned.

Fax server, fax server - a device or computer that sends and receives fax messages to local network users.

Server Reserve copy data, backup server - a device or computer that solves the problem of creating, storing and restoring copies of data located on file servers and workstations. One of the network file servers can be used as such a server.

It should be noted that all of the listed types of servers can operate on one computer dedicated for these purposes.

3) The communication nodes of the network include the following devices:

Repeaters;

Switches (bridges);

Routers;

The length of the network and the distance between stations are primarily determined by the physical characteristics of the transmission medium (coaxial cable, twisted pair, etc.). When transmitting data in any environment, signal attenuation occurs, which leads to distance limitations. To overcome this limitation and expand the network, special devices are installed - repeaters, bridges and switches. The part of the network that does not include the expansion device is usually called a network segment.

Repeater, repeater - a device that amplifies or regenerates a signal received by it. The repeater, having received a packet from one segment, transmits it to all others. In this case, the repeater does not decouple the segments attached to it. At any given time, in all segments connected by the repeater, data exchange is supported only between two stations.

Switch, switch, bridge, bridge is a device that, like a repeater, allows you to combine several segments. Unlike a repeater, a bridge decouples the segments connected to it, that is, it simultaneously supports several data exchange processes for each pair of stations of different segments.

Router- a device connecting networks of one or different types using one data exchange protocol. The router analyzes the destination address and routes the data along the optimal route.

Gateway is a device that allows you to organize data exchange between different network objects using different data exchange protocols.

The main hardware components of the network are the following:

1. Subscriber systems: computers (workstations or clients and servers); printers; scanners, etc.

2. Network hardware: network adapters; concentrators (hubs); bridges; routers, etc.

3. Communication channels: cables; connectors; devices for transmitting and receiving data in wireless technologies.

The main software components of the network are the following:

1. Network operating systems, where the most famous of them are: MS Windows; LANtastic; NetWare; Unix; Linux, etc.

2. Network software(Network Services): network client; LAN card; protocol; service remote access.

LAN (Local computer network) is a collection of computers, communication channels, network adapters running a network operating system and network software.

In a LAN, each PC is called a workstation, with the exception of one or more computers that are designed to serve as servers. Each workstation and server have network cards (adapters) that are connected to each other through physical channels. In addition to the local operating system, each workstation runs network software that allows the station to communicate with the file server.

The computers included in the LAN client-server architecture are divided into two types: workstations, or clients, intended for users, and servers, which, as a rule, are inaccessible to ordinary users and are designed to manage network resources.

Workstations

A workstation is subscriber system, specialized to solve certain tasks and using network resources. The workstation network software includes the following services:

Client for networks;

File and Printer Access Service;

Network protocols for of this type networks;

Network card;

Remote access controller.

The workstation is different from a regular standalone personal computer as follows:

Availability network card(network adapter) and communication channel;

Additional messages appear on the screen while the OS is loading, informing you that the network operating system is loading;

Before you begin, you must provide your network software with a username and password. This is called the network logon procedure;

After connecting to the LAN, additional network disk drives appear;

it becomes possible to use network equipment, which may be located far from the workplace.

Network adapters

To connect a PC to a network, you need an interface device called a network adapter, interface, module, or card. It fits into the slot motherboard. Network adapter cards are installed on each workstation and on the file server. The workstation sends a request through the network adapter to the file server and receives a response through the network adapter when the file server is ready.

Network adapters, together with network software, are able to recognize and handle errors that may occur due to electrical interference, collisions, or poor hardware performance.

Different types of network adapters differ not only in their methods of accessing the communication channel and protocols, but also in the following parameters:

Transmission speed;

Packet buffer size;

Tire type;

Bus performance;

Compatible with various microprocessors;

Using direct memory access (DMA);

Addressing I/O ports and interrupt requests;

connector design.

The combination of the components discussed above into a network can be done different 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);

A local area network (LAN) is a communications system that supports one or more high-speed transmission channels within a building or some other limited area digital information, provided to connected devices for short-term exclusive use. The areas covered by the drug may vary significantly.
The length of communication lines for some networks can be no more than 1000 m, while other networks are able to serve an entire city. The serviced areas can be factories, ships, airplanes, as well as institutions, universities, and colleges. As a rule, coaxial cables are used as a transmission medium, although networks on twisted pair and optical fiber are becoming increasingly widespread, and recently the technology of wireless local networks has also been rapidly developing, which uses one of three types of radiation: broadband radio signals, low-power radiation ultrahigh frequencies (microwave radiation) and infrared rays.
Short distances between network nodes, the transmission medium used and the associated low probability of errors in the transmitted data make it possible to maintain high exchange rates - from 1 Mbit/s to 100 Mbit/s (at present there are already industrial designs of LANs with speeds of the order of 1 Gbit/s ).

City networks, as a rule, cover a group of buildings and are implemented on fiber optic or broadband cables. According to their characteristics, they are intermediate between local and global networks. Recently, in connection with the laying of high-speed and reliable fiber optic cables in urban and intercity areas, and new promising network protocols, for example, ATM (Asynchronous Transfer Mode), which in the future can be used 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 wired channels are used as a transmission medium in global networks, as well as satellite channels communications (usually for communication between continents). Limitations on transmission speed (up to 28.8 Kbps on analog channels and up to 64 Kbps 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. For example:

According to the scope of operation, networks can be divided into banking networks, networks of scientific institutions, university networks;

Based on the form of operation, we can distinguish commercial networks and free networks, corporate and public networks;

Based on the nature of the implemented functions, networks are divided into computational ones, designed to solve control problems based on computational processing of initial information; informational, intended to obtain reference data at the request of users; mixed, in which computing and information functions are implemented;

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 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, under centralized control, tasks that have the highest priority and, as a rule, are associated with processing large volumes of information, are solved;

According to software compatibility, networks can be homogeneous or homogeneous (consisting of software-compatible computers) and heterogeneous or heterogeneous (if the computers included in the network are software incompatible).

. Specify the main purpose of a computer network

2016-02-17

Specify the main purpose of a computer network

Computer networks. Lecture notes

1. Basic software and hardware components of the network. The concepts of “client”, “server”, “network service”.

Computer network is a complex set of interconnected and coordinated software and hardware components.

The main purpose of a computer network is:

Information sharing;

sharing of equipment and software;

centralized administration and maintenance.

Main components of a computer network:

Computers (hardware layer);

Communication equipment;

Network operating systems;

Network applications.

The entire network hardware and software complex can be described multilayer model. At the core any network lies hardware layer standardized computer platforms. The second layer is communication equipment. Although computers are central to data processing in networks, communications devices have recently begun to play an equally important role. Cabling systems, repeaters, bridges, switches, routers, and modular hubs have gone from being ancillary network components to becoming essential components, along with computers and system software, in both their impact on network performance and cost.

Third layer, forming the software platform of the network, are operating systems (OS). The efficiency of the entire network depends on which concepts for managing local and distributed resources form the basis of the network OS.

The topmost layer are different network applications such as network databases, postal systems, data archiving tools, teamwork automation systems, etc.

The network application is distributed program, i.e. a program that consists of several interacting parts, each of which runs on a separate computer on the network.

Server program– a special program designed to service requests for access to the resources of a given computer from other computers on the network. The server module is constantly in standby mode for requests coming over the network.

Client program- a special program designed to compose and send requests for access to remote resources, as well as receive and display information on the user’s computer.

Network Service- a pair of client-server modules that provide shared user access to a certain type of resource. Typically, a network operating system supports several types of network services for its users - file service, print service, service Email, remote access service, etc. (Examples of network services - WWW, FTP, UseNet).

The terms “client” and “server” are used not only to refer to software modules, but also to computers connected to the network. If a computer provides its resources to other computers on the network, then it is called a server, and if it consumes them, it is called a client. Sometimes the same computer can simultaneously play the roles of both server and client.

2. Classification of computer networks.

When classifying networks by territorial basis, they distinguish between local (LAN), global (WAN) and metropolitan (MAN) networks.

LAN - concentrated in an area of ​​no more than 1-2 km; built using expensive high-quality communication lines, which allow, using simple data transmission methods, to achieve high data exchange rates of the order of 100 Mbit/s. The services provided are varied and usually involve on-line implementation.

WAN - connects computers dispersed over hundreds and thousands of kilometers. Often existing, low-quality communication lines are used. Lower data transfer rates than in local networks (tens of kilobits per second) limit the range of services provided to file transfer, mainly not online, but background, using email. For stable transmission of discrete data, more complex methods and equipment are used than in local networks.

MAN - occupy an intermediate position between local and global networks. With sufficiently large distances between nodes (tens of kilometers), they have high-quality communication lines and high exchange rates, sometimes even higher than in classical local networks. As in the case of local networks, when building a MAN, existing communication lines are not used, but are laid anew.

Depending on the scale of the production unit within which the network operates, departmental networks, campus networks and corporate networks are distinguished.

Departmental networks are used by a small group of employees primarily for the purpose of sharing expensive peripherals, applications, and data; have one or two file servers and no more than thirty users; usually not divided into subnets; are created on the basis of any one network technology; can work on the basis of peer-to-peer network operating systems.

Campus networks combine networks of departments within a single building or a single area of ​​several square kilometers, without using global connections. At the campus network level, challenges arise in integrating and managing heterogeneous hardware and software.

Corporate networks connect a large number of computers in all areas of an individual enterprise. A corporate network is characterized by:

o scale - thousands of user computers, hundreds of servers, huge volumes of data stored and transmitted over communication lines, many different applications;

o high degree of heterogeneity - types of computers, communications equipment, operating systems and applications are different;

o use of global connections - branch networks are connected using telecommunications means, including telephone channels, radio channels, and satellite communications.

3. Main characteristics of modern computer networks.

The quality of network operation is characterized by the following properties: performance, reliability, compatibility, manageability, security, extensibility and scalability.

There are two main approaches to ensuring network quality. The first is that the network guarantees the user compliance with a certain numerical value of the quality of service indicator. For example, frame relay and ATM networks can guarantee the user a given level of throughput. In the second approach (best effort), the network tries to serve the user as efficiently as possible, but does not guarantee anything.

The main characteristics of network performance include: response time, which is defined as the time between the occurrence of a request for a network service and the receipt of a response to it; throughput, which reflects the amount of data transmitted by the network per unit time, and transmission delay, which is equal to the interval between the moment a packet arrives at the input of any network device and the moment of its appearance at the output of this device.

Various characteristics are used to assess the reliability of networks, including: availability factor, which means the proportion of time during which the system can be used; security, that is, the ability of the system to protect data from unauthorized access; fault tolerance - the ability of a system to operate under conditions of failure of some of its elements.

Extensibility means the ability to relatively easily add individual network elements (users, computers, applications, services), increase the length of network segments and replace existing equipment with more powerful ones.

Scalability means that the network allows you to increase the number of nodes and the length of connections within a very wide range, while the network performance does not deteriorate.

Transparency is the ability of a network to hide details of its internal structure from the user, thereby simplifying his work on the network.

Network manageability implies the ability to centrally monitor the status of the main elements of the network, identify and resolve problems that arise during network operation, perform performance analysis and plan network development.

Interoperability means that the network can accommodate a wide variety of software and hardware.

4. The concept of “topology”. Physical and logical topology of the CS. Basic CS topologies.

Topology – configuration of physical connections between network nodes. Network characteristics depend on the type of topology installed. In particular, the choice of a particular topology affects:

The composition of the necessary network equipment;

Network equipment capabilities;

Possibility of network expansion;

Network management method.

The term “CS topology” can mean physical topology (configuration of physical connections) or logical topology– signal transmission routes between network nodes. The physical and logical topologies of the CS may be the same or different. Local networks are built on the basis three basic topologies, known as:

common bus (bus);

star

ring.

In topology common bus one cable is used to which all computers on the network are connected. It is easy to connect new nodes to such a network.

Only one computer can transmit at a time. Data is transmitted to all computers on the network; however, only the computer whose address matches the recipient's address receives the information.

The bus is a passive topology. This means that computers only “listen” to data transmitted over the network, but do not move it from sender to recipient. Therefore, if any computer fails, it will not affect the operation of the network.

To prevent electrical signals from being reflected, terminators are installed at each end of the cable to absorb these signals. If a cable breaks, one of its ends is disconnected, or there is no terminator, the entire network fails (“falls”).

With topology "star" All computers are connected using cable segments to a central component - a hub. Signals from the transmitting computer travel through the hub to everyone else.

In networks with a star topology, connecting computers to the network and managing the network are performed centrally. But there are also disadvantages: since all computers are connected to a central point, cable consumption increases significantly for large networks, the cost of the network (plus hub) is higher, the number of plug-in modules is limited by the number of hub ports. Additionally, if a central component fails, the entire network will shut down. If only one computer (or the cable connecting it to the hub) fails, then only this computer will not be able to transmit or receive data over the network. This failure will not affect other computers on the network. With topology "ring" computers are connected to a cable closed in a ring. Signals are transmitted along the ring in one direction and pass through each computer. Unlike a passive bus topology, each computer acts as a repeater, amplifying the signals and passing them on to the next computer. Therefore, if one computer fails, the entire network stops functioning. Consequently, it is difficult to isolate problems, and changing the configuration requires shutting down the entire network. Equipment for networks with ring topology is more expensive.

The advantages include: network stability to overloads (no collisions, no central node) and the ability to cover a large area. Additionally, the number of users does not have a big impact on network performance.

The configuration of physical connections is determined by the electrical connections between computers and may differ from the configuration of logical connections between network nodes. Logical connections are data transmission routes between network nodes.

Typical physical link topologies are: mesh, mesh, bus, ring, and star topologies.

A fully connected topology (Fig. 1.10, a) corresponds to a network in which each computer on the network is connected to all the others.

A mesh topology (mesh) is obtained from a fully connected one by removing some possible connections (Fig. 1.10, b). In a network with a mesh topology, only those computers between which intensive data exchange occurs are directly connected, and for data exchange between computers that are not directly connected, transit transmissions through intermediate nodes are used. The mesh topology allows the connection of a large number of computers and is typically characteristic of global networks.

In networks with a ring configuration (Fig. 1.10, e), data is transmitted along the ring from one computer to another, usually in one direction. If the computer recognizes the data as “its own,” then it copies it to its internal buffer. 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.

5. Principles of naming and addressing in computer networks.

One of the problems that must be taken into account when connecting three or more computers is the problem of addressing them. Several requirements can be placed on the network node address and its destination scheme.

The address must uniquely identify a computer on a network of any size.

The address assignment scheme should minimize the manual labor of the administrator and the likelihood of duplicate addresses.

The address must have a hierarchical structure, convenient for building large networks. This problem is well illustrated by international postal addresses, which allow the postal service organizing the delivery of letters between countries to use only the name of the recipient’s country and not take into account the name of his city, much less the street. IN large networks, consisting of many thousands of nodes, the lack of address hierarchy can lead to large overheads - end nodes and communication equipment will have to operate with address tables consisting of thousands of entries.

The address must be convenient for network users, which means that it must have a symbolic representation, for example, Servers or www.cisco.com.

The address should be as compact as possible so as not to overload the memory of communication equipment - network adapters, routers, etc.

Hardware addresses. These addresses are intended for a small to medium sized network, so they do not have a hierarchical structure. A typical representative of this type of address is the address of a local network adapter. This address is usually used only by equipment, so they try to make it as compact as possible and write it as a binary or hexadecimal value, for example 0081005e24a8. When setting hardware addresses, manual work is usually not required, since they are either built into the equipment by the manufacturer or are generated automatically with each new start of the equipment, and the uniqueness of the address within the network is ensured by the equipment.

Symbolic addresses or names. These addresses are intended to be remembered by people and therefore usually carry a semantic load. Symbolic addresses are easy to use in both small and large networks.

Numeric compound addresses. Symbolic names are convenient for humans, but due to their variable format and potentially large length, they are not very economical to transmit over a network. Therefore, in many cases, to work in large networks, numeric composite addresses of fixed and compact formats are used as node addresses. Typical representatives of this type of address are IP and IPX addresses.

The problem of establishing correspondence between addresses of different types, which is dealt with by the name resolution service, can be solved either by completely centralized or distributed means. In the case of a centralized approach, one computer (name server) is allocated on the network, which stores a table of how names of different types correspond to each other, for example, symbolic names and numeric numbers. All other computers contact the name server to use the symbolic name to find the numeric number of the computer with which they need to exchange data.

In another, distributed approach, each computer itself solves the problem of establishing correspondence between names. For example, if the user specified a numeric number for the destination node, then before the data transfer begins, the sending computer sends a message to all computers on the network (this message is called a broadcast message) asking them to identify this numeric name. All computers, having received this message, compare the given number with their own. The computer that has a match sends a response containing its hardware address, after which it becomes possible to send messages over the local network.

The distributed approach is good because it does not involve allocation special computer, which also often requires manually setting up a name matching table. The disadvantage of the distributed approach is the need for broadcast messages - such messages overload the network, since they require mandatory processing by all nodes, and not just the destination node. Therefore, the distributed approach is used only in small local networks. In large networks, the distribution of broadcast messages across all its segments becomes almost impossible, so they are characterized by a centralized approach. The most well-known centralized name resolution service is the Internet's Domain Name System (DNS).

6. Multi-level approach to standardization in computer networks. The concepts of “protocol”, “interface”, “protocol stack”. Characteristics of standard communication protocol stacks.

Along with autonomous operation a significant increase in the efficiency of using computers can be achieved by combining them into computer networks(network).

A computer network in the broad sense of the word refers to any set of computers connected to each other by communication channels for data transmission.

There are a number of good reasons for connecting computers together in a network. First, resource sharing allows multiple computers or other devices to share access to a single disk (file server), CD-ROM drive, tape drive, printers, plotters, scanners and other equipment, which reduces the cost of each individual user.

Secondly, in addition to sharing expensive peripheral devices, it is possible to similarly use network versions of application software. Thirdly, computer networks provide new forms of interaction between users in one team, for example, when working on a common project.

Fourthly, it becomes possible to use common means of communication between various application systems (communication services, data and video transmission, speech, etc.). Of particular importance is the organization of distributed data processing. In the case of centralized storage of information, the processes of ensuring its integrity, as well as backup, are significantly simplified.

2. Basic software and hardware components of the network

Computer network is a complex complex of interconnected and coordinated software and hardware components.

Studying the network as a whole presupposes knowledge of the operating principles of its individual elements:

Computers;

Communication equipment;

Operating systems;

Network applications.

The entire network hardware and software complex can be described by a multilayer model. At the heart of any network is a hardware layer of standardized computer platforms, i.e. the system of the end user of the network, which can be a computer or a terminal device (any input/output or information display device). Computers on network nodes are sometimes called host machines or simply hosts.

Currently, computers of various classes are widely and successfully used in networks - from personal computers to mainframes and supercomputers. The set of computers on the network must correspond to the variety of tasks solved by the network.

The second layer is communications equipment. Although computers are central to data processing in networks, communications devices have recently begun to play an equally important role.

Cabling systems, repeaters, bridges, switches, routers, and modular hubs have gone from being ancillary network components to becoming essential components, along with computers and system software, in both their impact on network performance and cost. Today, a communications device may be a complex, specialized multiprocessor that must be configured, optimized, and managed.

The third layer that forms the network software platform is operating systems (OS). The efficiency of the entire network depends on which concepts for managing local and distributed resources form the basis of the network OS.

When designing a network, it is important to consider how easily a given operating system can interact with other operating systems on the network, how secure and secure it is for data, the extent to which it can increase the number of users, whether it can be transferred to a different type of computer, and many other considerations.

The topmost layer of network tools are various network applications, such as network databases, mail systems, data archiving tools, teamwork automation systems, etc.

It is important to understand the range of capabilities that applications provide for different applications and how compatible they are with other network applications and operating systems.

Even as a result of a fairly superficial examination of networking, it becomes clear that a computer network is a complex set of interconnected and coordinated software and hardware components. Studying the network as a whole presupposes knowledge of the operating principles of its individual elements:

computers;

communication equipment;

operating systems;

network applications.

The entire network hardware and software complex can be described by a multilayer model. At the heart of any network is a hardware layer of standardized computer platforms. Currently, computers are widely and successfully used in networks. various classes- from personal computers to mainframes and supercomputers. The set of computers on the network must correspond to the variety of tasks solved by the network.

The second layer is communications equipment. Although computers are central to data processing in networks, communications devices have recently begun to play an equally important role. Cabling systems, repeaters, bridges, switches, routers, and modular hubs have gone from being ancillary network components to becoming essential components, along with computers and system software, in both their impact on network performance and cost. Today, a communications device may be a complex, specialized multiprocessor that must be configured, optimized, and managed. Learning how communications equipment works requires familiarity with a large number of protocols used in both local and wide area networks.

The third layer that forms the network software platform is operating systems (OS). The efficiency of the entire network depends on which concepts for managing local and distributed resources form the basis of the network OS. When designing a network, it is important to consider how easily a given operating system can interact with other operating systems on the network, how secure and secure it is for data, the extent to which it can increase the number of users, whether it can be transferred to a different type of computer, and many other considerations.

The topmost layer of networking tools are various network applications, such as network databases, mail systems, data archiving tools, collaboration automation systems, etc. It is very important to understand the range of capabilities provided by applications for different application areas, as well as to know how compatible they are with other network applications and operating systems.

The simplest case of interaction between two computers

In the simplest case, the interaction of computers can be realized using the same means that are used to interact a computer with peripherals, for example, through the RS-232C serial interface. Unlike the interaction of a computer with peripheral device, when a program works, as a rule, only on one side - on the computer side, in this case there is an interaction between two programs running on each computer.

A program running on one computer cannot gain direct access to the resources of another computer - its disks, files, printer. She can only “ask” the program running on the computer to which these resources belong. These "requests" are expressed as messages transmitted over communication channels between computers. Messages can contain not only commands to perform certain actions, but also actual information data (for example, the contents of a file).

Consider the case when a user working with text editor on personal computer A, you need to read part of a file located on the disk of personal computer B (Fig. 4). Let's assume that we connected these computers via a communication cable through COM ports, which, as is known, implement the RS-232C interface (such a connection is often called a null modem). To be sure, let the computers run MS-DOS, although this is not of fundamental importance in this case.

Rice. 4. Interaction between two computers

The COM port driver together with the COM port controller work in approximately the same way as in the case of interaction between the control unit and the computer described above. However, in this case, the role of the PU control device is performed by the controller and driver of the COM port of another computer. Together they ensure the transfer of one byte of information over a cable between computers. (In “real” local networks, similar functions of transferring data to the communication line are performed by network adapters and their drivers.)

The driver of computer B periodically polls the reception completion sign, set by the controller when the data is transferred correctly, and when it appears, reads the received byte from the controller buffer into RAM, thereby making it available to programs on computer B. In some cases, the driver is called asynchronously, by interrupts from the controller.

Thus, the programs of computers A and B have the means to transmit one byte of information. But the task considered in our example is much more complicated, since it is necessary to transfer not one byte, but a certain part of a given file. All additional problems associated with this must be solved by higher-level programs than COM port drivers. For definiteness, we will call such programs of computers A and B application A and application B, respectively. So, application A must generate a request message for application B. The request must specify the file name, the type of operation (in this case, reading), the offset and the size of the file area containing the required data.

To transmit this message to computer B, application A contacts the COM port driver, telling it the address in RAM, where the driver finds the message and then transmits it byte by byte to application B. Application B, having received the request, executes it, that is, reads it the required area of ​​the file from the disk using local OS tools to the buffer area of ​​its random access memory, and then, using the COM port driver, transmits the read data via a communication channel to computer A, where it reaches application A.

The described functions of application A could be performed by the text editor program itself, but it is not very rational to include these functions in every application - text editors, graphic editors, database management systems and other applications that need access to files. It is much more profitable to create a special software module that will perform the functions of generating request messages and receiving results for all computer applications. As mentioned earlier, such a service module is called a client. On the side of computer B, another module must operate - a server, constantly waiting for requests for remote access to files located on the disk of this computer. The server, having received a request from the network, contacts local file and performs specified actions with it, possibly with the participation of the local OS.

The software client and server perform system functions for servicing requests from applications on computer A for remote access to files on computer B. In order for applications on computer B to be able to use files on computer A, the described scheme must be symmetrically supplemented with a client for computer B and a server for computer A.

The interaction diagram of the client and server with applications and the operating system is shown in Fig. 5. Despite the fact that we have considered a very simple hardware communication scheme for computers, the functions of programs that provide access to remote files are very similar to the functions of modules of a network operating system operating on a network with more complex hardware connections of computers.

Rice. 5. Interaction of software components when connecting two computers

Very convenient and useful function client program is the ability to distinguish a request to remote file from a request to a local file. If the client program can do this, then applications do not have to care about which file they are working with (local or remote), the client program itself recognizes and redirects request to a remote machine. Hence the name often used for the client part of a network OS - redirector. Sometimes recognition functions are separated into a separate software module; in this case, not the entire client part is called a redirector, but only this module.