Even as a result of a rather superficial consideration of network operation, 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 requires knowledge of the principles of operation of its individual elements:
Computers;
Communication equipment;
operating systems;
network applications.
The whole complex of software and hardware of the network can be described by a multilayer model. At the heart of any network is the hardware layer of standardized computer platforms. At present, computers of various classes are widely and successfully used in networks - from personal computers to mainframes and supercomputers. The set of computers in the network should correspond to the set of various tasks solved by the network.
The second layer is the communications equipment. Although computers are central to the processing of data in networks, communication devices have recently begun to play an equally important role. Cabling, repeaters, bridges, switches, routers, and modular hubs have evolved from ancillary network components to being essential, along with computers and system software, both in terms of impact on network performance and cost. Today, a communication device can
represent a complex specialized multiprocessor that needs to be controlled, optimized and administered. Learning how communication equipment works requires familiarity with a large number of protocols used in both local and wide area networks.
The third layer that forms the software platform of the network is operating systems (OS). The efficiency of the entire network depends on what concepts of managing local and distributed resources are the basis of the network operating system. When designing a network, it is important to consider how easily a given operating system can interact with other network operating systems, how much it provides security and data protection, to what extent it allows you to 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 the various networking applications such as network databases, mail systems, archiving tools, collaboration automation systems, etc. other network applications and operating systems.
1.1.4. What gives the enterprise the use of networks?
This question can be clarified as follows: in what cases is the deployment of computer networks on the subject preferable to using stand-alone computers or multi-machine systems? What new opportunities appear in the enterprise with the advent of a computer network there? And finally, does an enterprise always need a network?
Without going into particular, the ultimate goal of using computer networks in an enterprise is to increase the efficiency of its work, which can be expressed, for example, in increasing the profits of the enterprise. Indeed, if computerization reduced the cost of producing an existing product, reduced the development time for a new model, or accelerated the service of customer orders, this means that this enterprise really needed a network.
Answering in detail the question of why an enterprise needs a network, let's start by considering the fundamental advantages of networks that arise from their belonging to distributed systems.
The conceptual advantage of distributed systems (and hence networks) over centralized systems is their ability to perform parallel calculations . Due to this, in a system with several processing nodes, performance exceeding the maximum currently possible performance of any single processor can in principle be achieved. Distributed systems potentially have the best performance-cost ratio, than centralized systems.
Another obvious and important advantage of distributed systems is their higher fault tolerance. Fault tolerance is understood as the ability of the system to perform its functions (maybe not in full) in case of failures of individual hardware elements and incomplete availability
data. Redundancy is the basis of increased fault tolerance of distributed systems. The redundancy of processing nodes (processors in multiprocessor systems or computers in networks) makes it possible, in the event of a failure of one node, to reassign the tasks assigned to it to other nodes. To this end, dynamic or static reconfiguration procedures may be provided in a distributed system. In computer networks, some sets of data can be duplicated on the external storage devices of several computers on the network, so that if one of them fails, the data remains available.
The use of geographically distributed computing systems is more in line with distributed nature of applied tasks in some subject areas, such as automation of technological processes, banking, etc. In all these cases, there are individual consumers of information dispersed over a certain territory - employees, organizations or technological installations. These consumers solve their tasks autonomously enough, therefore it is more rational to provide them with their own computing tools, but at the same time, since the tasks they solve are closely interconnected, their computing tools should be combined into a single system. An adequate solution in such a situation is the use of a computer network.
For the user, in addition to the above mentioned, distributed systems also provide such advantages as the ability to share data and devices, as well as the possibility of flexible distribution of work throughout the system. This separation of expensive peripherals - such as high capacity disk arrays, color printers, plotters, modems, optical disks - is in many cases the primary reason for enterprise networking. The user of a modern computer network works at his computer, often not realizing that he is using the data of another powerful computer located hundreds of kilometers away. He sends e-mail via a modem connected to a communications server shared by several departments in his enterprise. The user is given the illusion that these resources are connected directly to their computer, or are "almost" connected, since their use requires little additional steps compared to using their own resources. Such a property is called transparency networks.
Recently, another motive for deploying networks has become dominant, much more important in modern conditions than cost savings due to the division of expensive equipment or programs among employees of a corporation. This motive was the desire to provide employees quick access to extensive corporate information.| 3 conditions of fierce competition in any sector of the market, in the end, the company whose employees can quickly and correctly answer any question of the client - about the possibilities of their products, about the conditions for its use, about solving any possible problems, etc. wins In a large corporation, it is unlikely that even a good manager can know all the subtleties of each of the products manufactured by the company, especially since their nomenclature is now updated every quarter, if not a month. Therefore, it is very important that the manager has the opportunity from his computer connected to the corporate network, say in Magadan, to transfer the client's question to the server located in the central office of the enterprise in Novosibirsk, and promptly receive a high-quality answer that satisfies the client. In this case, the client will not turn to another company, but will continue to use the services of this manager.
To make such work possible, it is necessary not only to have fast and reliable connections in the corporate network, but also to have structured information on the company's servers, as well as the ability to effectively search for the necessary data. This aspect of networking has always been a bottleneck in organizing the delivery of information to employees - even with the existence of powerful DBMS, the information they received was not the “freshest” and not in the amount that was needed. Recently, there has been some progress in this area associated with the use of the WWW hypertext information service - the so-called technology intranet. This technology supports a fairly simple way of presenting textual and graphical information in the form of hypertext pages, which allows you to quickly place the latest information on the corporation's WWW servers. In addition, it unifies the viewing of information using standard programs - Web browsers, which are easy to work with even for a non-specialist. Now, many large corporations have already transferred huge piles of their documents to the pages of WWW servers, and employees of these companies, scattered around the world, use the information from these servers via the Internet or intranet. With easier and more complete access to information, employees make decisions faster, and the quality of that decision is generally higher.
Network use leads to improving communications, that is, to improve the process of information exchange and interaction between employees of the enterprise, as well as its customers and suppliers. Networks reduce the need for businesses to use other forms of information transfer, such as the telephone or regular mail. Often, it is the possibility of organizing e-mail that is the main reason and economic justification for deploying a computer network in an enterprise. New ones are becoming more and more widespread;
technologies that allow transmitting not only computer data, but voice and video information over network communication channels. A corporate network that integrates data and multimedia information can be used to organize audio and video conferences, in addition, it can be used to create its own internal telephone network.
Of course, computer networks have their own problems. These problems are mainly related to the organization of effective interaction of individual parts of a distributed system.
First, there are the complexities associated with software - operating systems and applications. Programming for distributed systems is fundamentally different from programming for centralized systems. So, the network operating system, performing in the general case all the functions of managing the local resources of the computer, in addition, solves numerous tasks of providing network services. The development of network applications is complicated by the need to organize the joint work of their parts running on different machines. A lot of worries are about ensuring software compatibility.
Secondly, many problems are associated with the transport of messages through communication channels between computers. The main tasks here are to ensure reliability (so that the transmitted data is not lost or distorted) and performance (so that data exchange occurs with acceptable delays). In the structure of the total costs for a computer network, the costs of solving “transport issues” make up a significant part, while in centralized systems these problems are completely absent.
Thirdly, these are issues related to security, which are much more difficult to solve in a computer network than in a centralized system. In some cases, when security is especially important, it is better not to use the network at all.
There are many more pros and cons to the use of networks, but the main proof of their effectiveness is the indisputable fact of their ubiquity. It is difficult to find any large enterprise that did not have at least a one-segment network of personal computers; more and more large networks appear with hundreds of workstations and dozens of servers, some large organizations and enterprises are acquiring private global networks that unite their branches thousands of kilometers away. In each specific case, there were reasons for creating a network, but the general statement is also true: there is still something in these networks.
Computing networks were the result of the evolution of computer technology.
* A computer network is a collection of computers connected by communication lines. Communication lines are formed by cables, network adapters and other communication devices. All network equipment operates under the control of system and application software.
* The main purpose of the network is to provide network users with the potential to share the resources of all computers.
* A computer network is one of the varieties of distributed systems, the advantage of which is the possibility of parallelizing calculations, due to which an increase in performance and fault tolerance of the system can be achieved.
The most important stage in the development of networks is the emergence of standard network technologies such as Ethernet, which allow you to quickly and efficiently connect computers of various types.
* The use of computer networks gives the enterprise the following opportunities:
Sharing expensive resources;
Improving communications;
Improving access to information;
Fast and high-quality decision making;
Freedom in the territorial placement of computers.
When creating computer networks, their developers had to solve many problems. In this section, we will consider only the most important of them, and in the order in which they naturally arose in the process of development and improvement of network technologies.
The mechanisms of interaction of computers in the network have borrowed a lot from the scheme of interaction between a computer and peripheral devices, so we will begin our consideration of the principles of network operation from this “pre-network” case.
1.2.1. Communication of the computer with peripheral devices
To exchange data between a computer and peripheral device(PU) the computer is provided with an external interface(Fig. 1.6), that is, a set of wires connecting a computer and a peripheral device, as well as a set of rules for exchanging information over these wires (sometimes instead of the term interface the term is used protocol - We'll talk more about these important terms later.) Examples of interfaces used in computers are parallel:
a Centronics interface, usually for connecting printers, and an RS-232C serial interface, through which a mouse, modem, and many other devices are connected. The interface is implemented on the computer side by a combination of hardware and software: the PU controller and special program, which controls this controller, which is often called driver corresponding peripheral.
From the PU side, the interface is most often implemented by a hardware control device, although there are also software-controlled peripheral devices.
The program executed by the processor can exchange data using input/output commands with any modules connected to the computer's internal bus, including PU controllers.
Peripheral devices can receive from the computer both data, such as bytes of information to be printed on paper, and control commands, in response to which the PU can perform special actions, such as moving the disk head to the desired track or ejecting a sheet of paper from the printer. The peripheral device uses the external interface of the computer not only to receive information, but also to transmit information to the computer, that is, data exchange over the external interface is usually bidirectional. So, for example, even a printer, which by its nature is an information output device, returns information about its state to the computer.
PU controllers accept commands and data from the processor into their internal buffer, which is often called a register or port, then perform the necessary transformations of these data and commands in accordance with the formats understandable PU, and issue them to the external interface. The distribution of responsibilities between the controller and the PU driver can vary, but usually the controller executes a set of simple commands to control the PU, and the driver uses these commands to make the device perform actions.
1.2. The main problems of building networks
more complex actions according to some algorithm. For example, a printer controller can support such elementary commands as “Print a character”, “Line feed”, “Carriage return”, etc. The printer driver, using these commands, organizes the printing of character strings, splitting a document into pages, and other higher-level operations . For the same controller, my but to develop different drivers that will control the given PU ps in different ways - some better, others worse - depending on the experience and ability of the programmers who developed them.
Controller commands:
"Set the beginning of the sheet", "Move the magnetic head", "Report device status", etc.
Rice. 1.6. Communication between a computer and a peripheral device
Consider a scheme for transferring one byte of information from an application program to a peripheral device. The program that needed to exchange data with the PU addresses the driver of this device, telling it, as a parameter, the address of the memory byte to be transferred. The driver loads the value of this byte into the buffer of the PU controller, which begins to sequentially transfer bits to the communication line, representing each bit with the corresponding electrical signal. To make it clear to the PU control device that a byte transfer is starting, before transmitting the first bit of information, the PU controller generates a start signal of a specific form, and after transmitting the last information bit, a stop signal. These signals synchronize byte transfer.
In addition to information bits, the controller can transmit a parity bit to improve the reliability of the exchange. The control device, having found a start bit on the corresponding line, performs preparatory actions and begins to receive information bits, forming a byte out of them in its receive buffer. If the transfer is accompanied by a parity bit, then the correctness of the transfer is checked: if the transfer is correctly performed, the sign of the completion of information reception is set in the corresponding register of the control device.
Typically, the most complex protocol functions are assigned to the driver (for example, calculating the checksum of the sequence of transmitted bytes, analyzing the state of the peripheral device, checking the correct execution of the command). But even the most primitive controller driver must support at least two operations: “Take data from the controller into RAM” and “Transfer data from random access memory to the controller.
There are both very specialized interfaces suitable for connecting a narrow class of devices (for example, Vista's high-resolution graphic monitors), and interfaces general purpose, which are standard and allow you to connect various peripheral devices. An example of such an interface is the RS-232C interface, which is supported by many terminals, printers, plotters, mice, and many other devices.
1.2.2. The simplest case of interaction between two computers
In the simplest case, the interaction of computers can be implemented using the same means that are used to interact with a computer with peripherals, for example, through a serial RS-232C interface! In contrast to the interaction of a computer with a peripheral device, when the program usually works only on one side - from the side of the computer, in this case there is an interaction between two programs running on each computer.
A program running on one computer cannot directly access the resources of another computer - its disks, files, printer. She can only "ask" for this program running on the computer that owns these resources. These "requests" are expressed in the form messages, transmitted over communication channels between computers. Messages can contain not only commands to perform certain actions, but also informational data itself (for example, the contents of a certain file).
Consider the case when a user working with a text editor on personal computer A needs to read a part of some file located on the disk of personal computer B. (Fig. 1.7). Let's assume that we have connected these computers via a communication cable through COM ports, which, as you know, implement the RS-232C interface (such a connection is often called a null-modem connection). Let, for definiteness, computers operate under MS-DOS, although this is of no fundamental importance in this case.
1.2. The main problems of building networks
The COM port driver together with the COM port controller work in much the same way as in the case of interaction between the PU and the computer described above. However, in this case, the role of the control device of the PU is performed by the controller and driver of the COM port of another computer. Together they provide the transmission of one byte of information over the cable between computers. (In "real" LANs, these line transfer functions are handled by network adapters and their drivers.)
The driver of computer B periodically polls the sign of the completion of the reception, 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 the programs of computer B. In some cases, the driver is called asynchronously, by interrupts from the controller.
Rice. 1.7. Interaction of two computers
Thus, the programs of computers A and B have a means to transfer 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 the given file. Any additional problems associated with this should be solved by programs at a higher level than the 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 calls the COM port driver, telling it the address in RAM where the driver finds the message and then passes it byte by byte to application B. Application B, having received the request, executes it, that is, reads the required area of the disk file with the help of local OS tools to the buffer area of its RAM, and then, using the COM port driver, transfers the read data via a communication channel to computer A, where they get to 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 with each application - text editors, graphic editors, database management systems and other applications that need access to files (although there are a large number programs that really independently solve all the tasks of machine-to-machine data exchange, for example Kermit - a file exchange program via COM ports, implemented for various operating systems , Norton Commander 3.0 with its Link feature. 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 work - a server that is constantly waiting for requests for remote access to files located on the diskettes of this computer. The server, having received a request from the network, accesses the local file and performs the specified actions with it, possibly with the participation of the local OS.
The software client and server perform system functions to service requests from applications on computer A for remote access to files on computer B. To allow applications on computer B to use computer files
And, the described scheme needs to be symmetrically supplemented with a client for the computer Vi
server for computer A.
The scheme of interaction between the client and server with applications and the operating system is shown in fig. 1.8. Despite the fact that we have considered a very simple scheme of hardware communication of computers, the functions of programs that provide access to remote files are very similar to the functions of the modules of a network operating system operating on a network with more complex hardware communication of computers.
Rice. 1.8. Interaction of software components when connecting two computers
1.2. Basic construction problems networks
A very convenient and useful feature of the client program is the ability to distinguish a request for a remote file from a request for a local file. If the client program knows how to do this, then applications should not care what file they work 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 the network OS, -redirector. Sometimes the recognition functions are separated into a separate program module, in which case not the entire client part is called the redirector, but only this module.
1.2.3. Problems of physical data transmission over communication lines
Even when considering the simplest network, consisting of only two machines, one can see many problems inherent in any computer network, including problems associated with the physical transmission of signals over communication lines, without which any kind of communication is impossible.
In computing, binary code is used to represent data. Inside the computer, discrete electrical signals correspond to data ones and zeros. The representation of data in the form of electrical or optical signals is called coding. There are various ways to encode binary digits 1 and 0, for example, a potential way in which one voltage level corresponds to one, and another voltage level corresponds to zero, or a pulse way, when pulses of different or the same polarity are used to represent numbers.
Similar approaches can be used to encode data and transfer it between two computers over communication lines. However, these communication lines differ in their electrical characteristics from those that exist inside a computer. The main difference between external communication lines and internal ones is their much greater length, as well as the fact that they pass outside the shielded housing through spaces often subject to strong electromagnetic interference. All this leads to much greater distortion of rectangular pulses (for example, “filling up” of fronts) than inside a computer. Therefore, for reliable recognition of pulses at the receiving end of the communication line, when transmitting data inside and outside the computer, it is not always possible to use the same speeds and coding methods. For example, the slow rise of the pulse front due to the high capacitive load of the line requires the transmission of pulses at a lower speed (so that the leading and trailing edges of neighboring pulses do not overlap and the pulse has time to grow to the required level).
In computer networks, both potential and impulse coding of discrete data are used, as well as a specific way of representing data-JVD, which is never used inside a computer - modulation(Fig. 1.9). When modulating, discrete information is represented by a sinusoidal signal of the frequency that the existing communication line transmits well.
Potential or pulse coding is used on high quality channels, while sinusoidal modulation is preferred when the channel introduces severe distortion into the transmitted signals. Usually
|
modulation is used in WANs to transmit data over analog telephone circuits, which were designed to transmit voice in analog form and are therefore not well suited for direct transmission of pulses.
Rice. 1.9. Examples of Discrete Information Representation
The method of signal transmission is also affected by the number of wires in the communication lines between computers. To reduce the cost of communication lines in networks, they usually strive to reduce the number of wires and because of this they use not parallel transmission of all the bits of one byte or even several bytes, as is done inside a computer, but serial, bit-by-bit transmission, requiring only one pair of wires.
Another problem to be solved in signaling is the problem of mutual synchronization transmitter of one computer from the receivers of another. When organizing the interaction of modules inside the computer, this problem is solved very simply, since in this case all modules are synchronized;
from a common clock generator. The problem of synchronization when connecting computers can be solved different ways, both by exchanging special clock pulses over a separate line, and by using periodic synchronization with predetermined codes or pulses of a characteristic shape that differs from the shape of the data pulses.
Despite the measures taken - the choice of an appropriate data exchange rate, communication lines with certain characteristics, a method for synchronizing the receiver and transmitter - there is a possibility of distorting some bits of the transmitted data. To improve the reliability of data transfer between computers, a standard technique is often used - counting checksum and transmitting it over the communication lines after each byte or after some block of bytes. It is often included in the data exchange protocol as a mandatory element! a signal receipt that confirms the correctness of data reception and is sent from the recipient to the sender.
The tasks of reliable exchange of binary signals represented by the corresponding electromagnetic signals in computer networks are solved by a certain class of equipment. In local networks, this network adapters, a in global networks - data transmission equipment, which includes, for example, devices that perform modulation and demodulation of discrete signals, - modems. This equipment encodes and decodes each information bit, synchronizes the transmission of electromagnetic signals over communication lines, checks the correctness of the transmission by the checksum, and can perform some other operations. Network adapters are designed, as a rule, to work with a certain transmission medium - coaxial cable, twisted pair, optical fiber, etc. Each type of transmission medium has certain electrical characteristics that affect the way the medium is used, and determines the signal transmission rate, how they are encoded, and some other parameters.
1.2.4. Problems joining multiple computers
So far, we have considered a degenerate network consisting of only two machines. When more computers are networked together, a whole new set of problems arises.
Topology of physical links
First of all, it is necessary to choose a method of organizing physical connections, that is, topology. The topology of a computer network is understood as the configuration of a graph, the vertices of which correspond to the network computers (sometimes other equipment, such as hubs), and the edges to the physical connections between them. Computers connected to a network are often referred to as stations or network nodes.
Note that the configuration physical connections determined by the electrical connections of computers to each other and may differ from the configuration logical connections between network nodes. Logical links are data transfer routes between network nodes and are formed by appropriately configuring the communication equipment.
The choice of the topology of electrical connections significantly affects many characteristics of the network. For example, the presence of redundant links increases the reliability of the network and makes it possible to balance the load of individual channels. The ease of adding new nodes, inherent in some topologies, makes the network easily expandable. Economic considerations often lead to the choice of topologies, which are characterized by a minimum total length of communication lines.
Consider some of the most common topologies.
Fully Connected the topology (Fig. 1.10, a) corresponds to a network in which each computer on the network is connected to all the others. Despite the logical simplicity, this option turns out to be cumbersome and inefficient. Indeed, each computer on the network must have a large number of communication ports, sufficient to communicate with each of the other computers on the network. For each pair of computers, a separate electrical communication line must be allocated. Fully connected topologies are rarely used because they do not satisfy any of the above requirements. More often this type of topology is used in multi-machine complexes or global networks with a small number of computers.
All other options are based on non-full mesh topologies, when data exchange between two computers may require intermediate data transmission through other network nodes.
Cellular 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 takes place are directly connected, and for data exchange between computers that are not connected by direct connections, transit transmissions through intermediate nodes are used. Mesh topology allows the connection of a large number of computers and is typical, as a rule, for wide area networks.
. Specify the main purpose of a computer network2016-02-17
Specify the main purpose of a computer network
Computer networks. Lecture notes1.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:
Sharing information;
sharing of equipment and software;
centralized administration and maintenance.
The main components of a computer network:
Computers (hardware layer);
Communication equipment;
Network OS;
network applications.
The whole complex of software and hardware of the network can be described layered model. At the core any network lies hardware layer standardized computer platforms. The second layer is communication equipment. Although computers are central to the processing of data in networks, communication devices have recently begun to play an equally important role. Cabling, repeaters, bridges, switches, routers, and modular hubs have evolved from ancillary network components to being essential, along with computers and system software, both in terms of 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 what concepts of managing local and distributed resources are the basis of the network operating system.
The topmost layer are different network applications, such as network databases, mail 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 is executed on a separate computer on the network.
Server program- a special program designed to serve requests for access to the resources of this computer from other computers on the network. The server module is constantly in the mode of waiting 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 users with joint access to a certain type of resource. Typically, a network operating system supports several types of network services for its users - a file service, a print service, a Email, remote access service, etc. (Examples of network services are 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 play the roles of both the server and the client at the same time.
2. Classification of computer networks.
Classifying networks on a territorial basis, there are local (LAN), global (WAN) and urban (MAN) networks.
LAN - concentrated on the territory of no more than 1-2 km; are 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 Mbps. The services provided are varied and usually involve on-line implementation.
WAN - unite computers dispersed at a distance of hundreds and thousands of kilometers. Often existing low-quality communication lines are used. Data transfer rates lower than in local networks (tens of kilobits per second) limit the range of services provided to file transfer, mainly not online, but in the background, using e-mail. For stable transmission of discrete data, more sophisticated 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 laid anew.
Depending on the scale of the production unit within which the network operates, a distinction is made between departmental networks, campus networks and corporate networks.
Departmental networks are used by a small group of employees primarily for the purpose of separating expensive peripherals, applications, and data; have one or two file servers and no more than thirty users; usually not subnetted; 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 departmental networks within a single building or a single area of several square kilometers, while global connections are not used. At the campus network level, there are challenges in integrating and managing heterogeneous hardware and software.
Corporate networks unite a large number of computers in all areas of a single enterprise. The corporate network is characterized by:
o scale - thousands of user computers, hundreds of servers, huge amounts of data stored and transmitted over communication lines, a wide variety of applications;
o high degree of heterogeneity - types of computers, communication equipment, operating systems and applications are different;
o use of global connections - networks of branches are connected using telecommunications, including telephone channels, radio channels, satellite communications.
3. Main characteristics of modern computer networks.
The quality of the network is characterized by the following properties: performance, reliability, compatibility, manageability, security, extensibility and scalability.
There are two main approaches to network quality assurance. The first one is that the network guarantees to the user that a certain numerical value of the quality of service indicator is observed. For example, frame relay and ATM networks can guarantee a given level of throughput to the user. In the second approach (best effort), the network tries to serve the user as best 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 to a network service and receiving a response to it; bandwidth, which reflects the amount of data transmitted by the network per unit of time, and transmission delay, which is equal to the interval between the moment a packet arrives at the input of a network device and the moment it appears at the output of this device.
Various characteristics are used to assess the reliability of networks, including: availability factor, which means the fraction 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 the system to work in conditions of failure of some of its elements.
Expandability 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 links in a very wide range, while the performance of the network does not deteriorate.
Transparency is the property of a network to hide the details of its internal device 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 the operation of the network, perform performance analysis and plan the development of the network.
Compatibility means that the network is capable of including 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 links between network nodes. The characteristics of the network depend on the type of topology being installed. In particular, the choice of a particular topology affects:
On the composition of the necessary network equipment;
Capabilities of network equipment;
Network expansion options;
Network management method.
The term "CS topology" may mean the physical topology (configuration of physical links) or logical topology– signaling routes between network nodes. The physical and logical topologies of the CS may coincide or differ. 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, the information is received only by the computer whose address matches the address of the recipient.
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 receiver. Therefore, if any computer fails, it will not affect the operation of the network.
To prevent the reflection of electrical signals, terminators are installed at each end of the cable to absorb these signals. If the 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 via cable segments to a central component - a hub. Signals from the transmitting computer go through the hub to everyone else.
In networks with a star topology, connecting computers to the network and managing the network is performed centrally. But there are also disadvantages: since all computers are connected to a central point, cable consumption increases significantly for large networks, higher network cost (plus hub), the number of plug-ins is limited by the number of hub ports. In addition, if the central component fails, the entire network will stop. If only one computer (or the cable connecting it to the hub) fails, then only that computer will not be able to transmit or receive data over the network. Other computers on the network will not be affected by this failure. With topology "ring" computers are connected to a cable closed in a ring. Signals travel around the ring in one direction and pass through each computer. Unlike a passive bus topology, here 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 a ring topology is more expensive.
The advantages include: network resistance to congestion (no collisions, no central node) and the ability to cover a large area. In addition, the number of users does not have much impact on network performance.
The configuration of physical connections is determined by the electrical connections of computers to each other and may differ from the configuration of logical connections between network nodes. Logical links are data transfer 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 topology by removing some possible connections (Fig. 1.10, b). In a network with a mesh topology, only those computers between which intensive data exchange takes place are directly connected, and for data exchange between computers that are not connected by direct connections, transit transmissions through intermediate nodes are used. Mesh topology allows the connection of a large number of computers and is typical, as a rule, for wide area networks.
In networks with a ring configuration (Fig. 1.10, e), data is transferred around 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 itself in the internal buffer. In a network with a ring topology, special measures must be taken so that in the event of a failure or disconnection of a station, the communication channel between the other stations is not interrupted.
5. Principles of naming and addressing in computer networks.
One of the issues to consider when merging three or more computers is the issue of their addressing. Several requirements can be made to the address of the network node and the scheme of its assignment.
The address must uniquely identify a computer on a network of any size.
The address assignment scheme should minimize administrator manual labor and the possibility of duplicate addresses.
The address must have a hierarchical structure that is convenient for building large networks. This problem is well illustrated by international postal addresses, which allow the postal service that organizes the delivery of letters between countries to use only the name of the country of the addressee and not take into account the name of his city, and even more so the street. IN large networks, consisting of many thousands of nodes, the lack of an address hierarchy can lead to high costs - end nodes and communication equipment will have to operate with address tables consisting of thousands of entries.
The address must be user-friendly, which means it must have a character representation such as 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 LAN network adapter. Such an address is usually used only by hardware, so it is tried to be as compact as possible and written 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 automatically generated each time the equipment is restarted, and the uniqueness of the address within the network is ensured by the equipment.
Symbolic addresses or names. These addresses are designed to be remembered by people and therefore usually carry a semantic load. Symbolic addresses are easy to use on both small and large networks.
Numeric compound addresses. Symbolic names are convenient for humans, but because of the variable format and potentially long length, their transmission over the network is not very economical. Therefore, in many cases, for working in large networks, numeric composite addresses of fixed and compact formats are used as node addresses. Typical representatives of this type of addresses are IP and IPX addresses.
The problem of establishing correspondence between addresses of different types, which is handled by the name resolution service, can be solved by both completely centralized and distributed means. In the case of a centralized approach, one computer (name server) is allocated on the network, in which a table of correspondence of different types of names, such as symbolic names and numeric numbers, is stored. All other computers use the name server to find the numerical number of the computer with which they want to communicate using the symbolic name.
In a different, distributed approach, each computer itself solves the problem of establishing a correspondence between names. For example, if the user specified a numeric number for the destination host, then before starting data transmission, the sending computer sends a message (such a message is called a broadcast) to all computers on the network asking them to recognize this numeric name. All computers, having received this message, compare the given number with their own. The computer with a match sends a response containing its hardware address, after which it becomes possible to send messages over the local network.
The advantage of the distributed approach is that it does not involve the allocation of a special computer, which also often requires manually setting up a name lookup 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 to all its segments becomes almost unrealistic, so they are characterized by a centralized approach. The most well-known centralized name resolution service is the Domain Name System (DNS) service on the Internet.
6. Multilevel approach to standardization in computer networks. The concepts of "protocol", "interface", "protocol stack". Characteristics of standard communication protocol stacks.
Combining computers into one system allows you to have access to shared resources:
- equipment, for example, printers, disks, which saves money and time allocated for device maintenance;
- programs and data, which provides ease of maintenance and reduces the cost of purchasing software;
- information services.
Combining the resources of computers involved in the processing, transmission, storage of information allows you to increase the speed of these processes, reliability, organize the interaction of participants in joint data processing.
In this case, the user gets the opportunity to work with equipment, network services and application processes located on other computers.
An important advantage of combining computers is the transfer of information from one computer to another located at any remote distance from each other.
The network equipment operates under the control of system and application software.
Computers on a network communicate with each other using hardware and network software. The main hardware components of the network form nodes - workstations and servers. Workstations are computers installed at users' workstations and equipped with specialized software for a specific subject area. Servers, as a rule, are sufficiently powerful computers, the functions of which are to provide all the processes for managing the network.
To connect the nodes, communication systems are used, including communication lines, transmitting equipment, and various communication equipment.
7.1.2. Network hardware components
Main hardware components
The main hardware components of a computer network (Fig. 1) are:
- Servers;
- Workstations;
- Channels (lines) of communication;
- Data transmission equipment.
Rice. 1. The main hardware components of a computer network
Servers and workstations
Servers are quite powerful computers, as they must provide a high speed of data transfer and request processing. The server is a source of network resources, a computer with a large capacity of RAM, large hard drives and additional storage media. There can be many servers in the network.
The server runs under the control of a network operating system, which provides simultaneous access of network users to the data located on it. The requirements for the server are determined by the tasks that are assigned to it in a particular network. The success of the server tasks depends on the installed software. Servers can store data, forward mail messages, manage databases, remotely process jobs, access web pages, print jobs, and a number of other functions that network users may need.
A computer that is connected to a network and has access to its resources is called workstation.
The roles of the server and workstation can be different in networks.
For example, a file server performs the following functions:
- data storage;
- data archiving;
- synchronization of data changes by different users;
- data transfer.
The file server receives a file access request from the workstation. The file is sent to the workstation. The user at the workstation processes the data. The file is then returned to the server.
There is another division of roles between computers on a network, such as a Client/Server network.
Client called a workstation on which software is installed that provides a solution to the problems formed in the course of the user's work.
In the process of processing data, the client forms a request to the server to perform various tasks: forwarding a message, browsing web pages, etc.
The server fulfills the request from the client. The results of the request are sent to the client. Some tasks can be performed on the client side. Communication, request processing, and data processing continue between the server and client until they complete the task. Data processing can be performed by both the server and the client.
The server provides storage of public data, organizes access to this data and transmits data to the client.
The client processes the received data and presents the processing results in a user-friendly way.
Channels of connection
Link(or communication line) - the physical medium through which the information signals of the data transmission equipment are transmitted.
The communication medium can be based on various physical principles of operation. For example, it can be a cable and connectors. The physical medium for data transmission can be the earth's atmosphere or outer space through which information signals propagate.
In telecommunications systems, data is transmitted using electric current, radio signals or light signals. All these physical processes are oscillations of the electromagnetic field of various frequencies and nature. The main characteristic of physical channels is baud rate, measured in bits (Kbps, Mbps) per second.
Depending on the physical environment, communication lines can be classified into the following groups: wired lines, cable lines, terrestrial and satellite radio channels.
wire lines- these are unshielded wires laid above the ground through the air. They mainly carry telephone or telegraph signals, but they can also be used to transmit data sent from one computer to another. The data transfer rate on such lines is measured in tens of Kbps.
cable lines- this is a set of conductors insulated with different layers. Basically, fiber-optic cables and cables based on copper wires are used: twisted pair (speed from 100 Mbps to 1 Gbps) and coaxial cable(speed - tens of Mbps). Cables are used for internal and external wiring. External cables are divided into underground, underwater and overhead cables.
The best quality cable is fiber optic cable. It consists of flexible glass fibers through which light signals propagate. It provides signal transmission at a very high speed (up to 10 Gbps and above). This type of cable is reliable, as it protects data well from external interference.
Radio channels of terrestrial and satellite communications, are a channel formed between the transmitter and receiver of radio waves. Radio channels differ in the frequency ranges used and the channel range. They provide different data transfer rates. Satellite channels and radio communications are used in cases where a cable channel cannot be used, for example, in sparsely populated areas, to communicate with users of a mobile radio network.
In computer networks, all the described types of physical data transmission media are used, but fiber-optic cable seems to be the most promising. It has already begun to be widely used as backbones of territorial, city networks, and is also used in high-speed sections of local networks.
Data communication equipment
Data transmission equipment is used for direct connection of computers to the communication line. It includes data transmission devices that are responsible for transmitting information to the physical medium (communication line) and receiving data from it: a network card (adapter), modems, devices for connecting to digital channels, terminal adapters for ISBN networks, bridges, routers, gateways and etc.
Network card (adapter) specifies the address of the computer. A computer on the network must be correctly identified, that is, its address must be unique. Therefore, manufacturers of network cards are allocated a number of different addresses that do not match.
Rice. 2. Network adapter (card)
Modems- devices for converting digital computer signals into analog telephone line signals and vice versa. The common data transfer rate is 56 Kbps.
Network terminal adaptersISBN(Integrated Services Digital Network) - a telephone network with integrated services. The basis of such a network is digital signal processing. The subscriber is provided with two channels for voice communication and data transmission at a speed of 64 Kbps.
Digital connection devices designed to improve the quality of signals and create a permanent composite channel between two network subscribers. They are mainly used on long-distance communication lines.
Bridges- devices connecting two networks and using the same data transmission methods.
Routers or routers - devices that connect networks of different types, but use the same operating system.
Gateways- devices that allow organizing data exchange between two networks using different rules of interaction, for example, connecting a local area network to a global one.
Bridges, routers, gateways can operate both in the mode of full allocation of functions, and in the mode of combining them with the functions of a computer network workstation.
Data transmission equipment also includes:
- Amplifiers - devices that increase the power of signals;
- Regenerators that restore the shape of pulse signals distorted during transmission over long distances;
- Switches - equipment for creating a long-term continuous composite channel between two subscribers of a network from segments of the physical medium with amplifiers.
Invisible to users, the network with the intermediate equipment of the communication channel forms a complex network, which is called the primary network. It does not support any services for the user, but only serves as the basis for building other networks.
7.1.3. Types of networks
Computer networks are usually classified according to different criteria. The most common is the classification by size depending on the territory occupied (Fig. 3):
- local computer network - LAN (Local Area Network);
- regional computer network - MAN (M e tropolitan Area Network);
- global computer network - WAN (Wide Area Network).
Local computing network unites subscribers located at short distances. Typically, a local area network is used to solve the problems of individual enterprises, for example, the local area network of a clinic, store or educational institution. Local network resources are not available to users on other networks.
Regionalcomputernetworks connect nodes at considerable distances from each other. They may include local networks and other subscribers within a large city, an economic region, a separate country. Usually, the distances between subscribers of a regional computer network are tens - hundreds of kilometers. An example of such a network is the regional network of regional libraries.
global computernetworks combine the resources of computers remote over long distances. The global computer network unites subscribers located in different countries on different continents. Interaction between subscribers of such a network can be carried out on the basis of telephone lines, radio communications and satellite communications systems.
Rice. 3. Combining computer networks of various types
Global Computing Networks Will Solve the Unification Problem information resources of all mankind and the organization of access to these resources.
Networks have a hierarchical organization (Fig. 3). They can enter one into another, uniting local networks into regional ones, and regional ones into global ones. Global area networks include regional networks and may connect other global networks. An example of such a combination of networks is the Internet, where network users have a single interface for accessing the resources of global networks. Currently widespread corporate networks, which, on the one hand, solve the problems of local networks, connecting computers for the exchange of intracorporate information, on the other hand, they use global network technologies. Corporate network - a network of mixed topology, which includes several local area networks. It unites the branches of the corporation and is the property of the enterprise. A corporate network that uses unified network technologies, unified interaction methods and applications for accessing global networks and for solving internal problems is called Intranet.
7.1.4. Topologies of computer networks
The topology of networks is understood as the configuration of the physical links of the network. There are several types of topologies: fully connected, ring, star, bus, mixed.
Fully connected topology involves the interconnection of each computer (Fig. 4). A fully meshed topology is rarely used, since it requires a separate physical channel for each pair of computers.
Rice. 4. Fully connected network topology
Rice. 5. Ring network topology
Ring topology(Fig. 5) provides data transfer around the ring from one computer to another. Any pair of computers is connected in this configuration in two ways - clockwise and counterclockwise. However, in such a network, the failure of one computer breaks the communication channel between other computers.
Star topology(Fig. 6) is formed by connecting each computer to a common central device, which can be a computer, repeater or router, hub. The star topology is currently the most common.
Rice. 6. Star network topology
Bus topology(Fig. 7) ensures the dissemination of information over a common bus. If this is a wireless connection, then the radio environment plays the role of a common bus instead of a cable. Information transmitted over the bus is available simultaneously to all computers connected to it. The implementation of this topology is inexpensive and easy to scale. The disadvantage is the unreliability of the cable.
Rice. 7. Bus topology
Mixed topology– use of all topologies in one network. Typical topologies (star, ring, bus) are used in small networks. In large networks, separate sections can be distinguished with an arbitrarily chosen typical topology. Therefore, the topology of large networks can be called mixed. Figure 8 schematically shows a section of a network with a mixed topology.
Rice. 8. Mixed network topology
7.1.5. Types of switching in networks
Messages can be transmitted from computer to computer not directly, but in transit - through special nodes.
If the network topology is not fully connected, then data exchange between an arbitrary pair of end nodes (subscribers) should generally go through transit nodes.
The sequence of transit nodes on the way from the sender to the recipient is called route.
The connection of end nodes through a network of transit nodes is called switching.
At the same time, switching tasks are solved such as:
- determination of information flows for which data exchange is required;
- formation of addresses of workstations;
- determination of routes for flows and selection of the optimal one;
- recognition of flows and their switching at each transit node.
Information flow forms a sequence of bytes, united by a set of common features. A sign can be computer addresses.
Switch node- this is a special device or a universal computer with a built-in software switching mechanism (software switch). By type of switching, networks are distinguished as follows:
- circuit-switched network;
- packet-switched network;
- message switched network.
Circuit Switched Networks originate from the first telephone networks. Circuit switching is the process of organizing the connection of a sequence of channels between a pair of subscriber systems.
Circuit switching forms a continuous physical channel between end nodes from intermediate channel sections connected in series by switches with equal data transfer rates. A connection is established between the end nodes and data transfer begins. At the end of the transmission, the channel is terminated. Switches are used for network switching.
Figure 9 shows a circuit switched network. Switching nodes (UK1–UK5) serve the workstations connected to them. (PC1–PC5). For example, to transfer data from workstation 1 (PC1) to workstation 2 (PC2), a channel must be established between nodes 1 (UC1) and 4 (UC4). This channel can be established along the routes UK1-UK3-UK2-UK4 or UK1-UK5-UK4. To organize data transfer, RS1 sends a request to establish a connection to the switching node (UC1) indicating the destination address (RS2). The switching node (ST1) must choose the route for the formation of a composite channel, and then transfer the request to the next node, for example, ST3, and that one to the next one, until the request is transmitted from the node ST4 to RS2. If the request is accepted by the destination computer, then a response is sent to the source computer via the already established channel, for example, UK1-UK2-UK4. It is considered that the channel between PC1 and PC2 is established. After that, data can be sent through it. At the end of the data transfer, the channel is terminated.
Rice. 9. Switching network
Packet networks appeared as a result of experiments in global computer networks. Packet switching is a technology for delivering messages that are divided into portions (individual packets) for data transmission, which can be sent from source to destination by different routes. The specific route is chosen by the sending and receiving computers based on the availability of the connection and the amount of traffic.
Message-switched networks. This type of switching establishes a logical channel for transmitting a message from one computer to another through switching nodes. Each intermediate device on the path of this route receives the message, stores it locally until the next section of the link becomes free, and sends it to the next device as soon as the link becomes free.
7.1.6. Reference Model for Open Systems Interconnection
The emergence of networks in which different types of computers functioned led to the need to develop standards for the exchange of information. The functioning of computers in networks is possible due to the rules of interaction, called protocols. When information is transmitted, they interact at different levels.
Communications and processes in open networks occur according to the ISO OSI standard model, which describes the rules for the interaction of systems with an open architecture from various manufacturers.
ISO - International Standard Organization - International Organization of Standards.
OSI is an abbreviation that stands for two variants:
- Open System Interconnection - Interaction of Open Systems - VOS;
- Optimum Scale Integration - An information system with an optimal degree of integration.
The interaction is based on a set of structures, rules, and programs that ensure the processing of events in networks. These sets are called in the OSI model levels. Each layer is described by protocols (a set of transmission rules). In the OSI model, seven levels of interaction are distinguished to perform a certain set of exchange functions on each of them.
Level 1- physical. Describes the transmission of binary information over a communication line: voltages, frequencies, the nature of the transmission medium. Protocols of this layer provide communication, reception and transmission of the bit stream.
Level 2- channel. Provides access to the medium, communication channel control, data transmission in blocks (frames). At this level, blocks are formed, the beginning and end of the frame in the bit stream is determined, the correctness of their transmission is checked, the presence and correction of errors.
Level 3- network. Provides a connection between any two points in the network. Routing takes place at this level, i.e. determination of the path along which data is transmitted through different communication lines, address processing.
At this level, the information is converted into packets for transmission to the destination. Data transfer occurs after the establishment of a virtual communication channel. After the data is transmitted, the channel is closed. Packets are transmitted over different physical routes, i.e. the channel is determined dynamically. The address is determined during connection establishment. Data can also be transmitted not only by packets, but also by other methods.
Widespread network layer protocol IP (Internet Protocol).
Level 4- transport. The task of the transport layer is to transfer information from one point of the network to another and ensure the quality of transportation. This level controls the data flow, the correctness of the transmission of blocks, the correctness of delivery to the destination, the sequence order, collects information from the blocks in its previous form. Can confirm receipt and correct delivery when transmitted by other methods.
The common transport protocol is TCP (Transmission Control Protocol). Often, network and transport layer protocols are collectively referred to as TCP / IP, meaning by this a whole family of protocols, because they implement internetworking technology.
TCP divides the transmitted information into several parts and numbers each part to restore their order when received. The TCP packet is placed inside the IP packet. Upon receipt, the IP packet is decompressed first, and then the TCP packet. The data is then collected according to the packet numbers.
Other standard protocols also operate at this level.
Level 5- session. Establishes, maintains, terminates connections. Coordinates interactions during a communication session: starts a session, ends it, restores crashed sessions. At this level, domain network names are converted to numbers and vice versa.
Level 6– representative (representation of data). Responsible for the syntax and semantics of the transmitted information, encryption, encoding and data compression. For example, at this stage, textual information, images are recoded, compressed, and decompressed.
Level 7- applied. Provides information transfer between programs. This layer connects the user to the network, making various services available, such as transferring files, e-mails, browsing the Internet. The following protocols are used at this level: FTP (file transfer), HTTP (HyperText Transfer Protocol) – hypertext transfer protocol.
Each layer provides a service to the upper layer adjacent to it, receives a service from the lower layer adjacent to it, exchanges blocks of data to perform its tasks.
Interactions are carried out sequentially level by level. The transmitted information coming from the user must be processed first by the application (seventh) level of rules, then must be processed at the representative, then session, transport level. Then, sequentially, the information is processed by the network, link level and is transmitted to the physical environment of the network. After processing at the physical layer and transferring it to another computer, the information is processed in reverse order from the lower layers to the next, and finally, after the application layer of processing, it is received by the user.
The task of each level when transmitting information is to prepare data in accordance with the standard and transfer it to the next lower level. Upon receipt of information - to the next top.
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Basic concepts
Computer network (computer network, data transmission network)- communication system of computers or computer equipment (servers, routers and other equipment). Various physical phenomena can be used to transmit information, as a rule, various types of electrical signals, light signals or electromagnetic radiation.
Data transfer(data exchange, digital transmission, digital communications) - the physical transfer of data (digital bit stream) in the form of signals from point to point or from point to several points by means of telecommunication over a communication channel, as a rule, for subsequent processing by means of computer technology. Examples of such channels are copper wires, optical fiber, wireless links, or a storage device.
Data transfer can be analog or digital (i.e. a bitstream) and also modulated by analog modulation or by digital coding.
server a computer is called a computer allocated from a group of personal computers (or workstations) to perform some service task without the direct participation of a person. The server and the workstation may have the same hardware configuration, as they differ only in the participation of the person behind the console in their work.
Some service tasks can run on the workstation in parallel with the user's work. Such a workstation is conditionally called a non-dedicated server.
Console(usually a monitor/keyboard/mouse) and human participation are required for servers only at the initial setup stage, during hardware maintenance and management in emergency situations (normally, most servers are controlled remotely). For emergency situations, servers are typically provided with one console kit per group of servers (with or without a switch, such as a KVM switch).
router- a specialized network computer that has at least two network interfaces and forwards data packets between different network segments, making decisions about forwarding based on information about the network topology and certain rules set by the administrator.
electromagnetic radiation e (electromagnetic waves) - a disturbance (change of state) of the electromagnetic field propagating in space (that is, electric and magnetic fields interacting with each other).
Signal(in the theory of information and communication) - a material carrier of information used to transmit messages in a communication system. A signal can be generated, but it is not required to receive it, unlike a message, which must be received by the receiving party, otherwise it is not a message. A signal can be any physical process whose parameters change in accordance with the transmitted message.
Principles of formation and types of networks
A computer connected to a network is called workstation(Workstation); the computer providing FAULTS resources, - server; a computer that has access to shared resources is a client.
Several computers located in the same room or functionally performing the same type of work (accounting or planned accounting, registration of incoming products, etc.) are connected to each other and combined into a working group so that they can share various resources: programs, documents, printers, fax, etc.
The working group is organized so that the computers included in it contain all the resources necessary for normal work. As a rule, a workgroup that includes more than 10-15 computers includes a dedicated server - enough powerful computer, which contains all shared directories and special software for managing access to all or part of the network.
Computer networks are of two types - peer-to-peer and server-based networks.
A peer-to-peer network is more suitable for those people who do not have the opportunity to organize a large network, but want to check how it still works and what benefits it brings. As for the server-based network, it is usually used to control all jobs.
In fact, these two types of computer networks practically do not differ in the basics of functioning, and this makes it possible to make transitions from a peer-to-peer network to a server-based network quite easily and quickly.
peer-to-peer network
A peer-to-peer network is actually several computers that are interconnected through one of the common types of communication. It is precisely because of the lack of a server in this type of network that it is considered simpler and more affordable. But it should also be noted that in a peer-to-peer network, computers should be as powerful as possible, since they will have to independently cope not only with the main work, but also with various problems.
In such a network, there is no computer that plays the role of a server, and therefore any of the working computers can be one. It is usually monitored by the user himself, and this is the main drawback of the peer-to-peer network: the user must not only work on the computer, but also perform administrator functions. He should also be responsible for troubleshooting the computer, to ensure maximum protection of the computer from virus attacks.
Peer-to-peer networking supports any operating system, so it could be Windows 95, for example.
Typically, a peer-to-peer network is built to connect a small number of computers (up to 10) via cable and in cases where there is no need for strict data protection. And yet, one incompetent user of the network can endanger not only its performance, but also its existence!
Server based network
A server-based network is the most common type of network.
It can use one or more servers that control jobs. The server is distinguished by its power and speed, it processes user requests very quickly and is usually monitored by one person, called a system administrator. The system administrator keeps the anti-virus databases up to date, troubleshoots the network, and manages shared resources.
As for the number of jobs in such a network, it is unlimited. Only to maintain the normal operation of the network, additional servers are installed as necessary.
Servers differ depending on the kind of work they do.
File - the server is used to store various information in files and folders. Such a server is controlled by any operating system like Windows NT 4.0.
The print server maintains network printers and provides access to them.
The database server provides maximum speed for searching and writing the necessary data to the database.
The application server executes requests that require high performance.
There are also other servers: mail, communication, etc.
A server-based network provides many more features and services than a peer-to-peer network, it is characterized by high performance and reliability.
Purpose of computer networks
All computer networks, without exception, have one purpose - to provide shared access to common resources.
The word resource is very convenient. Depending on the purpose of the network, one or another meaning can be invested in it. There are three types of resources: hardware, software and information. For example, a printing device (printer) is a hardware resource. Hard drive capacities are also a hardware resource. When all members of a small computer network share the same printer, this means that they share a common hardware resource. The same can be said about a network that has one computer with increased capacity. hard drive(file server), where all network members store their archives and work results.
In addition to hardware resources, computer networks allow the sharing of software resources. So, for example, to perform very complex and lengthy calculations, you can connect to a remote mainframe computer and send a computational task to it, and at the end of the calculations you can get the result back in the same way. .
Data stored on remote computers form an information resource. The role of this resource today is most clearly seen on the example of the Internet, which is perceived primarily as a giant information and reference system.
Examples with the division of resources into hardware, software and information are rather arbitrary. In fact, when working in a computer network of any type, all types of resources are shared at the same time. So, for example, turning to the Internet for information about the content of an evening television program, we certainly use someone else's hardware, on which other people's programs are running, providing the delivery of the data we requested.
Main software and hardware components of the network
Computer network- a complex set of interconnected and coordinated functioning software and hardware components.
Studying the network as a whole requires knowledge of the principles of operation of its individual elements:
– computers;
– communication equipment;
– operating systems;
– network applications.
The whole complex of software and hardware of the network can be described by a multilayer model:
1. At the heart of any network is 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 at the nodes of a network are sometimes called host machines, or simply hosts.
At present, computers of various classes are widely and successfully used in networks - from personal computers to mainframes and supercomputers. The set of computers in the network should correspond to the set of various tasks solved by the network.
2. Second layer - communication equipment. Although computers are central to the processing of data in networks, communication devices have recently begun to play an equally important role.
Cabling, repeaters, bridges, switches, routers, and modular hubs have evolved from ancillary network components to core ones along with computers and system software. Today, a communications device can be a complex, dedicated multiprocessor that needs to be configured, optimized, and administered.
3. The third layer, which forms the software platform of the network, are OS(OS). The efficiency of the entire network depends on what concepts of managing local and distributed resources are the basis of the network operating system.
When designing a network, it is important to consider how easily this OS can interact with other network OSes, how secure and secure data it provides, to what extent it allows you to increase the number of users, whether it can be transferred to a different type of computer, and much more.
4. The topmost layer of network media are various network applications, such as network databases, mail systems, data archiving tools, teamwork automation systems, etc.
It is important to be aware of the range of capabilities provided by applications for different applications, as well as to know how they are compatible with other network applications and operating systems.
Classification of computer networks
By area distribution
- PAN (Personal Area Network)- a personal network designed for interaction various devices owned by the same owner.
- LAN (Local Area Network)- local networks with a closed infrastructure before reaching service providers. The term "LAN" can describe both a small office network and a large factory network covering several hundred hectares. Foreign sources even give a close estimate - about six miles (10 km) in radius. Local networks are networks of a closed type, access to them is allowed only to a limited circle of users for whom work in such a network is directly related to their professional activities.
- CAN (Campus Area Network)- campus network - unites local networks of closely spaced buildings.
- MAN (Metropolitan Area Network)- urban networks between institutions within one or more cities, linking many local area networks.
- WAN (Wide Area Network)- covering large geographic regions, including both local networks and other telecommunication networks and devices. An example of a WAN is a packet-switched network (Frame relay), through which various computer networks can “talk” to each other. Global networks are open and focused on serving any user.
- Term "corporate network" also used in the literature to refer to the combination of several networks, each of which can be built on different technical, software and information principles.
By type of functional interaction
- Point to point network- the simplest type of computer network, in which two computers are directly connected to each other through communication equipment. The advantage of this type of connection is simplicity and cheapness, the disadvantage is that only 2 computers can be connected in this way and no more.
- client-server- a computing or network architecture in which tasks or network load are distributed between service providers (services), called servers, and service customers, called clients. It is not uncommon for clients and servers to interact over a computer network and may be different physical devices or software.
Fig.1 - Scheme of network architecture "client-server"
- Peer-to-peer network (decentralized, peer-to-peer, P2P) is an overlay computer network based on the equality of participants. Often in such a network there are no dedicated servers, and each node (peer) is both a client and performs the functions of a server. Unlike the client-server architecture, such an organization allows the network to remain operational with any number and any combination of available nodes. Network members are called peers.
Fig.2 - Scheme of a peer-to-peer network
- multi-level network is a network that includes one or more dedicated servers. The remaining computers in such a network (workstations) act as clients.
- mixed network- network architecture, which has a number of servers that form a peer-to-peer network. End users connect each to their own server according to the "client-server" scheme. Searching for information is possible online, both on your own server and (through it) on other network servers. The advantage of mixed networks is the ability to perform simultaneous searches on a large number of computers. The main disadvantage is the reduced reliability of the network.
By type of network topology
- Tire- The physical transmission medium consists of a single cable, called a common bus, to which all computers on the network are connected in parallel. The disadvantages are the connection of a small number of workstations (no more than 20) and the complete cessation of the network if the common cable is damaged. Failures of individual computers do not affect the operation of the network. To prevent signal distortion, it is necessary to install terminators at the ends of the cable.
Fig.3 - Bus topology
- Ring- this is a topology in which each computer is connected by communication lines with only two others: from one it only receives information, and only transmits to the other. On each communication line, as in the case of a star, only one transmitter and one receiver operate. This eliminates the need for external terminators. The computers in the ring are not completely equal (unlike, for example, a bus topology). Some of them necessarily receive information from the computer that is transmitting at this moment, earlier, while others - later. Each computer retransmits (restores) the signal coming to it, that is, it acts as a repeater, so the attenuation of the signal in the entire ring does not matter, only the attenuation between neighboring computers of the ring is important.
Fig.4 - Topology "ring"
- double ring- topology built on two rings. The first ring is the main path for data transfer. The second is a backup path that duplicates the main one. During the normal functioning of the first ring, data is transmitted only through it. When it fails, it combines with the second one and the network continues to function. In this case, data is transmitted on the first ring in one direction, and on the second in the opposite direction. An example is the FDDI network.
- Star- all computers are connected to the central node. The entire exchange of information goes exclusively through the central computer, which in this way has a very large load, so it cannot do anything other than the network. As a rule, it is the central computer that is the most powerful, and it is on it that all the functions of managing the exchange are assigned. No conflicts in a network with a star topology are in principle impossible, because management is completely centralized.
Fig.5 - Star topology
- Cellular- Each workstation on the network connects to several other workstations on the same network. It is characterized by high fault tolerance, configuration complexity and excessive cable consumption, allows connection of a large number of computers and is typical, as a rule, for large networks. Each computer has many possible ways to connect to other computers. A cable break will not result in a loss of connection between the two computers.
- Lattice is a topology in which the nodes form a regular multidimensional lattice. In this case, each edge of the lattice is parallel to its axis and connects two adjacent nodes along this axis. When connecting both external nodes of a one-dimensional lattice, the "ring" topology is obtained. Two- and three-dimensional lattices are used in the architecture of supercomputers. It is characterized by high reliability and complexity of implementation.
Fig.6 - Lattice topology
- Tree- characterized by the fact that between any pair of network nodes with such a topology there is only one path. The number of communication channels in the n-node tree network is minimal and equal to (n - 1). The reliability of the network is low, since the failure of even one of the links can lead to the division of the network into two isolated subnets.
Fig.7 - Topology "tree"
- fat tree- Unlike the classic tree topology, in which all links between nodes are the same, links in Fat Tree become wider (thick, bandwidth efficient) with each level as it approaches the root of the tree. It is often used to double the throughput at each level.
Fig.8 - "Fat tree" topology
By type of transmission medium
- Wired (telephone wire, coaxial cable, twisted pair, fiber optic cable)
- Wireless (transmission of information via radio waves in a certain frequency range, WI-FI)
The main types of transmission media used in computer networks are:
– analog public telephone channels;
– digital channels;
– narrowband and broadband cable channels;
– radio channels and satellite communication channels;
– fiber optic communication channels.
By function
- Storage Area Networks
- Server farms
- Process control networks
- SOHO networks, home networks
By transmission speed
- low-speed (up to 10 Mbps),
- medium-speed (up to 100 Mbps),
- high-speed (over 100 Mbps);
By network operating systems
- Windows based
- UNIX based
- Based on NetWare
- Based on Cisco
Need to maintain a permanent connection
- Packet network like Fidonet and UUCP
- Online network such as Internet and GSM
Local computer networks
A local network unites computers installed in one room (for example, a school computer class consisting of 8-12 computers) or in one building (for example, several dozen computers installed in different subject rooms can be combined into a local network in a school building).
Fig.9 - Diagram of a local area network (LAN)
In small local networks, all computers are usually equal, that is, users independently decide which resources of their computer (disks, directories, files) to make publicly available over the network. Such networks are called peer-to-peer.
If more than ten computers are connected to the local network, then the peer-to-peer network may not be efficient enough. To increase performance, as well as to ensure greater reliability when storing information on the network, some computers are specially allocated for storing files or application programs. Such computers are called servers, and the local area network is called a server-based network.
Each computer connected to the local network must have a special board (network adapter). Computers (network adapters) are connected to each other using cables.
Global computer network Internet.
Currently, a huge amount of information is stored on tens of millions of computers connected to (hundreds of millions of files, documents, etc.) and hundreds of millions of people use the information services of the global network.
is a global computer network that unites many local, regional and corporate networks and includes tens of millions of computers.
Each local or corporate network usually has at least one computer that has a permanent connection to the Internet using a high-bandwidth communication line (Internet server).
Fig.10 - Global network - Internet
The reliability of the functioning of the global network is ensured by the redundancy of communication lines: as a rule, servers have more than two communication lines connecting them to the Internet.
The basis, "framework" of the Internet is more than one hundred million servers permanently connected to the network.
Hundreds of millions of network users can connect to Internet servers using local area networks or dial-up telephone lines.
Basic network protocols
Simply connecting one computer to another is a necessary step to create a network, but not sufficient. To start transmitting information, you need to make sure that computers "understand" each other. How do computers communicate over a network? To provide this possibility, special tools have been developed, called "protocols". A protocol is a set of rules by which information is transmitted over a network. The concept of protocol is applicable not only to the computer industry. Even those who have never dealt with the Internet, most likely worked in everyday life with any devices whose functioning is based on the use of protocols. So, the ordinary public telephone network also has its own protocol, which allows the devices, for example, to establish the fact that the receiver is off-hook at the other end of the line or to recognize the disconnect signal and even the caller's number.
Based on this natural necessity, the world of computers needed a single language (that is, a protocol) that would be understandable to each of them.
network protocol is a set of rules and standards by which data is exchanged in a computer network.
The most common classification system network protocols is the so-called OSI model, according to which protocols are divided into 7 levels according to their purpose - from physical (formation and recognition of electrical or other signals) to application (application programming interface for transmitting information by applications).
Network protocols prescribe rules for the operation of computers that are connected to a network. They are built on a multi-level principle. A layer protocol defines one of the technical communication rules. Currently, the OSI (Open System Interconnection) model is used for network protocols.
The OSI model is a 7-layer logical model for how a network works. The OSI model is implemented by a group of protocols and communication rules organized into several layers:
In a computer network, there are 7 levels of interaction between computers:
1) physical;
2) logical (or channel);
3) network;
4) transport;
5) the level of communication sessions;
6) representative;
7) application layer.
1. Physical Layer defines electrical, mechanical, procedural, and functional specifications and enables the data link layer to establish, maintain, and terminate a physical connection between two computer systems that are directly connected to each other via a transmission medium, such as an analog telephone line, radio link, or fiber optic link.
2. Data Link Layer manages data transmission over the communication channel. The main functions of this layer are the division of transmitted data into portions called frames, the separation of data from the bit stream transmitted at the physical layer for processing at the network layer, the detection of transmission errors and the recovery of incorrectly transmitted data.
3. Network Layer provides communication between two computer systems on a network that exchange information with each other. Another function of the network layer is the routing of data (called packets at this layer) within and between networks (internet protocol).
4. Transport Layer provides reliable transmission (transportation) of data between computer systems of the network for higher levels. To do this, mechanisms are used to establish, maintain and break virtual channels (analogous to dedicated telephone channels), identify and correct transmission errors, and control data flow (in order to prevent overflow or data loss).
5. Session Layer provides for the establishment, maintenance and termination of a communication session for the presentation layer, as well as the resumption of an abnormally interrupted session.
6. Presentation Layer converts data from a representation used in an application program on one computer system to a representation used on another computer system. The functions of the presentation layer also include the transformation of data codes, their encryption / decryption, as well as the compression of transmitted data.
7. Application Level differs from the other layers of the model in that it provides services for application tasks. This level determines the availability of application tasks and resources for communication, synchronizes interacting application tasks, establishes agreements on procedures for error recovery and data integrity management. Important functions of the application layer are network management, as well as the execution of the most common system application tasks: e-mail, file sharing, and others.
Since each layer of the ISO/OSI model has its own characteristics, it is not possible to implement all these features within a single protocol.
The main protocols used in the work of the Internet:
- IMAP4
- Gorpher
Brief description of the protocols
The most common transport layer protocol in both local and wide area networks, developed by the US Department of Defense over 20 years ago.
is not one protocol, but a whole set of protocols working together. It consists of two levels. The top-level protocol, TCP, is responsible for correctly converting messages into information packets, from which the original message is assembled on the receiving side. The lower layer protocol, IP, is responsible for correctly delivering messages to the specified address. Sometimes packets of the same message can be delivered in different ways.
The standards are open and continuously improved.
Fig.11 - The principle of operation of the TCP / IP protocol
POP (Post Office Protocol)
Standard mail connection protocol. POP servers handle incoming mail, and the POP protocol is designed to handle requests to receive mail from client mailers.
SMTP (Simple Mail Transfer Protocol)
A protocol that specifies a set of rules for mail transmission. The SMTP server either returns an acknowledgment, an error message, or requests additional information.
The HTTP (Hypertext Transfer Protocol) protocol is a higher layer protocol than the TCP/IP protocol, an application layer protocol. HTTP was designed to efficiently transmit Web pages over the Internet. It is thanks to HTTP that we have the opportunity to contemplate the pages of the Web in all its splendor. The HTTP protocol is the backbone of the World Wide Web.
You issue HTTP commands using the browser interface, which is an HTTP client. When a link is clicked, the browser queries the Web server for the resource that the link points to, such as the next Web page.
In order for the text that makes up the content of Web pages to be displayed on them in a certain way - in accordance with the intention of the page creator - it is marked up using special text labels - HyperText Markup Language (HTML) tags.
The addresses of the Internet resources that you access via the HTTP protocol look something like this: http://www.tut.by
Using this protocol, you can connect to a remote computer as a user (if you have the appropriate rights, that is, you know the username and password) and perform actions on its files and applications in the same way as if you were working on your computer.
Telnet is a terminal emulation protocol. It is being worked on from command line. If you need to use the services of this protocol, you should not scour the wilds of the Internet in search of a suitable program. The Telnet client is supplied, for example, with Windows 98.
To instruct the Telnet client to connect to a remote computer, connect to the Internet, select the Start command Run, and type in the input line, for example, the following: telnet lib.ru
(Instead of lib.ru, you can, of course, enter a different address.) After that, the Telnet program will start, and a communication session will begin.
WAIS stands for Wide-Area Information Servers. This protocol was developed for searching information in databases. The WAIS information system is a distributed database system where individual databases are stored on different servers. Information about their content and location is stored in a special database - the catalog of servers. Viewing information resources is carried out with the help of the WAIS client program.
Information is searched for by keywords specified by the user. These words are entered for a certain database, and the system finds all the text fragments corresponding to them on all servers where the data of this database is located. The result is presented as a list of references to documents indicating how often the search word and all search words in the aggregate occur in this document.
Even today, when the WAIS system can be considered obsolete, specialists in many fields, when conducting scientific research, nevertheless turn to it in search of specific information that they cannot find by traditional means.
The address of a WAIS resource on the Internet looks something like this: wais://site.edu
The Gopher protocol is an application layer protocol developed in 1991. Prior to the ubiquity of the hypertext system, the World Wide Web Gopher was used to extract information (mostly text) from a hierarchical file structure. Gopher was the forerunner of the WWW, allowing menus to move from one page to another, gradually narrowing the range of information displayed. Gopher client programs had a text interface. However, Gopher menu items could point not only to text files, but also, for example, to telnet connections or WAIS databases.
Gopher is translated as "gopher", which reflects the glorious university past of the developers of this system. The student sports teams at the University of Minnesota were called the Golden Gophers.
Gopher resources can now be viewed with a regular Web browser, as modern browsers support this protocol.
Gopher information resource addresses look something like this: gopher://gopher.tc.umn.edu
WAP (Wireless Application Protocol) was developed in 1997 by a group of companies Ericsson, Motorola, Nokia and Phone.com (formerly Unwired Planet) in order to provide access to Internet services to users of wireless devices such as mobile phones, pagers, electronic organizers and others using different communication standards.
For example, if your mobile phone supports the WAP protocol, then by typing the address of the desired Web page on its keyboard, you can see it (in a simplified form) right on the phone's display. Currently, the vast majority of device manufacturers have already moved to the release of WAP-enabled models, which also continues to improve.
Network devices and equipment
Technical means of communication are cables (shielded and unshielded twisted pair, coaxial, fiber optic), connectors and terminators, network adapters, repeaters, splitters, bridges, routers, gateways, as well as modems that allow the use of various protocols and topologies in a single heterogeneous system.
Network card (adapter)- a device for connecting a computer to a network cable.
As the physical medium for information exchange are commonly used: thick (thick) coaxial cable, thin (thin) coaxial cable, fiber optic cable and unshielded twisted pair (Unshielded Twisted-Pair, UTP).
To solve the problem of interworking, equipment manufacturers offer various interface devices - repeaters (repeater), bridges (bridge), routers (router), bridges / routers (bridge / router) and gateways (gateway).
The main difference between these devices is that repeaters operate at layer 1 (physical), bridges operate at layer 2, routers are devices that operate at layer 3 (network) and gateways operate at layer 4. -7 levels.
Routers- devices for connecting network segments, operating at the network level and using routing information of the network level. Routers exchange information about the properties, network status, link health and node availability in order to select the best path for packet transmission. This process of choosing a route to the address of the subscriber system that receives the packet is called routing.
Distinguish single protocol And multiprotocol routers that can support multiple protocols at the same time, such as IPX/SPX, TCP/IP, and others. Since there are protocols that do not contain network layer information, routers must also perform bridging functions. Therefore, modern multiprotocol routers are called "bridge routers". Among the advantages of routers, one should note the possibility of choosing a route, splitting long messages into several short ones and using alternative paths for their transmission, which leads to the alignment of traffic along parallel paths, thereby allowing networks with packets of different lengths to be connected and facilitating network aggregation.
Bridges- devices for connecting network segments, functioning at the sublevel of media access control (Media Access Control) of the link level of the OSI / ISO model. Bridges have the property of transparency for higher level protocols, that is, they transfer a frame from one segment to another at the physical address of the recipient's station, which is extracted from the link layer header, analyze the integrity of frames and filter out corrupted ones. These devices can have the property of self-learning, that is, as frames pass through the bridge, it fills two tables with the addresses of stations sending messages, physically placing them on opposite sides of the bridge and writing them into different tables.
Network segments that are connected by a bridge can use either the same or different channel protocols. In the latter case, the bridge translates a frame of one format into a frame of another format.
Bridges automatically adapt to changing network configurations and can connect networks with different network layer protocols. Unfortunately, these devices cannot share the load using alternative paths in the network, which sometimes leads to traffic congestion (traffic flow in the communication line).
Repeater- a device operating at the physical layer, designed to compensate for attenuation in the data transmission medium by amplifying signals in order to increase their propagation distance. One of the varieties of repeaters are media converters. They allow you to convert signals, for example, when connecting coaxial and fiber optic cables, when moving from one transmission medium to another.
Splitter- a passive device for connecting more than two cable segments.
Gateways- devices operating at the upper levels of the OSI model (session, presentation and applications). They represent a method of connecting network segments and computer networks to the central computer. The need for gateways arises when two systems with completely different architectures are combined to translate the flow of data passing between these systems.
Modems are used to connect to other communication lines. The most widespread are modems focused on connecting to a dial-up telephone line.
Modem- a device designed to exchange information between remote computers via communication channels. A modem for connecting to a dial-up telephone line converts computer data into an analog audio signal for transmission over a telephone line (modulation), as well as the reverse conversion (demodulation).
Modems are internal and external. Internal modems are inserted inside the computer system unit. External modems are presented as a separate device, which is connected by cable to the computer's serial port, the same one that is often connected to a mouse. Internal modems contain a built-in serial port and are powered by a computer, external modems have a separate power supply. Internal modems are cheaper than external ones, other things being equal, the main of which is speed.
fax modem- a device that provides electronic transmission of plain text, drawings, photographs, diagrams, documents, converting information into a form suitable for transmission via an existing communication channel, and forming a duplicate - facsimile - of the original document on paper on the receiving side. Generally speaking, any telefax includes a scanner for reading a document, a modem that transmits and receives information over a telephone line, and a printer that prints the received message on thermal or plain paper. Of course, there are no components such as a scanner and a printer on fax modem cards. Information is presented only in "electronic" form.
FAQ
What is an IP address (IP address)?
Each computer on the network has its own unique address (number) - the so-called IP address - it is a number like aaa.bbb.ccc.ddd, (for example 10.240.51.23), where the first and second digits (10.240.) are the same for of all DOM networks, the third digit indicates the network segment to which the computer is connected, the fourth digit indicates the computer number itself.
Each computer has two IP addresses: internal (local) and external (when connected to the Internet).
How to find out the IP address?
What is a gateway (server)?
This is a computer in our network through which you access the Internet. The request from your computer is transmitted through the network to the server, it checks your data (ip-address, MAC-address, login and password) and after that you get access to the Internet.
What is a DNS server?
DNS server(pronounced "de-en-es") - a special server containing information about IP addresses. The Domain Name System (DNS), which is used on the Internet, maps host and domain names on the one hand to IP addresses on the other. DNS uses a hierarchical name database distributed across multiple computers.
What is traffic?
Traffic is the amount of information coming to your computer from the network and sent from it to the network. Every time you browse the Internet, you receive a certain amount of information on your computer, measured in bytes.
The fact is that any Internet resource, whether it be www pages, music videos, www-chats, IRC, news servers, etc., is traffic. You are viewing a www page - it means that some information has come to your computer from the network, you are listening to music from the Internet - it means that information is being transmitted to the computer from the network.
What is "incoming" and "outgoing" traffic?
Incoming traffic is the amount of information coming to your computer from the network, and outgoing, respectively, the amount leaving your computer to the network.
How to connect two computers in a network (network bridge)?
Answer: One of the computers connects to the Internet, the second computer connects to the first. The main disadvantage in this case is that in order to access the network of the second computer, it is necessary that the first computer is also on the network. And also, if your Internet connection goes through a network card, then you need an additional network card to connect the second computer to the first one, because. the built-in network card is already busy (it accepts the Internet).
Help, please, most competently to choose network topology.
Answer: First of all, decide on the carrier type. The fact is that the use of coaxial cable or twisted pair implies fundamentally different architectures of the local network. In the first case, the network will be built on the principle of a "common bus" - all the computers included in it are connected in series to each other in a chain using cable segments, forming a single backbone. This is quite convenient if all users of your network live on the same landing or in apartments located one below the other. However, if computers are scattered throughout the entrance (or house), the coaxial cable will loop, which is inconvenient already at the stage of the initial laying of the network. If you need to connect a few more new users to it, the problems will increase exponentially. In addition, the "common bus" is dangerous: if the network segment between two computers is damaged, then the entire network is turned off. Twisted pair allows you to create a completely different network architecture. A twisted-pair cable is similar to a regular telephone cable, but instead of 2 (or 4) wires, it uses 8, divided into 4 pairs. Twisted pair is a more flexible and practical cable, easy to install and well protected from external influences. However, the main advantage of this option is different: a local network of the "star" or "tree" type is based on a twisted pair - in its center there is a communication device (in the simplest case, a hub) with several ports, each of which is connected via a cable to the end computer. using this architecture, the failure of one or more sections of the network will not bring it to a halt, and the rest of the users will be able to continue working. The only danger lies only in the failure of the communication equipment.
We stretched a network cable between houses and are afraid of a network failure during a thunderstorm. How do you deal with thunderstorms?
Answer: Thunderstorms are generally the scourge of networks. In a large network, not a single thunderstorm passes without loss. There are many devices to protect network equipment from this scourge. Basically, these are adapters between devices and a network cable. The adapter is grounded, and when lightning strikes the cable, only the adapter burns out. According to advertising, the effectiveness of their work reaches 90%. Which device to choose is up to you. A more reliable means in case of a thunderstorm is the use of fiber-optic network technology, at least in open sections of the network.
Along with offline work 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 is understood as any set of computers interconnected by communication channels for data transmission.
There are a number of good reasons for networking computers together. 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 per individual user.
Secondly, in addition to sharing expensive peripherals, it is possible to similarly use network versions of application software. Thirdly, computer networks provide new forms of user interaction in the same 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 data 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 greatly simplified.
2. Main software and hardware components of the network
Computer network is a complex set of interconnected and coordinated software and hardware components.
Studying the network as a whole requires knowledge of the principles of operation of its individual elements:
Computers;
Communication equipment;
operating systems;
network applications.
The whole complex of software and hardware of the network can be described by a multilayer model. At the heart of any network lies the 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 at the nodes of a network are sometimes referred to as host machines or simply hosts.
At present, computers of various classes are widely and successfully used in networks - from personal computers to mainframes and supercomputers. The set of computers in the network should correspond to the set of various tasks solved by the network.
The second layer is the communications equipment. Although computers are central to the processing of data in networks, communication devices have recently begun to play an equally important role.
Cabling, repeaters, bridges, switches, routers, and modular hubs have evolved from ancillary network components to being essential, along with computers and system software, both in terms of impact on network performance and cost. Today, a communications device can be a complex, dedicated multiprocessor that needs to be configured, optimized, and administered.
The third layer that forms the software platform of the network is operating systems (OS). The efficiency of the entire network depends on what concepts of managing local and distributed resources are the basis of the network operating system.
When designing a network, it is important to consider how easily a given operating system can interact with other network operating systems, how secure and secure data it is, to what extent it allows you to 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 facilities are various network applications, such as network databases, mail systems, data archiving tools, teamwork automation systems, etc.
It is important to be aware of the range of capabilities provided by applications for different applications, as well as to know how they are compatible with other network applications and operating systems.