entranceXxxii. Communication equipment
Interlocutors. As a rule, in networks general access It is impossible to provide each pair of subscribers your own physical communication line, which they could monopolize "own" and use at any time. Therefore, a network always applies any way to switching subscribers, which ensures the separation of existing physical channels between several communication sessions and between network subscribers.
Switching in urban telephone networks
The city telephone network is a combination of linear and station structures. A network having one PBX is called necroshed. Linear structures of such a network consist only of subscriber lines. Typical value of the capacity of such a network of 8-10 thousand subscribers. With large capacities, due to a sharp increase in the length of Al, it is advisable to switch to the zoned construction of the network. In this case, the territory of the city is divided into areas, in each of which one district PBX (RATS) is constructed to which subscribers of this area are connected. The connections of the subscribers of the same area are carried out through one RATS, subscribers of different RATC - in two. RATS are associated with interconnecting lines in general, according to the principle "each with each". The total number of beams between the RATS is equal to the amount of RATS / 2. As an increase in the capacity of the network, the number of beams clutching the RATC among themselves according to the "each with each" principle begins to grow sharply, which leads to an excessive increase in the cable consumption and costs for the organization of communication and therefore, with the capacles of the network, more than 80,000 subscribers use additional switching node. On such a network, the connection between the PBX of different areas is carried out through the nodes of the incoming message (UVS), and the connection within its nodal area (ur is carried out according to the principle "Each with each" or through its UVS.
Consider in this article the main switching methods in networks.
In traditional telephone networks, the connection of subscribers is carried out with the help of switching communication channels. At the beginning, switching telephone communication channels was performed manually, then the switch was performed by automatic telephone stations (PBX).
The same principle is used in computing networks. As subscribers are geographically remote computing machines in a computer network. It is not physically possible to provide each computer to each computer its own non-commutable communication line, which they would use during the entire time. Therefore, in almost all computer networks, any method of switching subscribers (workstations) is always used, which allows access to existing communication channels for several subscribers to ensure simultaneously multiple communication sessions.
Switching - This is the process of connecting various subscribers of the communication network through transit nodes. Communication networks must ensure the connection of their subscribers among themselves. Subscribers can act as a computer, segments of local networks, fax devices or telephone interlocutors.
Workstations are connected to switches using individual communication lines, each of which is used at any time only one fixed by this line, subscriber. Switches are connected to each other using shared communication lines (shared by several subscribers).
Consider the three main most common ways of switching subscribers in networks:
- circuit Switching (Circuit Switching);
- packing Switching;
- switching messages (Message Switching).
Switching channels
Channel switching implies the formation of a continuous compound physical channel from sequentially connected individual channels for direct data transmission between nodes. Separate channels are connected to each other by special equipment - switches, which can establish links between any end network nodes. In the channel switching network, before transferring data, it is always necessary to perform a compound procedure in the process of which the composite channel is created.
The message time of the message is determined by the bandwidth of the channel, long communication and the message size.
Switches, as well as connecting their channels, should ensure simultaneously transmitting data from several subscriber channels. To do this, they must be high-speed and maintain any technique of multiplexing subscriber channels.
Advantages of switching channels:
- constant and known data transfer rate;
- the correct sequence of data arrival;
- Low and permanent level of data transfer over the network.
Disadvantages of switching channels:
- There is a network failure in servicing a connection to establish a connection;
- The irrational use of the bandwidth of physical channels, in particular the impossibility of using user equipment operating at different speeds. Separate parts of the composite channel work at the same speed, since the network switched networks do not buffer user data;
- Mandatory delay before transferring data due to the connection phase.
Switching messages - partitioning information on messages, each of which consists of a header and information.
This is a way of interaction in which a logical channel is created by consistently transmit messages through the communication nodes to the address specified in the message header.
At the same time, each node receives a message, writes to the memory, processes the title, selects the route and gives a message from memory to the next node.
The message delivery time is determined by the processing time in each node, the number of nodes and network bandwidth. When the transfer of information from the node A to the communication node is ends, the node A becomes free and can participate in the organization of another connection between subscribers, therefore the communication channel is used more efficiently, but the routing management system will be complex.
Today commuting messages in pure form Practically does not exist.
Package Switching is a special way to switch network nodes, which was specifically created for the best transmission of computer traffic (pulsating traffic). Experiments on the development of the very first computer networks, which were based on channel switching techniques, showed that this type of switching does not provide the opportunity to obtain high bandwidth of the computing network. The reason was rooted in the pulsating nature of the traffic that generate typical network applications.
When switching packets, all user-transmitted messages are broken in the source node for relatively small parts, called packages. You must specify that the message is the logically completed data portion - the file transfer request, the answer to this request containing the entire file, etc. Messages may have arbitrary length from several bytes to many megabytes. In contrast, packets usually can also have a variable length, but in narrow limits, for example from 46 to 1500 bytes (Ethernet). Each packet is supplied with the title, which indicates the address information required to deliver the destination node package, as well as the package number to be used by the destination node to build a message.
The batch network switches differ from channel switches by the fact that they have internal buffer memory for temporary storage packages if the output port of the switch at the time of acceptance of the package is engaged in the transmission of another package.
Package switching:
- more resistant to failures;
- High overall network bandwidth when transferring pulsating traffic;
- The ability to dynamically redistribute the bandwidth of physical communication channels.
Disadvantages of packet switching:
- uncertainty of data transfer rate between network subscribers;
- variable value packet delay;
- possible data loss due to buffer overflow;
- there are disorders of the sequence of packages.
Computer networks use packet switching.
Transferring packets in networks:
- Datagram method - Transmission is carried out as a set of independent packets. Each package moves over the network by its route and the user packages come in random order.
- Advantages: Simplicity of the transfer process.
- Disadvantages: Low reliability counted the possibility of packet loss and necessity software To build packages and recover messages.
- Logical canal - It is the transfer of sequences of the chain-related packages, accompanied by setting the pre-connection and confirmation of the reception of each package. If the i-th packet is not accepted, then all subsequent packets will not be accepted.
- Virtual Channel - This is a logical channel with the transfer to the fixed route of the sequence associated in the package chains.
- Advantages: the natural sequence of data is saved; sustainable traffic followers; perhaps reservation of resources.
- Disadvantages: the complexity of the hardware.
In this article, we considered the basic methods of switching in computing networks, with a description of each switching method, indicating the predominant and disadvantages.
See also:
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Topology relationship system between components windows networks. With reference to Active Directory replication, the topology is reduced to the set of connections used by the domain controllers to exchange data with each other.
(1) Computer networks implement information processing. M204, M205
parallel
lAN
● Distributed
bidirectional
(1) The address of the web page to view in the browser begins with:
Union of LAN
Causes of unification of the LAN
Created at a certain stage of development of the LAN system over time ceases to satisfy the needs of all users, and then the problem of expanding it functionality. There may be a need to unite within the company of various LANs that appeared in various departments and branches at different times, at least for the organization of data exchange with other systems. The problem of expanding the network configuration can be solved both within a limited space, and with exit to the external environment.
The desire to get a way to certain information resources may require the LAN connection to a higher level network.
In the simplest version, the LAN union is necessary to expand the network as a whole, but technical capabilities existing network Exhausted, new subscribers cannot be connected to it. One can only create another LAN and combine it with an existing one, using one of the following methods.
Methods of unification of the LAN
Bridge.The easiest option of the unification of the LAN is the combination of identical networks within a limited space. The physical transmitting medium imposes restrictions on the length of the network cable. Within permissible length, a network segment is built - a network segment. To combine network segments are used bridges.
Bridge- A device connecting two networks using the same data transfer methods.
The networks that combine the bridge must have the same network levels of the interaction of open systems, the lower levels may have some differences.
For the network personal computers The bridge is a separate computer with special software and additional equipment. The bridge can connect networks of different topologies, but running under the control of the same type of network operating systems.
Bridges can be local and remote.
Localbridges connect networks located on a limited area within the already existing system.
Remotebridges connect networks separated by geographically, using external communication channels and modems.
Local bridges, in turn, are divided into internal and external.
Internalbridges are usually located on one of the network of this network and combine the function of the bridge with the function of the subscriber computer. Expansion of functions is carried out by installing an additional network card.
Externalbridges provide for use to fulfill their features a separate computer with special software.
Router (router). A network of complex configuration, which is a connection of multiple networks, needs a special device. Task for this device - send a message to the destination to the desired network. Called such a device m ashruultizer.
Router, or Router, - Device connecting networks of different types, noispolyzing one operating system.
The router performs its functions on the network level, so it depends on the data exchange protocols, but does not depend on the type of network. With the help of two addresses - the network addresses and the address of the node, the router unambiguously selects a specific network station.
Example 6.7.You must establish a connection with the telephone network subscriber located in another city. First, the address of the telephone network of this city is recruited - the city code. Then - address of the node of this network - phone number subscriber. Functionsthe router performs PBX equipment.
The router can also select the best path to send a network to the network subscriber, filters the information passing through it, directing into one of the networks only the information that is addressed to it.
In addition, the router provides the load balancing on the network, redirecting the flow streams on free communication channels.
Gateway. To combine LAN perfectly different typesworking according to the protocols significantly different from each other, provide special "devices - gateways.
Gateway - device that allows you to organize data exchange between two networks using different interaction protocols.
The gateway performs its functions at levels above the network. It does not depend on the transmitted environment used, but depends on the data exchange protocols used. Typically, the gateway performs the transformation between two protocols.
Using gateways, you can connect a local computing network to the main computer, as well as the local network to connect to the global.
Example 6.8.It is necessary to combine local networks located in different cities. This task can be solved using a global data network. Such a network is the packet switching network based on the X.25 protocol. Using the gateway, the local computing network connects to the X.25 network. The gateway performs the necessary protocol conversion and provides data exchange between networks.
Bridges, routers and even gateways are performed in the form of boards that are installed in computers. The functions of their own can be performed both in the mode of complete allocation of functions and in the combination mode with the functions of the computer network workstation.
(1) A computer having 2 network cards and intended for connecting networks is called:
Router
Amplifier
Switch
(1) A device that switches several link channels to one by frequency separation, is called ...
repeater
● Concentrator
data Multiplexer
Hardware data transmission
Methods for transferring digital information
Digital data on the conductor are transmitted, by changing the current voltage: no voltage - "O", there is a voltage - "1". There are two ways to transfer information on a physical transmitting environment: digital and analog.
Notes:1. If all computer network subscribers lead data on the channel at one frequency, such a channel is called narrowband(passes one frequency).
2. If each subscriber operates on its own frequency on one channel, then such a channel is called broadband(skips many frequencies). The use of broadband channels saves on their quantity, but complicates the process of controlling the exchange of data.
For digitalor narrowband transmission method(Fig. 6.10) The data is transmitted in their natural form at a single frequency. A narrowband method allows you to transmit only digital information, ensures that only two users are able to use only two users and allows normal operation only at a limited distance (the length of the communication line is not more than 1000 m). At the same time, the narrowband transmission method ensures high data exchange rate - up to 10 Mbps and allows you to create easily configurable computing networks. The overwhelming number of local computing networks uses narrowband transmission.
Fig. 6.10. Digital transmission method
Analogthe method of transmitting digital data (Fig. 6.11) provides broadband transmission due to use in one channel signals of various carrier frequencies.
With analog transmission method, control of the parameters of the carrier frequency signal for transmission over the digital data channel.
The carrier signal is a harmonic oscillation described by the equation: "
A r \u003d a r max sin (ATF + 9 0),
where the qtah amplitude oscillations; co - the frequency of oscillations; t.- time; F 0 - the initial phase of oscillations.
You can pass the digital data on an analog channel by controlling one of the parameters of the carrier frequency signal: amplitude, frequency or phase. Since it is necessary to transmit data in binary form (sequence of units and zeros), then you can offer the following management methods (modulation):amplitude, frequency, phase.
The easiest way to understand the principle amplitudemodulations: "O" - Lack of signal, i.e. lack of carrier frequency oscillations; "1" - the presence of a signal, i.e. The presence of oscillations of the carrier frequency. There are oscillations - unit, no oscillations - zero (Fig. 6.11 but).
Frequencymodulation provides for the transmission of signals 0 and 1 at different frequency. When moving from 0 to 1 and from 1 to 0, a carrier frequency signal changes (Fig. 6.116).
The most difficult to understand is phasemodulation. Its essence is that when moving from 0 to 1 and from 1 to 0, the oscillation phase is changing, i.e. their direction (Fig. 6.11 in).
In the high-level networks of the hierarchy - global and regional are also used also broadband transmissionwhich provides for the work for each subscriber at its frequency within one channel. This ensures the interaction of a large number of subscribers at high data transfer rates.
Broadband allows you to combine digital data, images and sound in one channel, which is the necessary requirement of modern multimedia systems.
Example 6.5.A typical analog channel is a telephone channel. When the subscriber removes the tube, then hears uniform sound signal - This is the carrier frequency signal. Since it lies in the range of sound frequencies, it is called a tonal signal. To transmit on a telephone channel, you need to control the carrier frequency signal - modulate it. The sounds perceived by microphone are converted into electrical signals, and those, in turn, and modulate the carrier frequency signal. When transmitting digital information, the Office produces information bytes - the sequence of units and zeros.
Hardware
To ensure the transmission of information from the computer to the communication environment, it is necessary to coordinate the signals of the internal interface of the computer with the parameters of the signals transmitted through the communication channels. At the same time, both physical coordination (shape, amplitude and duration of the signal) and code can be performed.
Technical devices performing computer pairing functions with communication channels are called adaptersor network adapters.One adapter provides compilation with a computer of one communication channel.
Fig. 6.11. Methods for transferring digital information on analog signal: but- amplitude modulation; b.- frequency; in- Phase
In addition to single-channel adaptersmultichannel devices are used - data Multiplexersor simply multiplexers.
Data transfer multiplexer- A compilation device with multiple communication channels.
Data Multiplexers were used in data telework systems - the first step on the way to creating computing networks. In the future, in the appearance of networks with a complex configuration and with a large number of subscriber systems, special connection processors began to be applied to implement the interface functions.
As mentioned earlier, the bits flow to transform into analog signals to transmit digital information on the communication channel, and when receiving information from the communication channel to the computer, perform a reverse action - convert analog signals to the bits that can process computers. Such transforms performs a special device - mod eat.
Modem- A device that performs modulation and demodulation of information signals when transmitting them from computer to a communication channel and when receiving a computer from a communication channel.
The most expensive component of the computing network is a communication channel. Therefore, when building a number of computing networks, they are trying to save on communication channels by commuting several internal communication channels to one external. Special devices are used to perform switching functions - concentrators.
Concentrator- A device that switches several communication channels to one by frequency separation.
In LAN, where the physical transmission medium is a limited length cable, special devices are used to increase the length of the network - repeators.
Repeater- A device that ensures the preservation of the form and the amplitude of the signal when transmitting it to more than is provided by this type of physical transmitting medium, distance.
There are local and remote repeaters. Localrepeators allow you to connect the fragments of networks located at a distance of up to 50 m, and distance- up to 2000 m.
Characteristics of communication network
To assess the quality of the communication network, you can use the following characteristics:
■ data transfer rate over communication channel;
■ communication channel bandwidth;
■ accuracy of information transfer;
■ Reliability of communication channel and modems.
Data transfer ratethe communication channel is measured by the number of information bits transmitted per unit of time - a second.
Remember!Unit of data transfer speed - bit per second.
Note.Frequently used unit measurement unit - Bod. Bod - the number of changes in the state of the transfer medium per second. So aseach status change can correspond to several data bits, realspeed \u200b\u200bB. bitsper second may exceed the speed in the bodes.
The data transfer rate depends on the type and quality of communication channel, the type of modems used and the method of synchronization used.
So, for asynchronous modems and telephone communication channel, the speed range is 300 - 9600 bps, and for synchronous - 1200 - 19200 bits / s.
For users of computing networks, the value is not abstract bits per second, but information, the unit of measurement of which bytes or signs are used. Therefore, the more convenient characteristic of the channel is its bandwidth,which is estimated by the number of signs transmitted over the channel per unit of time - second. In this case, all service characters are included in the message. Theoretical bandwidth is determined by the data rate. Real bandwidth depends on a number of factors, among which and the transmission method, and the quality of the communication channel, and the conditions for its operation, and the message structure.
Remember!The unit of measurement of the bandwidth of the communication channel is a sign per second.
The essential characteristic of the communication system of any network is reliabilitytransmitted information. Since, based on the processing of information about the status of the control object, decisions are made about a different course of the process, then the fate of the object will eventually depend on the reliability of information. The accuracy of information transfer is assessed as the ratio of the number of erroneously transmitted signs to the total number of transmitted signs. The required level of reliability should provide both the equipment and the communication channel. It is impractical to use expensive equipment, if relative to the level of reliability, the communication channel does not provide the necessary requirements. *
Remember!Defense measurement unit: Number of errors for a sign - error / sign.
For computing networks, this indicator should lie within 10 -6 - 10 ~ 7 errors / sign, i.e. One error is allowed per million transmitted signs or ten million transmitted signs.
Finally, reliabilitythe communication system is determined either a share of a good condition in the total working time or average time of trouble-free operation. The second characteristic allows you to more effectively assess the reliability of the system.
Remember!Unit of reliability: average time of trouble-free operation - hour.
For computing networks, the average time of trouble-free operation should be quite large and form at least several thousand hours.
226 Chapter 6. Computer Networks
6.3. Local computing networks
Features of the organization LAN
Typical topology and LAN access methods
Union of LAN
Features of the organization LAN
Functional groups of devices on the network
The main purpose of any computer network is the provision of information and computing resources connected to it users.
From this point of view, a local computing network can be viewed as a set of servers and workstations.
Server- computer connected to the network and providing it highlighted services.
Serversdata storage, database management, remote task processing, print jobs and a number of other functions, the need for network users can occur. The server is a network resource source.
Work station- Personal computer connected to the network through which the user gets access to its resources.
Work stationnetwork functions both in the network and in local mode. It is equipped with its own operating system (MS DOS, Windows, etc.), provides a user by all required tools To solve applied tasks.
Particular attention should be paid to one of the types of servers - file server (File Server). In common terminology, the abbreviated name is adopted - file server.
The file server stores network user data and provides them with access to these data. This is a computer with a large capacity random access memory, high-capacity hard drives and additional magnetic tape drives (strimmers).
It works under the control of a special operating system that provides simultaneous access to network users to the data located on it.
The file server performs the following functions: data storage, data archiving, data change synchronization by various users, data transmission.
For many tasks, the use of one server file is insufficient. Then the network may include multiple servers. It is also possible to use mini-computer as file-servers.
Management of device interaction on the network
Information SystemsBuilt on the basis of computer networks, provide solving the following tasks: data storage, data processing, organization of user access to data, data transfer and data processing results.
In centralized processing systems, these functions performed the central computer (Mainframe, Host).
Computer networks implement distributed data processing. Data processing in this case is distributed between two objects: clientand server.
Client- Task, workstation or computer network user.
In the process of data processing, the client can form a request to the server to perform complex procedures, reading the file, search for information in the database, etc.
The server defined earlier executes the request received from the client. The results of the execution of the request are transmitted to the client. The server provides storage of public data, organizes access to this data and transmits data to the client.
The client processes the data obtained and represents the processing results in a convenient for the user. In principle, data processing can be performed on the server. For such systems, the terms are taken - systems client-server.or architecture client server.
Architecture The client-server can be used both in peer-to-peer local computing networks and networks with a dedicated server.
Single network.In such a network, there is no single operating station management center and no single device For data storage. The network operating system is distributed across all the workstations. Each network station can perform the functions of both the client and the server. It can serve requests from other workstations and send their network service requests.
The network user is available all devices connected to other stations (discs, printers).
Advantages of peer-to-peer networks: low cost and high reliability.
Disadvantages of peer-to-peer networks:
■ dependence of the efficiency of the network from the number of stations;
■ challenge network management;
■ the complexity of providing information protection;
■ Difficulties of updates and changing stations software. Peer-based networks based on networks are most popular.
lantastic operating systems, NetWare Lite.
Network S. dedicatedserver. On a network with a dedicated server, one of the computers performs data storage functions intended for all workstations, interaction management between workstations and a number of service functions.
Such a computer is usually called the network server. It establishes a network operating system, all partable external devices are connected to it - hard drives, printers and modems.
The interaction between workstations on the network is usually carried out through the server. The logical organization of such a network can be represented by topology star.The role of the central device performs the server. In centralized control networks, there is the possibility of exchanging information between workstations, bypassing the file server. To do this, you can use the NetLink program. After starting the program on two workstations, you can send files from a single station disk to the disk of another (similar to the copying of files from one directory to another using the Norton Commander).
Network advantages with a dedicated server:
■ reliable information security system;
■ high speed;
■ no restrictions on the number of workstations;
■ Easy control compared to peer-to-peer networks. Network Disadvantages:
■ High cost due to the allocation of one computer under the server;
■ dependence of the speed and reliability of the network from the server;
■ Ligger flexibility compared to peer-to-peer network.
Networks with a dedicated server are the most common in computer network users. Network operating systems for such networks - LanServer (IBM), Windows NT Server version 3.51 and 4.0 and NetWare (Novell).
(1) Local computing networks cannot be combined with ... M232
gateways, Bridges
● concentrators, modems
servers
routers
(1) BBS is ... M745
navigator
program for work in intranet
● System of electronic advertisements in the Internet
organization server service program
(1) Client-server data processing, it is processing. M227
parallel
localized
bidirectional
● Distributed
(1) The Bat program allows ...
download web pages
● Download and edit email
archive email
(1) one of search engines On the Internet is ...
(1) Internet Explorer allows ...
communicate in the chat on the IRC protocol
● Download HTTP Web Points and FTP Protocol
download NNTP Newsgroups
(1) The phone cable is an option ... M228
optical - high frequency
coaxial cable
fiber optic
● Twisted couple
(1) Usenet system is used ... M239
network Registration
● To move the news between computers around the world
information processing on the network
creating a workstation in the network
(1) Discussion Group, which is part of UseNet, is called ... M239
server group
group on the network
● Teleconference
(1) The message flow in the data network is determined ...
memory Channel Memory
● Traffic
6.1. Communication environment and data transmission
Appointment and classification of computer networks
Characteristics of data transfer process
Hardware data transmission
Data links
Appointment and classification of computer networks
Distributed data processing
Modern production requires high speed processing rates, convenient forms of its storage and transmission. It is also necessary to have dynamic ways to refer to information, ways to search for data into specified time intervals; Implement complex mathematical and logical data processing. The management of large enterprises, the management of the country at the country level requires participation in this process there are quite large teams. Such teams can be located in various parts of the city, in various regions of the country and even in different countries. To solve the management tasks, ensuring the implementation of the economic strategy, become important and relevant speed and ease of exchange of information, as well as the possibility of close interaction of all participating in the process of developing management decisions.
In the era of centralized use of computer with batch processing, users of computing equipment preferred to acquire computers on which almost all classes of their tasks could be solved. However, the complexity of solved problems is inversely proportional to their quantity, and this led to the inefficient use of computing power of the computer at a significant material cost. It is impossible not to take into account the fact that access to computer resources has been difficult due to the existing policies of the centralization of computational funds in one place.
Principle centralizeddata processing (Fig. 6.1) did not meet the high demands on the reliability of the processing process, it hampered the development of systems and could not provide the necessary temporary parameters when dialogue processing data in multiplayer mode. A short-term failure of the central computer led to the fatal consequences for the system as a whole, since it had to duplicate the functions of the central computer, significantly increasing the costs of creating and operating data processing systems.
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Fig.6.2. Distributed data processing system
The appearance of small computers, microevm and, finally, personal computers demanded a new approach to the organization of data processing systems, to the creation of new information technologies. The logically reasonable requirement of the transition from the use of individual computers in central data processing systems to distributeddata processing (Fig. 6.2).
Distributed data processing- Data processing performed on independent, but related computers representing a distributed system.
To implement distributed data processing were created multitarian Associationsthe structure of which is developed according to one of the following directions:
■ multi-gone computing complexes (MVK);
■ Computer (computing) network.
Multifunral computing complex- A group of computing machines installed by a number of computing machines, combined with the help of special means of interfacing and performing a jointly unified information and computing process.
Note .. Subprocessa certain sequence of actions is understood to solve the task defined by the program.
Multimaric computing complexes can be:
lANsubject to installing computers in one room that do not require special equipment and communication channels for the relationship; remoteif some computers of the complex are installed at a considerable distance from central computer and telephone communication channels are used to transfer data.
Example 6.1.A mini-computer packet processing mode is connected to a Mainframe type computer. Both computers are located in the same machine room. Mini-computer provides preparation and pre-processing of data, which are later used in solving complex tasks on the mainframe. This is a local multimecine complex.
Example 6.2.Three computers are combined into a complex for the distribution of tasks arriving for processing. One of them performs a dispatch function and distributes tasks depending on the employment of one of the two other processing computers. This is a local multimecine complex.
Example6.3. A computer collecting data for some region performs their pre-processing and transmits for further use on a central computer on a telephone communication channel. This is a remote multimecine complex.
Computer (computing) network- A set of computers and terminals connected using communication channels into a single system that meets the requirements of a well-to-process data.
Note.Under systemit is understood as an autonomous set consisting of one or more computer, software, peripheral equipment, terminals, data transmission tools, physical processes and operators capable of processing information and perform interaction with other systems.
Generalized Computer Network Structure
Computer networks are the highest form of multimaric associations. We highlight the main differences between the computer network from the multimarous computing complex.
The first difference is dimension. The composition of the multimarous computing complex usually includes two, maximum three computers located mainly in the same room. The computing network may consist of dozens and even hundreds of computers located at a distance from each other from several meters to dozen, hundreds and even thousand kilometers.
The second difference is the separation of functions between the computer. If the data processing function, data transmission and control system can be implemented in a single computer, then in computing networks, these functions are distributed between different computers.
The third difference is the need to solve the network routing task. A message from one computer to another on the network can be transmitted via various routes depending on the state of the communication channels connecting computers with each other.
Combining into one complex of computing equipment, communication equipment and data channels makes specific requirements from each element of the Multiple Association, and also requires the formation of a special terminology.
Subscribers of the network- Objects generating or consume information on the network.
Subscribersnetworks can be separate computers, computer complexes, terminals, industrial robots, numerical control machines, etc. Any network subscriber is connected to the station.
Station- Equipment that performs functions related to transmission and reception of information.
The totality of the subscriber and the station is customary called subscriber system.A physical transmitting environment is needed to organize the interaction of subscribers.
Physical transmitting medium - lines or space in which electrical signals are applied, and data transfer equipment.
Based on the physical transmission medium is built communication network,which ensures the transmission of information between subscriber systems.
This approach allows us to consider any computer network as a set of subscriber systems and a communications network. The generalized structure of the computer network is shown in Fig. 6.3.
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Fig.6.3. Generalized Computer Network Structure
Classification of computing networks
Depending on the territorial location of subscriber systems, computing networks can be divided into three main classes:
■ Global networks (WAN - WIDE AREA NETWORK);
■ Regional networks (MAN - Metropolitan Area Network);
■ Local networks (LAN - Local Area Network).
Globalthe computing network combines subscribers located in different countries, on various continents. The interaction between subscribers of such a network can be carried out on the basis of telephone lines of communication, radio communications and satellite communication systems. Global computing networks will solve the problem of combining the information resources of all mankind and organizing access to these resources.
Regionalthe computing network connects subscribers located at a considerable distance from each other. It may include subscribers inside a large city, economic region, a separate country. Typically, the distance between the subscribers of the regional computing network is tens of hundreds of kilometers.
Localthe computing network combines subscribers located within a small area. Currently, there are no exceptional restrictions on the territorial scatter of the subscribers of the local computing network. Usually such a network is associated with a specific place, to the class of local computing networks include networks of delometries, firms, banks, offices, etc. The length of such a network can be limited to 2 - 2.5 km.
Combining global, regional and local computing networks allows you to create multilayer hierarchies. They provide powerful, economically occurring means of processing huge information arrays and access to--bounded information resources. In fig. 6.4 shows one of the possible hierarchies of computing networks. . White computing networks can be included as components in the regional network, regional networks - to be combined as part of a global network and, finally, global networks can also form complex structures.
Fig. 6.4. Hierarchy of computer networks
Example 6.4. Computer internet It is the most popular global network. It includes many freely connected networks. Inside each network included in the Internet, there is a specific communication structure and a certain control discipline. Internally, the structure and methods of connections between different networks for a particular user have no value.
Personal computers who currently become an indispensable element of any control system led to a boom in the field of creating local computer networks. This, in turn, caused the need to develop new information technologies.
The practice of applying personal "computers in various branches of science, technology and production has shown that there are not separate autonomous PCs from the introduction of computing equipment, but local computing networks.
(1) Subscribers of the network are .. M205.
network administrators
users of personal computers
● Objects generating or network consumes
communication equipment
(1) Network subscribers can not be ... M205
● ECM complexes (may)
Terminals (may)
separate computers (may)
end users
(1) The network server is a computer ... M226 (server - network resource source)
with the highest processor frequency
providing access to the keyboard and monitor
with the greatest memory
● Providing access to resources
(1) FTP - Server - this ... M240
the computer that contains files intended for the network administrator
the computer containing information for organizing the work of teleconferences
corporate server
● The computer that contains files intended for open access.
(1) SMTP protocol is designed for ...
(SMTP protocol component TCP / IP protocol components; This protocol controls the exchange of email messages between message transfer agents.
POP3 Protocol Popular Protocol for Email Messages. This protocol is often used by Internet service providers. On the POP3 servers allowed only to one mailboxUnlike IMAP servers that provide access to multiple folders on the server.
The set of widely used network protocols that supports the connection between the combined networks consisting of computers of various architecture and with different operating systems. The TCP / IP protocol includes standards for communication between computers and network connection agreements and rules of message routing.)
Chatting
● Email sending
Browsing web pages
Email reception
(1) the most effective way Communications for computer traffic transmission are ...
● Packages M220.
messages
all equally effective
Circuit Switching and Packet - IT "S Methods for Solving The Generalized Problem of Switching Data in Any Network Technology. Complex Technical Solutions of the Generalized Switching Tasks in Its Entirety Consists of the Particular Problems of Data Transmission Networks.
By The Special Problems of Data Networks Include:
- dEFINE FLOWS AND APPROPRIATE ROUTES;
- fixation Route Configuration Parameters, and Tables of Network Devices;
- rECOGNITION FLOWS AND DATA TRANSFER BETWEEN ONE DEVICE INTERFACE;
- multiplexing / Demultiplexing Streams;
- separation medium.
Among The Many Possible Approaches to the Solution of the Generalized Problem of Subscribers Switching Networks Allocate Two Basic, Which Include Channel Switching and Packet Switching. Thus There Are Traditional Applications of Each Switching Techniques, For example, Telephone Networks Continue to Be Built and Constructed using Circuit Switched Technology, Computer Networks and The Vast Majority Are Based on Packet Switching Technique.
Theraefore, AS Information Flow in Circuit-Switched Networks Are The Data Exchanged Between A Pair of Subscribers. Accordingly, The Global Flow Feature Is A Pair of Addresses (Telephone Numbers) Subscribers Communicate with Each Other. One Feature of Circuit-Switched Networks Is The Concept of An Elementary Channel.
Elementary Channel.
Elemental Channel (Or Channel) - Is a Basic Technical Characteristics of the Circuit Switched Network, Which Is Fixed Within A Given Type of Network ThroughPut Value. Every Link In The Circuit Switched Network Has A Capacity Of A MultiPle Channel Elementary Adopted for This Type of Network.
In Traditional Telephone Systems The Value of the Elementary Channel Speed \u200b\u200bIS Equal to 64 Kbit / S, Which Is Sufficient for High-Quality Digital Voice.
For High-Quality Voice Use the Frequency of Sound Vibrations Amplitude Quantization 8000 HZ (Sampling Time 125 MS Intervals). To Represent A Measure of the Amplitude is Most Often Used 8-bit Code, Which Makes 256 Tone Gradation (by sampling values).
In This Case, The Transmission of One Voice Channel Is Needed Bandwidth 64 Kbit / S:
8000 x 8 \u003d 64000 BITS / S or 64 kbit / s.
SUCH A VOICE CHANNEL IS CALLED AN ELEMENTARY CHANNEL DIGITAL TELEPHONE NETWORKS. A Feature of the Circuit Switched Network is That The Bandwidth of Each Link Must Be Equal To An Integer Number of Elementary Channels.
THE COMPOSITE CHANNEL
Communication Constructed by Switching (Connection) of Elementary Channels, Called A Composite Channel.
Composite Channel
THE PROPERTIES OF THE COMPOSIE CHANNEL:
- composite Channel Throughout ITS Length Is Made Up of The Same Number of Elementary Channels;
- composite Channel Has a Constant and Fixed Bandwidth Throughout Its Length;
- composite Channel Is Created Temporarily for The PERIOD OF THE SESSION TWO SUBSCRIBERS;
- at the Session, All The Basic Channels That Are Included in The Composite Channel, ENTER THE EXCLUSIVE USE OF SUBSCRIRS, FOR WHICH THE CONPOSE CHANNEL HAS BEEN CREATED;
- during The Communication Session In The Subscribers CAN Send Network Data Rate Not Exceeding A CHANNEL CAPACITY OF THE COMPOSITE;
- data Received in A Composite Channel, The Called Subscriber Is Guaranteed to Be Delivered Without Delay, Losses, and at the Same Rate (Source Rate) Regardless of Whether Here Is At This Time in the Other Network Connection or Not;
- after the End of the Session The Basic Cronesponding Composite Channel, Declared Free and Returned to the Pool of Resources Allocated for Use by Other Users.
Connection refused
Connection refused
Connection Requests Are Not Always Successful.
If The Path Between The Calling and Called Subscribers Are No Free Calnels or Called Basic Node Is Busy, The Malfunction Occurs in the Connection Setup.
THE ADVANTAGE OF CIRCUIT SWITCHING
Circuit Switching Technology Is Aimed At Minimizing The Accidental Events in The Network, Ie a Technology. In Order to Avoid Any Possible UNCERTAINTY MUCH OF THE WORK ON INFORMATION EXCHAND IS CARRIED OUT IN ADVANCE, EVEN BEFORE THE START THE DATA TRANSFER. FIRST, FOR A GIVEN ADDRESS, THE AVAILABILITY OF THE REQUIRED BASIC CHANNELS ALL THE WAY FROM THE SENDER TO THE RECIPIENT. But in the Case of Bursty, This Approach Is Inefficient, Since 80% of The Time Channel May Be IDle.
Packet Switching.
The most Important Principle of The Networks with a Packet Switched Data Submission IS Transmitted Over The Network in The Form of Structurally Separated from the Each Other Pieces of Data Called Packets. Each Packet Has a Header, Which CONTAINS THE DESTINATION ADDRESS, AND OTHER SUPPORTING INFORMATION (LENGTH OF THE DATA FIELD, A CHECKSUM, AND OTHERS.), USED FOR THE DELIVERY TO THE ADDRESSEE OF THE PACKAGE.
Having Address in Each Packet Is One of the Most Important Features of Packet Switching Technology, Since Each Packet Can Be Processed Independently of The Other Switch Packets Constituting The Network Traffic. INDITION TO TITLE IN THE PACKAGE MAY HAVE ONE ADDITIONAL FIELD TO BE PACKED AT THE END OF THE PACKAGE AND SO-CALLED TRAILER. In The Trailer Is Usally Placed Checksum, Which Allows You to Check Whether The Information Has Been Corrupted During Transmission Over The Network or Not.
Partitioning the Data Into Packets
Partitioning the Data Into Packets Takes Place in Several Stages. Chain Sender Node Generates Transmission Data, Which Is Divided Into Equal Parts. Aftert That Occurs The Formation of A Package by Adding the Header Overhead. And The Last Stage Is Assembled Packets Into The Original Message To the Destination Node.
Partitioning the Data Into Packets
TRANSFERRING DATA OVER A NETWORK AS A PACKET
Packet Transmission Network.
AS IN THE CIRCUIT SWITCHED NETWORKS, PACKET SWITCHED NETWORKS, FOR EACH OF THE STREAMS IS DETERMINED MANUALLY OR AUTOMATICALLY ROUTE FIXED IN THE STORED TABLES FOR COMMUTION SWITCHES. Packets Entering The Switch Are Processed and Sent On a Particular Route
UNCERTAINTY AND ASYNCHRONOUS MOVEMENT OF DATA IN PACKET-SWITCHED NETWORKS MAKES SPECIAL DEMANDS ON THE SWITCHES IN SUCH NETWORKS.
The Main Difference Between A Paccuit Switch of The Switches In The Circuit-Switched Networks Is That They An Internal Buffer Memory for Temporarily String Packets. Switch Buffers Need to Harmonize Data Rates In Communication Links Connected to Its Interfaces, As Well As to Harmonize The Rate of Arrival Packets with ther Switching Speed.
Methods of TRANSFER PACKAGES
A Switch Can Operate On The Basis of One Of Three Methods Promote Packages:
- datagram Transmission;
- TRANSFER TO THE ESTABLISHMENT OF A LOGICAL CONNECTION;
- TRANSFER TO THE ESTABLISHMENT OF A VIRTUAL CHANNEL.
Datagram Transmission
Datagram Transfer. Method Based on the Promotion of Packet Independent from Each Other. Packet Processing Procedure Is Only Determined by The Values \u200b\u200bof Parameters That It Carries and The Current State of the Network. AND EVERY SINGLE PACKET NETWORK IS CONSIDERED AS A COMPLEYLY INDEPENDENT UNIT TRANSFER - DATAGRAM.
Illustration Datagram Packet Principle
TRANSFER TO THE ESTABLISHMENT OF A LOGICAL CONNECTION
TRANSFER TO THE ESTABLISHMENT OF A LOGICAL CONNECTION
Procedure for Harmonization of The Two End Nodes of A Network of Some Paramers of Packet Exchange Process IS Called The Establishment of a Logical Connection. Options NEGOTIATED by The Two Interacting Nodes, Called A Logical Connection Parameters.
Virtual Channel
Virtual Channel
The Only Pre-Padded Fixed Route Connecting End Nodes to the Packet-Switched Network, Referred to As A Virtual Channel. (Virtual Circuit or Virtual Channel) Virtual Channels Are Laid for Sustainable Information Flow. In Order to Isolate The Data Flow of the Total Traffic Flow of Each Packet Is Marked With A Special Kind of Sign - Label. As With the Establishment of A Logical Network Connections, The Virtual Channel Begins with a Special Package - The Connection Request.
Table Switching Networks using Virtual Channels Is Different from the Switching Table in Datagram Networks. IT Contains Entries Only Passing Through The Switch Virtual Channels, And Not All The Possible Destination Address, As Is the Case in Networks with Datagram Algorithm Transfer.
COMPARISON CIRCUIT-SWITCHED AND PACKET
| Switching Channels. | Packet Switching. |
| You Must First Establish A Connection | No Stage of Establishing A Connection (Datagram Method) |
| Location Is Only Required WHEN ESTABLISHING A Connection | ADDRESS AND OTHER SERVICE INFORMATION ARE TRANSMITTED WITH EACH PACKET |
| The Network May Refuse a Connection to the Subscriber | The Network Is Always Ready To Receive Data From Subscriber |
| Guaranteed Bandwidth (Bandwidth) for Interacting Subscribers | Network Bandwidth for Users Is Unknown, Transmission DELAYS ARE RANDOM |
| Real-Time Traffic Is Transferred Wort Delay | Network Resources Are Used Effectively WHEN TRANSMITTING BURSTY TRAFFIC |
| High Transmission Reliability | Possible Data Loss Due to Buffer Overflow |
| IRRATIONAL USE OF CHANNEL CAPACITY, REDUCING THE OVERALL EFFICIENCY OF THE NETWORK | Automatic Dynamic Bandwidth Allocation Of A Physical Channel Between Subscribers |
Methods for transferring digital information
Digital data on the conductor are transmitted by changing the current voltage: no voltage - "O", there is a voltage - "1". There are two ways to transfer information on a physical transmitting environment: digital and analog.
Notes: 1. If all computer network subscribers lead data on the channel at one frequency, such a channel is called narrowband (skips one frequency).
2. If each subscriber operates at its own frequency on one channel, then such a channel is called broadband (skips many frequencies). The use of broadband channels saves on their quantity, but complicates the process of controlling the exchange of data.
For digital or narrowband transmission method (Fig. 6.10) The data is transmitted in their natural form at a single frequency. A narrowband method allows you to transmit only digital information, ensures that only two users are able to use only two users and allows normal operation only at a limited distance (the length of the communication line is not more than 1000 m). At the same time, the narrowband transmission method ensures high data exchange rate - up to 10 Mbps and allows you to create easily configurable computing networks. The overwhelming number of local computing networks uses narrowband transmission.
Fig. 6.10. Digital transmission method
Analog The method of transmitting digital data (Fig. 6.11) provides broadband transmission due to use in one channel signals of various carrier frequencies.
With analog transmission method, control of the parameters of the carrier frequency signal for transmission over the digital data channel.
The carrier frequency signal is a harmonic oscillation described by the equation:
X \u003d x max sin (ωt + φ 0),
where x Max is the amplitude of oscillations;
ω - frequency of oscillations;
φ - the initial phase of oscillations.
You can pass the digital data on an analog channel by controlling one of the parameters of the carrier frequency signal: amplitude, frequency or phase. Since it is necessary to transmit data in binary form (sequence of units and zeros), then the following management methods can be offered ( modulations): amplitude, frequency, phase.
The easiest way to understand the principle amplitude Modulations: "0" - no signal, i.e. lack of carrier frequency oscillations; "1" - the presence of a signal, i.e. The presence of oscillations of the carrier frequency. There are oscillations - unit, no oscillations - zero (Fig. 6.11a).
Frequency Modulation provides for the transmission of signals 0 and 1 at different frequency. When moving from 0 to 1 and from 1 to 0, a carrier frequency signal changes (Fig. 6.116).
The most difficult to understand is phase modulation. Its essence is that when moving from 0 to 1 and from 1 to 0, the oscillation phase is changing, i.e. Their direction (Fig. 6.11b).
In the high-level networks of the hierarchy - global and regional are also used also broadband transferwhich provides for the work for each subscriber at its frequency within one channel. This ensures the interaction of a large number of subscribers at high data transfer rates.
Broadband allows you to combine digital data, images and sound in one channel, which is the necessary requirement of modern multimedia systems.
Example 6.5. A typical analog channel is a telephone channel. When the subscriber removes the tube, then hears the uniform beep - this is the carrier frequency signal. Since it lies in the range of sound frequencies, it is called a tonal signal. To transmit on a telephone channel, you need to control the carrier frequency signal - modulate it. The sounds perceived by microphone are converted into electrical signals, and those, in turn, and modulate the carrier frequency signal. When transmitting digital information, the Office produces information bytes - the sequence of units and zeros.
Hardware
To ensure the transmission of information from the computer to the communication environment, it is necessary to coordinate the signals of the internal interface of the computer with the parameters of the signals transmitted through the communication channels. At the same time, both physical coordination (shape, amplitude and duration of the signal) and code can be performed.
Technical devices that perform computer pairing functions with communication channels are called adapters or network adapters. One adapter provides compilation with a computer of one communication channel.
Fig. 6.11. Methods for transferring digital information on analog signal:
a - amplitude modulation; b - frequency; V - Phase
In addition to single-channel adapters, multichannel devices are used - data Multiplexers or simply multiplexers.
Data transfer multiplexer - A compilation device with multiple communication channels.
Data Multiplexers were used in data telework systems - the first step on the way to creating computing networks. In the future, in the appearance of networks with a complex configuration and with a large number of subscriber systems, special connection processors began to be applied to implement the interface functions.
As mentioned earlier, the bits flow to transform into analog signals to transmit digital information on the communication channel, and when receiving information from the communication channel to the computer, perform a reverse action - convert analog signals to the bits that can process computers. Such transforms performs a special device - modem.
Modem - A device that performs modulation and demodulation of information signals when transmitting them from computer to a communication channel and when receiving a computer from a communication channel.
The most expensive component of the computing network is a communication channel. Therefore, when building a number of computing networks, they are trying to save on communication channels by commuting several internal communication channels to one external. Special devices are used to perform switching functions - hubs.
Concentrator - A device that switches several communication channels to one by frequency separation.
In LAN, where the physical transmission medium is a limited length cable, special devices are used to increase the length of the network - repeators.
Repeater - A device that ensures the preservation of the shape and amplitude of the signal when transmitting it to more than is provided by this type of physical transmitting medium, distance.
There are local and remote repeaters. Local Repeators allow you to connect network fragments located at a distance of up to 50m, and distance - up to 2000 m.