Independent work : pp. 646–651, 720–722, pp. 67–79, 542–544, –651, pp. 48–58; pp. 408–431
Repeater (repeater) – transmits electrical signals from one section of the cable to another, pre-amplifying them and restoring their shape. Used in local networks to increase their length. In terminology OSI operates on a physical level.
Switches – multiport repeaters that read the destination address of each incoming packet and transmit it only through the port that is connected to the recipient computer. Can function on different OSI levels. (another version - duct level)
Hub (hub) – a multiport device for amplifying signals during data transmission. Used to add workstations to the network or to increase the distance between the server and the workstation (the total capacity of the input channels is higher than the capacity of the output channel). It works like a switch, but in addition can amplify the signal.
Multiplexer (device or program) – allows you to transmit several different signals simultaneously over one communication line.
Gateway – transfers data between networks or application programs that use different protocols (encoding methods, physical media for data transfer), for example, connecting a local network to a global one. Operates on applied level.
Bridge – connects two networks with the same protocols, amplifies the signal and passes only those signals that are addressed to the computer located on the other side of the bridge. Other edition : A computer with two network cards designed to connect networks.
Router – (connects different LANs, like a bridge, passes only that information that is intended for the segment to which it is connected.) Responsible for choosing the route for transmitting packets between nodes. The route is selected based on: – a routing protocol containing information about the network topology;
– a special routing algorithm.
Operates on network OSI level.
Unclear questions :
A device for connecting a computer with several communication channels is called:
– hub/repeater/multiplexer/modem
A device that switches several communication channels is called:
– data multiplexer/hub/repeater/modem
Basic Cryptography Concepts
Independent work : pp. 695–699
Cryptography (encryption) – encoding data sent to the network so that it can only be read by parties involved in a specific transaction. The reliability of the protection depends on the encryption algorithm and the length of the key in bits.
Encryption method – an algorithm that describes the procedure for converting the original message into the resulting message. Example . Method gamming – replacing letters with notes according to a certain algorithm.
Encryption key – a set of parameters necessary to apply the method. Another edition: – a sequence of characters stored on a hard or removable disk.
Static key – does not change when working with different messages.
Dynamic key – changes for each message.
Types of encryption methods .
– Symmetrical : The same key is used for both encryption and decryption. Inconvenient in e-commerce, since the seller and buyer must have different rights to access information. The seller sends all buyers the same catalogs, but buyers return confidential credit card information to the seller, and orders and payments cannot be mixed between different buyers.
– Asymmetrical (asymmetrical ): are based on special mathematical methods that create a pair of keys so that what is encrypted with one key can only be decrypted with another, and vice versa. One of the keys is called open , anyone can get it. The key developer keeps the second key for himself, it is called closed (secret) .
Orders, contracts are encrypted with the public key, but can only be read by the owner of the private key. If a client receives a file to which his key does not match, then it was not sent by his company.
Methods of transmitting digital information
Digital data is transmitted along the conductor by changing the current voltage: no voltage - “O”, voltage present - “1”. There are two ways to transmit information over a physical transmission medium: digital and analog.
Notes: 1. If all subscribers of a computer network transmit data over a channel at the same frequency, such a channel is called narrowband (passes one frequency).
2. If each subscriber operates on his own frequency using one channel, then such a channel is called broadband (passes many frequencies). The use of broadband channels allows you to save on their quantity, but complicates the process of managing data exchange.
At digital or narrowband transmission method(Fig. 6.10) data is transmitted in its natural form on a single frequency. The narrowband method allows only digital information to be transmitted, ensures that only two users can use the transmission medium at any given time, and allows normal operation only over a limited distance (communication line length no more than 1000 m). At the same time, the narrowband transmission method provides high data exchange speeds - up to 10 Mbit/s and allows you to create easily configurable computer networks. The overwhelming majority of local area networks use narrowband transmission.
Rice. 6.10. Digital transmission method
Analog The method of transmitting digital data (Fig. 6.11) provides broadband transmission through the use of signals of different carrier frequencies in one channel.
With the analog transmission method, the parameters of the carrier frequency signal are controlled for transmitting digital data over the communication channel.
The carrier frequency signal is a harmonic oscillation described by the equation:
Х=Х max sin (ωt +φ 0),
where X max is the amplitude of oscillations;
ω - oscillation frequency;
φ - initial phase of oscillations.
You can transmit digital data over 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 (a sequence of ones and zeros), the following control methods can be proposed ( modulation): amplitude, frequency, phase.
The easiest way to understand the principle is amplitude modulation: "0" - no signal, i.e. no carrier frequency oscillations; "1" - presence of a signal, i.e. presence of carrier frequency oscillations. There are oscillations - one, no oscillations - zero (Fig. 6.11a).
Frequency modulation involves transmitting signals 0 and 1 at different frequencies. When moving from 0 to 1 and from 1 to 0, the carrier frequency signal changes (Fig. 6.116).
The most difficult thing to understand is phase modulation. Its essence is that when moving from 0 to 1 and from 1 to 0, the phase of oscillations changes, i.e. their direction (Fig. 6.11c).
In high-level hierarchical networks - global and regional - it is also used broadband transmission, which provides for each subscriber to operate on its own frequency within one channel. This ensures the interaction of a large number of subscribers at high data transfer rates.
Wideband transmission allows you to combine the transmission of digital data, image and sound in one channel, which is a necessary requirement of modern multimedia systems.
Example 6.5. A typical analog channel is a telephone channel. When the subscriber picks up the handset, he hears a uniform sound signal - this is the carrier frequency signal. Since it lies in the audio frequency range, it is called a tone signal. To transmit speech over a telephone channel, it is necessary to control the carrier frequency signal - to modulate it. The sounds received by the microphone are converted into electrical signals, which, in turn, modulate the carrier frequency signal. When transmitting digital information, control is carried out by information bytes - a sequence of ones and zeros.
Hardware
To ensure the transfer of information from a 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 via communication channels. In this case, both physical matching (shape, amplitude and duration of the signal) and code matching must be performed.
Technical devices that perform the functions of interfacing a computer with communication channels are called adapters or network adapters. One adapter provides pairing with a computer of one communication channel.
Rice. 6.11. Methods of transmitting digital information over an analog signal:
a – amplitude modulation; b – frequency; c - phase
In addition to single-channel adapters, multi-channel devices are also used - data transmission multiplexers or simply multiplexers.
Data transmission multiplexer- a device for interfacing a computer with several communication channels.
Data transmission multiplexers were used in teleprocessing systems - the first step towards the creation of computer networks. Later, with the emergence of networks with complex configurations and a large number of subscriber systems, special communication processors began to be used to implement interfacing functions.
As mentioned earlier, to transmit digital information over a communication channel, it is necessary to convert a stream of bits into analog signals, and when receiving information from a communication channel to a computer, perform the opposite action - convert analog signals into a stream of bits that the computer can process. Such transformations are performed by a special device - modem.
Modem- a device that modulates and demodulates information signals when transmitting them from a computer to a communication channel and when receiving them from a communication channel into a computer.
The most expensive component of a computer network is the communication channel. Therefore, when building a number of computer networks, they try to save on communication channels by switching several internal communication channels to one external one. To perform switching functions, special devices are used - hubs.
Hub- a device that switches several communication channels into one through frequency division.
In a LAN, where the physical transmission medium is a cable of limited length, special devices are used to increase the length of the network - repeaters.
Repeater- a device that ensures the preservation of the shape and amplitude of the signal when transmitting it over a distance greater than that provided by this type of physical transmission medium.
There are local and remote repeaters. Local repeaters allow you to connect network fragments located at a distance of up to 50m, and remote- up to 2000 m.
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 ones, which include channel switching and packet switching. 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.
Therefore, 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 is equal to 64 kbit/s, which is sufficient for high-quality digital voice.
For high-quality voice uses 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 = 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 composite 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 subscribers, for which the composite 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 there is at this time in the other network connection or not;
- after the end of the session the basic channels that were included with the corresponding 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 channels 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 exchange is carried out in advance, even before the start of 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 ineffective, 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 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. In addition to the title in the package may have one additional field to be placed at the end of the package and so-called trailer. In the trailer is usually 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. After 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 commutation 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 packet switch of the switches in the circuit-switched networks is that they have an internal buffer memory for temporarily storing 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 their 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 of parameters that it carries, and the current state of the network. And every single packet network is considered as a completely 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 parameters 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 gasket from the source node 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 without 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 |
In this article we will consider the main methods of switching in networks.
In traditional telephone networks, communication between subscribers is carried out using switching communication channels. At the beginning, switching of telephone communication channels was performed manually, then switching was performed by automatic telephone exchanges (ATS).
A similar principle is used in computer networks. Geographically remote computers in a computer network act as subscribers. It is physically impossible to provide each computer with its own non-switched communication line, which they would use at all times. Therefore, in almost all computer networks, some method of switching subscribers (workstations) is always used, which makes it possible for several subscribers to access existing communication channels in order to provide several communication sessions simultaneously.
Switching is the process of connecting various subscribers of a communication network through transit nodes. Communication networks must ensure that their subscribers communicate with each other. Subscribers can be computers, local network segments, fax machines or telephone interlocutors.
Workstations are connected to switches using individual communication lines, each of which is used at any time by only one subscriber assigned to this line. Switches are connected to each other using shared communication lines (shared by several subscribers).
Let's look at the three main most common methods of switching subscribers in networks:
- circuit switching;
- packet switching;
- message switching.
Circuit switching
Circuit switching involves the formation of a continuous composite physical channel from individual channel sections connected in series for direct data transfer between nodes. Individual channels are connected to each other by special equipment - switches, which can establish connections between any end nodes of the network. In a circuit-switched network, before transmitting data, it is always necessary to perform a connection establishment procedure, during which a composite channel is created.
The message transmission time is determined by the channel capacity, the length of the connection and the size of the message.
Switches, as well as the channels connecting them, must ensure the simultaneous transmission of data from several subscriber channels. To do this, they must be high-speed and support some kind of subscriber channel multiplexing technique.
Advantages of circuit switching:
- constant and known data transfer rate;
- correct sequence of data arrival;
- low and constant latency of data transmission through the network.
Disadvantages of circuit switching:
- the network may refuse to service the request to establish a connection;
- irrational use of the capacity of physical channels, in particular the inability to use user equipment operating at different speeds. Individual parts of a composite circuit operate at the same speed because circuit-switched networks do not buffer user data;
- mandatory delay before data transmission due to the connection establishment phase.
Message switching is the division of information into messages, each of which consists of a header and information.
This is a method of interaction in which a logical channel is created by sequentially transmitting messages through communication nodes to the address specified in the message header.
In this case, each node receives a message, writes it to memory, processes the header, selects a route, and issues a message from memory to the next node.
The message delivery time is determined by the processing time at each node, the number of nodes, and the network capacity. When the transfer of information from node A to communication node B ends, node A becomes free and can participate in organizing other communications between subscribers, so the communication channel is used more efficiently, but the routing control system will be complex.
Today, message switching in its pure form practically does not exist.
Packet switching is a special method of switching network nodes, which was specially created for the best transmission of computer traffic (pulsating traffic). Experiments in the development of the very first computer networks, which were based on circuit switching technology, showed that this type of switching does not provide the opportunity to obtain high throughput of a computer network. The reason lay in the bursty nature of traffic that typical network applications generate.
When packet switching occurs, all messages transmitted by a network user are broken up at the source node into relatively small parts called packets. It is necessary to clarify that a message is a logically completed piece of data - a request to transfer a file, a response to this request containing the entire file, etc. Messages can have an arbitrary length, from several bytes to many megabytes. On the contrary, packets can usually also have a variable length, but within narrow limits, for example from 46 to 1500 bytes (EtherNet). Each packet is provided with a header that specifies the address information needed to deliver the packet to the destination node, as well as the packet number that will be used by the destination node to assemble the message.
Packet network switches differ from circuit switches in that they have internal buffer memory to temporarily store packets if the switch's output port is busy transmitting another packet when a packet is received.
Advantages of packet switching:
- more resistant to failures;
- high overall network throughput when transmitting bursty traffic;
- the ability to dynamically redistribute the bandwidth of physical communication channels.
Disadvantages of packet switching:
- uncertainty of data transfer speed between network subscribers;
- variable delay of data packets;
- possible data loss due to buffer overflow;
- There may be irregularities in the sequence of packet arrivals.
Computer networks use packet switching.
Methods of transmitting packets in networks:
- Datagram method– transmission is carried out as a set of independent packets. Each packet moves through the network along its own route and the user receives the packets in random order.
- Advantages: simplicity of the transfer process.
- Disadvantages: low reliability due to the possibility of packet loss and the need for software to assemble packets and restore messages.
- Logical channel is the transmission of a sequence of packets linked in a chain, accompanied by the establishment of a preliminary connection and confirmation of the receipt of each packet. If the i-th packet is not received, then all subsequent packets will not be received.
- Virtual channel– this is a logical channel with the transmission of a sequence of packets connected in a chain along a fixed route.
- Advantages: the natural sequence of data is preserved; sustainable traffic routes; resource reservation is possible.
- Disadvantages: hardware complexity.
In this article, we reviewed the main switching methods in computer networks, with a description of each switching method indicating the advantages and disadvantages.
Read also:
|
Topology The system of relationships between components of a Windows network. When applied to Active Directory replication, topology comes down to the set of connections used by domain controllers to communicate with each other.
(1) Computer networks implement information processing. M204, M205
parallel
local
●distributed
bidirectional
(1)The web page address for viewing in a browser begins with:
LAN COMBINATION
Reasons for combining LANs
A LAN system created at a certain stage of development over time ceases to satisfy the needs of all users, and then the problem of expanding its functionality arises. It may be necessary to combine within a company various LANs that appeared in its various departments and branches at different times, at least to organize data exchange with other systems. The problem of expanding the network configuration can be solved both within a limited space and with access to the external environment.
The desire to gain access to certain information resources may require connecting a LAN to higher-level networks.
In the simplest version, LAN consolidation is necessary to expand the network as a whole, but the technical capabilities of the existing network are exhausted, and new subscribers cannot be connected to it. You can only create another LAN and combine it with an existing one, using one of the methods listed below.
Methods for combining LANs
Bridge. The simplest option for combining a LAN is to combine identical networks within a limited space. The physical transmission medium imposes restrictions on the length of the network cable. Within the permissible length, a network segment is built - a network segment. To combine network segments, they are used bridges.
Bridge- a device that connects two networks using the same data transfer methods.
The networks that the bridge connects must have the same network levels of the open systems interaction model; the lower levels may have some differences.
For a network of personal computers, a bridge is a separate computer with special software and additional equipment. A bridge can connect networks of different topologies, but running the same type of network operating systems.
Bridges can be local or remote.
Local Bridges connect networks located in a limited area within an existing system.
Deleted Bridges connect geographically dispersed networks using external communication channels and modems.
Local bridges, in turn, are divided into internal and external.
Domestic bridges are usually located on one of the computers of a given network and combine the function of a bridge with the function of a subscriber computer. Expansion of functions is carried out by installing an additional network card.
External Bridges require the use of a separate computer with special software to perform their functions.
Router (router). A complex network, which is a connection of several networks, requires a special device. The task of this device is to send a message to the recipient on the desired network. This device is called m router.
A router, or router, is a device that connects networks of different types, but uses the same operating system.
A router performs its functions at the network layer, so it depends on communication protocols, but does not depend on the type of network. Using two addresses - the network address and the host address, the router uniquely selects a specific network station.
Example 6.7. It is necessary to establish a connection with a telephone network subscriber located in another city. First, the address of the telephone network of this city is dialed - the area code. Then - the node address of this network - telephone number subscriber Functions The router is performed by PBX equipment.
The router can also choose the best path for transmitting a message to a network subscriber, filters the information passing through it, sending to one of the networks only the information that is addressed to it.
In addition, the router provides load balancing in the network by redirecting message flows over free communication channels.
Gateway. To combine LANs of completely different types, operating under significantly different protocols, special “devices are provided - gateways.
A gateway is a device that allows you to organize data exchange between two networks using different communication protocols.
The gateway performs its functions at levels above the network level. It does not depend on the transmission medium used, but depends on the data exchange protocols used. Typically a gateway converts between two protocols.
Using gateways, you can connect a local area network to the host computer, as well as connect a local network to a global one.
Example 6.8. It is necessary to unite local networks located in different cities. This problem can be solved using a global data network. Such a network is a packet switching network based on the X.25 protocol. Using a gateway, the local area network is connected to the X.25 network. The gateway performs the necessary protocol conversions and ensures data exchange between networks.
Bridges, routers and even gateways are constructed in the form of boards that are installed in computers. They can perform their functions both in the mode of completely separating functions, and in the mode of combining them with the functions of a computer network workstation.
(1) A computer that has 2 network cards and is designed to connect networks is called:
Router
Amplifier
Switch
(1) A device that switches several communication channels into one by frequency division is called...
repeater
●hub
data transmission multiplexer
HARDWARE IMPLEMENTATION OF DATA TRANSFER
Methods of transmitting digital information
Digital data is transmitted along the conductor by changing the current voltage: no voltage - “O”, voltage present - “1”. There are two ways to transmit information over a physical transmission medium: digital and analog.
Notes: 1. If all subscribers of a computer network transmit data over a channel at the same 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(passes many frequencies). The use of broadband channels allows you to save on their quantity, but complicates the process of managing data exchange.
At digital or narrowband transmission method(Fig. 6.10) data is transmitted in its natural form on a single frequency. The narrowband method allows only digital information to be transmitted, ensures that only two users can use the transmission medium at any given time, and allows normal operation only over a limited distance (communication line length no more than 1000 m). At the same time, the narrowband transmission method provides high data exchange speeds - up to 10 Mbit/s and allows you to create easily configurable computer networks. The overwhelming majority of local area networks use narrowband transmission.
Rice. 6.10. Digital transmission method
Analog The method of transmitting digital data (Fig. 6.11) provides broadband transmission through the use of signals of different carrier frequencies in one channel.
With the analog transmission method, the parameters of the carrier frequency signal are controlled for transmitting digital data over the communication channel.
The carrier frequency signal is a harmonic oscillation described by the equation: "
A r =A r max sin(atf+9 0),
where Xmax is the amplitude of oscillations; co - oscillation frequency; t- time; f 0 - initial phase of oscillations.
You can transmit digital data over 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 (a sequence of ones and zeros), the following control methods can be proposed (modulation): amplitude, frequency, phase.
The easiest way to understand the principle is amplitude modulation: "O" - no signal, i.e. no carrier frequency oscillations; "1" - presence of a signal, i.e. presence of carrier frequency oscillations. There are oscillations - one, no oscillations - zero (Fig. 6.11 A).
Frequency modulation involves transmitting signals 0 and 1 at different frequencies. When moving from 0 to 1 and from 1 to 0, the carrier frequency signal changes (Fig. 6.116).
The most difficult thing to understand is phase modulation. Its essence is that when moving from 0 to 1 and from 1 to 0, the phase of oscillations changes, i.e. their direction (Fig. 6.11 V).
In high-level hierarchical networks - global and regional - it is also used broadband transmission, which provides for each subscriber to operate on its own frequency within one channel. This ensures the interaction of a large number of subscribers at high data transfer rates.
Wideband transmission allows you to combine the transmission of digital data, image and sound in one channel, which is a necessary requirement of modern multimedia systems.
Example 6.5. A typical analog channel is a telephone channel. When the subscriber picks up the handset, he hears a uniform sound signal - this is the carrier frequency signal. Since it lies in the audio frequency range, it is called a tone signal. To transmit speech over a telephone channel, it is necessary to control the carrier frequency signal - to modulate it. The sounds received by the microphone are converted into electrical signals, which, in turn, modulate the carrier frequency signal. When transmitting digital information, control is carried out by information bytes - a sequence of ones and zeros.
Hardware
To ensure the transfer of information from a 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 via communication channels. In this case, both physical matching (shape, amplitude and duration of the signal) and code matching must be performed.
Technical devices that perform the functions of interfacing a computer with communication channels are called adapters or network adapters. One adapter provides pairing with a computer of one communication channel.
Rice. 6.11. Methods of transmitting digital information over an analog signal: A- amplitude modulation; b- frequency; V- phase
In addition to single-channel adapters multi-channel devices are also used - data transmission multiplexers or simply multiplexers.
Data transmission multiplexer- a device for interfacing a computer with several communication channels.
Data transmission multiplexers were used in teleprocessing systems - the first step towards the creation of computer networks. Later, with the emergence of networks with complex configurations and a large number of subscriber systems, special communication processors began to be used to implement interfacing functions.
As mentioned earlier, to transmit digital information over a communication channel, it is necessary to convert a stream of bits into analog signals, and when receiving information from a communication channel to a computer, perform the opposite action - convert analog signals into a stream of bits that the computer can process. Such transformations are performed by a special device - mod eat.
Modem- a device that modulates and demodulates information signals when transmitting them from a computer to a communication channel and when receiving them from a communication channel into a computer.
The most expensive component of a computer network is the communication channel. Therefore, when building a number of computer networks, they try to save on communication channels by switching several internal communication channels to one external one. To perform switching functions, special devices are used - hubs.
Hub- a device that switches several communication channels into one through frequency division.
In a LAN, where the physical transmission medium is a cable of limited length, special devices are used to increase the length of the network - repeaters.
Repeater- a device that ensures the preservation of the shape and amplitude of the signal when transmitting it over a distance greater than that provided by this type of physical transmission medium.
There are local and remote repeaters. Local repeaters allow you to connect network fragments located at a distance of up to 50 m, and remote- up to 2000 m.
Communication network characteristics
To assess the quality of a communication network, you can use the following characteristics:
■ data transmission speed over the communication channel;
■ communication channel capacity;
■ reliability of information transfer;
■ reliability of the communication channel and modems.
Data transfer rate over a communication channel is measured by the number of bits of information transmitted per unit of time - a second.
Remember! The unit of data transfer speed is bits per second.
Note. The frequently used unit of speed measurement is baud. Baud is the number of state changes of the transmission medium per second. So How each state change can correspond to several bits of data, then real speed in bits per second can exceed the baud rate.
The data transfer speed depends on the type and quality of the communication channel, the type of modems used and the synchronization method adopted.
Thus, for asynchronous modems and a telephone communication channel, the speed range is 300 - 9600 bps, and for synchronous modems - 1200 - 19200 bps.
For users of computer networks, what matters is not abstract bits per second, but information, the unit of measurement of which is bytes or characters. Therefore, a more convenient characteristic of a channel is its throughput, which is estimated by the number of characters transmitted over the channel per unit of time - a second. In this case, all service characters are included in the message. Theoretical throughput is determined by the data transfer rate. The actual throughput depends on a number of factors, including the transmission method, the quality of the communication channel, its operating conditions, and the message structure.
Remember! The unit of measurement for communication channel capacity is digit per second.
An essential characteristic of any network communication system is reliability transmitted information. Since based on the processing of information about the state of the control object, decisions are made about one or another course of the process, the fate of the object may ultimately depend on the reliability of the information. The reliability of information transmission is assessed as the ratio of the number of erroneously transmitted characters to the total number of transmitted characters. The required level of reliability must be provided by both the equipment and the communication channel. It is inappropriate to use expensive equipment if the communication channel does not meet the necessary requirements regarding the level of reliability. *
Remember! Reliability unit: number of errors per sign - errors/sign.
For computer networks, this indicator should be within 10 -6 - 10~ 7 errors/sign, i.e. One error is allowed per million characters transmitted or per ten million characters transmitted.
Finally, reliability of a communication system is determined either by the proportion of uptime in the total operating time, or by the average time between failures. The second characteristic allows you to more effectively assess the reliability of the system.
Remember! Unit of measurement of reliability: average time between failures - hour.
For computer networks, the mean time between failures must be quite large and amount to at least several thousand hours.
226 CHAPTER 6. COMPUTER NETWORKS
6.3. LOCAL COMPUTER NETWORKS
Features of LAN organization
Typical LAN topologies and access methods
LAN merger
FEATURES OF LAN ORGANIZATION
Functional groups of devices on the network
The main purpose of any computer network is to provide information and computing resources to users connected to it.
From this point of view, a local area network can be considered as a collection of servers and workstations.
Server- a computer connected to the network and providing its benefits providers of certain services.
Servers can perform data storage, database management, remote job processing, job printing and a number of other functions that network users may need. The server is the source of network resources.
Work station- a personal computer connected to a network through which the user gains access to its resources.
Work station The network operates in both network and local modes. It is equipped with its own operating system (MS DOS, Windows, etc.) and provides the user with all the necessary tools for solving applied problems.
Particular attention should be paid to one type of server - a file server. In common terminology, the abbreviated name is accepted for it - file server.
The file server stores the data of network users and provides them with access to this data. This is a computer with a large capacity of RAM, high-capacity hard drives and additional magnetic tape drives (streamers).
It runs under a special operating system that provides simultaneous access to the data located on it by network users.
The file server performs the following functions: data storage, data archiving, synchronization of data changes by different users, data transfer.
For many tasks, using a single file server is not enough. Then several servers can be included in the network. It is also possible to use mini-computers as file servers.
Managing the interaction of devices on the network
Information systems built on the basis of computer networks provide solutions to the following tasks: data storage, data processing, organizing user access to data, transferring data and data processing results to users.
In centralized processing systems, these functions were performed by the central computer (Mainframe, Host).
Computer networks implement distributed data processing. Data processing in this case is distributed between two objects: client And server.
Client- a task, workstation or computer network user.
During data processing, the client can create a request to the server to perform complex procedures, read a file, search for information in a database, etc.
The server defined earlier fulfills the request received from the client. The results of the request are transmitted to the client. The server provides storage of public data, organizes access to this data, and transmits the data to the client.
The client processes the received data and presents the processing results in a form convenient for the user. In principle, data processing can also be performed on the server. For such systems, the terms adopted are systems client-server or architecture client-server.
The client-server architecture can be used both in peer-to-peer local area networks and in a network with a dedicated server.
Peer-to-peer network. In such a network there is no single center for managing the interaction of workstations and there is no single device for storing data. The network operating system is distributed across all workstations. Each network station can perform the functions of both a client and a server. It can service requests from other workstations and forward its own service requests to the network.
The network user has access to all devices connected to other stations (disks, printers).
Advantages of peer-to-peer networks: low cost and high reliability.
Disadvantages of peer-to-peer networks:
■ dependence of network efficiency on the number of stations;
■ complexity of network management;
■ difficulty in ensuring information security;
■ difficulties in updating and changing station software. The most popular are peer-to-peer networks based on network
operating systems LANtastic, NetWare Lite.
Network with highlighted server. In a network with a dedicated server, one of the computers performs the functions of storing data intended for use by all workstations, managing interaction between workstations, and a number of service functions.
Such a computer is usually called a network server. A network operating system is installed on it, and all shared external devices are connected to it - hard drives, printers and modems.
Interaction between workstations on a network is usually carried out through a server. The logical organization of such a network can be represented by the topology star. The role of the central device is performed by the server. In networks with centralized management, it is possible to exchange information between workstations, bypassing the file server. To do this, you can use the NetLink program. After running the program on two workstations, you can transfer files from the disk of one station to the disk of another (similar to the operation of copying files from one directory to another using Norton Commander).
Advantages of a network with a dedicated server:
■ reliable information security system;
■ high performance;
■ no restrictions on the number of workstations;
■ ease of management compared to peer-to-peer networks. Network disadvantages:
■ high cost due to the allocation of one computer for the server;
■ dependence of network speed and reliability on the server;
■ less flexibility compared to a peer-to-peer network.
Dedicated server networks are the most common among computer network users. Network operating systems for such networks are LANServer (IBM), Windows NT Server versions 3.51 and 4.0, and NetWare (Novell).
(1)Local area networks cannot be interconnected using...M232
gateways, bridges
●hubs, modems
servers
routers
(1)BBS is...M745
navigator
software for working on the Intranet
●system of electronic bulletin boards on the Internet
organization server maintenance program
(1) Client-server data processing, this is processing. M227
parallel
localized
bidirectional
●distributed
(1)The Bat program allows...
load web pages
●upload and edit email
archive email
(1) One of the search engines on the Internet is...
(1)Internet Explorer allows...
chat via IRC protocol
●download web pages via http protocol and files via FTP protocol
download newsgroups via NNTP protocol
(1)Telephone cable is option...M228
optical - high frequency
coaxial cable
fiber optic
●twisted pair
(1)The Usenet system is used...M239
registration of users on the network
●to move news between computers around the world
processing information on the network
creating a workstation on the network
(1)The Usenet discussion group is called...M239
server group
group online
●teleconference
(1)The flow of messages in a data network is determined...
message channel memory capacity
●traffic
6.1. COMMUNICATION ENVIRONMENT AND DATA TRANSMISSION
Purpose and classification of computer networks
Characteristics of the data transfer process
Hardware implementation of data transfer
Data Links
PURPOSE AND CLASSIFICATION OF COMPUTER NETWORKS
Distributed Data Processing
Modern production requires high speeds of information processing, convenient forms of its storage and transmission. It is also necessary to have dynamic ways of accessing information, ways of searching for data in given time intervals; implement complex mathematical and logical data processing. Managing large enterprises and managing the economy at the country level require the participation of fairly large teams in this process. Such groups can be located in different areas of the city, in different regions of the country, and even in different countries. To solve management problems that ensure the implementation of economic strategy, the speed and convenience of information exchange, as well as the possibility of close interaction between all those involved in the process of developing management decisions, become important and relevant.
In the era of centralized use of computers with batch information processing, computer users preferred to purchase computers that could solve almost all classes of their problems. However, the complexity of the problems being solved is inversely proportional to their number, and this led to inefficient use of computer computing power at significant material costs. One cannot ignore the fact that access to computer resources was difficult due to the existing policy of centralizing computing resources in one place.
Principle centralized data processing (Fig. 6.1) did not meet high requirements for the reliability of the processing process, hampered the development of systems and could not provide the necessary time parameters for interactive data processing in multi-user mode. A short-term failure of the central computer led to fatal consequences for the system as a whole, since it was necessary to duplicate the functions of the central computer, significantly increasing the costs of creating and operating data processing systems.
 |
Rice. 6.2. Distributed data processing system
The emergence of small computers, microcomputers and, finally, personal computers required a new approach to the organization of data processing systems and the creation of new information technologies. A logically justified requirement has arisen to transition from the use of individual computers to centralized data processing systems to distributed data processing (Fig. 6.2).
Distributed Data Processing- data processing performed on independent but interconnected computers representing a distributed system.
To implement distributed data processing, we created multi-machine associations, the structure of which is developed in one of the following directions:
■ multi-machine computing systems (MCC);
■ computer (computer) networks.
Multi-machine computing complex- a group of computers installed nearby, united using special interface tools and jointly performing a single information and computing process.
Note: Belowprocess a certain sequence of actions to solve a problem, determined by the program, is understood.
Multi-machine computing systems can be:
local provided that computers are installed in the same room and do not require special equipment and communication channels for interconnection; remote, if some computers of the complex are installed at a considerable distance from the central computer and telephone communication channels are used for data transmission.
Example 6.1. It is connected to a mainframe-type computer that provides batch information processing mode using a mini-computer interface device. Both computers are located in the same computer room. The mini-computer provides preparation and preliminary processing of data, which is subsequently used to solve complex problems on the mainframe. This is a local multi-machine complex.
Example 6.2. Three computers are combined into a complex to distribute tasks received for processing. One of them performs a dispatching function and distributes tasks depending on the occupancy of one of the other two processing computers. This is a local multi-machine complex.
Example 6.3. A computer that collects data for a certain region performs preliminary processing and transmits it for further use to the central computer via a telephone communication channel. This is a remote multi-machine complex.
Computer (computer) network- a set of computers and terminals connected via communication channels into a single system that meets the requirements of distributed data processing.
Note. Under system is understood as an autonomous set consisting of one or more computers, software, peripheral equipment, terminals, data transmission facilities, physical processes and operators, capable of processing information and performing functions of interaction with other systems.
Generalized structure of a computer network
Computer networks are the highest form of multi-machine associations. Let us highlight the main differences between a computer network and a multi-machine computing complex.
The first difference is the dimension. A multi-machine computing complex usually includes two, maximum three computers, located mainly in one room. A computer network can consist of tens and even hundreds of computers located at a distance from each other from several meters to tens, hundreds and even thousands of kilometers.
The second difference is the division of functions between computers. If in a multi-machine computing complex the functions of data processing, data transfer and system management can be implemented in one computer, then in computer networks these functions are distributed between different computers.
The third difference is the need to solve the problem of message routing in the network. A message from one computer to another in the network can be transmitted along different routes depending on the state of the communication channels connecting the computers to each other.
Combining computer technology, communication equipment and data transmission channels into one complex places specific requirements on the part of each element of a multi-machine association, and also requires the formation of a special terminology.
Network subscribers- objects that generate or consume information on the network.
Subscribers networks can be individual computers, computer complexes, terminals, industrial robots, numerically controlled machines, etc. Any network subscriber connects to the station.
Station- equipment that performs functions related to the transmission and reception of information.
The set of subscriber and station is usually called subscriber system. To organize the interaction of subscribers, a physical transmission medium is required.
Physical transmission medium - communication lines or space in which electrical signals propagate and data transmission equipment.
Based on the physical transmission medium, it is built communication network, which ensures the transfer of information between subscriber systems.
This approach allows us to consider any computer network as a set of subscriber systems and a communication network. The generalized structure of a computer network is shown in Fig. 6.3.
 |
Rice. 6.3. Generalized structure of a computer network
Classification of computer networks
Depending on the territorial location of subscriber systems, computer 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).
Global The computer network unites subscribers located in different countries and on different continents. Interaction between subscribers of such a network can be carried out on the basis of telephone communication lines, radio communications and satellite communication systems. Global computer networks will solve the problem of uniting the information resources of all humanity and organizing access to these resources.
Regional A computer network connects subscribers located at a considerable distance from each other. It may include subscribers within a large city, economic region, or individual country. Typically, the distance between subscribers of a regional computer network is tens to hundreds of kilometers.
Local A computer network unites subscribers located within a small area. Currently, there are no specific restrictions on the territorial dispersion of local area network subscribers. Typically, such a network is connected to a specific location. The class of local computer networks includes networks of business enterprises, firms, banks, offices, etc. The length of such a network can be limited to 2 - 2.5 km.
The combination of global, regional and local computer networks makes it possible to create multi-network hierarchies. They provide powerful, cost-effective means of processing huge amounts of information and access to limited information resources. In Fig. Figure 6.4 shows one of the possible hierarchies of computer networks. .Local computer networks can be included as components in a regional network, regional networks can be united as part of a global network, and, finally, global networks can also form complex structures
Rice. 6.4. Hierarchy of computer networks
Example 6.4. The Internet computer network is the most popular global network. It consists of many loosely connected networks. Within each network that is part of the Internet, there is a specific communication structure and a specific management discipline. Within the Internet, the structure and methods of connections between different networks have no meaning for a particular user.
Personal computers, which have now become an indispensable element of any control system, have led to a boom in the creation of local computer networks. This, in turn, necessitated the development of new information technologies.
The practice of using personal computers in various branches of science, technology and production has shown that the greatest efficiency from the introduction of computer technology is provided not by individual autonomous PCs, but by local computer networks.
(1)The network subscribers are.. M205.
network administrators
PC users
●objects that generate or consume network information
communications equipment
(1) Network subscribers cannot be...M205
●computer complexes (can)
Terminals (can)
individual computers (may)
end users
(1) The network server is a computer...M226 (the server is the source of network resources)
with the highest processor frequency
providing access to the keyboard and monitor
with the largest amount of memory
●providing access to resources
(1)FTP server is...M240
a computer that contains files intended for the network administrator
a computer that contains information for organizing teleconferences
corporate server
●a computer that contains files intended for public access
(1)SMTP protocol is designed for...
(SMTP protocol A component of the TCP/IP protocol suite; this protocol manages the exchange of email messages between message transfer agents.
POP3 Protocol A popular protocol for receiving email messages. This protocol is often used by Internet service providers. POP3 servers only allow access to one mailbox, unlike IMAP servers which allow access to multiple folders on the server.
A set of network protocols widely used on the Internet that supports communication between interconnected networks consisting of computers of different architectures and operating systems. The TCP/IP protocol includes standards for communication between computers and conventions for connecting networks and rules for routing messages.)
Chatting
●Sending emails
Web browsing
Receive email
(1) The most effective communication method for transmitting computer traffic is...
●M220 packages
messages
all equally effective
