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Xxxii. Communication equipment

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.

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 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 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

Interlocutors. As a rule, in general access networks it is impossible to provide each couple of subscribers their 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 packet loss and need to build packages and recovery 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.

Lecture number 8.

Characteristics of information channels

Information channel can also be characterized by three relevant parameters: channel use time T K. , bandwidth bands transmitted by the canalF K., and dynamic channel rangeD K. It characterizes its ability to transmit different signal levels.

Validates capacity Channel.

Unlisted signaling is possible only if the signal is "fitted" in its volume into the channel capacity.

Consequently, the overall condition for negotiating the signal with information channel is determined by the ratio

However, the ratio expresses the necessary, but insufficient condition for matching the signal with the channel. A sufficient condition is the coordination in all respects:

For the information channel uses concepts: information entry speed, information transfer rate and channel bandwidth.

Under the speed of information entry (information flow) I. ( X. ) Understand the average amount of information entered from the source of messages to the information channel per unit of time. This feature of the message source is determined only by the statistical properties of messages.

Information transfer rate I. ( Z. , Y. ) - The average amount of information transmitted via channel per unit time. It depends on the statistical properties of the transmitted signal and on the properties of the channel.

Bottomy S. - The largest theoretically achievable information transfer rate for this channel. This is the characteristic of the channel and does not depend on the statistics of the signal.

The bandwidth of the information channel is determined by two parameters: bit and frequency. It is proportional to their work.

Bottomy Call the maximum amount of information that can be simultaneously placed in the channel.

Frequency Shows how many times the information can be placed in the channel for a unit of time.

The discharge of the postal channel is enormous. So, shipping by mail, for example, a laser disk, can be placed simultaneously in the channel of more than 600 MB of information. At the same time, the mail channel frequency is very low - the removal of mail from the boxes occurs more often than five times a day.

Telephone channel information SingleBody: Simultaneously on the telephone wire you can send or unit (current, impulse), or zero. The frequency of this channel can reach tens and hundreds of thousands of cycles per second. This property of the telephone network allows you to use it for communication between computers.

In order to make the most efficient use of the information channel, it is necessary to take measures to ensure that the information transfer rate is as close as possible to the channel bandwidth. At the same time, the speed of entering information should not exceed the channel bandwidth, otherwise not all information will be transferred via the channel.

This is the basic condition for the dynamic coordination of the source of messages and the information channel.

One of the main issues in the theory of information transfer is to determine the dependence of the speed of transmitting information and bandwidth from the channel parameters and characteristics of signals and interference. These questions were for the first time deeply studied by K. Shannon.

1. Methods of increasing noise immunity

In the basics of all methods of increasing the noise immunity of information systems, the use of certain differences between the useful signal and an interference. Therefore, a priori information about interference and signal properties is necessary to combat interference.

Currently, a large number of ways to increase the noise immunity of systems are known. These ways are convenient to break into two groups.

I. The Group is based on choosing a messaging method.

II. The Group is associated with the construction of noise-resistant receivers.

Simple and applied method of increasing noise immunity is increase signal / interference ratio By increasing the power of the transmitter. But this method may be economically unable, as it is associated with a significant increase in the complexity and cost of equipment. In addition, an increase in the transmission capacity is accompanied by the enhancement of the interfering effect of this channel to others.

An important way to increase the noise immunity of transmission of continuous signals is rational choice of modulation type signals. Applying modulation types that provide a significant extension of the signal frequency band, one can achieve a significant increase in the noise immunity of the transmission.

Radical way to increase the noise immunity of transmission of discrete signals is to use special noise-resistant codes . At the same time there are two ways to increase the noise immunity of the codes:

1. The choice of transmission methods that provide a smaller probability of code distortion;

2. Increase the correction properties of code combinations. This path is associated with the use of codes that allow you to detect and eliminate distortions in code combinations. This encoding method is associated with the introduction of additional, redundant symbols into the code, which is accompanied by an increase in the transmission time or frequency of transmission of code characters.

An increase in the noise immunity of transmission can also be achieved by re-transmitting the same message. On the receiving side, the received messages are compared and those that have the greatest number of coincidences are taken as true. To eliminate uncertainty when processing adopted information and ensure the selection by the criterion of the majority, the message should be repeated at least three times. This method of increasing noise immunity is associated with increasing transmission time.

Systems with repetition of discrete information are divided into systems with group summation, in which the comparison is made according to code combinations, and on the system with seductive summation, in which the comparison is carried out by code combinations in symbols. Suitable check is more efficient than groupage.

A variety of systems that have an increase in noise immunity due to an increase in transmission time are feedback systems. If there are distortion in the transmitted messages, the information coming over the reverse channel provides a repetition of the transmission. The presence of a reverse channel leads to a complication of the system. However, in contrast to the transmission repetition systems in feedback systems, the transmission repetition will take place only in case of distortion in the transmitted signal, i.e. Redundancy generally turns out to be less.

Noise resistant reception It consists in using redundancy, as well as a priori information about signals and interference to solve the optimal method of receiving task: signal detection, signal differences or message recovery. Currently, the apparatus of the theory of statistical solutions is widely used for the synthesis of optimal receivers.

The receiver errors decrease with an increase in the signal / interference ratio at the receiver input. In this regard, it often produces preliminary processing of the received signal in order to increase the relationship of the useful component to interference. Such methods of pre-processing signals include the show method (combination of broadband amplifier, limiter and narrowband amplifier), the selection of signal signals, the measurement method, the filtering method, the correlation method, the accumulation method, etc.

2. Modern technical means of exchanging data and channel-forming equipment


The receiver can be a computer, terminal or any digitally device.


To ensure the transfer of information from the computer to communication

This may be a database file, a table, response to a request, text or image.


For messaging in computing networks, various types of communication channels are used. The most common selected telephone channels and special channels for transmitting digital information are most common. Radio channels and satellite channels are also used.

A mansion in this respect is a LAN, where twisted pair of wires are used as a transmitted medium, coaxial cable and fiber optic cable.

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 to ensure compilation with a computer of one communication channel. In addition to single-channel adapters, multichannel 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, it is necessary to transform the bits to the analog channels to transmit digital information on the communication channel, and when receiving information from the communication channel to the computer, perform the opposite action - convert analog signals into the bits that can process computers. Such transforms performs a special device - modem.

Modem- device performing modulation and demodulation of information signals when transmitting them from computer to 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 Repears allow you to connect network fragments located at a distance of up to 50 m., and distance - up to 2000 m.