Transfer methods at the physical layer. Transfer of discrete data at the physical layer

Topic 2. Physical layer

Plan

Theoretical foundations of data transmission

Information can be transmitted over wires by changing some physical quantity, such as voltage or current. By representing the value of voltage or current as a single-valued function of time, it is possible to model the behavior of the signal and subject it to mathematical analysis.

Fourier series

At the beginning of the 19th century, the French mathematician Jean-Baptiste Fourier proved that any periodic function with period T can be expanded into a series (possibly infinite) consisting of sums of sines and cosines:
(2.1)
where is the fundamental frequency (harmonic), and are the amplitudes of the sines and cosines of the nth harmonic, and c is a constant. Such an expansion is called a Fourier series. The function expanded in the Fourier series can be restored by the elements of this series, that is, if the period T and the amplitudes of the harmonics are known, then the original function can be restored using the sum of the series (2.1).
An information signal that has a finite duration (all information signals have a finite duration) can be expanded into a Fourier series if we imagine that the entire signal repeats indefinitely over and over again (that is, the interval from T to 2T completely repeats the interval from 0 to T, and etc.).
Amplitudes can be calculated for any given function. To do this, you need to multiply the left and right sides of equation (2.1) by, and then integrate from 0 to T. Since:
(2.2)
only one member of the series remains. The line disappears completely. Similarly, by multiplying equation (2.1) by and integrating over time from 0 to T, one can calculate the values. If we integrate both parts of the equation without changing it, we can get the value of the constant With. The results of these actions will be as follows:
(2.3.)

Managed storage media

The purpose of the physical layer of a network is to transfer the raw bit stream from one machine to another. Various physical media, also called signal propagation media, can be used for transmission. Each of them has a characteristic set of bandwidths, delays, prices, and ease of installation and use. Media can be divided into two groups: steerable media such as copper wire and fiber optic cable, and unsteered media such as radio and laser beam transmission without cable.

Magnetic media

One of the easiest ways to transfer data from one computer to another is to write it to tape or other removable media (such as a rewritable DVD), physically transfer those tapes and disks to the destination, and read them there.
High throughput. A standard Ultrium tape cartridge holds 200 GB. About 1000 of these cassettes are placed in a 60x60x60 box, which gives a total capacity of 1600 Tbit (1.6 Pbit). A box of cassettes can be shipped within the US within 24 hours by Federal Express or another company. The effective bandwidth for this transmission is 1600 Tbps/86400 s, or 19 Gbps. If the destination is only an hour away, then the throughput will be over 400 Gbps. Not a single computer network is yet able to even come close to such indicators.
Profitability. The wholesale price of the cassette is about $40. A box of ribbons will cost $4,000, and the same ribbon can be used dozens of times. Let's add $1000 for shipping (actually, much less) and get about $5000 for transferring 200 TB, or 3 cents per gigabyte.
Flaws. Although the speed of data transfer using magnetic tapes is excellent, however, the amount of delay in such a transfer is very large. Transfer time is measured in minutes or hours, not milliseconds. Many applications require immediate response from the remote system (in connected mode).

twisted pair

A twisted pair consists of two insulated copper wires with a typical diameter of 1 mm. The wires twist one around the other in the form of a spiral. This allows you to reduce the electromagnetic interaction of several adjacent twisted pairs.
Application - telephone line, computer network. It can transmit a signal without attenuation of power over a distance of several kilometers. Repeaters are required for longer distances. They are combined into a cable, with a protective coating. A pair of wires are twisted in the cable to avoid signal overlap. They can be used to transmit both analog and digital data. The bandwidth depends on the diameter and length of the wire, but in most cases, several megabits per second can be achieved over distances of several kilometers. Due to the rather high bandwidth and low cost, twisted pair cables are widely used and will most likely continue to be popular in the future.
Twisted-pair cables come in several forms, two of which are particularly important in the field of computer networking. Category 3 twisted pair (CAT 3) consists of two insulated wires twisted together. Four such pairs are usually placed together in a plastic shell.
Category 5 twisted pair (CAT 5) is similar to Category 3 twisted pair, but has more turns per centimeter of wire length. This makes it possible to further reduce interference between different channels and provide improved signal transmission quality over long distances (Fig. 1).

Rice. 1. UTP category 3 (a), UTP category 5 (b).
All these types of connections are often referred to as UTP (unshielded twisted pair - unshielded twisted pair)
Shielded twisted-pair cables from IBM did not become popular outside of IBM.

Coaxial cable

Another common means of data transmission is coaxial cable. It is better shielded than twisted pair, so it can carry data over longer distances at higher speeds. Two types of cables are widely used. One of them, 50-ohm, is usually used for transmission of exclusively digital data. Another type of cable, 75-ohm, is often used to transmit analog information, as well as in cable television.
The sectional view of the cable is shown in Figure 2.

Rice. 2. Coaxial cable.
The design and special type of shielding of the coaxial cable provide high bandwidth and excellent noise immunity. The maximum throughput depends on the quality, length and signal-to-noise ratio of the line. Modern cables have a bandwidth of about 1 GHz.
Application - telephone systems (mains), cable television, regional networks.

fiber optics

Current fiber optic technology can reach data rates up to 50,000 Gb/s (50 Tb/s), and many people are looking for better materials. Today's practical limit of 10 Gbps is due to the inability to convert electrical signals to optical signals and vice versa faster, although 100 Gbps on a single fiber has already been achieved in laboratory conditions.
An optical fiber data transmission system consists of three main components: a light source, a carrier through which the light signal propagates, and a signal receiver, or detector. A light pulse is taken as one, and the absence of a pulse is taken as zero. Light propagates in an ultra-thin glass fiber. When light hits it, the detector generates an electrical impulse. By attaching a light source to one end of an optical fiber and a detector to the other, a unidirectional data transmission system is obtained.
When transmitting a light signal, the property of reflection and refraction of light during the transition from 2 media is used. Thus, when light is supplied at a certain angle to the media boundary, the light beam is completely reflected and locked in the fiber (Fig. 3).

Rice. 3. Property of light refraction.
There are 2 types of fiber optic cable: multi-mode - transmits a beam of light, single-mode - thin to the limit of several wavelengths, acts almost like a waveguide, the light moves in a straight line without reflection. Today's single-mode fiber links can operate at 50 Gbps over distances up to 100 km.
Three wavelength ranges are used in communication systems: 0.85, 1.30 and 1.55 µm, respectively.
The structure of fiber optic cable is similar to that of coaxial wire. The only difference is that the first one does not have a screening grid.
In the center of the fiber optic core is a glass core through which light propagates. Multimode fiber has a core diameter of 50 µm, which is about the thickness of a human hair. The core in a single-mode fiber has a diameter of 8 to 10 µm. The core is covered with a layer of glass with a lower refractive index than that of the core. It is designed to more reliably prevent light from escaping the core. The outer layer is a plastic shell that protects the glazing. Fiber optic cores are usually grouped into bundles protected by an outer sheath. Figure 4 shows a three-core cable.

Rice. 4. Three-core fiber optic cable.
In the event of a break, the connection of cable segments can be carried out in three ways:

A special connector can be attached to the end of the cable, with which the cable is inserted into an optical socket. The loss is 10-20% of the light intensity, but it makes it easy to change the system configuration.

Splicing - two neatly cut ends of the cable are laid next to each other and clamped with a special sleeve. Improved light transmission is achieved by aligning the ends of the cable. Loss - 10% of light power.

Fusion. There is practically no loss.

Two types of light source can be used to transmit a signal over a fiber optic cable: light emitting diodes (LED, Light Emitting Diode) and semiconductor lasers. Their comparative characteristics are given in table 1.

Table 1.
Comparison table of LED and semiconductor laser usage
The receiving end of an optical cable is a photodiode that generates an electrical pulse when light falls on it.

Comparative characteristics of fiber optic cable and copper wire.

Optical fiber has several advantages:

High speed.

Less signal attenuation, fewer repeaters output (one per 50km, not 5)

Inert to external electromagnetic radiation, chemically neutral.

Lighter in weight. 1000 copper twisted pairs 1 km long weighs about 8000 kg. A pair of fiber optic cables weighs only 100 kg with more bandwidth

Low laying costs

Flaws:

Difficulty and competence in installation.

transmission in simplex mode, a minimum of 2 wires is required between networks.

Wireless connection

electromagnetic spectrum

The movement of electrons generates electromagnetic waves that can propagate in space (even in a vacuum). The number of oscillations of electromagnetic oscillations per second is called frequency, and is measured in hertz. The distance between two successive highs (or lows) is called the wavelength. This value is traditionally denoted by the Greek letter (lambda).
If an antenna of a suitable size is included in the electrical circuit, then electromagnetic waves can be successfully received by the receiver at a certain distance. All wireless communication systems are based on this principle.
In a vacuum, all electromagnetic waves travel at the same speed, regardless of their frequency. This speed is called the speed of light, - 3*108 m/s. In copper or glass, the speed of light is about 2/3 of this value, and it also depends slightly on frequency.
Relationship of quantities, and:

If the frequency () is measured in MHz, and the wavelength () in meters then.
The totality of all electromagnetic waves forms the so-called continuous spectrum of electromagnetic radiation (Fig. 5). Radio, microwave, infrared, and visible light can be used to transmit information using amplitude, frequency, or phase modulation of waves. Ultraviolet, X-ray and gamma radiation would be even better due to their high frequencies, but they are difficult to generate and modulate, they do not pass through buildings well, and, in addition, they are dangerous for all living things. The official name of the ranges is given in Table 6.

Rice. 5. Electromagnetic spectrum and its application in communications.
Table 2.
Official ITU band names
The amount of information that an electromagnetic wave can carry is related to the frequency range of the channel. Modern technologies make it possible to encode several bits per hertz at low frequencies. Under certain conditions, this number can increase eightfold at high frequencies.
Knowing the width of the wavelength range, it is possible to calculate the corresponding frequency range and data rate.

Example: For a 1.3 micron fiber optic cable range, then. Then at 8 bps it turns out you can get a transfer rate of 240 Tbps.

Radio communication

Radio waves are easy to generate, travel long distances, pass through walls, go around buildings, propagate in all directions. The properties of radio waves depend on the frequency (Fig. 6). When operating at low frequencies, radio waves pass through obstacles well, but the signal strength in the air drops sharply as you move away from the transmitter. The ratio of power and distance from the source is expressed approximately as follows: 1/r2. At high frequencies, radio waves generally tend to travel in a straight line only and bounce off obstructions. In addition, they are absorbed, for example, by rain. Radio signals of any frequency are subject to interference from spark brush motors and other electrical equipment.

Rice. 6. Waves of the VLF, LF, MF bands go around the roughness of the earth's surface (a), the waves of the HF and VHF bands are reflected from the ionosphere and absorbed by the earth (b).

Communication in the microwave range

At frequencies above 100 MHz, radio waves propagate almost in a straight line, so they can be focused into narrow beams. The concentration of energy in the form of a narrow beam using a parabolic antenna (like the well-known satellite TV dish) leads to an improvement in the signal-to-noise ratio, but for such a connection, the transmitting and receiving antennas must be fairly accurately pointed at each other.
Unlike radio waves with lower frequencies, microwaves do not pass well through buildings. Microwave radio became so widely used in long-distance telephony, cell phones, television broadcasts, and other areas that there was a severe shortage of spectrum.
This connection has a number of advantages over optical fiber. The main one is that there is no need to lay a cable, and accordingly, there is no need to pay for the lease of land along the signal path. It is enough to buy small plots of land every 50 km and install relay towers on them.

Infrared and millimeter waves

Infrared and millimeter radiation without the use of a cable is widely used for communication over short distances (for example, remote controls). They are relatively directional, cheap and easy to install, but will not pass through solid objects.
Communication in the infrared range is used in desktop computing systems (for example, to connect laptops with printers), but still does not play a significant role in telecommunications.

Communications satellites

E types of satellites are used: geostationary (GEO), medium altitude (MEO) and low orbit (LEO) (Fig. 7).

Rice. 7. Communication satellites and their properties: orbit height, delay, number of satellites required to cover the entire surface of the globe.

Public Switched Telephone Network

Telephone system structure

The structure of a typical telephone communication route over medium distances is shown in Figure 8.

Rice. 8. Typical communication route with an average distance between subscribers.

Local lines: modems, ADSL, wireless

Since the computer works with a digital signal, and the local telephone line is the transmission of an analog signal, a modem device is used to convert digital to analog and vice versa, and the process itself is called modulation / demodulation (Fig. 9).

Rice. 9. Use of a telephone line when transmitting a digital signal.
There are 3 modulation methods (Fig. 10):

amplitude modulation - 2 different signal amplitudes are used (for 0 and 1),

frequency - several different signal frequencies are used (for 0 and 1),

phase - phase shifts are used during the transition between logical units (0 and 1). Shear angles - 45, 135, 225, 180.

In practice, combined modulation systems are used.

Rice. 10. Binary signal (a); amplitude modulation (b); frequency modulation (c); phase modulation.
All modern modems allow you to transfer data in both directions, this mode of operation is called duplex. A connection with serial transmission capability is called half-duplex. A connection in which transmission occurs in only one direction is called simplex.
The maximum modem speed that can be achieved at the moment is 56Kb/s. V.90 standard.

Digital subscriber lines. xDSL technology.

After the speed through modems reached its limit, telephone companies began to look for a way out of this situation. Thus, many proposals appeared under the general name xDSL. xDSL (Digital Subscribe Line) - digital subscriber line, where instead of x there may be other letters. The most well-known technology from these proposals is ADSL (Asymmetric DSL).
The reason for the speed limit of modems was that they used the transmission range of human speech for data transmission - 300 Hz to 3400 Hz. Together with the boundary frequencies, the bandwidth was not 3100 Hz, but 4000 Hz.
Although the spectrum of the local telephone line is 1.1 Hz.
The first proposal of ADSL technology used the entire spectrum of the local telephone line, which is divided into 3 bands:

POTS - the range of the conventional telephone network;

outgoing range;

input range.

A technology that uses different frequencies for different purposes is called frequency multiplexing or frequency multiplexing.
An alternative method called discrete multitone modulation, DMT (Discrete MultiTone) consists in dividing the entire spectrum of a 1.1 MHz wide local line into 256 independent channels of 4312.5 Hz each. Channel 0 is POTS. Channels 1 to 5 are not used so that the voice signal cannot interfere with the information signal. Of the remaining 250 channels, one is occupied with transmission control towards the provider, one - towards the user, and all others are available for transmitting user data (Fig. 11).

Rice. 11. ADSL operation using discrete multitone modulation.
The ADSL standard allows you to receive up to 8 Mb / s, and send up to 1 Mb / s. ADSL2+ - outgoing up to 24Mb/s, incoming up to 1.4 Mb/s.
A typical ADSL equipment configuration contains:

DSLAM - DSL access multiplexer;

NID is a network interface device that separates the ownership of the telephone company and the subscriber.

A splitter (splitter) is a frequency splitter that separates the POTS band and ADSL data.

Rice. 12. Typical configuration of ADSL equipment.

Lines and seals

Saving resources plays an important role in the telephone system. The cost of laying and maintaining a high-capacity backbone and a low-quality line is almost the same (that is, the lion's share of this cost is spent on digging trenches, and not on the copper or fiber optic cable itself).
For this reason, telephone companies have collaborated to develop several schemes for carrying multiple conversations over a single physical cable. Multiplexing schemes (compression) can be divided into two main categories FDM (Frequency Division Multiplexing - frequency division multiplexing) and TDM (Time Division Multiplexing - time division multiplexing) (Fig. 13).
With frequency multiplexing, the frequency spectrum is divided between logical channels, and each user receives exclusive ownership of his subband. In time division multiplexing, users take turns (cyclically) using the same channel, and each is given the full capacity of the channel for a short period of time.
Fiber optic channels use a special variant of frequency multiplexing. It is called spectral division multiplexing (WDM, Wavelength-Division Multiplexing).

Rice. 13. An example of frequency multiplexing: original spectra of 1 signals (a), frequency-shifted spectra (b), multiplexed channel (c).

Switching

From the point of view of the average telephone engineer, the telephone system consists of two parts: external equipment (local telephone lines and trunks, outside the switches) and internal equipment (switchboards) located at the telephone exchange.
Any communication networks support some way of switching (communication) of their subscribers among themselves. It is practically impossible to provide each pair of interacting subscribers with their own non-switched physical communication line, which they could monopolize "own" for a long time. Therefore, in any network, some method of subscriber switching is always used, which ensures the availability of available physical channels simultaneously for several communication sessions between network subscribers.
Two different techniques are used in telephone systems: circuit switching and packet switching.

Circuit switching

Circuit switching implies the formation of a continuous composite physical channel from serially connected individual channel sections for direct data transmission between nodes. In a circuit-switched network, before data transmission, it is always necessary to perform a connection establishment procedure, during which a composite channel is created (Fig. 14).

Packet switching

In packet switching, all messages transmitted by the network user are broken up at the source node into relatively small parts, called packets. Each packet is provided with a header that specifies the address information needed to deliver the packet to the destination host, as well as the packet number that will be used by the destination host to assemble the message. Packets are transported on the network as independent information units. Network switches receive packets from end nodes and, based on address information, transmit them to each other, and ultimately to the destination node (Fig. 14).
etc.................

The initial information that needs to be transmitted over a communication line can be either discrete (computer output data) or analog (speech, television image).

The transmission of discrete data is based on the use of two types of physical encoding:

a) analog modulation, when coding is carried out by changing the parameters of a sinusoidal carrier signal;

b) digital coding by changing the levels of the sequence of rectangular information pulses.

Analog modulation leads to a much smaller spectrum of the resulting signal than with digital coding, at the same information transfer rate, but its implementation requires more complex and expensive equipment.

Currently, the original data, which has an analog form, is increasingly transmitted over communication channels in a discrete form (in the form of a sequence of ones and zeros), i.e., discrete modulation of analog signals is carried out.

analog modulation. It is used to transmit discrete data over channels with a narrow bandwidth, a typical representative of which is a voice frequency channel provided to users of telephone networks. Signals with a frequency of 300 to 3400 Hz are transmitted over this channel, i.e., its bandwidth is 3100 Hz. Such a band is quite sufficient for speech transmission with acceptable quality. The bandwidth limitation of the tone channel is associated with the use of multiplexing and circuit switching equipment in telephone networks.

Before the transmission of discrete data on the transmitting side using a modulator-demodulator (modem) modulation of the carrier sinusoid of the original sequence of binary digits is carried out. The inverse conversion (demodulation) is performed by the receiving modem.

There are three ways to convert digital data to analog form, or three methods of analog modulation:

Amplitude modulation, when only the amplitude of the carrier of sinusoidal oscillations changes in accordance with the sequence of transmitted information bits: for example, when transmitting one, the oscillation amplitude is set large, and when transmitting zero, it is small, or there is no carrier signal at all;

Frequency modulation, when under the influence of modulating signals (transmitted information bits) only the carrier frequency of sinusoidal oscillations changes: for example, when zero is transmitted, it is low, and when one is transmitted, it is high;

Phase modulation, when, in accordance with the sequence of transmitted information bits, only the phase of the carrier of sinusoidal oscillations changes: when switching from signal 1 to signal 0 or vice versa, the phase changes by 180 °. In its pure form, amplitude modulation is rarely used in practice due to low noise immunity. Frequency modulation does not require complex circuitry in modems and is typically used in low speed modems operating at 300 or 1200 bps. Increasing the data rate is provided by the use of combined modulation methods, more often amplitude modulation in combination with phase.

The analog method of discrete data transmission provides wideband transmission by using signals of different carrier frequencies in one channel. This guarantees the interaction of a large number of subscribers (each pair of subscribers operates at its own frequency).

Digital coding. When digitally encoding discrete information, two types of codes are used:

a) potential codes, when only the value of the signal potential is used to represent information units and zeros, and its drops are not taken into account;

b) pulse codes, when binary data is represented either by pulses of a certain polarity, or by potential drops of a certain direction.

The following requirements are imposed on the methods of digital coding of discrete information when using rectangular pulses to represent binary signals:

Ensuring synchronization between transmitter and receiver;

Ensuring the smallest spectrum width of the resulting signal at the same bit rate (since a narrower spectrum of signals allows one to

networks with the same bandwidth achieve higher speeds

data transmission);

Ability to recognize errors in transmitted data;

Relatively low implementation cost.

By means of the physical layer, only the recognition of corrupted data (error detection) is carried out, which saves time, since the receiver, without waiting for the received frame to be completely placed in the buffer, immediately rejects it when it recognizes erroneous bits in the frame. A more complex operation - the correction of corrupted data - is performed by higher-level protocols: channel, network, transport or application.

Synchronization of the transmitter and receiver is necessary so that the receiver knows exactly when to read the incoming data. Clock signals tune the receiver to the transmitted message and keep the receiver synchronized with the incoming data bits. The synchronization problem is easily solved when transmitting information over short distances (between blocks inside a computer, between a computer and a printer) by using a separate timing communication line: information is read only at the moment the next clock pulse arrives. In computer networks, the use of clock pulses is abandoned for two reasons: for the sake of saving conductors in expensive cables and due to the heterogeneity of the characteristics of conductors in cables (over long distances, the uneven speed of signal propagation can lead to desynchronization of clock pulses in the clock line and information pulses in the main line , as a result of which the data bit will either be skipped or reread).

Currently, the synchronization of the transmitter and receiver in networks is achieved by using self-synchronizing codes (SC). The coding of the transmitted data using the SC is to ensure regular and frequent changes (transitions) of the levels of the information signal in the channel. Each signal level transition from high to low or vice versa is used to trim the receiver. The best are those SCs that provide a signal level transition at least once during the time interval required to receive one information bit. The more frequent the signal level transitions, the more reliable the synchronization of the receiver is and the more confident the identification of the received data bits is.

These requirements for the methods of digital coding of discrete information are mutually contradictory to a certain extent, therefore, each of the coding methods considered below has its advantages and disadvantages compared to others.

Self-synchronizing codes. The most common are the following SCs:

Potential code without return to zero (NRZ - Non Return to Zero);

Bipolar pulse code (RZ code);

Manchester code;

Bipolar code with alternate level inversion.

On fig. 32 shows the coding schemes for message 0101100 using these CKs.

The following indicators are used to characterize and compare the SC:

Level (quality) of synchronization;

Reliability (confidence) of recognition and selection of received information bits;

The required rate of change of the signal level in the communication line when using the SC, if the line bandwidth is set;

The complexity (and hence the cost) of the equipment that implements the SC.

The NRZ code is easy to code and low cost to implement. It received such a name because when transmitting a series of bits of the same name (ones or zeros), the signal does not return to zero during the cycle, as is the case in other coding methods. The signal level remains unchanged for each series, which significantly reduces the quality of synchronization and the reliability of recognition of the received bits (the receiver's timer may misalign with the incoming signal and untimely polling of the lines may occur).

For the N^-code, the following relations hold:

where VI is the rate of change of the signal level in the communication line (baud);

Y2 - throughput of the communication line (bit / s).

In addition to the fact that this code does not have the property of self-synchronization, it also has another serious drawback: the presence of a low-frequency component that approaches zero when transmitting long runs of ones or zeros. As a result, the NRZ code in its pure form is not used in networks. Its various modifications are applied, in which poor self-synchronization of the code and the presence of a constant component are eliminated.

The RZ-code, or bipolar pulse code (return-to-zero code), differs in that during the transmission of one information bit, the signal level changes twice, regardless of whether a series of bits of the same name or alternating bits are transmitted. A unit is represented by an impulse of one polarity, and a zero is represented by another. Each pulse lasts half a cycle. Such code has excellent self-synchronizing properties, but the cost of its implementation is quite high, since it is necessary to ensure the ratio

The spectrum of an RZ code is wider than that of potential codes. Due to its too wide spectrum, it is rarely used.

The Manchester code provides a change in the signal level when presenting each bit, and when transmitting a series of bits of the same name, a double change. Each measure is divided into two parts. Information is encoded by potential drops that occur in the middle of each cycle. A unit is encoded by a low-to-high transition, and a zero is encoded by a reverse edge. The speed ratio for this code is:

The Manchester code has good self-clocking properties, since the signal changes at least once per cycle of transmission of one data bit. Its bandwidth is narrower than that of the RZ code (one and a half times on average). In contrast to the bipolar pulse code, where three signal levels are used for data transmission (which is sometimes very undesirable, for example, only two states are consistently recognized in optical cables - light and darkness), the Manchester code has two levels.

Manchester code is widely used in Ethernet and Token Ring technologies.

The Alternate Level Inversion Bipolar Code (AMI code) is a modification of the NRZ code. It uses three levels of potential - negative, zero and positive. The unit is encoded either by a positive potential or a negative one. Zero potential is used to encode zero. The code has good synchronizing properties when transmitting series of units, since the potential of each new unit is opposite to the potential of the previous one. When transmitting runs of zeros, there is no synchronization. The AMI code is relatively easy to implement. For him

When transmitting various combinations of bits on the line, the use of the AMI code leads to a narrower signal spectrum than for the NRZ code, and hence to a higher line throughput.

Note that improved potential codes (upgraded Manchester code and AMI code) have a narrower spectrum than pulse codes, so they are used in high-speed technologies, such as FDDI, Fast Ethernet, Gigabit Ethernet.

Discrete modulation of analog signals. As already noted, one of the trends in the development of modern computer networks is their digitalization, i.e., the transmission of signals of any nature in digital form. The sources of these signals can be computers (for discrete data) or devices such as telephones, camcorders, video and audio equipment (for analog data). Until recently (before the advent of digital communication networks), in territorial networks all types of data were transmitted in analog form, and computer data, discrete in nature, were converted into analog form using modems.

However, the transmission of information in analog form does not improve the quality of the received data if there was a significant distortion during transmission. Therefore, the analogue technique for recording and transmitting sound and images has been replaced by digital technology, which uses discrete modulation of analog signals.

Discrete modulation is based on the sampling of continuous signals both in amplitude and in time. One of the widely used methods for converting analog signals into digital ones is pulse code modulation (PCM), proposed in 1938 by A.Kh. Reeves (USA).

When using PCM, the conversion process includes three stages: mapping, quantization and encoding (Fig. 33).

The first stage is display. The amplitude of the original continuous signal is measured with a given period, due to which time discretization occurs. At this stage, the analog signal is converted into pulse amplitude modulation (PAM) signals. The execution of the stage is based on the Nyquist-Kotelnikov mapping theory, the main position of which is: if the analog signal is displayed (i.e., represented as a sequence of its discrete-time values) on a regular interval with a frequency of at least twice the frequency of the highest harmonic spectrum of the original continuous signal, then the display will contain information sufficient to restore the original signal. In analog telephony, the range from 300 to 3400 Hz is chosen for voice transmission, which is sufficient for high-quality transmission of all the main harmonics of the interlocutors. Therefore, in digital networks where the PCM method is implemented for voice transmission, a display frequency of 8000 Hz is adopted (this is more than 6800 Hz, which provides some margin of quality).

In the quantization step, each IAM signal is given a quantized value corresponding to the nearest quantization level. The entire range of IAM signal amplitude variation is divided into 128 or 256 quantization levels. The more quantization levels, the more accurately the IAM signal amplitude is represented by the quantized level.

At the encoding stage, each quantized mapping is assigned a 7-bit (if the number of quantization levels is 128) or 8-bit (if the number of quantization levels is 128) binary code. On fig. 33 shows the signals of the 8-element binary code 00101011 corresponding to a quantized signal with level 43. When encoding with 7-element codes, the data rate over the channel should be 56 Kbps (this is the product of the display frequency and the bit depth of the binary code), and when encoding 8- element codes - 64 Kbps. The standard is a 64 kbit/s digital channel, which is also called the elementary channel of digital telephone networks.

The device that performs these steps of converting an analog value into a digital code is called an analog-to-digital converter (ADC). On the receiving side, using a digital-to-analog converter (DAC), an inverse conversion is carried out, i.e., the digitized amplitudes of a continuous signal are demodulated, and the original continuous function of time is restored.

In modern digital communication networks, other methods of discrete modulation are also used, which make it possible to represent voice measurements in a more compact form, for example, as a sequence of 4-bit numbers. The concept of converting analog signals into digital ones is also used, in which not the IAM signals themselves are quantized and then encoded, but only their changes, and the number of quantization levels is assumed to be the same. It is obvious that such a concept allows the conversion of signals with greater accuracy.

Digital methods for recording, reproducing and transmitting analog information provide the ability to control the reliability of data read from a carrier or received via a communication line. For this purpose, the same control methods are used as for computer data (see 4.9).

The transmission of a continuous signal in a discrete form imposes stringent requirements on the synchronization of the receiver. If synchronization is not observed, the original signal is restored incorrectly, which leads to distortion of the voice or transmitted image. If frames with voice measurements (or other analog values) arrive synchronously, then the voice quality can be quite high. However, in computer networks, frames can be delayed both in end nodes and in intermediate switching devices (bridges, switches, routers), which negatively affects the quality of voice transmission. Therefore, for high-quality transmission of digitized continuous signals, special digital networks (ISDN, ATM, digital television networks) are used, although Frame Relay networks are still used to transmit intracorporate telephone conversations, since the frame transmission delays in them are within acceptable limits.

Crosstalk at the near end of the line - determines the noise immunity of the cable to internal sources of interference. Usually they are evaluated in relation to a cable consisting of several twisted pairs, when the mutual pickups of one pair on another can reach significant values and create internal noise commensurate with the useful signal.

Reliability of data transmission(or bit error rate) characterizes the probability of distortion for each transmitted data bit. The reasons for the distortion of information signals are interference on the line, as well as the limited bandwidth of its pass. Therefore, an increase in the reliability of data transmission is achieved by increasing the degree of noise immunity of the line, reducing the level of crosstalk in the cable, and using more broadband communication lines.

For conventional cable communication lines without additional error protection, the reliability of data transmission is, as a rule, 10 -4 -10 -6 . This means that, on average, out of 10 4 or 10 6 transmitted bits, the value of one bit will be corrupted.

Communication line equipment(data transmission equipment - ATD) is the edge equipment that directly connects computers to the communication line. It is part of the communication line and usually operates at the physical level, providing the transmission and reception of a signal of the desired shape and power. Examples of ADFs are modems, adapters, analog-to-digital and digital-to-analog converters.

The DTE does not include the user's data terminal equipment (DTE), which generates data for transmission over the communication line and is connected directly to the DTE. A DTE includes, for example, a LAN router. Note that the division of equipment into APD and OOD classes is rather conditional.

Intermediate equipment is used on long-distance communication lines, which solves two main tasks: improving the quality of information signals (their shape, power, duration) and creating a permanent composite channel (end-to-end channel) of communication between two network subscribers. In the LCN, intermediate equipment is not used if the length of the physical medium (cables, radio air) is not high, so that signals from one network adapter to another can be transmitted without intermediate restoration of their parameters.

In global networks, high-quality signal transmission over hundreds and thousands of kilometers is ensured. Therefore, amplifiers are installed at certain distances. To create a through line between two subscribers, multiplexers, demultiplexers and switches are used.

The intermediate equipment of the communication channel is transparent to the user (he does not notice it), although in reality it forms a complex network called primary network and serving as the basis for building computer, telephone and other networks.

Distinguish analog and digital communication lines, which use various types of intermediate equipment. In analog lines, intermediate equipment is designed to amplify analog signals that have a continuous range of values. In high-speed analog channels, a frequency multiplexing technique is implemented, when several low-speed analog subscriber channels are multiplexed into one high-speed channel. In digital communication channels, where rectangular information signals have a finite number of states, intermediate equipment improves the shape of the signals and restores their repetition period. It provides the formation of high-speed digital channels, working on the principle of time multiplexing of channels, when each low-speed channel is allocated a certain fraction of the time of the high-speed channel.

When transmitting discrete computer data over digital communication lines, the physical layer protocol is defined, since the parameters of the information signals transmitted by the line are standardized, and when transmitted over analog lines, it is not defined, since the information signals have an arbitrary shape and there are no there are no requirements.

The following are used in communication networks information transfer modes:

simplex, when the transmitter and receiver are connected by one communication channel, through which information is transmitted in only one direction (this is typical for television communication networks);

half-duplex, when two communication nodes are also connected by one channel, through which information is transmitted alternately in one direction, then in the opposite direction (this is typical for information-reference, request-response systems);

duplex, when two communication nodes are connected by two channels (forward communication channel and reverse), through which information is simultaneously transmitted in opposite directions. Duplex channels are used in systems with decision and information feedback.

Switched and dedicated communication channels. In the TSS, there are dedicated (non-switched) communication channels and those with switching for the duration of information transmission over these channels.

When using dedicated communication channels, the transceiver equipment of communication nodes is constantly connected to each other. This ensures a high degree of readiness of the system for information transfer, higher quality of communication, and support for a large amount of traffic. Due to the relatively high costs of operating networks with dedicated communication channels, their profitability is achieved only if the channels are fully loaded.

Switched communication channels, created only for the time of transmission of a fixed amount of information, are characterized by high flexibility and relatively low cost (with a small amount of traffic). The disadvantages of such channels are: loss of time for switching (for establishing communication between subscribers), the possibility of blocking due to the busyness of certain sections of the communication line, lower communication quality, high cost with a significant amount of traffic.

The initial information that needs to be transmitted over a communication line can be either discrete (computer output data) or analog (speech, television image).

Discrete data transmission is based on the use of two types of physical encoding:

a) analog modulation when encoding is performed by changing the parameters of the sinusoidal carrier signal;

b) digital coding by changing the levels of the sequence of rectangular information pulses.

At present, the initial data, which has an analog form, is increasingly transmitted over communication channels in a discrete form (in the form of a sequence of ones and zeros), i.e. discrete modulation analog signals.

Analog modulation. It is used to transmit discrete data over channels with a narrow bandwidth, a typical representative of which is a voice frequency channel provided to users of telephone networks. Signals with a frequency of 300 to 3400 Hz are transmitted over this channel, i.e., its bandwidth is 3100 Hz. Such a band is quite sufficient for speech transmission with acceptable quality. The bandwidth limitation of the tone channel is associated with the use of multiplexing and circuit switching equipment in telephone networks.

There are three ways to convert digital data to analog form, or three methods of analog modulation:

frequency modulation, when under the action of modulating signals (transmitted information bits) only the frequency of the carrier of sinusoidal oscillations changes: for example, when zero is transmitted, it is low, and when one is transmitted, it is high;

phase modulation, when, in accordance with the sequence of transmitted information bits, only the phase of the carrier of sinusoidal oscillations changes: when switching from signal 1 to signal 0 or vice versa, the phase changes by 180 °.

In its pure form, amplitude modulation is rarely used in practice due to low noise immunity. Frequency modulation does not require complex circuitry in modems and is typically used in low speed modems operating at 300 or 1200 bps. Increasing the data rate is provided by the use of combined modulation methods, more often amplitude modulation in combination with phase.

Digital coding. When digitally encoding discrete information, two types of codes are used:

a) potential codes, when only the value of the signal potential is used to represent information units and zeros, and its drops are not taken into account;

b) pulse codes, when binary data is represented either by pulses of a certain polarity, or by potential drops of a certain direction.

The following requirements are imposed on the methods of digital coding of discrete information when using rectangular pulses to represent binary signals:

ensuring synchronization between transmitter and receiver;

Ensuring the smallest spectrum width of the resulting signal at the same bit rate (since a narrower spectrum of signals allows a higher data rate to be achieved on a line with the same bandwidth);

the ability to recognize errors in transmitted data;

Relatively low cost of implementation.

Currently, synchronization of the transmitter and receiver in networks is achieved by using self-synchronizing codes(SK). The coding of the transmitted data using the SC is to ensure regular and frequent changes (transitions) of the levels of the information signal in the channel. Each signal level transition from high to low or vice versa is used to trim the receiver. The best are those SCs that provide a signal level transition at least once during the time interval required to receive one information bit. The more frequent the signal level transitions, the more reliable the synchronization of the receiver is and the more confident the identification of the received data bits is.

Self-synchronizing codes. The most common are the following SCs:

potential code without return to zero (NRZ - Non Return to Zero);

bipolar pulse code (RZ code);

The Manchester code

· bipolar code with alternating level inversion.

On fig. 32 shows the coding schemes for message 0101100 using these CKs.

Rice. 32. Message encoding schemes using self-synchronizing codes

For the transmission of discrete data over communication lines with a narrow frequency band, analog modulation. A typical representative of such lines is a voice-frequency communication line made available to users of public telephone networks. This communication line transmits analog signals in the frequency range from 300 to 3400 Hz (thus the bandwidth of the line is 3100 Hz). The strict bandwidth limitation of communication lines in this case is associated with the use of multiplexing and circuit switching equipment in telephone networks.

A device that performs the functions of modulating a carrier sinusoid on the transmitting side and demodulating on the receiving side is called modem (modulator-demodulator).

Analog modulation is a physical coding method in which information is encoded by changing amplitudes, frequencies or phases a sinusoidal signal of the carrier frequency. At amplitude modulation for a logical one, one level of the amplitude of the carrier frequency sinusoid is selected, and for a logical zero, another. This method is rarely used in practice in its pure form due to low noise immunity, but is often used in combination with other types of modulation. At frequency modulation the values 0 and 1 of the original data are transmitted by sinusoids with different frequencies . This modulation method does not require complex modem electronics and is typically used in low speed modems operating at 300 or 1200 bps. At phase modulation data values 0 and 1 correspond to signals of the same frequency but different phase, such as 0 and 180 degrees or 0, 90, 180 and 270 degrees. In high-speed modems, combined modulation methods are often used, as a rule, amplitude in combination with phase. Combined modulation methods are used to increase the data rate. The most common methods are Quadrature Amplitude Modulation-QAM). These methods are based on a combination of phase modulation with 8 phase shift values and amplitude modulation with 4 amplitude levels. However, not all of the possible 32 signal combinations are used. Such coding redundancy is required for the modem to recognize erroneous signals, which are the result of distortion due to interference, which on telephone channels (especially switched ones) are very significant in amplitude and long in time.

At digital coding discrete information is used potential and impulse codes. AT potential In codes, only the value of the signal potential is used to represent logical ones and zeros, and its drops, which form complete pulses, are not taken into account. Pulse codes allow binary data to be represented either by pulses of a certain polarity, or by a part of the pulse - a potential drop of a certain direction.

When using rectangular pulses to transmit discrete information, it is necessary to choose a coding method that would simultaneously achieve several goals: at the same bit rate, have the smallest width of the spectrum of the resulting signal; provided synchronization between transmitter and receiver; had the ability to recognize mistakes; had a low cost of implementation.

A narrower signal spectrum allows you to achieve a higher data transfer rate on the same line (with the same bandwidth). Synchronization of the transmitter and receiver is needed so that the receiver knows exactly at what point in time it is necessary to read new information from the communication line. This problem is more difficult to solve in networks than when communicating between devices in close proximity, such as between devices inside a computer or between a computer and a printer. At short distances, a scheme based on a separate clocking communication line works well, and information is removed from the data line only at the moment a clock pulse arrives. In networks, the use of this scheme causes difficulties due to the heterogeneity of the characteristics of the conductors in the cables. Over long distances, signal velocity ripples can cause the clock to arrive so late or too early for the corresponding data signal that a data bit is skipped or reread. Another reason why networks refuse to use clock pulses is to save conductors in expensive cables. Therefore, networks use the so-called self-synchronizing codes, the signals of which carry indications for the transmitter at what point in time it is necessary to recognize the next bit (or several bits, if the code is oriented to more than two signal states). Any sharp drop in signal - the so-called front- can serve as a good indication for synchronization of the receiver with the transmitter. When using sinusoids as a carrier signal, the resulting code has the property of self-synchronization, since a change in the amplitude of the carrier frequency allows the receiver to determine the moment the input code appears.

Recognition and correction of distorted data is difficult to implement by means of the physical layer, therefore, most often this work is undertaken by the protocols that lie above: channel, network, transport or application. On the other hand, error recognition at the physical layer saves time, since the receiver does not wait for the frame to be completely buffered, but rejects it immediately when erroneous bits are recognized within the frame.

The requirements for coding methods are mutually contradictory, so each of the popular digital coding methods discussed below has its own advantages and disadvantages compared to others.

One of the simplest methods potential coding is unipolar potential code, also called coding without returning to zero (Non Return to Zero-NRZ) (fig.7.1.a). The last name reflects the fact that when a sequence of ones is transmitted, the signal does not return to zero during the cycle. The NRZ method has good error detection (due to two sharply different potentials), but does not have the self-synchronization property. When transmitting a long sequence of ones or zeros, the line signal does not change, so the receiver is not able to determine from the input signal the time points when it is necessary to read the data again. Even with a highly accurate clock generator, the receiver can make a mistake with the moment of data acquisition, since the frequencies of the two generators are almost never completely identical. Therefore, at high data rates and long sequences of ones or zeros, a small mismatch of clock frequencies can lead to an error in a whole cycle and, accordingly, reading an incorrect bit value.

a B C D E F

Rice. 7.1. Binary data encoding methods: a-unipolar poten-

social code; b- bipolar potential code; in- unipolar im-

pulse code; G -bipolar pulse code; d-"Manchester" code;

e- potential code with four signal levels.

Another serious disadvantage of the NRZ method is the presence of a low frequency component that approaches zero when transmitting long sequences of ones or zeros. Because of this, many communication lines that do not provide a direct galvanic connection between the receiver and the source do not support this type of encoding. As a result, the NRZ code in its pure form is not used in networks, but its various modifications are used, in which both poor self-synchronization of the NRZ code and the presence of a constant component are eliminated.

One of the modifications of the NRZ method is the method bipolar potential coding with alternative inversion (Bipolar Alternate Mark Inversion-AMI). In this method ( rice. 7.1.b) three potential levels are used - negative, zero and positive. To encode a logical zero, a zero potential is used, and a logical unit is encoded either by a positive potential or a negative one (in this case, the potential of each new unit is opposite to the potential of the previous one). The AMI code partially eliminates the DC and lack of self-timing problems inherent in the NRZ code. This happens when sending long sequences of ones. In these cases, the signal on the line is a sequence of bipolar pulses with the same spectrum as the NRZ code transmitting alternating zeros and ones, that is, without a constant component and with a fundamental harmonic of N/2 Hz (where N is the data bit rate) . Long sequences of zeros are also dangerous for the AMI code, as well as for the NRZ code - the signal degenerates into a constant potential of zero amplitude. In general, for different combinations of bits on the line, the use of the AMI code leads to a narrower signal spectrum than for the NRZ code, and hence to a higher line throughput. For example, when transmitting alternating ones and zeros, the fundamental harmonic f 0 has a frequency of N/4 Hz. The AMI code also provides some features for recognizing erroneous signals. Thus, a violation of the strict alternation of the polarity of the signals indicates a false impulse or the disappearance of a correct impulse from the line. A signal with incorrect polarity is called forbidden signal (signal violation). Since the AMI code uses not two, but three signal levels per line, the additional level requires an increase in transmitter power to provide the same bit fidelity on the line, which is a general disadvantage of codes with multiple signal states compared to codes that only distinguish two states.

The simplest methods impulsive encodings are unipolar pulse code, in which one is represented by momentum and zero is represented by its absence ( rice. 7.1c), and bipolar pulse code, in which the unit is represented by a pulse of one polarity, and zero - the other ( rice. 7.1g). Each pulse lasts half a cycle. The bipolar pulse code has good self-clocking properties, but a DC pulse component may be present, for example, when transmitting a long sequence of ones or zeros. In addition, its spectrum is wider than that of potential codes. So, when transmitting all zeros or ones, the frequency of the fundamental harmonic of the code will be equal to N Hz, which is two times higher than the fundamental harmonic of the NRZ code and four times higher than the fundamental harmonic of the AMI code when transmitting alternating ones and zeros. Due to the too wide spectrum, the bipolar pulse code is rarely used.

In local networks, until recently, the most common coding method was the so-called " Manchester code"(rice. 7.1e). In the Manchester code, a potential drop, that is, the front of the pulse, is used to encode ones and zeros. In Manchester encoding, each clock is divided into two parts. Information is encoded by potential drops that occur in the middle of each cycle. A unit is encoded by a low-to-high transition, and a zero is encoded by a reverse edge. At the beginning of each cycle, a service signal edge can occur if you need to represent several ones or zeros in a row. Since the signal changes at least once per transmission cycle of one data bit, the Manchester code has good self-clocking properties. The bandwidth of the Manchester code is narrower than that of the bipolar pulse. It also does not have a constant component, and the fundamental harmonic in the worst case (when transmitting a sequence of ones or zeros) has a frequency of N Hz, and in the best case (when transmitting alternating ones and zeros) it is equal to N / 2 Hz, like in AMI codes or NRZ. On average, the bandwidth of the Manchester code is one and a half times narrower than that of the bipolar pulse code, and the fundamental harmonic oscillates around 3N/4. Another advantage of the Manchester code is that it has only two signal levels, while the bipolar pulse code has three.

There are also potential codes with a large number of signal levels for encoding data. Shown as an example ( fig 7.1e) potential code 2B1Q with four signal levels for data encoding. In this code, every two bits are transmitted in one cycle by a signal that has four states. A pair of bits "00" corresponds to a potential of -2.5 V, a pair of bits "01" - a potential of -0.833 V, a pair of bits "11" - a potential of +0.833 V, and a pair of bits "10" - a potential of +2.5 V. This coding method requires additional measures to deal with long sequences of identical bit pairs, since then the signal turns into a constant component. With random bit interleaving, the signal spectrum is twice as narrow as that of the NRZ code (at the same bit rate, the cycle time is doubled). Thus, using the presented 2B1Q code, it is possible to transfer data over the same line twice as fast as using the AMI code. However, for its implementation, the transmitter power must be higher so that the four levels are clearly distinguished by the receiver against the background of interference.

To improve potential codes like AMI and 2B1Q, logical coding. Logic coding is designed to replace long sequences of bits, leading to a constant potential, interspersed with ones. Two methods are characteristic for logical coding - redundant codes and scrambling.

Redundant codes are based on splitting the original sequence of bits into portions, which are often called characters. Then each original character is replaced with a new one that has more bits than the original. For example, a 4B/5B logic code replaces the original 4-bit characters with 5-bit characters. Since the resulting symbols contain redundant bits, the total number of bit combinations in them is greater than in the original ones. So, in the 4B / 5B code, the resulting symbols can contain 32 bit combinations, while the original symbols - only 16. Therefore, in the resulting code, you can select 16 such combinations that do not contain a large number of zeros, and count the rest prohibited codes (code violation). In addition to removing DC and making the code self-synchronizing, redundant codes allow the receiver to recognize corrupted bits. If the receiver receives a forbidden code, it means that the signal has been distorted on the line. The 4V/5V code is transmitted over the line using physical coding using one of the potential coding methods that is sensitive only to long sequences of zeros. The 4V/5V code symbols, 5 bits long, guarantee that no more than three zeros in a row can occur on the line for any combination of them. The letter B in the code name means that the elementary signal has 2 states (from the English binary - binary). There are also codes with three signal states, for example, in the 8B / 6T code, to encode 8 bits of initial information, a code of 6 signals is used, each of which has three states. The redundancy of the 8B/6T code is higher than that of the 4B/5B code, since there are 729 (3 to the power of 6) resulting symbols for 256 source codes. Using the lookup table is a very simple operation, so this approach does not complicate the network adapters and interface blocks of switches and routers (see. sections 9,11).

To provide a given line capacity, a transmitter using a redundant code must operate at an increased clock frequency. So, to transmit 4V / 5V codes at a rate of 100 Mbps, the transmitter must operate at a clock frequency of 125 MHz. In this case, the spectrum of the signal on the line is expanded in comparison with the case when a pure, non-redundant code is transmitted over the line. Nevertheless, the spectrum of the redundant potential code turns out to be narrower than the spectrum of the Manchester code, which justifies the additional stage of logical coding, as well as the operation of the receiver and transmitter at an increased clock frequency.

Another way of logical coding is based on the preliminary "mixing" of the initial information in such a way that the probabilities of the appearance of ones and zeros on the line become close. Devices or blocks that perform this operation are called scramblers(scramble - dump, disorderly assembly). At scrambling a well-known algorithm is used, so the receiver, having received binary data, transmits them to descrambler, which restores the original bit sequence. Excess bits are not transmitted over the line. Improved potential redundancy and scrambled codes are used in modern high-speed network technologies instead of "Manchester" and bipolar pulse coding.

7.6. Communication Line Multiplexing Technologies

For multiplexing("compacting") of communication lines, several technologies are used. Technology frequencymultiplexing(Frequency Division Multiplexing - FDM) was originally developed for telephone networks, but is also used for other types of networks, such as cable television networks. This technology assumes the transfer of the signals of each subscriber channel to its own frequency range and the simultaneous transmission of signals from several subscriber channels in one broadband communication line. For example, the inputs of an FDM switch receive initial signals from telephone network subscribers. The switch performs a frequency translation of each channel in its own frequency band. Typically, the high-frequency range is divided into bands that are allocated for the transmission of data from subscriber channels. In the communication line between two FDM switches, the signals of all subscriber channels are simultaneously transmitted, but each of them occupies its own frequency band. The output FDM switch separates the modulated signals of each carrier frequency and transmits them to the corresponding output channel to which the subscriber telephone is directly connected. FDM switches can perform both dynamic and permanent switching. In dynamic switching, one subscriber initiates a connection with another subscriber by sending the called subscriber number to the network. The switch dynamically allocates one of the free bands to this subscriber. With constant switching, the band is assigned to the subscriber for a long time. The principle of switching based on frequency division remains unchanged in networks of a different type, only the boundaries of the bands allocated to a separate subscriber channel, as well as their number, change.

Multiplexing technologytime-sharing(Time Division Multiplexing - TDM) or temporary multiplexing is based on the use of TDM equipment (multiplexers, switches, demultiplexers) operating in the time-sharing mode, servicing all subscriber channels in turn during a cycle. Each connection is allocated one time slice of the hardware operation cycle, also called time slot. The duration of the time slot depends on the number of subscriber channels served by the equipment. TDM networks can support either dynamic, or constant switching, and sometimes both of these modes.

Networks with dynamic switching require a preliminary procedure for establishing a connection between subscribers. To do this, the address of the called subscriber is transmitted to the network, which passes through the switches and configures them for subsequent data transmission. The connection request is routed from one switch to another and eventually reaches the called party. The network may refuse to establish a connection if the capacity of the required output channel has already been exhausted. For an FDM switch, the output capacity is equal to the number of frequency bands, and for a TDM switch, it is equal to the number of time slots into which the channel operation cycle is divided. The network also refuses the connection if the requested subscriber has already established a connection with someone else. In the first case, they say that the switch is busy, and in the second - the subscriber. The possibility of connection failure is a disadvantage of the circuit switching method. If a connection can be established, then it is allocated a fixed bandwidth in FDM networks or a fixed bandwidth in TDM networks. These values remain unchanged throughout the connection period. Guaranteed network throughput after a connection is established is an important feature required for applications such as voice and video transmission or real-time object control.

If there is only one physical communication channel, for example, when exchanging data using modems over the telephone network, duplex operation is organized on the basis of dividing the channel into two logical subchannels using FDM or TDM technologies. When using FDM technology, modems for organizing duplex operation on a two-wire line operate at four frequencies (two frequencies - for encoding ones and zeros when transmitting data in one direction, and the other two frequencies - for encoding when transmitting in the opposite direction). In TDM technology, some time slots are used to transfer data in one direction, and some are used to transfer data in the other direction. Usually, time slots of opposite directions alternate.

In fiber-optic cables for the organization of duplex operation when using only one optical fiber, data transmission in one direction is carried out using a light beam of one wavelength, and in the opposite direction - a different wavelength. This technology is essentially related to the FDM method, but for fiber optic cables it is called wavelength multiplexing technologies(Wave Division Multiplexing - WDM) or wave multiplexing.

Technologydense wave(spectral)multiplexing(Dense Wave Division Multiplexing - DWDM) is designed to create a new generation of optical backbones operating at multi-gigabit and terabit speeds. Such a qualitative leap in performance is provided due to the fact that information in an optical fiber is transmitted simultaneously by a large number of light waves. DWDM networks operate on the principle of circuit switching, with each light wave representing a separate spectral channel and carrying its own information. One of the main advantages of DWDM technology is a significant increase in the utilization factor of the frequency potential of optical fiber, the theoretical bandwidth of which is 25,000 GHz.

Summary

In modern telecommunication systems, information is transmitted through electromagnetic waves - electrical, light or radio signals.

Communication lines, depending on the type of physical medium for information transmission, can be cable (wired) or wireless. As communication lines, telephone cables based on parallel non-twisted conductors, coaxial cables, cables based on twisted pairs of conductors (unshielded and shielded), fiber optic cables are used. The most effective today and promising in the near future are cables based on twisted pairs of conductors and fiber optic cables. Wireless communication lines are most often implemented by transmitting radio signals in various radio wave bands. Infrared wireless data transmission technology uses part of the electromagnetic spectrum between visible light and the shortest microwaves. The most high-speed and noise-resistant is the laser technology of wireless communication.

The main characteristics of communication lines are the frequency response, bandwidth and attenuation at a certain frequency.

The throughput of a communication line characterizes the maximum possible data transfer rate over it. The noise immunity of a communication line determines its ability to reduce the level of interference generated in the external environment on internal conductors. The reliability of data transmission characterizes the probability of distortion for each transmitted bit of data.

The representation of discrete information in one form or another of the signals applied to the communication line is called physical coding. Logical coding involves replacing bits of the original information with a new bit sequence that carries the same information but has additional properties.

To transmit discrete data over communication lines with a narrow frequency band, analog modulation is used, in which information is encoded by changing the amplitude, frequency, or phase of a sinusoidal carrier frequency signal. When digitally encoding discrete information, potential and impulse codes are used. For multiplexing of communication lines technologies of frequency, time and wave multiplexing are used.

Control questions and tasks

1. Give the classification of communication lines.

2. Describe the most common cable communication lines.

3. Present the main wireless communication lines and give their comparative characteristics.

4. Due to what physical factors do communication channels distort transmitted signals?

5. What is the amplitude-frequency characteristic of a communication channel?

6. In what units is the bandwidth of the communication channel measured?

7. Describe the concept of "noise immunity of the communication line."

8. What determines the characteristic "data transmission reliability" and in what units is it measured?

9. What is "analogue modulation" and what types of it are used to transmit discrete data?

10. What device performs the functions of modulating the carrier sinusoid on the transmitting side and demodulating it on the receiving side?

11. State the difference between potential and impulse coding of digital signals.