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Data transfer methods at the physical layer. Methods of transferring discrete data at the physical layer Methods of transferring at the physical layer

There are two main types of physical coding, one based on a sinusoidal carrier signal (analog modulation) and one based on a sequence of rectangular pulses (digital coding).

Analog modulation - for the transmission of discrete data over a channel with a narrow bandwidth - telephone networks, tone frequency channel (bandwidth from 300 to 3400 Hz) A device that performs modulation and demodulation - a modem.

Analog modulation techniques

n amplitude modulation (low noise immunity, often used in conjunction with phase modulation);

n frequency modulation (complex technical implementation, usually used in low-speed modems).

n phase modulation.

Modulated signal spectrum

Potential code- if discrete data is transmitted at a rate of N bits per second, then the spectrum consists of a constant component of zero frequency and an infinite series of harmonics with a frequency of f0, 3f0, 5f0, 7f0, ..., where f0 = N / 2. The amplitudes of these harmonics decrease slowly - with coefficients 1/3, 1/5, 1/7, ... from the amplitude f0. The spectrum of the resulting signal of the potential code when transmitting arbitrary data occupies a band from some value close to 0 to about 7f0. For a voice channel, the upper limit of the bit rate is reached for a data rate of 971 bps, and the lower limit is unacceptable for any rate, since the channel bandwidth starts at 300 Hz. That is, potential codes are not used on voice-frequency channels.

Amplitude modulation- the spectrum consists of a sinusoid of the carrier frequency fc and two side harmonics fc + fm and fc-fm, where fm is the frequency of change of the information parameter of the sinusoid, which coincides with the data transfer rate when using two amplitude levels. The frequency fm determines the bandwidth of the line at this way coding. With a low modulation frequency, the bandwidth of the transmitted spectrum will be also small (equal to 2fm), and the signals will not distort the line if the bandwidth is greater than or equal to 2fm. For a voice frequency channel, this method is acceptable at a data rate not exceeding 3100/2 = 1550 bits per second.

Phase and frequency modulation- the spectrum is more complex, but symmetrical, with a large number of rapidly decaying harmonics. These methods are suitable for transmission over a voice frequency channel.

Quadrate Amplitude Modulation - Phase modulation with 8 values of phase shift and AM with 4 values of amplitude. Not all 32 signal combinations are used.

Digital coding

Potential codes- to represent logical ones and zeros, only the value of the signal potential is used, and its drops, which form complete impulses, are not taken into account.

Pulse codes- represent binary data either by pulses of a certain polarity, or as part of a pulse - by a potential drop in a certain direction.

Requirements for the digital coding method:

At the same bit rate, it had the smallest spectrum width of the resulting signal (a narrower signal spectrum allows achieving a higher data transmission rate on the same line, the requirement is also made for the absence of a constant component, that is, direct current between transmitter and receiver);

Provided synchronization between the transmitter and the receiver (the receiver must know exactly at what point in time to read necessary information off the line, in local systems- timing lines, in networks - self-synchronizing codes, the signals of which carry for the transmitter an indication of at what point in time it is necessary to recognize the next bit);

Possessed the ability to recognize errors;

Possessed a low implementation cost.

Potential non-zero return code. NRZ (Non Retrurn to Zero). The signal does not return to zero during the clock cycle.

It is simple to implement, has good error recognition due to two sharply differing signals, but does not have the property of synchronization. When a long sequence of zeros or ones is transmitted, the signal on the line does not change, so the receiver cannot determine when to read the data again. Another drawback is the presence of a low-frequency component, which approaches zero when transmitting long sequences of ones and zeros. The code is rarely used in its pure form; modifications are used. Attractiveness - low fundamental frequency f0 = N / 2.

Bipolar Alternate Inversion Coding Method... (Bipolar Alternate Mark Inversion, AMI), a modification of the NRZ method.

To encode zero, a zero potential is used, a logical one is encoded either by a positive potential or by a negative potential, while the potential of each next one is opposite to the potential of the previous one. Partially eliminates the problems of the constant component and the lack of self-synchronization. In the case of transmission of a long sequence of ones, a sequence of bipolar pulses with the same spectrum as the NRZ code, transmitting a sequence of alternating pulses, that is, without a constant component and fundamental N / 2 harmonic. In general, the use of AMI results in a narrower spectrum than NRZ, and therefore a higher bandwidth lines. For example, when transmitting alternating zeros and ones, the fundamental f0 has a frequency of N / 4. It is possible to recognize erroneous transmissions, but to ensure the reliability of reception, it is necessary to increase the power by about 3 dB, since the signal level is used.

Potential code with inversion at one... (Non Return to Zero with ones Inverted, NRZI) AMI-like code with two signal levels. When transferring zero, the potential of the previous cycle is transferred, and when transferring one, the potential is inverted to the opposite one. The code is convenient in cases where the use of the third level is not desirable (optical cable).

There are two methods used to improve AMI, NRZI. The first is the addition of redundant units to the code. The self-synchronization property appears, the constant component disappears and the spectrum narrows, but the useful bandwidth decreases.

Another method is to “mix” the initial information in such a way that the probability of the appearance of ones and zeros on the line becomes close - scrambling. Both methods are logical coding, since they do not determine the shape of the signals on the line.

Bipolar Pulse Code... One is represented by a pulse of one polarity, and zero is the other. Each impulse lasts half a beat.

The code has excellent self-synchronization properties, but a DC component may be present when transmitting a long sequence of zeros or ones. The spectrum is wider than that of potential codes.

Manchester code... The most common code used in Ethernet networks, Token Ring.

Each measure is divided into two parts. The information is encoded by potential drops occurring in the middle of the cycle. One is coded by the slope from low to high signal level, and zero is coded by the reverse slope. At the beginning of each cycle, an overhead signal drop may occur if you need to represent several ones or zeros in a row. The code has excellent self-syncing properties. The bandwidth is narrower than that of a bipolar pulse, there is no constant component, and the fundamental harmonic has a frequency of N in the worst case, and N / 2 in the best case.

Potential code 2В1Q... Every two bits are transmitted in one clock cycle by a signal with four states. 00 - -2.5 V, 01 - -0.833 V, 11 - +0.833 V, 10 - +2.5 V. Additional tools are required to deal with long sequences of identical pairs of bits. With a random interleaving of bits, the spectrum is twice narrower than that of NRZ, since at the same bit rate, the cycle time is doubled, that is, data can be transmitted through the same line twice as fast as using AMI, NRZI , but you need a lot of transmitter power.

Logical coding

Designed to improve potential codes such as AMI, NRZI, 2B1Q, replacing long sequences of bits, leading to a constant potential, interspersed with ones. Two methods are used - redundant coding and scrambling.

Redundant codes are based on splitting the original bit sequence into chunks, which are often called symbols, after which each original symbol is replaced with a new one that has large quantity bit than the original.

The 4B / 5B code replaces 4-bit sequences with 5-bit sequences. Then, instead of 16 bit combinations, 32 are obtained. Of these, 16 are selected that do not contain a large number zeros, the rest are considered prohibited codes (code violation). In addition to eliminating the DC component and making the code self-synchronizing, redundant codes allow the receiver to recognize corrupted bits. If the receiver receives a prohibited code, then the signal is distorted on the line.

This code is transmitted over the line using physical coding using one of the potential coding methods, which is sensitive only to long sequences of zeros. The code ensures that there are no more than three zeros in a row on the line. There are other codes, for example 8B / 6T.

To ensure the specified bandwidth, the transmitter must operate with an increased clock frequency (for 100 Mb / s - 125 MHz). The spectrum of the signal expands in comparison with the original, but the spectrum of the Manchester code remains.

Scrambling - scrambling of data with a scrambler before transferring it from to line.

Scrambling methods consist in bit-by-bit calculation of the resulting code based on the bits of the source code and the bits of the resulting code received in the previous clock cycles. For instance,

B i = A i xor B i -3 xor B i -5,

where B i is the binary digit of the resulting code obtained on the i-th cycle of the scrambler operation, A i is the binary digit of the source code arriving at the i-th cycle at the input of the scrambler, B i -3 and B i -5 are the binary digits of the resulting code obtained in the previous steps of the work.

For the sequence 110110000001, the scrambler will give 110001101111, that is, there will be no sequence of six consecutive zeros.

After receiving the resulting sequence, the receiver will transmit it to the descrambler, which will apply reverse transformation

C i = B i xor B i-3 xor B i-5,

Different scrambling systems differ in the number of terms and the shift between them.

There are more simple methods combating sequences of zeros or ones, which are also referred to as scrambling methods.

To improve the Bipolar AMI, the following are used:

B8ZS (Bipolar with 8-Zeros Substitution) - Corrects only sequences of 8 zeros.

To do this, after the first three zeros, instead of the remaining five, he inserts five signals V-1 * -0-V-1 *, where V denotes a one signal prohibited for a given polarity cycle, that is, a signal that does not change the polarity of the previous one, 1 * - the signal is one of the correct polarity, and the asterisk marks the fact that in the source code in this cycle there was not a one, but a zero. As a result, at 8 clock cycles, the receiver observes 2 distortions - it is very unlikely that this happened due to noise on the line. Therefore, the receiver considers such violations to be encoding of 8 consecutive zeros. In this code, the constant component is zero for any sequences of binary digits.

HDB3 corrects any four consecutive zeros in the original sequence. Every four zeros are replaced by four signals that have one V signal. To suppress the DC component, the polarity of the V signal alternates with successive replacements. In addition, two samples of four-bar codes are used for replacement. If before replacing source contained an odd number of ones, then the sequence 000V is used, and if the number of ones was even, the sequence 1 * 00V.

Improved candidate codes have a sufficiently narrow bandwidth for any sequences of zeros and ones that occur in the transmitted data.

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 voltage or current value as a single-valued function of time, you can simulate 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 in a series (possibly infinite), consisting of the sums of sines and cosines:
(2.1)
where is the fundamental frequency (harmonic), and are the amplitudes of the sines and cosines of the n-th harmonic, and c is a constant. Such an expansion is called a Fourier series. The function expanded in a Fourier series can be reconstructed from 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 reconstructed 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 infinitely over and over again (that is, the interval from T to 2T completely repeats the interval from 0 to T, and etc.).
The 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 row disappears completely. Similarly, multiplying equation (2.1) by and integrating over time from 0 to T, you can calculate the values. If we integrate both sides of the equation without changing it, then 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 a raw bitstream from one machine to another. Various physical media, also called signal propagation media, can be used for transmission. Each has a characteristic set of bandwidths, latencies, prices, and ease of installation and use. Media can be divided into two groups: managed media, such as copper wire and fiber optic cable, and unmanaged media, such as radio and laser transmission without cable.

Magnetic media

One of the most simple ways transfer data from one computer to another - write it to magnetic tape or other removable media (for example, a rewritable DVD), physically transfer these tapes and disks to their destination and read them there.
High throughput. A standard Ultrium tape cartridge holds 200 GB. A 60x60x60 box holds about 1000 of these cassettes, giving a total storage capacity of 1600 Tbps (1.6 Pbps). A box of cassettes can be shipped within the United States within 24 hours by Federal Express or another company. The effective bandwidth for this transfer is 1,600 Tbps / 86,400 s, or 19 Gbps. If the destination is just 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. Add $ 1000 for transportation (and in fact, much less) and we get about $ 5000 for a transfer of 200 TB or 3 cents per gigabyte.
Flaws. Although the data transfer rate with magnetic tapes is excellent, the latency is very high. Transmission times are measured in minutes or hours, not milliseconds. Many applications require instant response from the remote system (connected mode).

Twisted pair

A twisted pair consists of two insulated copper wires, the usual diameter of which is 1 mm. The wires are wound around each other in a spiral. This reduces 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. For longer distances, repeaters are required. Combined into a cable, with protective coating... The cable has a pair of twisted wires to avoid signal overlapping. They can be used to transfer both analog and digital data. The bandwidth depends on the diameter and length of the wire, but in most cases a speed of several megabits per second can be achieved over distances of up to several kilometers. Due to their relatively high bandwidth and low price, twisted pairs are widespread and, most likely, will be popular in the future.
Twisted pairs are used in several variants, two of which are especially important in the field of computer networks. Category 3 (CAT 3) twisted pairs consist of two insulated wires twisted together. Four of these pairs are usually placed together in a plastic wrap.
Category 5 (CAT 5) twisted pairs are similar to Category 3 twisted pairs, but have more turns per centimeter of wire length. This makes it possible to further reduce crosstalk 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 called UTP (unshielded twisted pair)
Twisted pair shielded cables from IBM did not become popular outside of IBM.

Coaxial cable

Coaxial cable is another common data transmission medium. It is better shielded than twisted pair, so it can transfer data over longer distances at higher speeds. Two types of cables are widely used. One of them, 50-ohm, is usually used to transfer purely digital data. Another type of cable, 75-ohm, is often used to transmit analog information, as well as in cable television.
A cross-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 transmission capacity 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 (backbones), cable television, regional networks.

Fiber optics

The currently existing fiber-optic technology can develop data transfer rates up to 50,000 Gbps (50 Tbps), and at the same time many specialists are busy looking for better materials. Today's 10 Gbps practical limit is due to the inability to convert electrical signals to optical signals and vice versa faster, although in laboratory conditions the speed of 100 Gbps has already been achieved on single fiber.
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. The light pulse is taken as one, and the absence of the pulse is taken as zero. The light travels in an ultra-thin glass fiber. When light hits it, the detector generates an electrical pulse. By connecting 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 is used when passing from 2 media. Thus, when light is supplied at a certain angle to the interface between the media, 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: multimode - transmits a beam of light, single-mode - thin to the limit of several wavelengths, acts almost like a waveguide, light travels in a straight line without reflection. Today's single-mode fiber can operate at 50 Gbps over distances of up to 100 km.
Three wavelength ranges are used in communication systems: 0.85, 1.30 and 1.55 microns, respectively.
The structure of a fiber optic cable is similar to that of a coaxial wire. The only difference is that there is no screening mesh in the former.
At the center of the fiber optic core is a glass core through which light propagates. In multimode fiber, the core diameter is 50 microns, which is about the thickness of a human hair. The core in single-mode fiber has a diameter of 8 to 10 µm. The core is covered with a glass layer with a lower refractive index than the core. It is designed to more reliably prevent light from escaping outside the core. The outer layer is a plastic shell that protects the glazing. Fiber optic cores are usually bundled in bundles, protected by an outer jacket. Figure 4 shows a three-core cable.

Rice. 4. Three-core fiber optic cable.
In the event of a break, the connection of cable sections 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 outlet. The loss is 10-20% of the luminous intensity, but it makes it easy to change the configuration of the system.

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

Fusion. Loss is virtually nonexistent.

Two types of light sources can be used to transmit signals over fiber optic cables: Light Emitting Diodes (LEDs) and semiconductor lasers. Their comparative characteristics are shown in table 1.

Table 1.
LED vs Semiconductor Laser Usage Comparison Chart
The receiving end of the optical cable is a photodiode that generates an electrical pulse when light is incident 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 twisted copper pairs 1 km long weigh about 8000 kg. A pair of fiber optic cables weighs only 100 kg with more bandwidth

Low installation costs

Flaws:

Complexity and competence during installation.

transmission in simplex mode, a minimum of 2 cores are required between the networks.

Wireless connection

Electromagnetic spectrum

The movement of electrons generates electromagnetic waves that can propagate through space (even in a vacuum). The number 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 designated by the Greek letter (lambda).
If in electrical circuit turn on an antenna of a suitable size, 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 propagate 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, in addition, it is slightly dependent on frequency.
Relationship between quantities, and:

If the frequency () is measured in MHz, and the wavelength () is 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 rays 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 to all life. The official names of the ranges are shown in Table 6.

Rice. 5. Electromagnetic spectrum and its application in communication.
Table 2.
Official band names by ITU
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 some conditions, this number can increase eightfold at high frequencies.
Knowing the width of the wavelength range, the corresponding frequency range and data rate can be calculated.

Example: For a 1.3 micron fiber optic cable, the result is then. Then, at 8 bit / s, you can get a transfer rate of 240 Tbit / s.

Radio communication

Radio waves are easy to generate, travel long distances, pass through walls, bend around buildings, and travel in all directions. The property of radio waves depends on the frequency (Fig. 6). When operating at low frequencies, radio waves penetrate 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 like this: 1 / r2. At high frequencies, radio waves generally tend to travel in a purely straight line and bounce off obstacles. In addition, they are absorbed, for example, by rain. Radio signals of all frequencies are susceptible to interference from spark brush motors and other electrical equipment.

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

Microwave communication

At frequencies above 100 MHz, radio waves travel 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 television dish) leads to an improvement in the signal-to-noise ratio, however, for such a connection, the transmitting and receiving antennas must be fairly accurately directed towards each other.
Unlike radio waves with lower frequencies, microwaves do not penetrate buildings well. Microwave radio communication became so widely used in long distance telephony, cell phones, television broadcasting and other fields that the lack of spectrum bandwidth began to be felt.
This connection has a number of advantages over optical fiber. The main one is that there is no need to lay a cable, therefore, there is no need to pay for the lease of land on 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-wave 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 go through solid objects.
Infrared communication is used in desktop computing systems (for example, to connect laptops to printers), but still does not play a significant role in telecommunications.

Communication satellites

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

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

Public switched telephone network

Telephone system structure

The structure of a typical medium-haul telephony route 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 an analog signal transmission, a modem device is used to perform digital-to-analog conversion and vice versa, and the process itself is called modulation / demodulation (Fig. 9).

Rice. 9. Using 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 when switching between logical units (0 and 1). Shear angles - 45, 135, 225, 180.

In practice, combined modulation systems are used.

Rice. 10. Binary signal (s); amplitude modulation (b); frequency modulation (c); phase modulation.
All modern modems allow data transmission in both directions; this mode of operation is called duplex. An alternating connection is called half duplex. A connection in which only one direction is transmitted is called simplex.
The maximum speed of modems that can be reached at the current moment is 56Kb / s. Standard V.90.

Digital subscriber lines. XDSL technology.

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

POTS - POTS band;

outgoing range;

incoming range.

Technology that uses for different purposes different frequencies is called frequency division multiplexing or frequency multiplexing.
An alternative method called Discrete MultiTone (DMT) modulation consists of dividing the entire 1.1 MHz local link spectrum into 256 independent 4312.5 Hz channels. Channel 0 is POTS. Channels 1 through 5 are not used to prevent the voice signal from interfering with the data signal. Of the remaining 250 channels, one is busy controlling transmission towards the provider, one towards the user, and all the others are available for transferring user data (Fig. 11).

Rice. 11. ADSL operation using discrete multi-tone 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 hardware configuration contains:

DSLAM - DSL access multiplexer;

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

Splitter - A splitter separating 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-throughput backbone and a low-quality line is practically the same (that is, the lion's share of this cost goes to digging trenches, and not to the copper or fiber-optic cable itself).
For this reason telephone companies jointly developed several schemes for transmitting several conversations over one physical cable. Multiplexing schemes can be divided into two main categories FDM (Frequency Division Multiplexing) and TDM (Time Division Multiplexing) (Fig. 13).
In frequency division multiplexing, the frequency spectrum is divided between logical channels, while each user gets exclusive ownership of his sub-band. In time division multiplexing, users take turns (cyclically) to use the same channel, and each is given the full bandwidth of the channel for a short period of time.
In fiber optic channels, a special version of frequency division is used. It is called Wavelength Division Multiplexing (WDM).

Rice. 13. An example of frequency multiplexing: initial signal spectra 1 (a), frequency-shifted spectra (b), compressed channel (c).

Commutation

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

Channel switching

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

Packet switching

With packet switching, all messages transmitted by a network user are split at the source node into relatively small parts called packets. Each packet is provided with a header that specifies the address information required to deliver the packet to the destination node, as well as the package number that will be used by the destination node to assemble the message. Packets are transported across 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.................

2 Physical layer functions Bit representation by electrical / optical signals Bit coding Bit synchronization Bit synchronization / reception via physical communication channels Communication with the physical medium Transfer rate Range Signal levels, connectors In all network devices Hardware implementation (network adapters) Example: 10 BaseT - UTP cat 3, 100 ohm, 100m, 10Mbps, MII code, RJ-45

5 Data transmission equipment Transmitter Message - El. signal Encoder (compression, correction codes) Modulator Intermediate equipment Improving the quality of communication - (Amplifier) Creating a composite channel - (Switch) Channel compression - (Multiplexer) (PA may be absent in LAN)

6 Main characteristics of communication lines Throughput (Protocol) Reliability of data transmission (Protocol) Propagation delay Amplitude-frequency response (AFC) Bandwidth Attenuation Noise immunity Crosstalk at the near end of the line Specific cost

9 Attenuation A - one point on the frequency response A = log 10 Pout / Pin Bel A = 10 log 10 Pout / Pin deciBel (dB) A = 20 log 10 Uout / Uin deciBel (dB) q Example 1: Pin = 10 mW, Pout = 5 mW Attenuation = 10 log 10 (5/10) = 10 log 10 0.5 = - 3 dB q Example 2: UTP cat 5 Attenuation> = -23.6 dB F = 100MHz, L = 100 M Usually A is indicated for the fundamental frequency of the signal. = -23.6 dB F = 100MHz, L = 100 M Usually A is indicated for the fundamental frequency of the signal ">

11 Immunity Fiber optic cables Cable lines Wire overhead lines Radio links (Shielding, twisting) Immunity to external interference Immunity to internal interference Near-end crosstalk attenuation (NEXT) Far-end crosstalk attenuation (FEXT) (FEXT - Two pairs in one direction)

12 Near End Cross Talk loss (NEXT) For multi-pair cables NEXT = 10 log Pout / Pout dB NEXT = NEXT (L) UTP 5: NEXT

13 Data transmission reliability Bit Error Rate - BER Probability of data bit corruption Causes: external and internal interference, narrow bandwidth Fight: increased noise immunity, reduced NEXT interference, increased bandwidth Twisted pair BER ~ Fiber optic cable BER ~ No additional security features :: correction codes, protocols with repetition

16 Twisted pair Twisted Pair (TP) foil shield braided wire shield insulated wire outer sheath UTP Unshielded Twisted Pair category 1, UTP sheathed cat pairs STP Shielded Twisted Pair Types Type 1 ... 9 Each pair has its own shield Each pair has its own step twists, own color Noise immunity Cost Complexity of laying

18 Fiber Optics Total internal beam reflection at the interface between two media n1> n2 - (refractive index) n1 n2 n2 - (refractive index) n1 n2 "> n2 - (refractive index) n1 n2"> n2 - (refractive index) n1 n2 "title =" (! LANG: 18 Fiber Optics Total internal beam reflection at the interface between two media n1> n2 - (refractive index) n1 n2"> title="18 Fiber Optics Total internal beam reflection at the interface between two media n1> n2 - (refractive index) n1 n2"> !}

22 Fiber-optic cable Multi Mode Fiber MMF50 / 125, 62.5 / 125, Single Mode FiberSMF8 / 125, 9.5 / 125 D = 250 μm 1 GHz - 100 km BaseLH5000 km - 1 Gbps (2005) MMSM

23 Optical signal sources Channel: source - carrier - receiver (detector) Sources LED (LED- Light Emitting Diod) nm incoherent source - MMF Semiconductor laser coherent source - SMF - Power = f (t o) Detectors Photodiodes, pin diodes, avalanche diodes

25 Structured Cabling Systems - SCS Structured Cabling System - SCS First LANs - various cables and topologies Unification cable system SCS - open LAN cable infrastructure (subsystems, components, interfaces) - independence from network technology - LAN cables, TV, security systems, etc. - universal cabling without reference to a specific network technology -Constructor

27 SCS standards (basic) EIA / TIA-568A Commercial Building Telecommunications Wiring Standard (USA) CENELEC EN50173 Performance Requirements of Generic Cabling Schemes (Europe) ISO / IEC IS Information Technology - Generic cabling for customer premises cabling For each subsystem: Data transmission medium ... Topology Allowable distances (cable lengths) User connection interface. Cables and connecting equipment. Bandwidth (Performance). Installation practice (Horizontal subsystem - UTP, star, 100 m ...)

28 Wireless communication Wireless Transmission Advantages: good quality, inaccessible areas, mobility. fast deployment ... Disadvantages: high level of interference ( special means: codes, modulation ...), the complexity of using some bands Communication line: transmitter - medium - receiver LAN characteristics ~ F (Δf, fн);

34 2. Cellular telephony Territory division into cells Frequency reuse Low power (dimensions) In the center - base station Europe - Global System for Mobile - GSM Wireless telephone communications 1. Low-power radio station - (base tube, 300m) DECT Digital European Cordless Telecommunication Roaming - switching from one core network to another - base cellular

35 Satellite connection At the heart - a satellite (reflector-amplifier) Transceivers - transponders H ~ 50 MHz (1 satellite ~ 20 transponders) Frequency ranges: С. Ku, Ka C - Down 3.7 - 4.2 GHz Up 5.925-6.425 GHz Ku - Down 11.7-12.2 GHz Up 14.0-14.5 GHz Ka-Down 17.7-21.7 GHz Up 27.5-30.5 GHz

36 Satellite communications. Types of satellites Satellite communications: microwaves - line of sight Geostationary Large coverage Immobility, Low wear Satellite repeater, broadcast, low cost, cost does not depend on distance, Instant connection (Mil) Tz = 300ms Low security, Initially large antenna (but VSAT) Mid-orbit km Global Positioning System GPS - 24 satellites LEO km low coverage low latency Internet access

40 Spread Spectrum Technique Special methods modulation and coding for wirelessС (Bit / s) = Δ F (Hz) * log2 (1 + Ps / P N) Power reduction Noise immunity Stealth OFDM, FHSS (, Blue-Tooth), DSSS, CDMA

Near-End Crosstalk - Determines the cable's immunity to internal sources of interference. They are usually evaluated in relation to a cable consisting of several twisted pairs, when the mutual interference of one pair to another can reach significant values and create internal interference, commensurate with the useful signal.

Reliability of data transmission(or bit error rate) characterizes the probability of corruption 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 bandwidth. Therefore, an increase in the reliability of data transmission is achieved by increasing the level of noise immunity of the line, reducing the level of cross talk in the cable, using more broadband communication lines.

For conventional cable communication lines without additional means of protection against errors, the reliability of data transmission is, as a rule, 10 -4 -10 -6. This means that an average of 10 4 or 10 6 transmitted bits will distort the value of one bit.

Communication line equipment(data transmission equipment - ADF) is border equipment that directly connects computers with a communication line. It is part of the communication line and usually works at the physical layer, providing transmission and reception of a signal of the desired shape and power. Examples of ADFs are modems, adapters, A / D and D / A converters.

The ATM does not include the user's terminal data equipment (DTE), which generates data for transmission over the communication line and connects directly to the ATM. DTE includes, for example, a router local area networks... Note that the division of equipment into classes of APD and DTE is rather arbitrary.

On long-distance communication lines, intermediate equipment is used, 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 LKS, intermediate equipment is not used if the length of the physical medium (cables, radio air) is short, so that signals from one network adapter to another one can be transferred without intermediate restoration of their parameters.

V global networks high-quality signal transmission is provided for hundreds and thousands of kilometers. Therefore, amplifiers are installed at certain distances. To create an end-to-end 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 the construction of computer, telephone and other networks.

Distinguish analog and digital communication lines which use different types of intermediate equipment. In analog lines, intermediate equipment is designed to amplify analog signals with 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-division multiplexing, when each low-speed channel is allocated a certain fraction of the time of a high-speed channel.

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

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 only in 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 and reference, request-response systems);

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

Dial-up and dedicated communication channels... TCC distinguishes between dedicated (non-commutated) communication channels and those with commutation for the duration of information transmission over these channels.

When using dedicated communication channels, the transceiver equipment of the communication nodes is permanently connected to each other. This ensures a high degree of readiness of the system to transmit information, a higher quality of communication, and support for a large volume 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.

For switched communication channels, created only for the duration of the transmission of a fixed amount of information, high flexibility and relatively low cost (with a small volume of traffic) are characteristic. Disadvantages of such channels: 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 quality of communication, high cost with a significant volume of traffic.

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

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

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

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

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

Currently, the original data, which have an analog form, are increasingly transmitted via communication channels in a discrete form (in the form of a sequence of ones and zeros), i.e., discrete modulation analog signals.

Analog modulation... Used to transmit discrete data over narrow bandwidth channels, typical of which is a tone channel provided to users telephone networks... This channel transmits signals with a frequency of 300 to 3400 Hz, i.e., its bandwidth is 3100 Hz. This bandwidth is sufficient for transmitting speech with acceptable quality. Limiting the bandwidth 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, a modulator-demodulator (modem) modulates the carrier sinusoid of the original sequence of binary digits. The inverse transformation (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 a unit, the amplitude of oscillations is set large, and when transmitting zero, it is low, or there is no carrier signal at all;

· 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 transmitting zero, it is low, and when transmitting one, 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 passing 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 its low noise immunity. Frequency modulation does not require complex schemes in modems and is usually used in low speed modems operating at speeds of 300 or 1200 bps. An increase in the data transmission rate is provided by the use of combined modulation methods, more often amplitude in combination with phase.

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

Digital coding... When digital coding of 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 differences are not taken into account;

b) pulse codes, when binary data is represented either by pulses of a certain polarity, or by potential drops in 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 achieving a higher data transfer rate on a line with the same bandwidth);

· The ability to recognize errors in the transmitted data;

· Relatively low cost of implementation.

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

Synchronizing the transmitter and receiver is necessary so that the receiver knows exactly when to read the incoming data. Synchronization tune the receiver to the transmitted message and keep the receiver in sync with the incoming data bits. The synchronization problem is easily solved when transferring information to short distances(between blocks inside a computer, between a computer and a printer) by using a separate clocking communication line: information is read only at the time of the next clock pulse. V computer networks they refuse to use clock pulses for two reasons: for the sake of saving conductors in expensive cables and because of the inhomogeneity of the characteristics of the conductors in cables (at large distances, uneven signal propagation speed can lead to desynchronization of clock pulses in the clock line and information pulses in the main line, as a result of which data bit will either be skipped or re-read).

Currently, the 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 transition of the signal level from high to low or vice versa is used to trim the receiver. The best ones are considered to be those that ensure the transition of the signal level at least once during the time interval required to receive one information bit. The more frequent the signal level transitions, the more reliably the receiver synchronizes and the more confidently the received data bits are identified.

The specified requirements for digital coding methods for discrete information are to a certain extent mutually contradictory, therefore, each of the coding methods discussed below has its own 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.

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

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

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

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

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

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

Analog modulation. It is used for the transmission of discrete data over channels with a narrow frequency band, a typical representative of which is a voice frequency channel provided to users of telephone networks. This channel transmits signals with a frequency of 300 to 3400 Hz, i.e., its bandwidth is 3100 Hz. This bandwidth is quite sufficient for the transmission of speech with acceptable quality. Limiting the bandwidth 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 carrier frequency of sinusoidal oscillations changes: for example, when transmitting zero, it is low, and when transmitting one, 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 passing 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 its 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. An increase in the data transmission rate is provided by the use of combined modulation methods, more often amplitude in combination with phase.

Digital coding. When digital coding of 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 differences are not taken into account;

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

Providing the smallest spectrum width of the resulting signal at the same bit rate (since a narrower spectrum of signals allows for

with the same bandwidth to achieve higher speed

data transmission);

The ability to recognize errors in the transmitted data;

Relatively low cost of implementation.

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

Synchronizing the transmitter and receiver is necessary so that the receiver knows exactly when to read the incoming data. Synchronization tune the receiver to the transmitted message and keep the receiver in sync with the incoming data bits. The synchronization problem is easily solved when transferring information over short distances (between blocks inside a computer, between a computer and a printer) by using a separate clocking communication line: information is read only at the moment of the next clock pulse. In computer networks, they refuse to use clocking pulses for two reasons: for the sake of saving conductors in expensive cables and because of the inhomogeneity of the characteristics of the conductors in cables (at large distances, the unevenness of the signal propagation speed 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 be either skipped or re-read).

Currently, the 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 transition of the signal level from high to low or vice versa is used to trim the receiver. The best ones are considered to be those that ensure the transition of the signal level at least once during the time interval required to receive one information bit. The more frequent the signal level transitions, the more reliably the receiver synchronizes and the more confidently the received data bits are identified.

Self-timed 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 alternating level inversion.

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

To characterize and comparatively assess the UK, the following indicators are used:

The level (quality) of synchronization;

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

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

The complexity (and, therefore, the cost) of the equipment that implements the IC.

NRZ code is easy to code and low cost of implementation. It got this name because when transmitting a series of bits of the same name (ones or zeros), the signal does not return to zero during a clock cycle, as is the case in other encoding 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 timer may mismatch with respect to the incoming signal and untimely polling of lines).

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

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

U2 - communication line bandwidth (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, which approaches zero when transmitting long series 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.

RZ-code, or bipolar pulse code (code with return to zero), differs in that during the transmission of one information bit, the signal level changes twice, regardless of whether a series of like-named bits or alternately changing bits are transmitted. One is represented by a pulse of one polarity, and zero is the other. Each impulse lasts half a beat. Such a 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 the RZ code is wider than that of the potential codes. Due to its too wide spectrum, it is rarely used.

The Manchester code provides a change in the signal level at the presentation of 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 clock cycle. One is coded by the slope from low to high signal level, and zero is coded by the reverse slope. The speed ratio for this code is as follows:

The Manchester code possesses good self-timing properties, since the signal changes at least once per transmission cycle of one data bit. Its bandwidth is narrower than that of the RZ code (1.5 times on average). Unlike the bipolar pulse code, where three signal levels are used for data transmission (which is sometimes very undesirable, for example, in optical cables only two states are stably recognized - light and dark), in the Manchester code there are two levels.

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

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

When transmitting various combinations of bits on a line, the use of the AMI code results in a narrower signal spectrum than for the NRZ code, and therefore in a higher line capacity.

Note that improved potential codes (modernized Manchester code and AMI code) have a narrower spectrum than pulsed ones, therefore they are used in high-speed technologies, for example, in 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, that is, 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, video cameras, video and sound reproducing 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 discrete computer data 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 analog technology for recording and transmitting sound and image was replaced by digital technology, which uses discrete modulation of analog signals.

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

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

The first stage is display. The amplitude of the original continuous signal is measured with a specified period, due to which time sampling occurs. At this stage, the analog signal is converted into pulse-amplitude modulation (IAM) signals. The execution of the stage is based on the Nyquist-Kotelnikov mapping theory, the main position of which is: if an 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, the display will contain information sufficient to restore the original signal. In analog telephony, the range from 300 to 3400 Hz is selected for voice transmission, which is sufficient for high-quality transmission of all the fundamental 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 a certain quality margin).

At the quantization stage, each IAM signal is assigned a quantized value corresponding to the nearest quantization level. The entire range of changes in the amplitude of the IAM signals is divided into 128 or 256 quantization levels. The more quantization levels, the more accurate the IAM amplitude - the signal 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 (with 256-step quantization) binary code. In fig. 33 shows signals of an 8-element binary code 00101011, corresponding to a quantized signal with a level of 43. When encoding with 7-element codes, the data transfer rate over the channel should be 56 Kbit / s (this is the product of the display frequency and the width of the binary code), and when encoding 8- element codes - 64 Kbps. The standard is digital channel 64 kbps, which is also called the elementary channel of digital telephone networks.

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

In modern digital communication networks, other methods of discrete modulation are used, which make it possible to represent voice measurements in a more compact form, for example, in the form of 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. Obviously, this concept allows for 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 medium or received via a communication line. For this purpose, the same control methods are applied as for computer data (see clause 4.9).

The transmission of a continuous signal in discrete form imposes strict requirements on the synchronization of the receiver. If the synchronization is not observed, the original signal is reconstructed incorrectly, which leads to distortion of the voice or the transmitted image. If frames with voice measurements (or other analog value) arrive synchronously, then the voice quality can be quite high. However, in computer networks, frames can be delayed both at 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, networks digital television), although for the transfer of intracorporate telephone conversations Frame Relay networks are still used today because frame delays are within acceptable limits.