Physical basis of data transmission. Methods of data transmission at the physical level. Physical basis of data transmission

7. PHYSICAL DATA TRANSMISSION LEVEL

7.2. Discrete data transfer methods

When transmitting discrete data over communication channels, two main types of physical coding are used - based on a sinusoidal carrier signal and based on a sequence of rectangular pulses. The first method is often called modulation or analog modulation , emphasizing the fact that encoding is carried out by changing the parameters of the analog signal. The second method is called digital coding . These methods differ in the width of the spectrum of the resulting signal and the complexity of the equipment required for their implementation.

When using rectangular pulses, the spectrum of the resulting signal is very wide. The use of a sine wave results in a narrower spectrum at the same information transfer rate. However, to implement modulation, more complex and expensive equipment is required than to implement rectangular pulses.

Currently, increasingly, data that was originally in analog form - speech, television images - is transmitted over communication channels in discrete form, that is, as a sequence of ones and zeros. The process of representing analog information in discrete form is called discrete modulation .

Analog modulation is used to transmit discrete data over channels with a narrow frequency band - voice-frequency channel (public telephone networks). This channel transmits frequencies in the range from 300 to 3400 Hz, so its bandwidth is 3100 Hz.

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

Analog modulation is a physical coding method in which information is encoded by changing the amplitude, frequency or phase of a sinusoidal signal carrier frequency(Fig. 27).

At amplitude modulation (Fig. 27, b) for a logical unit one level of the amplitude of the carrier frequency sinusoid is selected, and for a logical zero - another. This method is rarely used in its pure form in practice due to low noise immunity, but is often used in combination with another type of modulation - phase modulation.

At frequency modulation (Fig. 27, c) the values ​​0 and 1 of the source data are transmitted by sinusoids with different frequencies - f 0 and f 1,. This modulation method does not require complex circuitry in modems and is typically used in low-speed modems operating at 300 or 1200 bps.

At phase modulation (Fig. 27, d) data values ​​0 and 1 correspond to signals of the same frequency, but with different phases, for example 0 and 180 degrees or 0, 90, 180, and 270 degrees.

High-speed modems often use combined modulation methods, usually amplitude combined with phase.

Rice. 27. Various types modulation

The spectrum of the resulting modulated signal depends on the type and speed of modulation.

For potential encoding, the spectrum is directly obtained from the Fourier formulas for the periodic function. If discrete data is transmitted at a bit rate of N bit/s, then the spectrum consists of a constant component of zero frequency and an infinite series of harmonics with frequencies f 0, 3f 0, 5f 0, 7f 0, ..., where f 0 = N/2. The amplitudes of these harmonics decrease quite slowly - with coefficients of 1/3, 1/5, 1/7, ... from the amplitude of the harmonic f 0 (Fig. 28, a). As a result, the spectrum of potential code requires a wide bandwidth for high-quality transmission. In addition, you need to take into account that in reality the signal spectrum is constantly changing depending on the nature of the data. Therefore, the spectrum of the resulting potential code signal when transmitting arbitrary data occupies a band from a certain value close to 0 Hz to approximately 7f 0 (harmonics with frequencies above 7f 0 can be neglected due to their small contribution to the resulting signal). For a voice channel, the upper limit for potential coding is achieved at a data rate of 971 bps. As a result, potential codes on voice channels are never used.

With amplitude modulation, the spectrum consists of a sine wave of the carrier frequency f with and two side harmonics: (f c + f m ) and ( f c – f m), where f m – frequency of change of the information parameter of the sinusoid, which coincides with the data transmission rate when using two amplitude levels (Fig. 28, b). Frequency f m determines the line capacity for a given coding method. At a low modulation frequency, the signal spectrum width will also be small (equal to 2f m ), so signals will not be distorted by a line if its bandwidth is greater than or equal to 2f m . For a voice frequency channel, this modulation method is acceptable at a data transfer rate of no more than 3100/2=1550 bps. If 4 amplitude levels are used to present data, then the channel capacity increases to 3100 bps.

Rice. 28. Spectra of signals during potential coding

and amplitude modulation

With phase and frequency modulation, the signal spectrum is more complex than with amplitude modulation, since more than two side harmonics are formed here, but they are also symmetrically located relative to the main carrier frequency, and their amplitudes quickly decrease. Therefore, these types of modulation are also well suited for data transmission over a voice channel.

When digitally encoding discrete information, potential and pulse codes are used. In potential codes, only the potential value of the signal is used to represent logical ones and zeros, and its edges are not taken into account. Pulse codes allow you to represent binary data either as pulses of a certain polarity, or as part of a pulse - a potential difference in a certain direction.

When using rectangular pulses to transmit discrete information, it is necessary to choose a coding method that simultaneously achieves several goals:

· had the smallest spectrum width of the resulting signal at the same bit rate;

· provided synchronization between the transmitter and receiver;

· had the ability to recognize mistakes;

· had a low selling price.

A narrower spectrum of signals allows for higher data transfer rates on the same line. Often the signal spectrum is required to have no DC component.

Synchronization of the transmitter and receiver is necessary 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 exchanging data between closely located devices, for example, between units inside a computer or between a computer and a printer. Therefore, networks use so-called self-synchronizing codes, the signals of which carry instructions for the transmitter about at what point in time the next bit (or several bits) should be recognized. Any sharp change in the signal - the so-called edge - can serve as a good indication for synchronizing the receiver with the transmitter.

When using sinusoids as a carrier signal, the resulting code has the property of self-synchronization, since changing the amplitude of the carrier frequency allows the receiver to determine the moment the input code appears.

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

In Fig. 29, a shows the potential encoding method, also called encoding without returning to zero (Non Return to Zero, NRZ) . The last name reflects the fact that when transmitting a sequence of ones, the signal does not return to zero during the clock cycle. The NRZ method is easy to implement, has good error recognition (due to two sharply different potentials), but does not have the property of self-synchronization. When transmitting a long sequence of ones or zeros, the signal on the line does not change, so the receiver is unable to determine from the input signal the moments in time when it is necessary to read data. Even with a high-precision clock generator, the receiver may make a mistake with the moment of data collection, since the frequencies of the two generators are never completely identical. Therefore, at high data rates and long sequences of ones or zeros, a small clock mismatch can lead to an error of a whole clock cycle and, accordingly, an incorrect bit value being read.

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 channels that do not provide a direct galvanic connection between the receiver and the source do not support this type of coding. As a result, the NRZ code in its pure form is not used in networks. Nevertheless, its various modifications are used, which eliminate both the poor self-synchronization of the NRZ code and the presence of a constant component. The attractiveness of the NRZ code, which makes it worthwhile to improve it, is the fairly low frequency of the fundamental harmonic f 0, which is equal to N/2 Hz. In other encoding methods, such as Manchester, the fundamental harmonic has a higher frequency.

Rice. 29. Methods of discrete data coding

One of the modifications of the NRZ method is the method bipolar coding with alternative inversion (Bipolar Alternate Mark Inversion, AMI). This method (Fig. 29, b) uses three potential levels - negative, zero and positive. To encode a logical zero, a zero potential is used, and a logical one is encoded either by a positive potential or a negative one, with the potential of each new unit being opposite to the potential of the previous one.

The AMI code partially eliminates the DC and lack of self-synchronization problems inherent in the NRZ code. This occurs when transmitting long sequences of ones. In these cases, the signal on the line is a sequence of oppositely polarized 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 bit rate of data transfer) . Long sequences of zeros are just as dangerous for the AMI code as for the NRZ code - the signal degenerates into a constant potential of zero amplitude. Therefore, the AMI code requires further improvement.

In general, for different bit combinations on a line, using the AMI code results in a narrower signal spectrum than the NRZ code, and therefore higher bandwidth lines. 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 capabilities for recognizing erroneous signals. Thus, a violation of the strict alternation of signal polarity indicates a false pulse or the disappearance of a correct pulse from the line. This signal is called prohibited signal (signal violation).

The AMI code uses not two, but three signal levels on the line. The additional layer requires an increase in transmitter power of about 3 dB to provide the same bit fidelity on the line, which is a common disadvantage of codes with multiple signal states compared to codes that distinguish only two states.

There is code similar to AMI, but with only two signal levels. When transmitting a zero, it transmits the potential that was set in the previous cycle (that is, does not change it), and when transmitting a one, the potential is inverted to the opposite one. This code is called potential code with inversion at one (Not Return to Zero with ones Inverted , NRZI ) . This code is useful in cases where the use of a third signal level is highly undesirable, for example, in optical cables, where two signal states are consistently recognized - light and shadow.

In addition to potential codes, pulse codes are also used in networks, when the data is represented by a full pulse or part of it - an edge. The simplest case of this approach is bipolar pulse code , in which one is represented by a pulse of one polarity, and zero by another (Fig. 29, c). Each pulse lasts half a beat. This code has excellent self-synchronizing properties, but a constant 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. Thus, 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 its 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 (Fig. 29, d). It is used in Ethernet and Token Ring technologies.

The Manchester code uses a potential difference, that is, the edge of a pulse, to encode ones and zeros. With Manchester encoding, each measure is divided into two parts. Information is encoded by potential drops that occur in the middle of each clock cycle. One is encoded by an edge from low to high signal level, and zero is encoded by a reverse edge. At the beginning of each clock cycle, an overhead signal drop may 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-synchronizing properties. The bandwidth of the Manchester code is narrower than that of the bipolar pulse. It also has no DC 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 AMI 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 fluctuates around the value of 3N/4. The Manchester code has another advantage over the bipolar pulse code. The latter uses three signal levels for data transmission, while the Manchester one uses two.

In Fig. 29, d shows a potential code with four signal levels for encoding data. This is a 2B1Q code, the name of which reflects its essence - every two bits (2B) are transmitted in one clock cycle by a signal having four states (1Q). Bit pair 00 corresponds to a potential of -2.5 V, bit pair 01 corresponds to a potential of -0.833 V, pair 11 corresponds to a potential of +0.833 V, and pair 10 corresponds to a potential of +2.5 V. With this coding method, additional measures are required to combat long sequences of identical bit pairs, since in this case the signal turns into a constant component. With random interleaving of bits, the signal spectrum is twice as narrow as that of the NRZ code, since at the same bit rate the clock duration is doubled. Thus, using the 2B1Q code, you can transfer data over the same line twice as fast as using the AMI or NRZI code. However, to implement it, the transmitter power must be higher so that the four levels are clearly distinguished by the receiver against the background of interference.

Page 27 from 27 Physical basis of data transmission(Communication lines,)

Physical basis of data transmission

Any network technology must ensure reliable and fast transmission of discrete data over communication lines. Although there are large differences between technologies, they are based on common principles of discrete data transfer. These principles are embodied in methods for representing binary ones and zeros using pulsed or sinusoidal signals in communication lines of various physical natures, error detection and correction methods, compression methods and switching methods.

Linescommunications

Primary networks, lines and communication channels

When describing technical system, which transmits information between network nodes, several names can be found in the literature: communication line, composite channel, channel, link. Often these terms are used interchangeably, and in many cases this does not cause problems. At the same time, there are specifics in their use.

Link(link) is a segment that provides data transfer between two neighboring network nodes. That is, the link does not contain intermediate switching and multiplexing devices.

Channel(channel) most often denote the part of the link bandwidth used independently during switching. For example, a primary network link may consist of 30 channels, each of which has a capacity of 64 Kbps.

Composite channel(circuit) is a path between two end nodes of a network. A composite channel is formed by individual intermediate links and internal connections in switches. Often the epithet “composite” is omitted and the term “channel” is used to refer to both a composite channel and a channel between neighboring nodes, that is, within a link.

Communication line can be used as a synonym for any of the other three terms.

In Fig. two communication line options are shown. In the first case ( A) the line consists of a cable segment several tens of meters long and is a link. In the second case (b), the communication line is a composite channel deployed in a circuit-switched network. Such a network could be primary network or telephone network.

However for computer network this line represents a link, since it connects two adjacent nodes, and all switching intermediate equipment is transparent to these nodes. The reason for mutual misunderstanding at the level of terms between computer specialists and primary network specialists is obvious here.

Primary networks are specifically created in order to provide data transmission channel services for computer and telephone networks, which in such cases are said to work “on top” of primary networks and are overlay networks.

Classification of communication lines

Communication line generally consists of a physical medium through which electrical information signals, data transmission equipment and intermediate equipment are transmitted. The physical medium for data transmission (physical storage media) can be a cable, that is, a set of wires, insulating and protective sheaths and connecting connectors, as well as the earth’s atmosphere or outer space through which electromagnetic waves propagate.

In the first case we talk about wired environment, and in the second - about wireless.

In modern telecommunication systems, information is transmitted using electric current or voltage, radio signals or light signals- all these physical processes represent oscillations of the electromagnetic field of various frequencies.

Wired (overhead) lines connections are wires without any insulating or shielding braiding, laid between poles and hanging in the air. Even in the recent past, such communication lines were the main ones for transmitting telephone or telegraph signals. Today, wired communication lines are quickly being replaced by cable lines. But in some places they are still preserved and, in the absence of other possibilities, continue to be used for transmitting computer data. The speed and noise immunity of these lines leave much to be desired.

Cable lines have a rather complex design. The cable consists of conductors enclosed in several layers of insulation: electrical, electromagnetic, mechanical and, possibly, climatic. In addition, the cable can be equipped with connectors that allow you to quickly connect various equipment to it. There are three main types of cable used in computer (and telecommunications) networks: cables based on twisted pairs of copper wires - unshielded twisted pair(Unshielded Twisted Pair, UTP) and shielded twisted pair(Shielded Twisted Pair, STP), coaxial cables with copper core, fiber optic cables. The first two types of cables are also called copper cables.

Radio channels Terrestrial and satellite communications are formed using a radio wave transmitter and receiver. There are a wide variety of types of radio channels, differing both in the frequency range used and in the channel range. Broadcast radio bands(long, medium and short waves), also called AM bands, or amplitude modulation ranges (Amplitude Modulation, AM), provide long-distance communication, but at a low data transfer rate. The fastest channels are those that use very high frequency ranges(Very High Frequency, VHF), for which frequency modulation (FM) is used. Also used for data transmission ultra high frequency ranges(Ultra High Frequency, UHF), also called microwave bands(over 300 MHz). At frequencies above 30 MHz, signals are no longer reflected by the Earth's ionosphere, and stable communication requires direct visibility between the transmitter and receiver. Therefore, such frequencies are used either by satellite channels, or radio relay channels, or local or mobile networks, where this condition is satisfied.

2 Functions of the physical layer Representation of bits by electrical/optical signals Coding of bits Synchronization of bits Transmission/reception of bits over physical communication channels Coordination with the physical environment Transmission speed Range Signal levels, connectors In all network devices Hardware implementation (network adapters) Example: 10 BaseT - UTP cat 3, 100 ohm, 100m, 10Mbit/s, MII code, RJ-45

5 Data transmission equipment Converter Message - El. signal Encoder (compression, correction codes) Modulator Intermediate equipment Improving communication quality - (Amplifier) ​​Creating a composite channel - (Switch) Channel multiplexing - (Multiplexer) (PA may be absent in a 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 Unit 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= 100 MHz, L= 100 M Typically A is indicated for the fundamental frequency of the signal = -23.6 dB F= 100 MHz, L= 100 M Typically A is indicated for the main signal frequency">

11 Noise immunity Fiber optic lines Cable lines Wired overhead lines Radio lines (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/Pin dB NEXT = NEXT (L) UTP 5: NEXT

13 Reliability of data transmission Bit Error Rate – BER Probability of data bit corruption Causes: external and internal interference, narrow bandwidth Struggle: increasing noise immunity, reducing NEXT interference, expanding bandwidth Twisted pair BER ~ Fiber optic cable BER ~ No additional means of protection:: corrective codes, protocols with repetition

16 Twisted pair Twisted Pair (TP) foil screen braided wire screen insulated wire outer sheath UTP Unshielded Twisted Pair category 1, UTP cat pairs in sheath STP Shielded Twisted Pair Types Type 1…9 Each pair has its own screen Each pair has its own step twists, your own color Noise immunity Cost Laying complexity

18 Fiber Optics Total internal reflection of a beam at the interface of two media n1 > n2 - (refractive index) n1 n2 n2 - (refractive index) n1 n2"> n2 - (refractive index) n1 n2"> n2 - (refractive index) n1 n2" title="18 Fiber Optics Total internal reflection of a beam at the boundary of two media n1 > n2 - (refractive index) n1 n2"> title="18 Fiber Optics Total internal reflection of a beam at the interface of 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 Gbit/s (2005) MMSM

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

25 Structured Cabling System - SCS First LANs – various cables and topologies Unification of the SCS cable system - 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. Throughput (Performance). Installation practice (Horizontal subsystem - UTP, star, 100 m...)

28 Wireless Communications Wireless Transmission Advantages: convenience, inaccessible areas, mobility. quick deployment... Disadvantages: high level of interference ( special means: codes, modulation...), complexity of using some ranges Communication line: transmitter - medium - receiver LAN characteristics ~ F(Δf, fн);

34 2. Cellular telephony Dividing the territory into cells Reuse of frequencies Low power (dimensions) In the center - base station Europe - Global System for Mobile - GSM Wireless telephone communications 1. Low-power radio station - (handset-base, 300m) DECT Digital European Cordless Telecommunication Roaming - switching from one core network to the other - the base cellular communication

35 Satellite connection Based on a satellite (reflector-amplifier) ​​Transceivers - transponders H~50 MHz (1 satellite ~ 20 transponders) Frequency ranges: C. 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: microwave - line of sight Geostationary Large coverage Fixed, Low wear Repeater satellite, broadcast, low cost, cost does not depend on distance, Instant connection establishment (Mil) Tz=300ms Low security, Initially large antenna (but VSAT) Mid-orbit km Global Positioning System GPS - 24 satellites Low-orbit km low coverage low latency Internet access

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

Two main types of physical encoding are used - based on a sinusoidal carrier signal (analog modulation) and based on a sequence of rectangular pulses (digital encoding).

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

Analog modulation methods

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 speed of N bits per second, then the spectrum consists of a constant component of zero frequency and an infinite series of harmonics with frequencies f0, 3f0, 5f0, 7f0, ..., where f0 = N/2. The amplitudes of these harmonics decrease slowly - with coefficients of 1/3, 1/5, 1/7, ... from the amplitude f0. The spectrum of the resulting potential code signal when transmitting arbitrary data occupies a band from a certain value close to 0 to approximately 7f0. For a voice frequency channel, the upper limit of the transmission rate is achieved for a data transfer rate of 971 bits per second, and the lower limit is unacceptable for any speed, 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 transmission rate when using two amplitude levels. The fm frequency determines the line capacity at this method coding. With a small modulation frequency, the signal spectrum width will also be small (equal to 2fm), and the signals will not be distorted by the line if the bandwidth is greater than or equal to 2fm. For a voice frequency channel, this method is acceptable at a data transfer rate of no higher than 3100 / 2 = 1550 bits per second.

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

Quadrate Amplitude Modulation - phase modulation with 8 phase shift values ​​and amplitude modulation with 4 amplitude values. 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 formulate completed pulses, are not taken into account.

Pulse codes– represent binary data either as pulses of a certain polarity, or as part of a pulse - as a potential difference 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 makes it possible to achieve a higher data transfer rate on the same line; there is also a requirement for the absence of a constant component, that is, the presence 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 the necessary information from the line, in local systems - clock lines, in networks - self-synchronizing codes, the signals of which carry instructions for the transmitter about at what point in time it is necessary to carry out recognition of the next bit);

Possessed the ability to recognize mistakes;

It had a low cost of implementation.

Potential code without returning to zero. NRZ (Non Return to Zero). The signal does not return to zero during the clock cycle.

It is easy to implement, has good error recognition due to two sharply different signals, but does not have the property of synchronization. When transmitting a long sequence of zeros or ones, the signal on the line does not change, so the receiver cannot determine when the data needs to be read again. Another disadvantage 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 frequency fundamental harmonic f0 = N /2.

Bipolar coding method with alternative inversion. (Bipolar Alternate Mark Inversion, AMI), modification of the NRZ method.

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

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

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

Another method is to “mix” the initial information so 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 by another. Each pulse lasts half a beat.

The code has excellent self-synchronizing properties, but when transmitting a long sequence of zeros or ones, there may be a constant component. 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. Information is encoded by potential drops that occur in the middle of a clock cycle. A one is encoded by a drop from a low signal level to a high one, and a zero is coded by a reverse drop. At the beginning of each clock cycle, a service signal drop may occur if several ones or zeros need to be represented in a row. The code has excellent self-synchronizing properties. The bandwidth is narrower than that of a bipolar pulse; there is no constant component, and the fundamental harmonic in the worst case has a frequency of N, and in the best - N/2.

Potential code 2B1Q. Every two bits are transmitted in one clock cycle by a four-state signal. 00 - -2.5 V, 01 - -0.833 V, 11 - +0.833 V, 10 - +2.5 V. Additional means are required to deal with long sequences of identical bit pairs. With random alternation of bits, the spectrum is twice as narrow as that of NRZ, since at the same bit rate the clock duration doubles, that is, it is possible to transmit data over the same line twice as fast as using AMI, NRZI , but needed high power transmitter.

Logic coding

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

Redundant codes are based on breaking the original sequence of bits into portions, which are often called symbols, after which each original symbol is replaced by a new one that has more bits than the original.

The 4B/5B code replaces sequences of 4 bits with sequences of 5 bits. Then, instead of 16 bit combinations, you get 32. Of these, 16 are selected that do not contain a large number of zeros, the rest are considered code violations. 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 prohibited codes, it means that the signal has been distorted on the line.

This code is transmitted over the line using physical encoding using a potential encoding method that is sensitive only to long sequences of zeros. The code guarantees that there will not be more than three zeros in a row on the line. There are other codes, such as 8B/6T.

To ensure the specified throughput, the transmitter must operate at a higher clock frequency (for 100 Mb/s - 125 MHz). The signal spectrum expands compared to the original one, but remains narrower than the Manchester code spectrum.

Scrambling - mixing data with a scrambler before transmission from the line.

Scrambling methods involve bit-by-bit calculation of the result code based on the bits of the source code and the bits of the result code obtained in previous clock cycles. For example,

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

where B i is the binary digit of the resulting code obtained at the i-th clock cycle of the scrambler, A i is the binary digit of the source code received at the i-th clock cycle at the input of the scrambler, B i -3 and B i -5 are the binary digits of the resulting code , obtained in previous cycles of 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 the inverse 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 classified as scrambling methods.

To improve Bipolar AMI the following are used:

B8ZS (Bipolar with 8-Zeros Substitution) – corrects only sequences consisting 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 that is prohibited for a given polarity cycle, that is, a signal that does not change the polarity of the previous one, 1* - the unit signal is of the correct polarity, and the asterisk sign marks the fact that in the source code there was not a unit in this clock cycle, 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 an encoding of 8 consecutive zeros. In this code, the constant component is zero for any sequence of binary digits.

The HDB3 code corrects any four consecutive zeros in the original sequence. Every four zeros are replaced by four signals, in which there is one V signal. To suppress the DC component, the polarity of the V signal is alternated in successive replacements. In addition, two patterns of four-cycle codes are used for replacement. If before replacing source contained an odd number of units, then the sequence 000V is used, and if the number of units was even, the sequence 1*00V is used.

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

When transmitting discrete data over communication channels, two main types of physical coding are used - based on sinusoidal carrier signal and based on a sequence of rectangular pulses. The first method is often also called modulation or analog modulation, emphasizing the fact that encoding is carried out by changing the parameters of the analog signal. The second method is usually called digital encoding. These methods differ in the width of the spectrum of the resulting signal and the complexity of the equipment required for their implementation.
Analog modulation used for transmitting discrete data over channels with a narrow frequency band, a typical representative of which is the voice-frequency channel provided to users of public telephone networks. A typical amplitude-frequency response of a voice frequency channel is shown in Fig. 2.12. This channel transmits frequencies in the range from 300 to 3400 Hz, so its bandwidth is 3100 Hz. A device that performs the functions of carrier sinusoid modulation on the transmitting side and demodulation on the receiving side is called a modem (modulator - demodulator).
Analog modulation methods
Analog modulation is a physical encoding method in which information is encoded by changing the amplitude, frequency or phase of a sinusoidal carrier signal.
The diagram (Fig. 2.13, a) shows a sequence of bits of the original information, represented by high-level potentials for a logical unit and a zero-level potential for logical zero. This encoding method is called potential code, which is often used when transferring data between computer units.
With amplitude modulation (Fig. 2.13, b), one level of the amplitude of the carrier frequency sinusoid is selected for a logical unit, and another for logical zero. This method is rarely used in its pure form in practice due to low noise immunity, but is often used in combination with another type of modulation - phase modulation.
With frequency modulation (Fig. 2.13, c), the values ​​0 and 1 of the source data are transmitted by sinusoids with different frequencies - f0 and f1. This modulation method does not require complex circuitry in modems and is typically used in low-speed modems operating at 300 or 1200 bps.
With phase modulation, data values ​​0 and 1 correspond to signals of the same frequency, but with different phases, for example 0 and 180 degrees or 0,90,180 and 270 degrees.
High-speed modems often use combined modulation methods, usually amplitude combined with phase.
When using rectangular pulses to transmit discrete information, it is necessary to choose a coding method that simultaneously achieves several goals:
· had the smallest spectrum width of the resulting signal at the same bit rate;
· ensured synchronization between the transmitter and receiver;
· had the ability to recognize mistakes;
· had a low cost of sale.
A narrower spectrum of signals allows one and the same line (with the same bandwidth) to achieve a higher data transfer rate. In addition, the signal spectrum is often required to have no DC component, that is, the presence of a DC current between the transmitter and receiver. In particular, the use of various transformer galvanic isolation circuits prevents the passage of direct current.
Synchronization of the transmitter and receiver is necessary so that the receiver knows exactly at what point in time it is necessary to read new information from the communication line.
Recognition and correction of distorted data is difficult to carry out using means of the physical layer, so most often this work is undertaken by the protocols that lie above: channel, network, transport or application. On the other hand, error recognition on physical level saves time, since the receiver does not wait for the frame to be completely placed in the buffer, but discards it immediately when it recognizes erroneous bits within the frame.
The requirements for encoding methods are mutually contradictory, therefore each of the popular digital encoding methods discussed below has its own advantages and disadvantages compared to others.