Two main types of physical coding are used - based on a sinusoidal carrier signal (analog modulation) and 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 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 (complicated technical implementation, usually used in low-speed modems).
n phase modulation.
Spectrum of the modulated signal
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 of 1/3, 1/5, 1/7, ... of the amplitude f0. The spectrum of the resulting potential code signal when transmitting arbitrary data occupies a band from some value close to 0 to approximately 7f0. For a voice-frequency channel, the upper limit of the transmission rate is reached at a data rate of 971 bits per second, and the lower limit is unacceptable for any speeds, 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 rate when using two amplitude levels. The frequency fm determines the line capacity for a given coding method. With a small modulation frequency, the signal spectrum width will be even 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 not 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 voice-frequency channel transmission.
Quadrature amplitude modulation (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 complete pulses, are not taken into account.
Pulse codes- represent binary data either by pulses of a certain polarity, or by a part of the pulse - by a potential drop of a certain direction.
Requirements for the digital coding method:
It had the smallest spectrum width of the resulting signal at the same bit rate (a narrower signal spectrum allows you to achieve a higher data rate on the same line, there is also a requirement for the absence of a constant component, that is, the presence of a direct current between the transmitter and receiver);
Provided synchronization between the transmitter and receiver (the receiver must know exactly at what point in time to read the necessary information from the line, in local systems- timing lines, in networks - self-synchronizing codes, the signals of which carry instructions for the transmitter about at what point in time the next bit should be recognized);
Had the ability to recognize mistakes;
Has a low cost of implementation.
Potential code without return to zero. NRZ (Non Return to Zero). The signal does not return to zero within a cycle.
It is easy to implement, has good error detection 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 drawback is the presence of a low-frequency component, which approaches zero when transmitting long sequences of ones and zeros. In its pure form, the code is rarely used, modifications are used. Attractiveness - low frequency of the fundamental harmonic f0 = N /2.
Bipolar coding method with alternative inversion. (Bipolar Alternate Mark Inversion, AMI), a modification of the NRZ method.
Zero potential is used to encode zero, a logical unit is encoded either by a positive potential or a negative one, while the potential of each next unit 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 transmitting a long sequence of ones, a sequence of pulses of different polarity with the same spectrum as the NRZ code transmitting a sequence of alternating pulses, that is, without a constant component and the fundamental harmonic N / 2. In general, the use of AMI results in narrower spectrum than NRZ, and thus 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, an increase in power of about 3 dB is necessary, since true signal levels are used.
Potential code with inversion at unity. (Non Return to Zero with ones Inverted, NRZI) AMI-like code but with two signal levels. When transferring zero, the potential of the previous cycle is transmitted, 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).
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 bandwidth decreases.
Another method is “mixing” 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. A one is represented by an impulse of one polarity, and a zero is represented by another. Each pulse lasts half a cycle.
The code has excellent self-timing properties, but there may be a DC component 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 is Token Ring.
Each measure is divided into two parts. Information is encoded by potential drops occurring in the middle of the cycle. A unit is encoded by a low-to-high transition, and a zero is encoded by a reverse transition. At the beginning of each cycle, an overhead signal edge may occur if several 1s or 0s 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 has a frequency of N in the worst case, and N / 2 in the best.
Potential code 2B1Q. Every two bits are transmitted in one cycle by a four-state signal. 00 - -2.5V, 01 - -0.833V, 11 - +0.833V, 10 - +2.5V. Required additional funds to deal with long sequences of identical bit pairs. With random bit interleaving, the spectrum is twice as narrow as that of NRZ, since at the same bit rate the cycle time is doubled, that is, it is possible to transmit data twice as fast on the same line than using AMI, NRZI, but more transmitter power is needed.
Logic 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 sequence of bits into portions, which are often called characters, after which each original character is replaced by a new one that has more bits than the original one.
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 of zeros, the rest are considered forbidden codes (code violation). In addition to removing DC and making the code self-synchronizing, redundant codes allow the receiver to recognize corrupted bits. If the receiver receives forbidden codes, then the signal has been distorted on the line.
This code is transmitted over the line using physical encoding using one of the potential encoding methods 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 8V/6T.
To ensure the specified bandwidth, the transmitter must operate at an increased clock frequency (for 100 Mb / s - 125 MHz). The spectrum of the signal expands compared to the original, but remains narrower than the spectrum of the Manchester code.
Scrambling - mixing data with a scrambler before transferring it from the 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 obtained in previous cycles. For example,
B i \u003d 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 cycle of the scrambler, A i is the binary digit of the source code that arrives 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 at the 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 pass it to the descrambler, which will apply the inverse transformation
C i \u003d 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 Bipolar AMI 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, it inserts five signals V-1*-0-V-1*, where V denotes a unit signal forbidden for a given polarity cycle, that is, a signal that does not change the polarity of the previous unit, 1* is a signal of a unit of correct polarity, and the asterisk sign marks the fact that in the source code in this cycle there was not a unit, but zero. As a result, the receiver sees 2 distortions on 8 cycles - it is very unlikely that this happened due to noise on the line. Therefore, the receiver treats such violations as encoding 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 that have one V signal. To suppress the DC component, the polarity of the V signal is reversed in successive changes. 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.
Improved candidate codes have a fairly narrow bandwidth for any sequences of zeros and ones that occur in the transmitted data.
The initial information that needs to be transmitted over a communication line can be either discrete (computer output data) or analog (speech, television image).
The transmission of discrete data is based on the use of two types of physical encoding:
a) analog modulation, when coding is carried out by changing the parameters of a sinusoidal carrier signal;
b) digital coding by changing the levels of the sequence of rectangular information pulses.
Analog modulation leads to a much smaller spectrum of the resulting signal than with digital coding, at the same information transfer rate, but its implementation requires more complex and expensive equipment.
Currently, the original data, which has an analog form, is increasingly transmitted over communication channels in a discrete form (in the form of a sequence of ones and zeros), i.e., discrete modulation of analog signals is carried out.
analog modulation. It is used to transmit discrete data over channels with a narrow bandwidth, a typical representative of which is a voice frequency channel provided to users of telephone networks. Signals with a frequency of 300 to 3400 Hz are transmitted over this channel, i.e., its bandwidth is 3100 Hz. Such a band is quite sufficient for speech transmission with acceptable quality. The bandwidth limitation of the tone channel is associated with the use of multiplexing and circuit switching equipment in telephone networks.
Before the transmission of discrete data on the transmitting side using a modulator-demodulator (modem) modulation of the carrier sinusoid of the original sequence of binary digits is carried out. The inverse conversion (demodulation) is performed by the receiving modem.
There are three ways to convert digital data to analog form, or three methods of analog modulation:
Amplitude modulation, when only the amplitude of the carrier of sinusoidal oscillations changes in accordance with the sequence of transmitted information bits: for example, when transmitting one, the oscillation amplitude is set large, and when transmitting zero, it is small, or there is no carrier signal at all;
Frequency modulation, when under the influence of modulating signals (transmitted information bits) only the carrier frequency of sinusoidal oscillations changes: for example, when zero is transmitted, it is low, and when one is transmitted, it is high;
Phase modulation, when, in accordance with the sequence of transmitted information bits, only the phase of the carrier of sinusoidal oscillations changes: when switching from signal 1 to signal 0 or vice versa, the phase changes by 180 °. In its pure form, amplitude modulation is rarely used in practice due to low noise immunity. Frequency modulation does not require complex circuitry in modems and is typically used in low speed modems operating at 300 or 1200 bps. Increasing the data rate is provided by the use of combined modulation methods, more often amplitude modulation in combination with phase.
The analog method of discrete data transmission provides wideband transmission by using signals of different carrier frequencies in one channel. This guarantees the interaction of a large number of subscribers (each pair of subscribers operates at its own frequency).
Digital coding. When digitally encoding discrete information, two types of codes are used:
a) potential codes when to present information units and zeros, only the value of the signal potential is applied, and its drops are not taken into account;
b) pulse codes, when binary data is represented either by pulses of a certain polarity, or by potential drops of a certain direction.
The following requirements are imposed on the methods of digital coding of discrete information when using rectangular pulses to represent binary signals:
Ensuring synchronization between transmitter and receiver;
Ensuring the smallest spectrum width of the resulting signal at the same bit rate (since a narrower spectrum of signals allows one to
networks with the same bandwidth achieve higher speeds
data transmission);
Ability to recognize errors in transmitted data;
Relatively low implementation cost.
By means of the physical layer, only the recognition of corrupted data (error detection) is carried out, which saves time, since the receiver, without waiting for the received frame to be completely placed in the buffer, immediately rejects it when it recognizes erroneous bits in the frame. A more complex operation - the correction of corrupted data - is performed by higher-level protocols: channel, network, transport or application.
Synchronization of the transmitter and receiver is necessary so that the receiver knows exactly when to read the incoming data. Clock signals tune the receiver to the transmitted message and keep the receiver synchronized with the incoming data bits. The synchronization problem is easily solved when transmitting information over short distances (between blocks inside a computer, between a computer and a printer) by using a separate timing communication line: information is read only at the moment the next clock pulse arrives. In computer networks, the use of clock pulses is abandoned for two reasons: for the sake of saving conductors in expensive cables and because of the heterogeneity of the characteristics of conductors in cables (on long distances uneven speed of propagation of signals can lead to desynchronization of clock pulses in the timing line and information pulses in the main line, as a result of which a 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 (SC). The coding of the transmitted data using the SC is to ensure regular and frequent changes (transitions) of the levels of the information signal in the channel. Each signal level transition from high to low or vice versa is used to trim the receiver. The best are those SCs that provide a signal level transition at least once during the time interval required to receive one information bit. The more frequent the signal level transitions, the more reliable the synchronization of the receiver is and the more confident the identification of the received data bits is.
These requirements for the methods of digital coding of discrete information are mutually contradictory to a certain extent, therefore, each of the coding methods considered below has its advantages and disadvantages compared to others.
Self-synchronizing codes. The most common are the following SCs:
Potential code without return to zero (NRZ - Non Return to Zero);
Bipolar pulse code (RZ code);
Manchester code;
Bipolar code with alternate level inversion.
On fig. 32 shows the coding schemes for message 0101100 using these CKs.
For characterization and comparative assessment SC uses the following indicators:
Level (quality) of synchronization;
Reliability (confidence) of recognition and selection of received information bits;
The required rate of change of the signal level in the communication line when using the SC, if the line bandwidth is set;
The complexity (and hence the cost) of the equipment that implements the SC.
The NRZ code is easy to code and low cost to implement. It received such a name because when transmitting a series of bits of the same name (ones or zeros), the signal does not return to zero during the cycle, as is the case in other coding methods. The signal level remains unchanged for each series, which significantly reduces the quality of synchronization and the reliability of recognition of the received bits (the receiver's timer may misalign with the incoming signal and untimely polling of the lines may occur).
For the N^-code, the following relations hold:
where VI is the rate of change of the signal level in the communication line (baud);
Y2 - throughput of the communication line (bit / s).
In addition to the fact that this code does not have the property of self-synchronization, it also has another serious drawback: the presence of a low-frequency component that approaches zero when transmitting long runs of ones or zeros. As a result, the NRZ code in its pure form is not used in networks. Its various modifications are applied, in which poor self-synchronization of the code and the presence of a constant component are eliminated.
The RZ-code, or bipolar pulse code (return-to-zero code), differs in that during the transmission of one information bit, the signal level changes twice, regardless of whether a series of bits of the same name or alternating bits are transmitted. A unit is represented by an impulse of one polarity, and a zero is represented by another. Each pulse lasts half a cycle. Such code has excellent self-synchronizing properties, but the cost of its implementation is quite high, since it is necessary to ensure the ratio
The spectrum of an RZ code is wider than that of potential codes. Due to its too wide spectrum, it is rarely used.
The Manchester code provides a change in the signal level when presenting each bit, and when transmitting a series of bits of the same name, a double change. Each measure is divided into two parts. Information is encoded by potential drops that occur in the middle of each cycle. A unit is encoded by a low-to-high transition, and a zero is encoded by a reverse edge. The speed ratio for this code is:
The Manchester code has good self-clocking properties, since the signal changes at least once per cycle of transmission of one data bit. Its bandwidth is narrower than that of the RZ code (one and a half times on average). In contrast to the bipolar pulse code, where three signal levels are used for data transmission (which is sometimes very undesirable, for example, only two states are consistently recognized in optical cables - light and darkness), the Manchester code has two levels.
Manchester code is widely used in Ethernet and Token Ring technologies.
The Alternate Level Inversion Bipolar Code (AMI code) is a modification of the NRZ code. It uses three levels of potential - negative, zero and positive. The unit is encoded either by a positive potential or a negative one. Zero potential is used to encode zero. The code has good synchronizing properties when transmitting series of units, since the potential of each new unit is opposite to the potential of the previous one. When transmitting runs of zeros, there is no synchronization. The AMI code is relatively easy to implement. For him 
When transmitting various combinations of bits on the line, the use of the AMI code leads to a narrower signal spectrum than for the NRZ code, and hence to a higher line throughput.
Note that improved potential codes (upgraded Manchester code and AMI code) have a narrower spectrum than impulse codes, so they are used in high speed technologies, such as FDDI, Fast Ethernet, Gigabit Ethernet.
Discrete modulation of analog signals. As already noted, one of the trends in the development of modern computer networks is their digitalization, i.e., digital transmission of signals of any nature. The sources of these signals can be computers (for discrete data) or devices such as telephones, camcorders, video and audio equipment (for analog data). Until recently (before the advent of digital communication networks), in territorial networks all types of data were transmitted in analog form, and computer data, discrete in nature, were converted into analog form using modems.
However, the transmission of information in analog form does not improve the quality of the received data if there was a significant distortion during transmission. Therefore, the analogue technique for recording and transmitting sound and images has been replaced by digital technology, which uses discrete modulation of analog signals.
Discrete modulation is based on the sampling of continuous signals both in amplitude and in time. One of the widely used methods for converting analog signals into digital ones is pulse code modulation (PCM), proposed in 1938 by A.Kh. Reeves (USA).
When using PCM, the conversion process includes three stages: mapping, quantization and encoding (Fig. 33).
The first stage is display. The amplitude of the original continuous signal is measured with a given period, due to which time discretization occurs. At this stage, the analog signal is converted into pulse amplitude modulation (PAM) signals. The implementation 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 values in time) on a regular interval with a frequency of at least twice the frequency of the highest harmonic of the spectrum of the original continuous signal, then the mapping will contain information sufficient to restore the original signal. In analog telephony, the range from 300 to 3400 Hz is chosen for voice transmission, which is sufficient for high-quality transmission of all the main harmonics of the interlocutors. Therefore, in digital networks where the PCM method is implemented for voice transmission, a display frequency of 8000 Hz is adopted (this is more than 6800 Hz, which provides some margin of quality).
In the quantization step, each IAM signal is given a quantized value corresponding to the nearest quantization level. The entire range of IAM signal amplitude variation is divided into 128 or 256 quantization levels. The more quantization levels, the more accurately the IAM signal amplitude is represented by the quantized level.
At the encoding stage, each quantized mapping is assigned a 7-bit (if the number of quantization levels is 128) or 8-bit (if the number of quantization levels is 128) binary code. On fig. 33 shows the signals of the 8-element binary code 00101011 corresponding to a quantized signal with level 43. When encoding with 7-element codes, the data rate over the channel should be 56 Kbps (this is the product of the display frequency and the bit depth of the binary code), and when encoding with 8-element codes - 64 Kbps. The standard is a 64 kbit/s digital channel, which is also called the elementary channel of digital telephone networks.
The device that performs these steps of converting an analog value into a digital code is called an analog-to-digital converter (ADC). On the receiving side, using a digital-to-analog converter (DAC), an inverse conversion is carried out, i.e., the digitized amplitudes of a continuous signal are demodulated, the original continuous function time.
In modern digital communication networks, other methods of discrete modulation are also used, which make it possible to represent voice measurements in a more compact form, for example, as a sequence of 4-bit numbers. The concept of converting analog signals into digital ones is also used, in which not the IAM signals themselves are quantized and then encoded, but only their changes, and the number of quantization levels is assumed to be the same. It is obvious that such a concept allows the conversion of signals with greater accuracy.
Digital methods for recording, reproducing and transmitting analog information provide the ability to control the reliability of data read from a carrier or received via a communication line. For this purpose, the same control methods are used as for computer data (see 4.9).
The transmission of a continuous signal in a discrete form imposes stringent requirements on the synchronization of the receiver. If synchronization is not observed, the original signal is restored incorrectly, which leads to distortion of the voice or transmitted image. If frames with voice measurements (or other analog values) arrive synchronously, then the voice quality can be quite high. However, in computer networks, frames can be delayed both in end nodes and in intermediate switching devices (bridges, switches, routers), which negatively affects the quality of voice transmission. Therefore, for high-quality transmission of digitized continuous signals, special digital networks (ISDN, ATM, networks digital television), although Frame Relay networks are still used to transmit intracorporate telephone conversations, since the frame transmission delays in them are within acceptable limits.
Plan
Table 1.
Comparison table of LED and semiconductor laser usage
The receiving end of an optical cable is a photodiode that generates an electrical pulse when light falls on it.
For the transmission of discrete data over communication lines with a narrow frequency band, analog modulation. A typical representative of such lines is a voice-frequency communication line made available to users of public telephone networks. This link transmits analog signals in the frequency range from 300 to 3400 Hz (thus the line bandwidth is 3100 Hz). Strict bandwidth limitation of communication lines in this case associated with the use of equipment for multiplexing and circuit switching in telephone networks.
A device that performs the functions of modulating a carrier sinusoid on the transmitting side and demodulating on the receiving side is called modem (modulator-demodulator).
Analog modulation is a physical coding method in which information is encoded by changing amplitudes, frequencies or phases a sinusoidal signal of the carrier frequency. At amplitude modulation for a logical one, one level of the amplitude of the carrier frequency sinusoid is selected, and for a logical zero, another. This method is rarely used in practice in its pure form due to low noise immunity, but is often used in combination with other types of modulation. At frequency modulation the values 0 and 1 of the original data are transmitted by sinusoids with different frequencies . This modulation method does not require complex electronic circuits in modems and is typically used in low speed modems operating at 300 or 1200 bps. At phase modulation data values 0 and 1 correspond to signals of the same frequency but different phase, such as 0 and 180 degrees or 0, 90, 180 and 270 degrees. In high-speed modems, combined modulation methods are often used, as a rule, amplitude in combination with phase. Combined modulation methods are used to increase the data rate. The most common methods are Quadrature Amplitude Modulation-QAM). These methods are based on a combination of phase modulation with 8 phase shift values and amplitude modulation with 4 amplitude levels. However, not all of the possible 32 signal combinations are used. Such coding redundancy is required for the modem to recognize erroneous signals, which are the result of distortion due to interference, which on telephone channels (especially switched ones) are very significant in amplitude and long in time.
At digital coding discrete information is used potential And impulse codes. IN potential In codes, only the value of the signal potential is used to represent logical ones and zeros, and its drops, which form complete pulses, are not taken into account. Pulse codes allow binary data to be represented either by pulses of a certain polarity, or by a part of the pulse - a potential drop of a certain direction.
When using rectangular pulses to transmit discrete information, it is necessary to choose a coding method that would simultaneously achieve several goals: at the same bit rate, have the smallest width of the spectrum of the resulting signal; provided synchronization between transmitter and receiver; had the ability to recognize mistakes; had a low cost of implementation.
A narrower signal spectrum allows you to achieve a higher data transfer rate on the same line (with the same bandwidth). Synchronization of the transmitter and receiver is needed so that the receiver knows exactly at what point in time it is necessary to read new information from the communication line. This problem is more difficult to solve in networks than when communicating between devices in close proximity, such as between devices inside a computer or between a computer and a printer. At short distances, a scheme based on a separate clocking communication line works well, and information is removed from the data line only at the moment a clock pulse arrives. In networks, the use of this scheme causes difficulties due to the heterogeneity of the characteristics of the conductors in the cables. Over long distances, signal velocity ripples can cause the clock to arrive so late or too early for the corresponding data signal that a data bit is skipped or reread. Another reason why networks refuse to use clock pulses is to save conductors in expensive cables. Therefore, networks use the so-called self-synchronizing codes, the signals of which carry indications for the transmitter at what point in time it is necessary to recognize the next bit (or several bits, if the code is oriented to more than two signal states). Any sharp drop in signal - the so-called front- can serve as a good indication for synchronization of the receiver with the transmitter. When using sinusoids as a carrier signal, the resulting code has the property of self-synchronization, since a change in the amplitude of the carrier frequency allows the receiver to determine the moment the input code appears.
Recognition and correction of distorted data is difficult to implement by means of the physical layer, therefore, most often this work is undertaken by the protocols that lie above: channel, network, transport or application. On the other hand, error detection physical level saves time, since the receiver does not wait for the frame to be completely buffered, but rejects it immediately upon recognition of erroneous bits within the frame.
The requirements for coding methods are mutually contradictory, so each of the popular digital coding methods discussed below has its own advantages and disadvantages compared to others.
One of the simplest methods potential coding is unipolar potential code, also called coding without returning to zero (Non Return to Zero-NRZ) (fig.7.1.a). The last name reflects the fact that when a sequence of ones is transmitted, the signal does not return to zero during the cycle. The NRZ method has good error detection (due to two sharply different potentials), but does not have the self-synchronization property. When transmitting a long sequence of ones or zeros, the line signal does not change, so the receiver does not have the ability to determine by input signal points in time when you need to read the data again. Even with a highly accurate clock generator, the receiver can make a mistake with the moment of data acquisition, since the frequencies of the two generators are almost never completely identical. Therefore, at high data rates and long sequences of ones or zeros, a small mismatch of clock frequencies can lead to an error in a whole cycle and, accordingly, reading an incorrect bit value.
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Rice. 7.1. Binary data encoding methods: a-unipolar poten-
social code; b- bipolar potential code; V- unipolar im-
pulse code; G -bipolar pulse code; d-"Manchester" code;
e- potential code with four signal levels.
Another serious disadvantage of the NRZ method is the presence of a low frequency component that approaches zero when transmitting long sequences of ones or zeros. Because of this, many communication lines that do not provide a direct galvanic connection between the receiver and the source do not support this type of encoding. As a result, the NRZ code in its pure form is not used in networks, but its various modifications are used, in which both poor self-synchronization of the NRZ code and the presence of a constant component are eliminated.
One of the modifications of the NRZ method is the method bipolar potential coding with alternative inversion (Bipolar Alternate Mark Inversion-AMI). In this method ( rice. 7.1.b) three potential levels are used - negative, zero and positive. To encode a logical zero, a zero potential is used, and a logical unit is encoded either by a positive potential or a negative one (in this case, the potential of each new unit is opposite to the potential of the previous one). The AMI code partially eliminates the DC and lack of self-timing problems inherent in the NRZ code. This happens when sending long sequences of ones. In these cases, the signal on the line is a sequence of bipolar pulses with the same spectrum as the NRZ code transmitting alternating zeros and ones, that is, without a constant component and with a fundamental harmonic of N/2 Hz (where N is the data bit rate). Long sequences of zeros are also dangerous for the AMI code, as well as for the NRZ code - the signal degenerates into a constant potential of zero amplitude. In general, for different combinations of bits on the line, the use of the AMI code leads to a narrower signal spectrum than for the NRZ code, and hence to a higher line throughput. For example, when transmitting alternating ones and zeros, the fundamental harmonic f 0 has a frequency of N/4 Hz. The AMI code also provides some features for recognizing erroneous signals. Thus, a violation of the strict alternation of the polarity of the signals indicates a false impulse or the disappearance of a correct impulse from the line. A signal with incorrect polarity is called forbidden signal (signal violation). Since the AMI code uses not two, but three signal levels per line, the additional level requires an increase in transmitter power to ensure the same bit fidelity on the line, which is a general disadvantage of codes with several signal states compared to codes that distinguish only two states.
The simplest methods impulsive encodings are unipolar pulse code, in which one is represented by momentum and zero is represented by its absence ( rice. 7.1v), And bipolar pulse code, in which the unit is represented by a pulse of one polarity, and zero is the other ( rice. 7.1g). Each pulse lasts half a cycle. The bipolar pulse code has good self-clocking properties, but a DC pulse component may be present, for example, when transmitting a long sequence of ones or zeros. In addition, its spectrum is wider than that of potential codes. So, when transmitting all zeros or ones, the frequency of the fundamental harmonic of the code will be equal to N Hz, which is two times higher than the fundamental harmonic of the NRZ code and four times higher than the fundamental harmonic of the AMI code when transmitting alternating ones and zeros. Due to the too wide spectrum, the bipolar pulse code is rarely used.
In local networks, until recently, the most common coding method was the so-called " Manchester code"(rice. 7.1d). In the Manchester code, a potential drop, that is, the front of the pulse, is used to encode ones and zeros. In Manchester encoding, each clock is divided into two parts. Information is encoded by potential drops that occur in the middle of each cycle. A unit is encoded by a low-to-high transition, and a zero is encoded by a reverse edge. At the beginning of each cycle, a service signal edge can occur if you need to represent several ones or zeros in a row. Since the signal changes at least once per transmission cycle of one data bit, the Manchester code has good self-clocking properties. The bandwidth of the Manchester code is narrower than that of the bipolar pulse. It also does not have a constant component, and the fundamental harmonic in the worst case (when transmitting a sequence of ones or zeros) has a frequency of N Hz, and in the best case (when transmitting alternating ones and zeros) it is equal to N / 2 Hz, like in AMI or NRZ codes. On average, the bandwidth of the Manchester code is one and a half times narrower than that of the bipolar pulse code, and the fundamental harmonic oscillates around 3N/4. Another advantage of the Manchester code is that it has only two signal levels, while the bipolar pulse code has three.
There are also potential codes with a large number signal levels for data encoding. Shown as an example ( fig 7.1e) potential code 2B1Q with four signal levels for data encoding. In this code, every two bits are transmitted in one cycle by a signal that has four states. A pair of bits "00" corresponds to a potential of -2.5 V, a pair of bits "01" - a potential of -0.833 V, a pair of bits "11" - a potential of +0.833 V, and a pair of bits "10" - a potential of +2.5 V. In this coding method, additional measures are required to combat long sequences of identical pairs of bits, since then the signal turns into a constant component. With random bit interleaving, the signal spectrum is twice as narrow as that of the NRZ code (at the same bit rate, the cycle time is doubled). Thus, using the presented 2B1Q code, it is possible to transfer data over the same line twice as fast as using the AMI code. However, for its implementation, the transmitter power must be higher so that the four levels are clearly distinguished by the receiver against the background of interference.
To improve potential AMI and 2B1Q type codes, logical coding. Logic coding is designed to replace long sequences of bits, leading to a constant potential, interspersed with ones. Two methods are characteristic for logical coding - redundant codes and scrambling.
Redundant codes are based on splitting the original sequence of bits into portions, which are often called characters. Then each original character is replaced with a new one that has more bits than the original. For example, a 4B/5B logic code replaces the original 4-bit characters with 5-bit characters. Since the resulting symbols contain redundant bits, the total number of bit combinations in them is greater than in the original ones. So, in the 4B / 5B code, the resulting symbols can contain 32 bit combinations, while the original symbols - only 16. Therefore, in the resulting code, you can select 16 such combinations that do not contain a large number of zeros, and count the rest prohibited codes (code violation). In addition to removing DC and making the code self-synchronizing, redundant codes allow the receiver to recognize corrupted bits. If the receiver receives a forbidden code, it means that the signal has been distorted on the line. The 4V/5V code is transmitted over the line using physical coding using one of the potential coding methods that is sensitive only to long sequences of zeros. The 4V/5V code symbols, 5 bits long, guarantee that no more than three zeros in a row can occur on the line for any combination of them. The letter B in the code name means that the elementary signal has 2 states (from the English binary - binary). There are also codes with three signal states, for example, in the 8B / 6T code, to encode 8 bits of initial information, a code of 6 signals is used, each of which has three states. The redundancy of the 8B/6T code is higher than that of the 4B/5B code, since there are 729 (3 to the power of 6) resulting symbols for 256 source codes. Using the lookup table is a very simple operation, so this approach does not complicate network adapters and interface blocks of switches and routers (see sections 9,11).
To provide a given line capacity, a transmitter using a redundant code must operate at an increased clock frequency. So, to transmit 4V / 5V codes at a rate of 100 Mbps, the transmitter must operate at a clock frequency of 125 MHz. In this case, the spectrum of the signal on the line is expanded in comparison with the case when a pure, non-redundant code is transmitted over the line. Nevertheless, the spectrum of the redundant potential code turns out to be narrower than the spectrum of the Manchester code, which justifies the additional stage of logical coding, as well as the operation of the receiver and transmitter at an increased clock frequency.
Another way of logical coding is based on the preliminary "mixing" of the initial information in such a way that the probabilities of the appearance of ones and zeros on the line become close. Devices or blocks that perform this operation are called scramblers(scramble - dump, disorderly assembly). At scrambling a well-known algorithm is used, so the receiver, having received binary data, transmits them to descrambler, which restores the original bit sequence. Excess bits are not transmitted over the line. Improved potential redundancy and scrambled codes are used in modern high-speed network technologies instead of "Manchester" and bipolar pulse coding.
7.6. Communication Line Multiplexing Technologies
For multiplexing("compacting") of communication lines, several technologies are used. Technology frequencymultiplexing(Frequency Division Multiplexing - FDM) was originally developed for telephone networks, but is also used for other types of networks, such as cable television networks. This technology assumes the transfer of the signals of each subscriber channel to its own frequency range and the simultaneous transmission of signals from several subscriber channels in one broadband communication line. For example, the inputs of an FDM switch receive initial signals from telephone network subscribers. The switch performs a frequency translation of each channel in its own frequency band. Typically, the high-frequency range is divided into bands that are allocated for the transmission of data from subscriber channels. In the communication line between two FDM switches, the signals of all subscriber channels are simultaneously transmitted, but each of them occupies its own frequency band. The output FDM switch separates the modulated signals of each carrier frequency and transmits them to the corresponding output channel to which the subscriber telephone is directly connected. FDM switches can perform both dynamic and permanent switching. In dynamic switching, one subscriber initiates a connection with another subscriber by sending the called subscriber number to the network. The switch dynamically allocates to this subscriber one of the free lanes. With constant switching, the band is assigned to the subscriber for a long time. The principle of switching based on frequency division remains unchanged in networks of a different type, only the boundaries of the bands allocated to a separate subscriber channel, as well as their number, change.
Multiplexing Technologytime-sharing(Time Division Multiplexing - TDM) or temporary multiplexing is based on the use of TDM equipment (multiplexers, switches, demultiplexers) operating in the time-sharing mode, servicing all subscriber channels in turn during a cycle. Each connection is allocated one time slice of the hardware operation cycle, also called time slot. The duration of the time slot depends on the number of subscriber channels served by the equipment. TDM networks can support either dynamic, or constant switching, and sometimes both of these modes.
Networks with dynamic switching require a preliminary procedure for establishing a connection between subscribers. To do this, the address of the called subscriber is transmitted to the network, which passes through the switches and configures them for subsequent data transmission. The connection request is routed from one switch to another and eventually reaches the called party. The network may refuse to establish a connection if the capacity of the required output channel has already been exhausted. For an FDM switch, the output capacity is equal to the number of frequency bands, and for a TDM switch, it is equal to the number of time slots into which the channel operation cycle is divided. The network also refuses the connection if the requested subscriber has already established a connection with someone else. In the first case, they say that the switch is busy, and in the second - the subscriber. The possibility of connection failure is a disadvantage of the circuit switching method. If a connection can be established, then it is allocated a fixed bandwidth in FDM networks or a fixed bandwidth in TDM networks. These values remain unchanged throughout the connection period. Guaranteed network throughput after a connection is established is an important feature required for applications such as voice and video transmission or real-time object control.
If there is only one physical communication channel, for example, when exchanging data using modems via telephone network, the duplex mode of operation is organized on the basis of dividing the channel into two logical subchannels using FDM or TDM technologies. When using FDM technology, modems for organizing duplex operation on a two-wire line operate at four frequencies (two frequencies - for encoding ones and zeros when transmitting data in one direction, and the other two frequencies - for encoding when transmitting in the opposite direction). In TDM technology, some time slots are used to transfer data in one direction, and some are used to transfer data in the other direction. Usually, time slots of opposite directions alternate.
In fiber-optic cables for the organization of duplex operation when using only one optical fiber, data transmission in one direction is carried out using a light beam of one wavelength, and in the opposite direction - a different wavelength. This technology is essentially related to the FDM method, but for fiber optic cables it is called wavelength multiplexing technologies(Wave Division Multiplexing - WDM) or wave multiplexing.
Technologydense wave(spectral)multiplexing(Dense Wave Division Multiplexing - DWDM) is designed to create a new generation of optical backbones operating at multi-gigabit and terabit speeds. Such a qualitative leap in performance is provided due to the fact that information in an optical fiber is transmitted simultaneously by a large number of light waves. DWDM networks operate on the principle of circuit switching, with each light wave representing a separate spectral channel and carrying its own information. One of the main advantages of DWDM technology is a significant increase in the utilization factor of the frequency potential of optical fiber, the theoretical bandwidth of which is 25,000 GHz.
Summary
In modern telecommunication systems, information is transmitted through electromagnetic waves - electrical, light or radio signals.
Communication lines, depending on the type of physical medium for information transmission, can be cable (wired) or wireless. As communication lines, telephone cables based on parallel non-twisted conductors, coaxial cables, cables based on twisted pairs of conductors (unshielded and shielded), fiber optic cables are used. The most effective today and promising in the near future are cables based on twisted pairs of conductors and fiber optic cables. Wireless communication lines are most often implemented by transmitting radio signals in various radio wave bands. Infrared wireless data transmission technology uses part of the electromagnetic spectrum between visible light and the shortest microwaves. The most high-speed and noise-resistant is the laser technology of wireless communication.
The main characteristics of communication lines are the frequency response, bandwidth and attenuation at a certain frequency.
The throughput of a communication line characterizes the maximum possible data transfer rate over it. The noise immunity of a communication line determines its ability to reduce the level of interference generated in the external environment on internal conductors. The reliability of data transmission characterizes the probability of distortion for each transmitted bit of data.
The representation of discrete information in one form or another of the signals applied to the communication line is called physical coding. Logical coding involves replacing bits of the original information with a new bit sequence that carries the same information but has additional properties.
To transmit discrete data over communication lines with a narrow frequency band, analog modulation is used, in which information is encoded by changing the amplitude, frequency, or phase of a sinusoidal carrier frequency signal. When digitally encoding discrete information, potential and impulse codes are used. For multiplexing of communication lines technologies of frequency, time and wave multiplexing are used.
Control questions and tasks
1. Give the classification of communication lines.
2. Describe the most common cable communication lines.
3. Present the main wireless communication lines and give their comparative characteristics.
4. Due to what physical factors do communication channels distort transmitted signals?
5. What is the amplitude-frequency characteristic of a communication channel?
6. In what units is the bandwidth of the communication channel measured?
7. Describe the concept of "noise immunity of the communication line."
8. What determines the characteristic "data transmission reliability" and in what units is it measured?
9. What is "analogue modulation" and what types of it are used to transmit discrete data?
10. What device performs the functions of modulating the carrier sinusoid on the transmitting side and demodulating it on the receiving side?
11. State the difference between potential and impulse coding of digital signals.
12. What are self-synchronizing codes?
13. What is the purpose of logical coding of digital signals and what methods are used?
14. Describe the technology frequency multiplexing communication lines.
15. What are the features of time division multiplexing technology?
16. What multiplexing technology is used in fiber optic cables to organize duplex operation when using only one optical fiber?
17. What is the purpose of dense wave multiplexing technology?
Information transmitted over a communication line is usually subjected to special coding, which improves the reliability of transmission. In this case, additional hardware costs for encoding and decoding are inevitable, and the cost of network adapters increases.
The coding of information transmitted over a network is related to the ratio of the maximum allowable transmission rate and the bandwidth of the transmission medium used. For example, with different codes, the maximum transmission rate over the same cable can differ by a factor of two. The complexity of the network equipment and the reliability of information transmission also directly depend on the chosen code.
To transmit discrete data over communication channels, two methods of physical encoding of initial discrete data are used - based on a sinusoidal carrier signal and based on a sequence of rectangular pulses. The first way is often called analog modulation, because coding is carried out by changing the parameters of the analog signal (amplitude, phase, frequency). The second way is called digital coding. At present, data having an analog form (speech, television image) is transmitted via communication channels in a discrete form. The process of representing analog information in discrete form is called discrete modulation.
5.1Analog modulation
The representation of discrete data as a sinusoidal signal is called analog modulation. Analog modulation allows you to represent information as a sinusoidal signal with different levels of amplitude, or phase, or frequency. You can also use combinations of changing parameters - amplitude and frequency, amplitude-phase. For example, if you form a sinusoidal signal with four amplitude levels and four frequency levels, this will give 16 states of the information parameter, which means 4 bits of information for one change.
There are three main types of analog modulation:
amplitude,
frequency,
Amplitude modulation.(AM) With amplitude modulation, for a logical one, one level of the amplitude of the carrier frequency sinusoid is selected, and for a logical zero, another (see Fig. 5.1). The frequency of the signal remains constant. 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.
Rice. 5.1 Various types modulation
Frequency modulation. ( World Cup) With frequency modulation, the values of logical 0 and logical 1 of the initial data are transmitted by sinusoids with different frequencies - f 1 and f 2 (see Fig. 5.1). The signal amplitude remains constant. This modulation method does not require complicated circuits in modems and is usually used in low speed modems.
Phase modulation. (FM) With phase modulation, the values of logical 0 and 1 correspond to signals of the same frequency, but with a different phase (inverted), for example, 0 and 180 degrees or 0.90,180 and 270 degrees. The resulting signal looks like a sequence of inverted sine waves (see Figure 5.1). The amplitude and frequency of the signal remain constant.
Combined modulation methods are used to increase the transmission rate (increase the number of bits per one cycle of the information parameter). The most common methods quadrature amplitude modulation (Quadrature Amplitude Modulation, QAM). These methods use a combination of phase modulation with 8 phase shift values and amplitude modulation with 4 amplitude levels. With this method, 32 signal combinations are possible. And although not all of them are used, the speed is still significantly increased, and due to redundancy, errors in data transmission can be controlled. For example, in some codes, only 6, 7 or 8 combinations are allowed to represent the original data, and the remaining combinations are prohibited. Such coding redundancy is required for the modem to recognize erroneous signals resulting from distortion due to interference, which on telephone channels, especially switched ones, are very significant in amplitude and long in time.
Let's determine on which lines analog modulation can work, and to what extent this method satisfies the bandwidth of one or another used transmission line, for which we consider the spectrum of the resulting signals. For example, take the amplitude modulation method. The spectrum of the resulting signal with amplitude modulation will consist of a sinusoid of the carrier frequency f With and two side harmonics:
(f With -f m ) And (f With +f m ), Where f m- modulation frequency (changes in the information parameter of the sinusoid), which will coincide with the data rate if two amplitude levels are used.
Rice. 5.2 Signal spectrum with amplitude modulation
Frequency f m determines the bandwidth of the line for a given coding method. With a low modulation frequency, the width of the signal spectrum will also be small (equal to 2f m see Figure 5.2), so signals will not be distorted by the line if its bandwidth is greater than or equal to 2f m .
Thus, with amplitude modulation, the resulting signal has a narrow spectrum.
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 decrease rapidly. Therefore, these types of modulation are also well suited for data transmission over lines with narrow bandwidths. A typical representative of such lines is the voice frequency channel, which is made available to users of public telephone networks.
From the typical frequency response of a voice-frequency channel, it can be seen that this channel transmits frequencies in the range from 300 to 3400 Hz, and thus its bandwidth is 3100 Hz (see Figure 5.3).
Rice. 5.3 frequency response of the voice frequency channel
Although the human voice has a much wider spectrum - from about 100 Hz to 10 kHz - for acceptable speech quality, a range of 3100 Hz is a good solution. The strict bandwidth limitation of the tone channel is associated with the use of multiplexing and circuit switching equipment in telephone networks.
Thus, for a voice frequency channel, amplitude modulation provides a data transfer rate of no more than 3100/2=1550 bit/s. If you use several levels of the information parameter (4 levels of amplitude), then the throughput of the voice frequency channel is doubled.
Most often, analog coding is used when transmitting information over a channel with a narrow bandwidth, for example, over telephone lines in wide area networks. In local networks, it is rarely used due to the high complexity and cost of both encoding and decoding equipment.
Currently, almost all equipment that works with analog signals is being developed on the basis of expensive microcircuits. DSP (Digital Signal Processor). In this case, after modulation and signal transmission, it is necessary to carry out demodulation upon reception, and this is again expensive equipment. To perform the function of modulating the carrier sinusoid on the transmitting side and demodulating on the receiving side, a special device is used, which is called modem (modulator-demodulator). A 56,000 bps modem costs $100, and LAN card for 100 Mbps costs $10.
In conclusion, we present the advantages and disadvantages of analog modulation.
Analog modulation has many different information parameters: amplitude, phase, frequency. Each of these parameters can take on multiple states per carrier change. And, therefore, the resulting signal can convey a large number of bits per second.
Analog modulation provides the resulting signal with a narrow spectrum, and therefore it is good where you need to work on poor lines (with a narrow bandwidth), it is able to provide high transmission speed there. Analog modulation can also work on good lines, here one more advantage of analog modulation is especially important - the ability to shift the spectrum in desired area, depending on the bandwidth of the line being used.
Analog modulation is difficult to implement and the equipment that does it is very expensive.
Analog modulation is used where it cannot be dispensed with, but in local networks other coding methods are used, for the implementation of which simple and cheap equipment is needed. Therefore, most often in local networks, when transmitting data in communication lines, the second method of physical coding is used - digital coding
5. 2.Digital coding
Digital coding- representation of information by rectangular pulses. For digital coding use potential And impulse codes.
Potential codes. In potential codes, only the value of the signal potential during the cycle period is used to represent logical ones and zeros, and its drops, which form complete pulses, are not taken into account. It is only important what value the resulting signal has during the cycle period.
impulse codes. Pulse codes represent a logical zero and a logical unit either by pulses of a certain polarity, or by part of the pulse - by a potential drop of a certain direction. The value of the pulse code includes the entire pulse along with its transitions.
Let's define the requirements for digital coding. For example, we need to transfer discrete data (a sequence of logical zeros and ones) from the output of one computer - the source - to the input of another computer - the receiver over the communication line.
1. For data transmission, we have communication lines that do not pass all frequencies, they have certain bandwidths depending on their type. Therefore, when encoding data, it must be taken into account that the encoded data is “passed through” by the communication line.
2. Sequences of discrete data must be encoded as digital pulses of a certain frequency. In this case, of course, it is best to achieve:
a) that the frequencies of the encoded signals be low to generally match the bandwidths of the communication links.
b) that the encoded signals provide a high transmission rate.
Thus, good code must have less Hertz and more bits per second.
3. The data to be transmitted is an unpredictably changing sequence of logical zeros and ones.
Let's encode this data in a certain way with digital pulses, then how can we determine what frequency the resulting signal has? In order to determine the maximum frequency of a digital code, it is enough to consider the resulting signal when encoding private sequences such as:
sequence of logical zeros
sequence of logical ones
alternating sequence of logical zeros and ones
Next, it is necessary to decompose the signal using the Fourier method, find the spectrum, determine the frequencies of each harmonic and find the total frequency of the signal, while it is important that the main spectrum of the signal falls within the bandwidth of the communication line. In order not to do all these calculations, it is enough to try to determine the fundamental harmonic of the signal spectrum, for this it is necessary to guess the first sinusoid from the signal shape, which repeats its contour of its shape, then find the period of this sinusoid. The period is the distance between two signal changes. Then you can also determine the frequency of the fundamental harmonic of the signal spectrum as F = 1/T, Where F- frequency, T- signal period. For the convenience of further calculations, we assume that the bit rate of signal change is equal to N.
Such calculations can be made for each digital encoding method to determine the frequency of the resulting signal. The resulting signal in digital coding is a specific sequence of rectangular pulses. To represent a sequence of rectangular pulses as a sum of sinusoids to find the spectrum, a large number of such sinusoids is needed. The spectrum of a square wave sequence will generally be much wider than that of modulated signals.
If a digital code is used to transmit data on a voice frequency channel, then the upper limit for potential coding is achieved for a data transfer rate of 971 bps, and the lower limit is unacceptable for any speeds, since the channel bandwidth starts at 300 Hz.
That's why digital codes on voice-frequency channels are simply never used. But on the other hand, they work very well in local networks that do not use telephone lines for data transmission.
Thus, digital coding requires a wide bandwidth for high-quality transmission.
4. When transmitting information over communication lines from a source node to a receiver node, it is necessary to provide such a transmission mode in which the receiver will always know exactly at what point in time it receives data from the source, i.e. it is necessary to provide synchronization source and receiver. In networks, the synchronization problem is more difficult to solve than when exchanging data between blocks within a computer or between a computer and a printer. At short distances, a scheme based on a separate clocking communication line works well. In such a scheme, information is removed from the data line only at the moment the clock pulse arrives (see Fig. 5.4).
Rice. 5.4 Synchronization of receiver and transmitter over short distances
This synchronization option is absolutely not suitable for any network due to the heterogeneity of the characteristics of the conductors in the cables. Over long distances, signal velocity ripples can cause the clock to arrive so late or too early for the corresponding data signal that a data bit is skipped or reread. Another reason why networks refuse to use clock pulses is to save conductors in expensive cables. Therefore, networks use the so-called self-synchronizing codes.
Self-synchronizing codes- signals that indicate to the receiver at what point in time it is necessary to recognize the next bit (or several bits, if the code is oriented to more than two signal states). Any sharp drop in signal - the so-called front- can serve as a good indication for synchronization of the receiver with the transmitter. An example of a self-synchronizing code would be a sine wave. Since the change in the amplitude of the carrier frequency allows the receiver to determine the moment the input code appears. But this applies to analog modulation. In digital coding, there are also methods that create self-synchronizing codes, but more on that later.
Thus, a good digital code should provide synchronization
Having considered the requirements for a good digital code, let's move on to the consideration of the digital coding methods themselves.
5. 2.1Potential code without return to zero NRZ
This code got its name because when a sequence of 1s is transmitted, the signal does not return to zero during the cycle (as we will see below, in other coding methods, a return to zero occurs in this case).
Code NRZ (Non Return to Zero)- without returning to zero - this is the simplest two-level code. The resulting signal has two potential levels:
Zero corresponds to the lower level, unit - the upper. Information transitions occur at a bit boundary.
Let us consider three special cases of data transmission by the code NRZ: an alternating sequence of zeros and ones, a sequence of zeros and a sequence of ones (see Fig. 5.5, a).
Rice. 5.5 NRZ code
Let's try to determine whether this code satisfies the listed requirements. To do this, it is necessary to determine the fundamental harmonic of the spectrum with potential coding in each of the presented cases in order to more accurately determine which NRZ code has requirements for the communication line used.
The first case - information is transmitted, consisting of an infinite sequence of alternating ones and zeros (see Fig. 5.5, b).
This figure shows that when alternating ones and zeros, two bits 0 and 1 will be transmitted in one cycle. With the shape of the sinusoid shown in fig. 4.22b N- bit rate, the period of this sinusoid is equal to T=2N. The frequency of the fundamental harmonic in this case is equal to f 0 = N/2.
As you can see, with such a sequence of this code, the data transfer rate is twice the signal frequency.
When transmitting sequences of zeros and ones, the resulting signal is direct current, the frequency of the signal change is zero f 0 = 0 .
The spectrum of a real signal is constantly changing depending on what data is transmitted over the communication line, and one should be wary of transmissions of long sequences of zeros or ones that shift the signal spectrum towards low frequencies. Because NRZ code when transmitting long sequences of zeros or ones has a constant component.
It is known from signal theory that, in addition to the requirements for width, another very important requirement is put forward for the spectrum of the transmitted signal - no constant component(the presence of direct current between the receiver and transmitter), because the use of various transformer interchanges does not pass in the communication line D.C..
Therefore, some of the information will simply be ignored by this link. Therefore, in practice, they always try to get rid of the presence of a constant component in the spectrum of the carrier signal already at the coding stage.
Thus, we have identified one more requirement for a good digital code the digital code should not have a constant component.
Another disadvantage of NRZ is - lack of synchronization. In this case, only additional methods of synchronization will help, which we will talk about later.
One of the main advantages of the NRZ code is simplicity. In order to generate rectangular pulses, two transistors are needed, and complex microcircuits are needed to implement analog modulation. The potential signal does not need to be encoded and decoded, since the same method is used for data transmission inside the computer.
As a result of everything shown above, we will draw several conclusions that will help us when considering other digital coding methods:
NRZ is very easy to implement, has good error detection (due to two sharply different potentials).
NRZ has a DC component when transmitting zeros and ones, which makes it impossible to transmit on transformer isolated lines.
NRZ is not a self-synchronizing code and this complicates its transmission on any line.
The attractiveness of the NRZ code, because of which it makes sense to improve it, lies in the rather low frequency of the fundamental harmonic fo, which is equal to N/2 Hz, as shown above. Thus the code NRZ operates at low frequencies from 0 to N/2 Hz.
As a result, in its pure form, the NRZ code is not used in networks. Nevertheless, its various modifications are used, in which both the poor self-synchronization of the NRZ code and the presence of a constant component are successfully eliminated.
The following digital coding methods have been developed with the goal of somehow improving the capability of the NRZ code
5. 2.2. AMI Alternate Inversion Bipolar Coding Method
Method of bipolar coding with alternative inversion (Bipolar Alternate Mark Inversion, AMI) is a modification of the NRZ method.
This method uses three levels of potential - negative, zero and positive. Three signal levels is a disadvantage of the code, because in order to distinguish between three levels, a better signal-to-noise ratio is needed at the input to the receiver. 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 general disadvantage of multi-state codes compared to bilevel codes. In the AMI code, a zero potential is used to encode a logical zero, a logical one is encoded either by a positive potential or a negative one, while the potential of each new one is opposite to the potential of the previous one.
Rice. 5.6 AMI code
This coding technique partially eliminates the problems of the DC component and the lack of self-synchronization inherent in the NRZ code when transmitting long sequences of ones. But the problem of the constant component remains for him when transmitting sequences of zeros (see Fig. 5.6).
Let's consider particular cases of the code operation and determine the fundamental harmonic of the resulting signal spectrum for each of them. With a sequence of zeros - signal - direct current - fo \u003d 0 (Fig. 5.7, a)
Rice. 5.7 Determining the fundamental frequencies of the AMI spectrum
For this reason, the AMI code also needs further improvement. When transmitting a sequence of units, the signal on the line is a sequence of bipolar pulses with the same spectrum as the NRZ code transmitting alternating zeros and ones, that is, without a constant component and with a fundamental harmonic fo = N/2 Hz.
When transmitting alternating ones and zeros, the fundamental harmonic fo = N/4 Hz, which is two times less than that of the NRZ code.
In general, for various combinations of bits on the line, the use of the AMI code leads to a narrower signal spectrum than for the NRZ code, and hence to a higher line throughput. The AMI code also provides some features for recognizing erroneous signals. Thus, a violation of the strict alternation of the polarity of the signals indicates a false impulse or the disappearance of a correct impulse from the line. A signal with incorrect polarity is called a forbidden signal. (signal violation).
The following conclusions can be drawn:
AMI cancels the DC component when transmitting a sequence of ones;
AMI has a narrow spectrum - from N/4 - N/2;
AMI partially eliminates synchronization problems
AMI uses not two, but three signal levels on the line and this is its drawback, but the following method managed to eliminate it.
5. 2.3 Potential code with inversion at unity NRZI
This code is completely similar to the AMI code, but only uses two signal levels. When zero is transmitted, it transmits the potential that was set in the previous cycle (that is, it does not change it), and when one is transmitted, the potential is inverted to the opposite.
This code is called potential code with inversion at one (Non Return to Zero with ones Inverted, NRZI).
It is convenient in cases where the use of the third signal level is highly undesirable, for example, in optical cables, where two signal states are reliably recognized - light and dark.
Rice. 5.8 NRZI code
The NRZI code differs in the shape of the resulting signal from the AMI code, but if you calculate the fundamental harmonics, for each case, it turns out that they are the same. For a sequence of alternating ones and zeros, the fundamental frequency of the signal is fo=N/4.(see Fig. 5.9, a). For with a sequence of units - fo=N/2. With a sequence of zeros, the same drawback remains fo=0- direct current in the line.
Rice. 5.9 Determining the fundamental frequencies of the spectrum for NRZI
The conclusions are as follows:
NRZI - provides the same capabilities as the AMI code, but uses only two signal levels for this and is therefore more suitable for further improvement. Disadvantages of NRZI are a DC component with a sequence of zeros, and lack of synchronization during transmission. The NRZI code became the basis for the development of more advanced coding methods at higher levels.
5. 2.4 Code MLT3
Code of three-level transmission MLT-3 (Multi Level Transmission - 3) has much in common with the NRZI code. Its most important difference is three signal levels.
One corresponds to the transition from one signal level to another. A change in the level of the linear signal occurs only if a unit is received at the input, however, unlike the NRZI code, the generation algorithm is chosen in such a way that two adjacent changes always have opposite directions.
Rice. 5.10 Potential MLT-3 code
Consider special cases, as in all previous examples.
When transmitting zeros, the signal also has a constant component, the signal does not change - fo = 0 Hz. (See Figure 5.10). When all ones are transmitted, information transitions are fixed at the bit boundary, and one signal cycle can accommodate four bits. In this case fo=N/4 Hz - maximum code frequency MLT-3 when transferring all units (Fig. 5.11, a).
Rice. 5.11 Determining the fundamental frequencies of the spectrum for MLT-3
In the case of an alternating sequence, the code MLT-3 has a maximum frequency equal to fo=N/8, which is two times less than the NRZI code, therefore, this code has a narrower bandwidth.
As you noticed, the disadvantage of the MLT-3 code, like the NRZI code, is the lack of synchronization. This problem is solved with an additional data transformation that eliminates long sequences of zeros and the possibility of desynchronization. The general conclusion can be drawn as follows - the use of three-level coding MLT-3 allows you to reduce the clock frequency of the line signal and thereby increase the transmission rate.
5. 2.5 Bipolar pulse code
In addition to potential codes, impulse codes are also used when the data is represented by a full impulse or its part - a front.
The simplest case of this approach is bipolar pulse code, in which the unit is represented by a pulse of one polarity, and zero is the other. Each pulse lasts half a cycle (Fig. 5.12). Bipolar pulse code - three-level code. Let us consider the resulting signals during data transmission by bipolar coding in the same particular cases.
Rice. 5.12 Bipolar pulse code
A feature of the code is that there is always a transition (positive or negative) in the center of the bit. Therefore, each bit is labeled. The receiver can extract a sync pulse (strobe) having a pulse repetition rate from the signal itself. Binding is made to each bit, which ensures synchronization of the receiver with the transmitter. Such codes, which carry a strobe, are called self-synchronizing. Consider the spectrum of signals for each case (Fig. 5.13). When transmitting all zeros or ones, the frequency of the fundamental harmonic of the code fo=N Hz, which is twice the fundamental of the NRZ code and four times the fundamental of the AMI code. When transmitting alternating ones and zeros - fo=N/2
Rice. 5.13 Determination of the main frequencies of the spectrum for a bipolar pulse code.
This shortcoming of the code does not give a gain in data transfer rate and clearly indicates that the impulse codes are slower than potential ones.
For example, a 10 Mbps link requires a carrier frequency of 10 MHz. When transmitting a sequence of alternating zeros and ones, the speed increases, but not much, because the frequency of the fundamental harmonic of the code fо=N/2 Hz.
The bipolar pulse code has a great advantage over previous codes - it is self-synchronizing.
The bipolar pulse code has a wide signal spectrum and is therefore slower.
The bipolar pulse code uses three levels.
5. 2.6 Manchester code
Manchester code was developed as an improved bipolar pulse code. The Manchester code also refers to self-synchronizing codes, but unlike the bipolar code, it has not three, but only two levels, which provides better noise immunity.
In the Manchester code, a potential drop, that is, the front of the pulse, is used to encode ones and zeros. In Manchester encoding, each clock is divided into two parts. Information is encoded by potential drops that occur in the middle of each cycle. It happens like this:
A unit is encoded by a low-to-high transition, and a zero is encoded by a reverse edge. At the beginning of each cycle, a service signal edge can occur if you need to represent several ones or zeros in a row.
Consider special cases of coding (sequences of alternating zeros and ones, some zeros, some ones), and then we will determine the main harmonics for each of the sequences (see Fig. 5.14). In all cases, it can be seen that with Manchester coding, the signal change at the center of each bit makes it easy to isolate the clock signal. Therefore, the Manchester code has good self-synchronizing properties.
Rice. 5.14 Manchester code
Self-synchronization always makes it possible to transmit large packets of information without loss due to differences in the clock frequency of the transmitter and receiver.
So, let's determine the fundamental frequency when transmitting only ones or only zeros.
Rice. 5.15 Determination of the main frequencies of the spectrum for the Manchester code.
As can be seen when transmitting both zeros and ones, there is no constant component. Fundamental Frequency fo=NHz, as in bipolar coding. Due to this, the galvanic isolation of signals in communication lines can be performed in the simplest ways, for example, using pulse transformers. When transmitting alternating ones and zeros, the frequency of the fundamental harmonic is equal to fo=N/2Hz.
Thus, the Manchester code is an improved bipolar code, improved by using only two signal levels for data transmission, and not three, as in bipolar. But this code is still slow compared to NRZI which is twice as fast.
Consider an example. Take for data transmission a communication line with a bandwidth 100 MHz and speed 100 Mbps. If earlier we determined the data rate at a given frequency, now we need to determine the frequency of the signal at a given line speed. Based on this, we determine that for data transmission by the NRZI code, the frequency range from N / 4-N / 2 is enough for us - these are frequencies from 25 -50 MHz, these frequencies are included in the bandwidth of our line - 100 MHz. For the Manchester code, we need a frequency range from N / 2 to N - these are frequencies from 50 to 100 MHz, in this range the main harmonics of the signal spectrum are located. For the Manchester code, it does not satisfy the bandwidth of our line, and, therefore, the line will transmit such a signal with large distortions (such a code cannot be used on this line).
5.2.7Differential Manchester code.
Differential Manchester code is a type of Manchester coding. It uses the middle of the clock interval of the line signal only for synchronization, and there is always a change in the signal level at it. Logic 0 and 1 are transmitted by the presence or absence of a signal level change at the beginning of the clock interval, respectively (Fig. 5.16)
Rice. 5.16 Differential Manchester code
This code has the same advantages and disadvantages as the Manchester one. But, in practice, it is the differential Manchester code that is used.
Thus, the Manchester code used to be very active in local networks (when high-speed lines were a great luxury for a local area network), due to its self-synchronization and lack of a constant component. It is still widely used in fiber optic and electrical networks. Recently, however, developers have come to the conclusion that it is still better to use potential coding, eliminating its shortcomings using the so-called logical coding.
5.2.8Potential code 2B1Q
Code 2B1Q- potential code with four signal levels for data encoding. Its name reflects its essence - every two bits (2B) are transmitted in one cycle by a signal that has four states (1Q).
Pare bit 00 corresponds potential (-2.5V), a couple of bits 01 corresponds potential (-0.833 V), couple 11 - potential (+0.833 V), and a couple 10 - potential ( +2.5 V).
Rice. 5.17 Potential code 2B1Q
As can be seen in Figure 5.17, this encoding method requires additional measures to deal with long sequences of identical bit pairs, as this turns the signal into a DC component. Therefore, when transmitting both zeros and ones fo=0Hz. When alternating ones and zeros, the signal spectrum is twice as narrow as that of the code NRZ, since at the same bit rate the duration of the cycle is doubled - fo=N/4Hz.
Thus, using the 2B1Q code, you can transfer data over the same line twice as fast as using the AMI or NRZI code. However, for its implementation, the transmitter power must be higher so that the four potential levels (-2.5V, -0.833 V, +0.833 V, +2.5 V) are clearly distinguished by the receiver against the background of interference.
5. 2.9 Code PAM5
All the signal coding schemes we have considered above were bit-based. With bit coding, each bit corresponds to a signal value determined by the protocol logic.
With byte encoding, the signal level is set by two or more bits. In a five level code PAM5 5 voltage levels (amplitudes) and two-bit coding are used. Each combination has its own voltage level. With two-bit coding, four levels are required to transmit information (two to the second degree - 00, 01, 10, 11 ). Transmitting two bits at the same time provides a halving of the signal change rate. The fifth level is added to create redundancy in the code used for error correction. This gives an additional margin of signal-to-noise ratio.
Rice. 5.18 Code PAM 5
5. 3. Logic coding
Logic coding runs until physical encoding.
At the stage of logical coding, the waveform is no longer formed, but the shortcomings of physical digital coding methods, such as the lack of synchronization, the presence of a constant component, are eliminated. Thus, first, corrected sequences of binary data are formed using logical coding tools, which are then transmitted over communication lines using physical coding methods.
Logical coding implies the replacement of bits of the original information with a new sequence of bits that carries the same information, but has, in addition, additional properties, for example, the ability for the receiving side to detect errors in the received data. Accompanying each byte of the original information with one parity bit is an example of a very commonly used method of logical coding when transmitting data using modems.
Separate two methods of logical coding:
Redundant codes
Scrambling.
5. 3.1 Redundant codes
Redundant codes are based on splitting the original sequence of bits into portions, which are often called characters. Then each original character is replaced with a new one that has more bits than the original. A clear example of redundant code is the 4V/5V logic code.
Logic code 4V/5V replaces the original 4-bit characters with 5-bit characters. Since the resulting symbols contain redundant bits, the total number of bit combinations in them is greater than in the original ones. Thus, the five-bit scheme gives 32 (2 5) two-digit alphanumeric characters, having a value in decimal code from 00 to 31. While the original data may contain only four bits or 16 (2 4) characters.
Therefore, in the resulting code, you can select 16 such combinations that do not contain a large number of zeros, and count the rest prohibited codes (code violation). In this case, the long strings of zeros are broken and the code becomes self-synchronizing for any transmitted data. The constant component also disappears, which means that the signal spectrum narrows even more. But this method reduces the useful bandwidth of the line, since the redundant units of user information do not carry, and only "occupy the airtime". The redundant codes allow the receiver to recognize corrupted bits. If the receiver receives a forbidden code, it means that the signal has been distorted on the line.
So let's look at work. logic code 4V/5V. The converted signal has 16 values for information transfer and 16 redundant values. In the receiver decoder, five bits are decoded as information and service signals.
Nine symbols are allocated for service signals, seven symbols are excluded.
Combinations with more than three zeros are excluded (01 - 00001, 02 - 00010, 03 - 00011, 08 - 01000, 16 - 10000 ) . Such signals are interpreted by the symbol V and the receiver team VIOLATION- failure. The command indicates an error due to high interference or transmitter failure. The only combination of five zeros (00 - 00000 ) refers to service signals, means the symbol Q and has the status QUIET- no signal on the line.
Such data encoding solves two problems - synchronization and noise immunity improvement. Synchronization occurs due to the elimination of a sequence of more than three zeros, and high noise immunity is achieved by the data receiver in a five-bit interval.
The price for these advantages with this method of data encoding is a decrease in the transmission rate. useful information. For example, As a result of adding one redundant bit to four information bits, the bandwidth efficiency in protocols with code MLT-3 and data encoding 4B/5B decreases respectively by 25%.
Encoding scheme 4V/5V presented in the table.
|
Binary code 4B |
Result code 5V |
So, according to this table, the code is formed 4V/5V, then transmitted over the line using physical coding using one of the potential coding methods that is sensitive only to long sequences of zeros - for example, using the NRZI digital code.
The 4V/5V code symbols, 5 bits long, guarantee that no more than three zeros in a row can occur on the line for any combination of them.
Letter IN in the code name means that the elementary signal has 2 states - from English binary- binary. There are also codes with three signal states, for example, in the code 8V/6T to encode 8 bits of the original information, a code of 6 signals is used, each of which has three states. Code Redundancy 8V/6T higher than code 4V/5V, since there are 3 6 = 729 resulting symbols for 256 source codes.
As we said, logical encoding occurs before physical, therefore, it is carried out by the network link-level equipment: network adapters and interface blocks of switches and routers. Since, as you yourself have seen, the use of a lookup table is a very simple operation, so the method of logical coding with redundant codes does not complicate the functional requirements for this equipment.
The only requirement is that the transmitter using the redundant code must operate at a higher clock rate to provide a given line capacity. Yes, to send codes 4V/5V with speed 100 Mb/s the transmitter must operate at a clock frequency 125 MHz. In this case, the spectrum of the signal on the line is expanded in comparison with the case when a pure, non-redundant code is transmitted over the line. However, the spectrum of the redundant potential code is narrower than the spectrum of the Manchester code, which justifies the additional stage of logical coding, as well as the operation of the receiver and transmitter at an increased clock frequency.
Thus, the following conclusion can be drawn:
Mainly for local networks simpler, more reliable, better, faster - use logical data coding using redundant codes, which will eliminate long sequences of zeros and ensure signal synchronization, then use a fast digital code for transmission at the physical level NRZI, rather than using a slow but self-synchronizing Manchester code.
For example, to transmit data over a line with a bandwidth of 100M bit / s and a bandwidth of 100 MHz, the NRZI code requires frequencies of 25 - 50 MHz, this is without coding 4V / 5V. And if applied to NRZI also 4V / 5V encoding, now the frequency band will expand from 31.25 to 62.5 MHz. But nevertheless, this range still "fits" into the line bandwidth. And for the Manchester code, without the use of any additional coding, frequencies from 50 to 100 MHz are needed, and these are the frequencies of the main signal, but they will no longer be passed by the 100 MHz line.
5. 3.2 Scrambling
Another method of logical coding is based on the preliminary "mixing" of the original information in such a way that the probability of occurrence of ones and zeros on the line becomes close.
Devices or blocks that perform this operation are called scramblers (scramble - dump, random assembly).
At scrambling the data is mixed according to a certain algorithm and the receiver, having received binary data, transmits it to descrambler, which restores the original bit sequence.
Excess bits are not transmitted over the line.
The essence of scrambling is simply a bit-by-bit change in the data stream passing through the system. Almost the only operation used in scramblers is XOR - "bitwise XOR", or else they say - addition by module 2. When two units are added by exclusive OR, the highest unit is discarded and the result is written - 0.
The scrambling method is very simple. First come up with a scrambler. In other words, they come up with what ratio to mix the bits in the original sequence using "exclusive OR". Then, according to this ratio, the values of certain bits are selected from the current sequence of bits and added up according to XOR between themselves. In this case, all bits are shifted by 1 bit, and the value just received ("0" or "1") is placed in the freed least significant bit. The value that was in the most significant bit before the shift is added to the coding sequence, becoming its next bit. Then this sequence is issued to the line, where, using physical encoding methods, it is transmitted to the recipient node, at the input of which this sequence is descrambled based on the inverse ratio.
For example, a scrambler might implement the following relationship:
Where Bi- binary digit of the resulting code obtained on the i-th cycle of the scrambler, AI- binary digit of the source code, coming at the i-th cycle to the input of the scrambler, B i-3 and B i-5- binary digits of the resulting code obtained in the previous cycles of the scrambler, respectively, 3 and 5 cycles earlier than the current cycle, - XOR operation (modulo 2 addition).
Now let's define the encoded sequence, for example, for such a source sequence 110110000001 .
The scrambler defined above will produce the following result code:
B 1 \u003d A 1 \u003d 1 (the first three digits of the resulting code will be the same as the original one, since there are no necessary previous digits yet)
Thus, the output of the scrambler will be the sequence 110001101111 . In which there is no sequence of six zeros that was present in the source code.
After receiving the resulting sequence, the receiver passes it to the descrambler, which reconstructs the original sequence based on the inverse relationship.
There are other different scrambling algorithms, they differ in the number of terms that give the digit of the resulting code, and the shift between the terms.
The main problem of coding based scramblers - synchronization of the transmitting (encoding) and receiving (decoding) devices. If at least one bit is omitted or erroneously inserted, all transmitted information is irreversibly lost. Therefore, in scrambler-based coding systems, much attention is paid to synchronization methods. .
In practice, a combination of two methods is usually used for these purposes:
a) adding synchronization bits to the information stream, which are known in advance to the receiving side, which allows it, if such a bit is not found, to actively start searching for synchronization with the sender,
b) the use of high-precision time pulse generators, which makes it possible to decode the received bits of information "from memory" without synchronization at times of loss of synchronization.
There are also simpler methods of dealing with sequences of ones, also classified as scrambling.
To improve the code Bipolar AMI two methods are used, based on the artificial distortion of the sequence of zeros by forbidden symbols.
Rice. 5.19 Codes B8ZS and HDB3
This figure shows the use of the method B8ZS (Bipolar with 8-Zeros Substitution) and method HDB3 (High-Density Bipolar 3-Zeros) to correct the AMI code. The source code consists of two long sequences of zeros (8- in the first case and 5 in the second).
Code B8ZS corrects only sequences consisting of 8 zeros. To do this, after the first three zeros, instead of the remaining five zeros, he inserts five digits: V-1*-0-V-1*.V here denotes a signal of a unit prohibited for a given cycle of polarity, that is, a signal that does not change the polarity of the previous unit, 1 * - a signal of the unit of correct polarity, and the asterisk sign marks the fact that in the source code in this cycle there was not a unit, but a zero. As a result, the receiver sees 2 distortions in 8 clock cycles - it is very unlikely that this happened due to noise on the line or other transmission failures. Therefore, the receiver considers such violations as coding of 8 consecutive zeros and, upon reception, replaces them with the original 8 zeros.
The B8ZS code is constructed in such a way that its constant component is zero for any sequence of binary digits.
Code HDB3 corrects any 4 consecutive zeros in the original sequence. The rules for generating the HDB3 code are more complex than the B8ZS code. Every four zeros are replaced by four signals that have one V signal. To suppress the DC component, the polarity of the signal V alternates with successive replacements.
In addition, two patterns of four-cycle codes are used for replacement. If the source code contained an odd number of ones before the replacement, then the sequence is used 000V, and if the number of units was even - the sequence 1*00V.
Thus, the use of logical coding in conjunction with potential coding provides the following advantages:
Enhanced candidate codes have a fairly narrow bandwidth for any sequences of 1s and 0s that occur in the transmitted data. As a result, codes derived from the potential by logical coding have a narrower spectrum than Manchester, even at an increased clock frequency.
