7. PHYSICAL LAYER

7.2. Discrete data transmission methods

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

When using rectangular pulses, the spectrum of the resulting signal is very wide. The use of a sinusoid results in a narrower spectrum at the same information rate. However, the implementation of modulation requires more complex and expensive equipment than the implementation of rectangular pulses.

At present, more and more often, data that initially has an analog form - speech, a television image - are transmitted over communication channels in a discrete form, that is, in the form of a sequence of ones and zeros. The process of representing analog information in discrete form is called discrete modulation .

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

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

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

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

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

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

Rice. 27. Different types of modulation

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

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

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

Rice. 28. Spectra of signals during potential coding

and amplitude modulation

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

When digitally encoding discrete information, potential and impulse codes are used. In potential codes, only the value of the signal potential is used to represent logical ones and zeros, and its drops 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 - by 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:

· had at the same bit rate 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 spectrum of signals allows you to achieve a higher data transfer rate on the same line. Often, the signal spectrum requires the absence of a constant component.

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 in the exchange of data between closely spaced devices, for example, between units within a computer or between a computer and a printer. Therefore, so-called self-synchronizing codes are used in networks, the signals of which carry instructions for the transmitter about at what point in time it is necessary to recognize the next bit (or several bits). Any sharp edge in the signal - the so-called front - can be 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.

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.

On fig. 29a shows a method of potential coding, also called coding no return to zero (Non Return to Zero, NRZ) . 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 is easy to implement, has good error recognition (due to two sharply different potentials), but does not have the self-synchronization property. When transmitting a long sequence of ones or zeros, the signal on the line does not change, so the receiver is unable to determine from the input signal the points in time when data should be read. 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 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.

Another serious disadvantage of the NRZ method is the presence of a low frequency component that approaches zero when transmitting long sequences of ones or zeros. Because of this, many communication channels that do not provide a direct galvanic connection between the receiver and the source do not support this type of encoding. 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 eliminated. 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 f 0, which is equal to N/2 Hz. Other coding methods, such as Manchester, have a higher fundamental frequency.

Rice. 29. Ways of discrete data coding

One of the modifications of the NRZ method is the method bipolar coding with alternative inversion (Bipolar Alternate Mark Inversion, AMI). This method (Fig. 29, b) uses three potential levels - negative, zero and positive. To encode a logical zero, a zero potential is used, and a logical unit is encoded either by a positive potential or a negative one, while 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. Therefore, the AMI code needs further improvement.

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. 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. Such a signal is called prohibited signal (signal violation).

The AMI code uses not two, but three signal levels per line. The extra 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 codes with multiple signal states compared to codes that only distinguish between two states.

There is code similar to AMI, but with only 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 unity (Not return to Zero with ones inverted , NRZI ) . This code is useful in cases where the use of a third signal level is highly undesirable, for example, in optical cables, where two signal states are reliably recognized - light and shadow.

In addition to potential codes, networks also use pulse codes, when the data is represented by a full pulse or its part - a front. The simplest case of this approach is bipolar pulse code , in which the unit is represented by an impulse of one polarity, and zero is the other (Fig. 29, c). Each pulse lasts half a cycle. This code has excellent self-synchronizing properties, but a constant component may be present, for example, when transmitting a long sequence of ones or zeros. In addition, its spectrum is wider than that of potential codes. 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 (Fig. 29, d). It is used in Ethernet and Token Ring technologies.

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 signal level, 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-synchronizing properties. The bandwidth of the Manchester code is narrower than that of the bipolar pulse. It also does not have a constant component, and the fundamental harmonic in the worst case (when transmitting a sequence of ones or zeros) has a frequency of N Hz, and in the best case (when transmitting alternating ones and zeros) it is equal to N / 2 Hz, like in AMI codes or NRZ. On average, the bandwidth of the Manchester code is one and a half times narrower than that of the bipolar pulse code, and the fundamental harmonic oscillates around 3N/4. The Manchester code has another advantage over the bipolar pulse code. The latter uses three signal levels for data transmission, while Manchester uses two.

On fig. 29, e shows a potential code with four signal levels for data encoding. This is a 2B1Q code, the name of which reflects its essence - every two bits (2B) are transmitted in one cycle by a signal that has four states (1Q). Bit 00 is -2.5V, bit 01 is -0.833V, bit 11 is +0.833V, and bit 10 is +2.5V. sequences of identical pairs of bits, since in this case the signal is converted into a constant component. With random bit interleaving, the spectrum of the signal is twice as narrow as that of the NRZ code, since at the same bit rate the clock duration is doubled. Thus, using the 2B1Q code, you can transfer data over the same line twice as fast as using the AMI or NRZI code. However, 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.

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

When using rectangular pulses, the spectrum of the resulting signal is very wide. This is not surprising if we remember that the spectrum of an ideal momentum has an infinite width. The use of a sinusoid results in a much smaller spectrum at the same information rate. However, the implementation of sinusoidal modulation requires more complex and expensive equipment than the implementation of rectangular pulses.

Currently, more and more often, data that initially has an analog form - speech, a television image - are transmitted over communication channels in a discrete form, that is, in the form of a sequence of ones and zeros. The process of representing analog information in discrete form is called discrete modulation. The terms "modulation" and "coding" are often used interchangeably.

At digital coding discrete information, potential and impulse codes are used. In potential 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.

Networks use 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 edge in the signal - the so-called edge - can serve as a good indication for the synchronization of the receiver with the transmitter. Recognition and correction of distorted data is difficult to implement by means of the physical layer, therefore, most often this work is undertaken by the protocols that lie above: channel, network, transport or application. On the other hand, error recognition at the physical layer saves time, since the receiver does not wait for the frame to be completely placed in the buffer, but rejects it immediately upon placement. knowing bit errors within a frame.

Potential non-return-to-zero code, a method of potential coding, also called coding without returning to zero (Non return to Zero, NRZ). The last name reflects the fact that when transmitting a sequence of ones, the signal does not return to zero during the cycle (as we will see below, in other coding methods, a return to zero occurs in this case). The NRZ method is easy to implement, has good error recognition (due to two sharply different potentials), but does not have the self-synchronization property. When transmitting a long sequence of ones or zeros, the signal on the line does not change, so the receiver is unable to determine from the input signal the times when it is necessary to read the data again. Even with a highly accurate clock generator, the receiver can make a mistake with the moment of data acquisition, since the frequencies of the two generators are 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.

Bipolar coding method with alternative inversion. One of the modifications of the NRZ method is the method bipolar coding with alternative inversion (Bipolar Alternate Mark inversion, AMI). This method uses three levels of potential - 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, while the potential of each new unit is opposite to the potential of the previous one. 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 prohibited signal (signal violation). In the AMI code, not two, but three signal levels per line are used. The additional layer requires an increase in transmitter power of about 3dB to provide the same bit fidelity on the line, which is a general disadvantage of codes with multiple signal states compared to codes that only distinguish between two states.

Potential code with inversion at unity. There is code similar to AMI, but with only 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 unity (Non return to Zero with ones inverted, NRZI). This code is useful in cases where the use of a third signal level is highly undesirable, for example, in optical cables, where two signal states are reliably recognized - light and dark.

Bipolar pulse code In addition to potential codes, networks also use pulse codes when the data is represented by a full pulse 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. Such a code has excellent self-clocking properties, but a DC 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 NHz, 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.

Manchester code. In local networks, until recently, the most common coding method was the so-called Manchester code. It is used in Ethernet and TokenRing technologies. 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 signal level, 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-synchronizing properties. The bandwidth of the Manchester code is narrower than that of the bipolar pulse. 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. The Manchester code has another advantage over the bipolar pulse code. The latter uses three signal levels for data transmission, while Manchester uses two.

Potential code 2B 1Q. Potential code with four signal levels for encoding data. This is the code 2 IN 1Q, the name of which reflects its essence - every two bits (2B) are transmitted in one cycle by a signal that has four states (1Q). Bit 00 is -2.5V, bit 01 is -0.833V, 11 is +0.833V, and 10 is +2.5V. With this encoding method, additional measures are required to deal with long sequences of identical bit pairs, since the signal is then converted into a constant component. With random bit interleaving, the spectrum of the signal is twice as narrow as that of the NRZ code, since at the same bit rate the clock duration is doubled. Thus, using the 2B 1Q code, you can transfer data twice as fast on the same line than using the AMI or NRZI 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.

Logic coding Logical coding is used to improve potential codes like AMI, NRZI or 2Q.1B. Logic coding should replace long sequences of bits leading to a constant potential with interspersed ones. As noted above, two methods are characteristic of 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.

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 Mb / s, 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.

Scrambling. Shuffling the data with a scrambler before putting it on the line with a candid code is another way of logical coding. Scrambling methods consist in bit-by-bit calculation of the resulting code based on the bits of the source code and the bits of the resulting code received in previous cycles. For example, a scrambler might implement the following relationship:

Asynchronous and synchronous transmission

When data is exchanged at the physical layer, the unit of information is a bit, so the physical layer means always maintain bit-by-bit synchronization between the receiver and the transmitter. It is usually sufficient to ensure synchronization at these two levels - bit and frame - so that the transmitter and receiver can ensure a stable exchange of information. However, if the quality of the communication line is poor (this usually applies to switched telephone channels), additional means of synchronization at the byte level are introduced to reduce the cost of equipment and increase the reliability of data transmission.

This mode of operation is called asynchronous or start-stop. In asynchronous mode, each byte of data is accompanied by special start and stop signals. The purpose of these signals is, firstly, to notify the receiver of the arrival of data and, secondly, to give the receiver enough time to perform some timing-related functions before the next byte arrives. The start signal has a duration of one clock interval, and the stop signal can last one, one and a half, or two clocks, so one, one and a half, or two bits are said to be used as a stop signal, although these signals do not represent user bits.

In synchronous transfer mode, there are no start-stop bits between each pair of bytes. conclusions

When transmitting discrete data over a narrowband voice-frequency channel used in telephony, analog modulation methods are most suitable, in which the carrier sinusoid is modulated by the original sequence of binary digits. This operation is carried out by special devices - modems.

For low-speed data transmission, a change in the frequency of the carrier sinusoid is used. Higher speed modems operate on combined Quadrature Amplitude Modulation (QAM) methods, which are characterized by 4 levels of carrier sinusoid amplitude and 8 levels of phase. Not all of the possible 32 combinations of the QAM method are used for data transmission, the forbidden combinations make it possible to recognize distorted data at the physical level.

On broadband communication channels, potential and pulse coding methods are used, in which data is represented by different levels of a constant signal potential or pulse polarity or his front.

When using potential codes, the task of synchronizing the receiver with the transmitter is of particular importance, since when transmitting long sequences of zeros or ones, the signal at the receiver input does not change and it is difficult for the receiver to determine the moment of picking up the next data bit.

The simplest potential code is the non-return-to-zero (NRZ) code, but it is not self-clocking and creates a DC component.

The most popular pulse code is the Manchester code, in which the information is carried by the direction of the signal edge in the middle of each cycle. The Manchester code is used in Ethernet and TokenRing technologies.

To improve the properties of a potential NRZ code, logical coding methods are used that exclude long sequences of zeros. These methods are based on:

On the introduction of redundant bits into the original data (4V/5V type codes);

Scrambling of the original data (codes like 2B 1Q).

Improved potential codes have a narrower spectrum than pulse codes, so they are used in high-speed technologies such as FDDI, FastEthernet, GigabitEthernet.

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

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

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

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

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

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

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

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

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

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

The following are used in communication networks information transfer modes:

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

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

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

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

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

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

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

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

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

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

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.

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

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

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

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

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

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

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

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

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

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

ensuring synchronization between transmitter and receiver;

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

the ability to recognize errors in transmitted data;

Relatively low cost of implementation.

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

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

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

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);

The Manchester code

· bipolar code with alternating level inversion.

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

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

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

5 Data transmission equipment Converter Message - El. signal Encoder (compression, correction codes) Modulator Intermediary equipment Communication quality improvement - (Amplifier) ​​Composite channel creation - (Switch) Channel multiplexing - (Multiplexer) (PA may not be available in LAN)

6 Main characteristics of communication lines Bandwidth (Protocol) Reliability of data transmission (Protocol) Propagation delay Frequency response (AFC) Bandwidth Attenuation Noise immunity Crosstalk at the near end of the line Unit cost

9 Attenuation A - one point per frequency response A= log 10 Pout/Pin Bel A=10 log 10 Pout/Pin deciBel (dB) A=20 log 10 Uout/Uin deciBel (dB) q Example 1: Pin = 10 mW, Pout =5 mW Attenuation = 10 log 10 (5/10) = 10 log 10 0.5 = - 3 dB q Example 2: UTP cat 5 Attenuation >= -23.6 dB F= 100MHz, L= 100M Usually A is indicated for the fundamental frequency of the signal. \u003d -23.6 dB F \u003d 100 MHz, L \u003d 100 M Usually A is indicated for the main frequency of the signal ">

11 Noise Immunity Optical Fiber Lines Cable Lines Wired Overhead Lines Radio Links (Shielding, Twisting) External Interference Immunity Internal Interference Immunity Near End Crosstalk Attenuation (NEXT) Far End Crosstalk Attenuation (FEXT) (FEXT - Two Pairs in One Direction)

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

13 Reliability of data transmission Bit Error Rate - BER Probability of data bit distortion Causes: external and internal interference, narrow bandwidth Struggle: increased noise immunity, reduced interference NEXT, increased bandwidth Twisted pair cable BER ~ Fiber optic cable BER ~ Without additional protection:: corrective codes, protocols with repetition

16 Twisted Pair Twisted Pair (TP) foil shield braided wire shield insulated wire outer sheath UTP Unshielded Twisted Pair category 1, UTP sheathed pair category STP Shielded Twisted Pair Types Type 1…9 Each pair has its own shield Each pair has its own pitch twisting, own color Interference immunity Cost Laying complexity

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

22 Fiber optic cable Multi Mode Fiber MMF50/125, 62.5/125, Single Mode FiberSMF8/125, 9.5/125 D = 250 µm 1 GHz - 100 km BaseLH5000km - 1 Gbps (2005) MMSM

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

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

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

28 Wireless Communication Wireless Transmission Benefits: Convenience, inaccessible areas, mobility. fast deployment ... Disadvantages: high level of interference (special means: codes, modulation ...), difficulty in using some ranges Communication line: transmitter - medium - receiver Characteristics of LAN ~ F (Δf, fn);

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

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

36 Satellite communication. Satellite types Satellite communication: microwave - line of sight Geostationary Large coverage Fixed, Low wear Follower satellite, broadcast, low cost, cost independent of distance, Instantaneous link establishment (Mil) T3=300ms Low security, Initially large antenna (but VSAT) MEO km Global Positioning System GPS - 24 satellites LEO km low coverage low latency Internet access

40 Spread Spectrum Technique Special Modulation and Coding Techniques for Wireless Communication C (Bit/s) = Δ F (Hz) * log2 (1+Ps/P N) Power Reduction Noise Immunity Stealth OFDM, FHSS (, Blue-Tooth), DSSS, CDMA

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

Physical basis of data transmission

Any network technology must provide reliable and fast transmission of discrete data over communication lines. And although there are big differences between technologies, they are based on the general principles of discrete data transmission. These principles are embodied in methods for representing binary ones and zeros using pulsed or sinusoidal signals in communication lines of various physical nature, error detection and correction methods, compression methods, and switching methods.

linesconnections

Primary networks, lines and communication channels

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

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

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

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

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

On fig. two variants of the communication line are shown. In the first case ( a) the line consists of a cable segment with a length of several tens of meters and is a link. In the second case (b), the link is a composite link deployed in a circuit-switched network. Such a network could be primary network or telephone network.

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

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

Classification of communication lines

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

In the first case, one speaks of wired environment, and in the second - wireless.

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

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

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

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