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 also often called modulationor analog modulation,emphasizing the fact that coding is carried out by changing the parameters of an 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 pulse has an infinite width. The use of a sinusoid results in a much narrower spectrum at the same bit rate. However, the implementation of sinusoidal modulation requires more complex and expensive equipment than the implementation of rectangular pulses.
Nowadays, more and more often, data that initially had an analog form - speech, television image - is transmitted via communication channels in a discrete form, that is, in the form of a sequence of ones and zeros. The process of presenting analog information in discrete form is called discrete modulation.The terms "modulation" and "encoding" are often used interchangeably.
When digital codingpotential and impulse codes are used for discrete information. 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 as pulses of a certain polarity, or as part of a pulse - a potential drop in a certain direction.
When using rectangular pulses for transmitting discrete information, it is necessary to choose a coding method that would simultaneously achieve several goals: at the same bit rate, it had the smallest spectrum width of the resulting signal; provided synchronization between transmitter and receiver;
Possessed the ability to recognize errors; had a low implementation cost.
The networks use the so-called self-synchronizing codes,whose signals carry for the transmitter an indication of at what point in time it is necessary to recognize the next bit (or several bits, if the code is oriented towards more than two signal states). Any sharp drop in the signal - a so-called front - can be a good indication for synchronizing 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 complete frame to be placed in the buffer, but rejects it immediately upon allocation. knowing the erroneous bits within the frame.
Potential non-return-to-zero code, potential encoding technique, also called encoding without returning 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 a clock cycle (as we will see below, in other coding methods, a return to zero occurs in this case). The NRZ method is simple to implement, has good error recognition (due to two sharply differing potentials), but does not have the property of self-synchronization. When a long sequence of ones or zeros is transmitted, the signal on the line does not change, so the receiver is deprived of the ability to determine from the input signal the moments of time when it is necessary to read the data again. Even with a high-precision clock generator, the receiver can make a mistake when picking up data, since the frequencies of the two oscillators are never completely identical. Therefore, at high data rates and long sequences of ones or zeros, a slight mismatch in clock frequencies can lead to an error in a whole cycle and, accordingly, reading an incorrect bit value.
Alternative inversion bipolar coding method. 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 one is encoded either by a positive potential or negative, with the potential of each new unit being opposite to the potential of the previous one. Thus, a violation of the strict alternation of the polarity of the signals indicates a false pulse or the disappearance of the correct pulse from the line. A signal with incorrect polarity is called prohibited signal (signal violation). The AMI code uses not two, but three signal levels on the line. The additional layer requires an increase in transmitter power of about 3dB to ensure the same reliability of receiving bits on the line, which is a common disadvantage of codes with multiple signal states compared to codes that distinguish only two states.
Potential code with inversion at one. There is a code similar to AMI, but with only two signal levels. When transferring zero, it transfers the potential that was set in the previous cycle (that is, does not change it), and when transferring one, 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). This code is convenient in cases where the use of the third signal level is highly undesirable, for example, in optical cables, where two signal states, light and dark, are steadily recognized.
Bipolar Pulse CodeIn addition to potential codes, pulse codes are also used in networks when data is represented by a full pulse or part of it by a front. The simplest case of this approach is bipolar pulse code,in which one is represented by an impulse of one polarity, and zero is represented by the other . Each impulse lasts half a beat. Such a code has excellent self-synchronizing properties, but a DC component can be present, for example, when transmitting a long sequence of ones or zeros. In addition, its spectrum is wider than that of potential codes. Thus, when transmitting all zeros or ones, the frequency of the fundamental harmonic of the code will be equal to NHz, which is twice 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. Because of the too wide spectrum, bipolar pulse code is rarely used.
Manchester code.Until recently, in local networks, the most common coding method was the so-called manchester code.It is used in the Ethernet and TokenRing technologies. The Manchester code uses the potential drop, that is, the pulse front, to encode ones and zeros. In Manchester encoding, each bar is divided into two parts. The information is encoded by potential drops that occur in the middle of each cycle. One is encoded by a drop from a low signal level to a high level, and a zero is encoded by a reverse edge. At the beginning of each cycle, an overhead signal 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-timing 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 oscillates around 3N / 4. The Manchester code has another advantage over the bipolar pulse code. In the latter, three signal levels are used for data transmission, and in Manchester, two.
Potential code 2B 1Q. Potential code with four signal levels for encoding data. This is the code 2 IN 1Q, whose name reflects its essence - every two bits (2B) are transmitted in one clock cycle by a signal that has four states (1Q). A pair of bits 00 corresponds to a potential of -2.5V, a pair of bits 01 corresponds to a potential of -0.833V, a pair of 11 has a potential of + 0.833V, and a pair of 10 has a potential of + 2.5V. With this coding method, additional measures are required to combat long sequences of identical pairs of bits, since this converts the signal into a DC component. With random interleaving of bits, the signal spectrum is twice narrower than that of the NRZ code, since at the same bit rate, the cycle time is doubled. Thus, using the 2B 1Q code, you can transmit data on 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.
Logical encodingBoolean coding is used to enhance potential codes like AMI, NRZI, or 2Q.1B. Logic coding should replace long bit sequences leading to constant potential with interspersed ones. As noted above, logical coding is characterized by two methods -. redundant codes and scrambling.
Redundant codesare based on breaking the original bit sequence into chunks, which are often called symbols. Then each original character is replaced with a new one that has more bits than the original.
A transmitter using a redundant code must operate at an increased clock rate to maintain the specified line bandwidth. So, to transmit 4V / 5V codes at a speed of 100Mb / s, the transmitter must operate at a clock frequency of 125MHz. In this case, the spectrum of the signal on the line expands in comparison with the case when a clean, not redundant code is transmitted along 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 transmitting it to the line using the potential code is another way of boolean 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 the previous clock cycles. For example, a scrambler can implement the following relationship:
Asynchronous and synchronous transfers
When exchanging data at the physical layer, the unit of information is a bit, therefore, the means of the physical layer always maintain bit synchronization between the receiver and the transmitter. Usually, it is sufficient to provide synchronization at these two levels - bit and frame - so that the transmitter and receiver can provide a stable exchange of information. However, if the quality of the communication line is poor (usually this refers to telephone switched channels), additional synchronization means are introduced at the byte level to reduce the cost of equipment and increase the reliability of data transmission.
This mode of operation is called asynchronousor start-stop.In asynchronous mode, each data byte 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 of the timing-related functions before the next byte arrives. The start signal is 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 the user bits do not represent these signals.
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, the most suitable methods are analog modulation, in which a sinusoidal carrier 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 carrier frequency of a sinusoid is applied. Higher speed modems operate on combined Quadrature Amplitude Modulation (QAM) techniques, which are characterized by 4 levels of sinusoidal carrier amplitude and 8 levels of phase. Not all of the possible 32 combinations of the QAM method are used for data transmission, forbidden combinations make it possible to recognize corrupted data at the physical layer.
On wideband communication channels, potential and pulse coding methods are used, in which data are represented by different levels of constant potential of the signal or pulse polarities or himfront.
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 non-return-to-zero (NRZ) code, however it is not self-timing and creates a constant component.
The most popular pulse code is the Manchester code, in which the direction of the signal drop in the middle of each cycle carries information. Manchester code is used in Ethernet and TokenRing technologies.
To improve the properties of the potential NRZ code, logical coding methods are used that eliminate long sequences of zeros. These methods are based:
On the introduction of redundant bits into the original data (codes like 4B / 5B);
Scrambled raw data (codes of type 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.
The initial information that must be transmitted over the communication line can be either discrete (output data of computers) or analog (speech, television image).
Discrete data transmission is based on the use of two types of physical coding:
a) analog modulation, when encoding is carried out by changing the parameters of a sinusoidal carrier signal;
b) digital coding by changing the levels of a sequence of rectangular information pulses.
Analog modulation leads to a spectrum of the resulting signal of a much smaller width than with digital coding, at the same information transfer rate, but its implementation requires more complex and expensive equipment.
At present, the original data having an analog form is more and more often transmitted via 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 narrow bandwidth channels, a typical representative of which is a voice frequency channel provided to users of telephone networks. This channel transmits signals with a frequency of 300 to 3400 Hz, i.e., its bandwidth is 3100 Hz. This bandwidth is sufficient to transmit speech with acceptable quality. Limiting the bandwidth of the tone channel is associated with the use of multiplexing and circuit switching equipment in telephone networks.
Before the transmission of discrete data on the transmitting side, a modulator-demodulator (modem) modulates the carrier sinusoid of the original sequence of binary digits. The inverse transformation (demodulation) is performed by the receiving modem.
There are three ways to convert digital data to analog form, or three methods of analog modulation:
Amplitude modulation, when only the amplitude of the carrier of sinusoidal oscillations changes in accordance with the sequence of transmitted information bits: for example, when transmitting a unit, the amplitude of oscillations is set large, and when transmitting zero, it is low, or there is no carrier signal at all;
Frequency modulation, when under the action of modulating signals (transmitted information bits) only the frequency of the carrier of sinusoidal oscillations changes: for example, when transmitting zero, it is low, and when transmitting one, it is high;
Phase modulation, when, in accordance with the sequence of transmitted information bits, only the phase of the carrier of sinusoidal oscillations changes: when 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 its low noise immunity. Frequency modulation does not require complex circuitry in modems and is typically used in low speed modems operating at 300 or 1200 bps. An increase in the data transfer rate is provided by the use of combined modulation methods, more often amplitude in combination with phase.
The analogue method of transmitting discrete data provides broadband 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 digital coding of discrete information, two types of codes are used:
a) potential codes, when only the value of the signal potential is used to represent information units and zeros, and its differences are not taken into account;
b) pulse codes, when binary data are represented either by pulses of a certain polarity, or by potential drops in a certain direction.
The following requirements are imposed on the methods of digital coding of discrete information when using rectangular pulses to represent binary signals:
Ensuring synchronization between transmitter and receiver;
Providing the smallest spectrum width of the resulting signal at the same bit rate (since a narrower spectrum of signals allows for
with the same bandwidth to achieve higher speed
data transmission);
The ability to recognize errors in the transmitted data;
Relatively low cost of implementation.
By means of the physical layer, only the recognition of distorted data (error detection) is carried out, which saves time, since the receiver, without waiting for the complete placement of the received frame in the buffer, immediately rejects it when recognizing erroneous bits in the frame. A more complex operation - correction of corrupted data - is performed by higher-level protocols: channel, network, transport, or application.
Synchronizing the transmitter and receiver is necessary so that the receiver knows exactly when to read the incoming data. Synchronization tune the receiver to the transmitted message and keep the receiver in sync with the incoming data bits. The synchronization problem is easily solved when transmitting information over short distances (between blocks inside a computer, between a computer and a printer) by using a separate clocking communication line: information is read only at the moment of the next clock pulse. In computer networks, they refuse to use clock pulses for two reasons: for the sake of saving conductors in expensive cables and because of the inhomogeneity of the characteristics of the conductors in cables (at large distances, uneven signal propagation speed can lead to desynchronization of clock pulses in the clock line and information pulses in the main line , as a result of which the data bit will either be skipped or re-read).
Currently, the synchronization of the transmitter and receiver in networks is achieved by using self-synchronizing codes (SK). The coding of the transmitted data using the SC is to ensure regular and frequent changes (transitions) of the levels of the information signal in the channel. Each transition of the signal level from high to low or vice versa is used to trim the receiver. The best ones are considered to be those that ensure the transition of the signal level at least once during the time interval required to receive one information bit. The more frequent the signal level transitions, the more reliably the receiver synchronizes and the more confidently the received data bits are identified.
The specified requirements for digital coding methods for discrete information are to a certain extent mutually contradictory, therefore, each of the coding methods considered below has its own advantages and disadvantages compared to others.
Self-timed codes. The most common SCs are:
Potential code without return to zero (NRZ - Non Return to Zero);
Bipolar Pulse Code (RZ Code);
Manchester code;
Bipolar code with alternate level inversion.
In fig. 32 shows the coding schemes for message 0101100 using these CKs.
To characterize and comparatively assess the UK, the following indicators are used:
The level (quality) of synchronization;
Reliability (confidence) of recognition and selection of received information bits;
The required rate of change in the signal level in the communication line when using the SC, if the line capacity is specified;
The complexity (and, therefore, the cost) of the equipment that implements the IC.
NRZ code is easy to code and low cost of implementation. It got this name because when transmitting a series of bits of the same name (ones or zeros), the signal does not return to zero during a clock cycle, as is the case in other encoding methods. The signal level remains unchanged for each series, which significantly reduces the quality of synchronization and the reliability of recognition of the received bits (the receiver timer may mismatch with respect to the incoming signal and untimely polling of lines).
For the L ^ -code, the following relations hold:
where VI is the rate of change of the signal level in the communication line (baud);
U2 - communication line bandwidth (bit / s).
In addition to the fact that this code does not have the property of self-synchronization, it also has another serious drawback: the presence of a low-frequency component that approaches zero when transmitting long series of ones or zeros. As a result, the NRZ code in its pure form is not used in networks. Its various modifications are applied, in which poor self-synchronization of the code and the presence of a constant component are eliminated.
RZ-code, or bipolar pulse code (code with return to zero), differs in that during the transmission of one information bit, the signal level changes twice, regardless of whether a series of like-named bits or alternately changing bits are transmitted. One is represented by a pulse of one polarity, and zero is the other. Each impulse lasts half a beat. Such a code has excellent self-synchronizing properties, but the cost of its implementation is quite high, since it is necessary to ensure the ratio
The spectrum of the RZ code is wider than that of the potential codes. Due to its too wide spectrum, it is rarely used.
The Manchester code provides a change in the signal level when each bit is represented, and when transmitting a series of bits of the same name, a double change. Each measure is divided into two parts. The information is encoded by potential drops that occur in the middle of each cycle. One is encoded by the slope from low to high signal level, and zero is encoded by the reverse slope. The speed ratio for this code is as follows:
The Manchester code has good self-timing properties, since the signal changes at least once per transmission cycle of one data bit. Its bandwidth is narrower than that of the RZ code (1.5 times on average). Unlike the bipolar pulse code, where three signal levels are used for data transmission (which is sometimes very undesirable, for example, in optical cables only two states are stably recognized - light and dark), in the Manchester code there are two levels.
Manchester code is widely used in Ethernet and Token Ring technologies.
Bipolar Alternate Level Inversion (AMI) code is one of the modifications of the NRZ code. It uses three levels of potential - negative, zero and positive. The unit is coded either by a positive potential or by a negative one. Zero potential is used to encode zero. The code has good synchronizing properties when transmitting a series of units, since the potential of each new unit is opposite to the potential of the previous one. There is no synchronization when transmitting series of zeros. AMI code is relatively simple to implement. For him 
When transmitting various combinations of bits on a line, the use of the AMI code results in a narrower signal spectrum than for the NRZ code, and therefore in a higher line capacity.
Note that improved potential codes (modernized Manchester code and AMI code) have a narrower spectrum than pulsed ones, therefore they find application in high-speed technologies, for example, in FDDI, Fast Ethernet, Gigabit Ethernet.
Discrete modulation of analog signals. As already noted, one of the trends in the development of modern computer networks is their digitalization, that is, the transmission of signals of any nature in digital form. The sources of these signals can be computers (for discrete data) or devices such as telephones, video cameras, video and sound reproducing equipment (for analog data). Until recently (before the advent of digital communication networks) in territorial networks, all types of data were transmitted in analog form, and discrete computer data were converted into analog form using modems.
However, the transmission of information in analog form does not improve the quality of the received data, if there was a significant distortion during transmission. Therefore, the analog technology for recording and transmitting sound and image was replaced by digital technology, which uses discrete modulation of analog signals.
Discrete modulation is based on sampling continuous signals in both amplitude and time. One of the widespread methods of converting analog signals into digital is pulse-code modulation (PCM), proposed in 1938 by A.Kh. Reeves (USA).
When using PCM, the transformation process includes three stages: display, quantization and encoding (Fig. 33).
The first stage is display. The amplitude of the original continuous signal is measured with a specified period, due to which time sampling occurs. At this stage, the analog signal is converted into pulse-amplitude modulation (IAM) signals. The execution of the stage is based on the Nyquist-Kotelnikov mapping theory, the main provision of which is: if an analog signal is displayed (i.e., represented as a sequence of its discrete time values) on a regular interval with a frequency of at least twice the frequency of the highest harmonic spectrum of the original continuous signal, the display will contain information sufficient to restore the original signal. In analog telephony, the range from 300 to 3400 Hz is selected for voice transmission, which is sufficient for high-quality transmission of all the fundamental harmonics of the interlocutors. Therefore, in digital networks, where the PCM method is implemented for voice transmission, a display frequency of 8000 Hz is adopted (this is more than 6800 Hz, which provides some quality margin).
At the quantization stage, each IAM signal is assigned a quantized value corresponding to the nearest quantization level. The entire range of changes in the amplitude of the IAM signals is divided into 128 or 256 quantization levels. The more quantization levels, the more accurate the IAM amplitude - the signal is represented by the quantized level.
At the encoding stage, each quantized mapping is assigned a 7-bit (if the number of quantization levels is 128) or 8-bit (with 256-step quantization) binary code. In fig. 33 shows signals of an 8-element binary code 00101011, corresponding to a quantized signal with a level of 43. When encoding with 7-element codes, the data transfer rate over the channel should be 56 Kbit / s (this is the product of the display frequency and the width of the binary code), and when encoding 8- element codes - 64 Kbit / s. The standard is a 64 kbps digital channel, which is also called the elementary channel of digital telephone networks.
A 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), the inverse conversion is carried out, i.e., the digitized amplitudes of the continuous signal are demodulated, the original continuous function of time is restored.
In modern digital communication networks, other methods of discrete modulation are used, which allow representing voice measurements in a more compact form, for example, in the form of a sequence of 4-bit numbers. The concept of converting analog signals to digital is also used, in which not the IAM signals themselves are quantized and then encoded, but only their changes, and the number of quantization levels is assumed to be the same. Obviously, this concept allows for signal conversion with greater accuracy.
Digital methods of recording, reproducing and transmitting analog information provide the ability to control the reliability of data read from a medium or received via a communication line. For this purpose, the same control methods are applied as for computer data (see paragraph 4.9).
The transmission of a continuous signal in discrete form imposes strict requirements on the synchronization of the receiver. If the synchronization is not observed, the original signal is reconstructed incorrectly, which leads to distortion of the voice or the transmitted image. If frames with voice measurements (or other analog value) arrive synchronously, the voice quality can be quite high. However, in computer networks, frames can be delayed both at end nodes and in intermediate switching devices (bridges, switches, routers), which negatively affects the quality of voice transmission. Therefore, for high-quality transmission of digitized continuous signals, special digital networks (ISDN, ATM, digital television networks) are used, although Frame Relay networks are still used for the transmission of intra-corporate telephone conversations today, since frame transmission delays in them are within acceptable limits.
2 Physical layer functions Bit representation by electrical / optical signals Bit coding Bit synchronization Bit synchronization / reception of bits via physical communication channels Transmission speed Range Signal levels, connectors In all network devices Hardware implementation (network adapters) Example: 10 BaseT - UTP cat 3, 100 ohm, 100m, 10Mbps, MII code, RJ-45
5 Data transmission equipment Transmitter Message - El. signal Encoder (compression, correction codes) Modulator Intermediate equipment Improving the quality of communication - (Amplifier) \u200b\u200bCreating a composite channel - (Switch) Channel compression - (Multiplexer) (PA may be absent in LAN)
6 Main characteristics of communication lines Bandwidth (Protocol) Reliability of data transmission (Protocol) Propagation delay Amplitude-frequency response (AFC) Bandwidth Attenuation Noise immunity Near-end crosstalk Specific cost
9 Attenuation A - one point on the frequency response A \u003d log 10 Pout / Pin Bel A \u003d 10 log 10 Pout / Pin deciBel (dB) A \u003d 20 log 10 Uout / Uin deciBel (dB) q Example 1: Pin \u003d 10 mW, Pout \u003d 5 mW Attenuation \u003d 10 log 10 (5/10) \u003d 10 log 10 0.5 \u003d - 3 dB q Example 2: UTP cat 5 Attenuation\u003e \u003d -23.6 dB F \u003d 100MHz, L \u003d 100 M Usually A is indicated for the fundamental frequency of the signal. \u003d -23.6 dB F \u003d 100MHz, L \u003d 100 M Usually A is indicated for the fundamental frequency of the signal "\u003e 
11 Immunity Fiber optic cable lines Wire overhead lines Radio lines (Shielding, twisting) Immunity to external interference Immunity to internal interference Near-end crosstalk attenuation (NEXT) Far-end crosstalk attenuation (FEXT) (FEXT - Two pairs in one direction)
12 Near End Cross Talk loss (NEXT) For multi-pair cables NEXT \u003d 10 log Pout / Pout dB NEXT \u003d NEXT (L) UTP 5: NEXT 
13 Data transmission reliability Bit Error Rate - BER Probability of data bit corruption Causes: external and internal interference, narrow bandwidth Fight: increased noise immunity, reduced pickup NEXT, increased bandwidth Twisted pair BER ~ Fiber-optic cable BER ~ No 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 STP shielded Twisted Pair Types Type 1 ... 9 Each pair has its own shield Each pair has its own step twists, own color Noise immunity Cost Complexity of laying
18 Fiber Optics Total internal reflection of the beam at the interface between two media n1\u003e n2 - (refractive index) n1 n2 n2 - (refractive index) n1 n2 "\u003e n2 - (refractive index) n1 n2"\u003e n2 - (refractive index) n1 n2 "title \u003d" (! LANG: 18 Fiber Optics Total internal reflection of the beam at the interface between two media n1\u003e n2 - (refractive index) n1 n2"> title="18 Fiber Optics Total internal reflection of the beam at the interface between two media n1\u003e 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 \u003d 250 μm 1 GHz - 100 km BaseLH5000 km - 1 Gbps (2005) MMSM
23 Optical signal sources Channel: source - carrier - receiver (detector) Sources LED (LED- Light Emitting Diod) nm incoherent source - MMF Semiconductor laser coherent source - SMF - Power \u003d f (t o) Detectors Photodiodes, pin diodes, avalanche diodes
25 Structured cabling systems - SCS Structured Cabling System - SCS The first LAN - various cables and topologies Unification of the SCS cabling system - open cabling LAN 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 (basic) EIA / TIA-568A Commercial Building Telecommunications Wiring Standard (USA) CENELEC EN50173 Performance Requirements of Generic Cabling Schemes (Europe) ISO / IEC IS Information Technology - Generic cabling for customer premises cabling For each subsystem: Data transmission medium ... Topology Allowable distances (cable lengths) User connection interface. Cables and connecting equipment. Bandwidth (Performance). Installation practice (Horizontal subsystem - UTP, star, 100 m ...)
28 Wireless Transmission Advantages: good, inaccessible areas, mobility. fast deployment ... Disadvantages: high level of interference (special means: codes, modulation ...), the complexity of using some bands Communication line: transmitter - medium - receiver LAN characteristics ~ F (Δf, fн);
34 2. Cellular telephony Territory division into cells Frequency reuse Low power (dimensions) In the center - base station Europe - Global System for Mobile - GSM Wireless telephony 1. Low-power radio station - (handset-base, 300m) DECT Digital European Cordless Telecommunication Roaming - switching from one core network to another - the backbone of cellular
35 Satellite communications Basically - satellite (reflector-amplifier) \u200b\u200bTransceivers - transponders H ~ 50 MHz (1 satellite ~ 20 transponders) Frequency ranges: С. Ku, Ka C - Down 3.7 - 4.2 GHz Up 5.925-6.425 GHz Ku - Down 11.7-12.2 GHz Up 14.0-14.5 GHz Ka - Down 17.7-21.7 GHz Up 27.5-30.5 GHz
36 Satellite communications. Types of satellites Satellite communication: microwaves - line of sight Geostationary Large coverage Immobility, Low wear Satellite repeater, broadcast, low cost, cost does not depend on distance, Instant connection (Mil) Tz \u003d 300ms Low security, Initially large antenna (but VSAT) Mid-orbit km Global Positioning System GPS - 24 satellites LEO km low coverage low latency Internet access
40 Spread spectrum technique Special modulation and coding techniques for wireless communication С (Bit / s) \u003d Δ F (Hz) * log2 (1 + Ps / P N) Power reduction Noise immunity Stealth OFDM, FHSS (, Blue-Tooth), DSSS, CDMA
Physicalthe level deals with the actual transmission of raw bits over
communication channel.
Data transfer in computer networks from one computer to another is carried out sequentially, bit by bit. Physically, data bits are transmitted over data channels as analog or digital signals.
The set of means (communication lines, equipment for transmitting and receiving data), used to transmit data in computer networks, is called a data transmission channel. Depending on the form of the transmitted information, data transmission channels can be divided into analog (continuous) and digital (discrete).
Since the equipment for transmitting and receiving data operates with data in a discrete form (i.e., discrete electrical signals correspond to ones and zeros of data), then when they are transmitted through an analog channel, conversion of discrete data into analog (modulation) is required.
When receiving such analog data, the reverse conversion is necessary - demodulation. Modulation / demodulation - The processes of converting digital information into analog signals and vice versa. During modulation, information is represented by a sinusoidal signal of the frequency that the data channel transmits well.
Modulation methods include:
· Amplitude modulation;
· Frequency modulation;
· Phase modulation.
When transmitting discrete signals through a digital data transmission channel, coding is used:
· Potential;
· Pulse.
Thus, potential or impulse coding is used on high quality channels, and modulation based on sinusoidal signals is preferable in cases where the channel introduces strong distortions in the transmitted signals.
Typically, modulation is used in wide area networks to transmit data over analog telephone circuits, which were designed to carry voice in analog form and are therefore poorly suited for direct transmission of pulses.
Depending on the methods of synchronization, data transmission channels of computer networks can be divided into synchronous and asynchronous. Synchronization is necessary so that the transmitting data node can send some kind of signal to the receiving node, so that the receiving node knows when to start receiving incoming data.
Synchronous data transmission requires an additional communication line to transmit clock pulses. The transmission of bits by the transmitting station and their reception by the receiving station is carried out at the moments of the appearance of sync pulses.
An additional communication line is not required for asynchronous data transmission. In this case, data transfer is carried out in blocks of fixed length (bytes). Synchronization is carried out by additional bits (start bits and stop bits), which are transmitted before and after the transmitted byte.
When exchanging data between nodes of computer networks, three methods of data transfer are used:
simplex (unidirectional) transmission (television, radio);
half-duplex (reception / transmission of information is carried out in turn);
duplex (bi-directional), each node simultaneously transmits and receives data (for example, telephone conversations).
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