Lectures Computer networks. Physical layer
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 works 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 pulse coding is applied on channels high Quality, and modulation based on sinusoidal signals is preferable in cases where the channel introduces strong distortions in the transmitted signals.
Usually modulation is used in global networks when transmitting data over analog telephone communication channels, which were developed for the transmission of voice in analog form and therefore are 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 alternately);
duplex (bi-directional), each node simultaneously transmits and receives data (for example, telephone conversations).
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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 results in a spectrum of the resulting signal with a much smaller width than with digital coding, at the same information transfer rate, but its implementation requires more complex and expensive equipment.
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 channels with a narrow frequency band, a typical representative of which is the channel tone frequencyprovided to users of telephone networks. This channel transmits signals with a frequency of 300 to 3400 Hz, i.e., its bandwidth is 3100 Hz. This bandwidth is quite sufficient 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 transmission rate is provided by the use of combined modulation methods, more often amplitude in combination with phase.
The analog method of transmitting discrete data provides wideband transmission by using signals of different carrier frequencies in 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 is represented either by pulses of a certain polarity, or by potential drops in a certain direction.
The following requirements are imposed on the methods of digital coding of discrete information when using rectangular pulses to represent binary signals:
Ensuring synchronization between transmitter and receiver;
Providing the smallest spectrum width of the resulting signal at the same bit rate (since a narrower spectrum of signals allows for
with the same bandwidth to achieve higher speed
data transmission);
The ability to recognize errors in the transmitted data;
Relatively low cost of implementation.
By means of the physical layer, only the recognition of distorted data (error detection) is carried out, which saves time, since the receiver, without waiting for the complete placement of the received frame in the buffer, immediately rejects it when recognizing erroneous bits in the frame. A more complex operation - correction of corrupted data - is performed by higher-level protocols: channel, network, transport, or application.
Synchronizing the transmitter and receiver is necessary so that the receiver knows exactly when to read the incoming data. Synchronization tune the receiver to the transmitted message and keep the receiver in sync with the incoming data bits. The synchronization problem is easily solved when 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 clocking pulses for two reasons: for the sake of saving conductors in expensive cables and because of the inhomogeneity of the characteristics of 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 be either skipped or re-read).
Currently, the synchronization of the transmitter and receiver in networks is achieved by using self-synchronizing codes (SK). The coding of the transmitted data using the SC is to ensure regular and frequent changes (transitions) of the levels of the information signal in the channel. Each transition of the signal level from high to low or vice versa is used to trim the receiver. The best ones are considered to be those that ensure the transition of the signal level at least once during the time interval required to receive one information bit. The more frequent the signal level transitions, the more reliably the receiver synchronizes and the more confidently the received data bits are identified.
The specified requirements for digital coding methods for discrete information are to a certain extent mutually contradictory, therefore, each of the coding methods discussed below has its own advantages and disadvantages compared to others.
Self-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 the received information bits;
The required rate of change in the signal level in the communication line when using the SC, if the line capacity is specified;
The complexity (and, therefore, the cost) of the equipment that implements the IC.
NRZ code is easy to code and low cost of implementation. It got this name because when transmitting a series of bits of the same name (ones or zeros), the signal does not return to zero during a clock cycle, as is the case in other encoding methods. The signal level remains unchanged for each series, which significantly reduces the quality of synchronization and the reliability of recognition of the received bits (the receiver timer may mismatch with respect to the incoming signal and untimely polling of lines).
For the L ^ -code, the following relations hold:
where VI is the rate of change 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 the same bits or alternately changing bits are transmitted. One is represented by a pulse of one polarity, and zero is the other. Each impulse lasts half a beat. Such a code has excellent self-synchronizing properties, but the cost of its implementation is quite high, since it is necessary to ensure the ratio
The spectrum of the RZ code is wider than that of the potential codes. Due to its too wide spectrum, it is rarely used.
The Manchester code provides a change in the signal level at the presentation of each bit, and when transmitting a series of bits of the same name - a double change. Each measure is divided into two parts. The information is encoded by potential drops that occur in the middle of each cycle. One is coded by the slope from low to high signal level, and zero is coded by the reverse slope. The speed ratio for this code is as follows:
The Manchester code 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, only two states are stably recognized in optical cables - 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 positive potential or negative. 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 the line, using the AMI code leads to a narrower signal spectrum than for the NRZ code, and therefore to a higher bandwidth lines.
Note that improved potential codes (modernized Manchester code and AMI code) have a narrower spectrum than pulsed ones, therefore they are used in high-speed technologies, for example, in FDDI, Fast Ethernet, Gigabit Ethernet.
Discrete modulation of analog signals. As already noted, one of the development trends 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 position of which is: if an analog signal is displayed (i.e., represented as a sequence of its discrete time values) on a regular interval with a frequency of at least twice the frequency of the highest harmonic spectrum of the original continuous signal, the display will contain information sufficient to restore the original signal. In analog telephony, the range from 300 to 3400 Hz is selected for voice transmission, which is sufficient for high-quality transmission of all the fundamental harmonics of the interlocutors. Therefore, in digital networks, where the PCM method is implemented for voice transmission, a display frequency of 8000 Hz is adopted (this is more than 6800 Hz, which provides a certain quality margin).
At the quantization stage, each IAM signal is assigned a quantized value corresponding to the nearest quantization level. The entire range of changes in the amplitude of the IAM signals is divided into 128 or 256 quantization levels. The more quantization levels, the more accurate the IAM amplitude - the signal is represented by the quantized level.
At the coding 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 the 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 bit width of the binary code), and when encoding 8 element codes - 64 Kbps. 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, and the original continuous function of time is restored.
In modern digital communication networks, other methods of discrete modulation are used, which make it possible to represent voice measurements in a more compact form, for example, as 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, then the voice quality can be quite high. However, in computer networks, frames can be delayed both at end nodes and in intermediate switching devices (bridges, switches, routers), which negatively affects the quality of voice transmission. Therefore, for high-quality transmission of digitized continuous signals, special digital networks (ISDN, ATM, networks digital television), although Frame Relay networks are still used to transfer intra-corporate telephone conversations, since frame transmission delays in them are within acceptable limits.
Topic 2. Physical layer
Plan
Theoretical foundations of data transmission
Information can be transmitted over wires by changing some physical quantity, such as voltage or current. By representing the value of voltage or current as a single-valued function of time, you can simulate the behavior of the signal and subject it to mathematical analysis.Fourier series
At the beginning of the 19th century, the French mathematician Jean-Baptiste Fourier proved that any periodic function with period T can be expanded in a series (possibly infinite), consisting of the sums of sines and cosines:(2.1)
where is the fundamental frequency (harmonic), and are the amplitudes of the sines and cosines of the n-th harmonic, and c is a constant. Such an expansion is called a Fourier series. The function expanded in a Fourier series can be reconstructed from the elements of this series, that is, if the period T and the amplitudes of the harmonics are known, then the original function can be reconstructed using the sum of the series (2.1).
An information signal with a finite duration (all information signals have a finite duration) can be expanded into a Fourier series if we imagine that the entire signal repeats infinitely over and over again (that is, the interval from T to 2T completely repeats the interval from 0 to T, and etc.).
Amplitudes can be calculated for any given function. To do this, you need to multiply the left and right sides of equation (2.1) by, and then integrate from 0 to T. Since:
(2.2)
only one member of the series remains. The row disappears completely. Similarly, multiplying equation (2.1) by and integrating over time from 0 to T, you can calculate the values. If we integrate both sides of the equation without changing it, then we can get the value of the constant from... The results of these actions will be as follows:
(2.3.)
Managed storage media
The purpose of the physical layer of a network is to transfer a raw bitstream from one machine to another. For transmission, various physical media can be used, also called signal propagation media. Each has a distinctive set of bandwidths, latencies, prices, and ease of installation and use. Media can be divided into two groups: managed media, such as copper wire and fiber optic cable, and unmanaged media, such as radio communication and cableless laser transmission.Magnetic media
One of the easiest ways to transfer data from one computer to another is to write it to magnetic tape or other removable media (such as a rewritable DVD), physically transfer those tapes and discs to their destination, and read them there.High throughput. A standard Ultrium tape cassette holds 200 GB. A 60x60x60 box holds about 1000 of these cassettes, giving a total storage capacity of 1600 Tbit (1.6 Pbit). A box of cassettes can be shipped within the United States within 24 hours by Federal Express or another company. The effective bandwidth for this transmission is 1600 Tbps / 86 400 s, or 19 Gbps. If the destination is only an hour away, then the throughput will be over 400 Gbps. No computer network is yet able to even come close to such indicators.
Profitability. The wholesale price of the cassette is about $ 40. A box of ribbons will cost $ 4000, and the same ribbon can be used dozens of times. Add $ 1000 for transportation (and in fact, much less) and get about $ 5000 for the transfer of 200 TB or 3 cents per gigabyte.
Disadvantages. Although the data transfer rate with magnetic tapes is excellent, the latency is very high. Transmission times are measured in minutes or hours, not milliseconds. Many applications require immediate response from the remote system (connected mode).
Twisted pair
A twisted pair consists of two insulated copper wires, with a typical diameter of 1 mm. The wires are wound around one another in a spiral. This reduces the electromagnetic interference of several adjacent twisted pairs.Application - telephone line, computer network. It can transmit a signal without attenuation of power over a distance of several kilometers. For longer distances, repeaters are required. They are combined into a cable with a protective coating. The cable has a pair of twisted wires to avoid signal overlapping. They can be used to transfer both analog and digital data. The bandwidth depends on the diameter and length of the wire, but in most cases a speed of several megabits per second can be achieved over distances of up to several kilometers. Due to their relatively high bandwidth and low price, twisted pairs are widespread and, most likely, will be popular in the future.
Twisted pairs are used in several variants, two of which are especially important in the field of computer networks. Category 3 (CAT 3) twisted pairs consist of two insulated wires twisted together. Four of these pairs are usually placed together in a plastic wrap.
Category 5 (CAT 5) twisted pairs are similar to Category 3 twisted pairs, but have more turns per centimeter of wire length. This makes it possible to further reduce crosstalk between different channels and provide improved signal transmission quality over long distances (Fig. 1).
Figure: 1. UTP category 3 (a), UTP category 5 (b).
All of these types of connections are often referred to as UTP (unshielded twisted pair - unshielded twisted pair)
Twisted pair shielded cables from IBM have not become popular outside of IBM.
Coaxial cable
Coaxial cable is another common data transmission medium. It is better shielded than twisted pair, so it can transfer data over longer distances at higher speeds. Two types of cables are widely used. One of them, 50-ohm, is usually used for purely digital data transmission. Another type of cable, 75-ohm, is often used to transmit analog information, as well as in cable television.A cross-sectional view of the cable is shown in Figure 2.
Figure: 2. Coaxial cable.
The design and special type of shielding of the coaxial cable provide high bandwidth and excellent noise immunity. The maximum throughput depends on the quality, length and signal-to-noise ratio of the line. Modern cables have a bandwidth of about 1 GHz.
Application - telephone systems (highways), cable television, regional networks.
Fiber optics
The existing fiber-optic technology can develop data transfer rates up to 50,000 Gbps (50 Tbps), and at the same time many specialists are busy looking for better materials. Today's 10 Gbps practical limit is due to the inability to convert electrical signals to optical signals and vice versa faster, although in laboratory conditions the speed of 100 Gbps has already been achieved on single fiber.A fiber optic data transmission system consists of three main components: a light source, a carrier through which the light signal propagates, and a signal receiver or detector. The light pulse is taken as one, and the absence of the pulse is taken as zero. The light travels in an ultra-thin glass fiber. When light hits it, the detector generates an electrical pulse. By attaching a light source to one end of an optical fiber and a detector to the other, a unidirectional data transmission system is obtained.
When transmitting a light signal, the property of reflection and refraction of light is used when passing from 2 media. Thus, when light is supplied at a certain angle to the interface between the media, the light beam is completely reflected and locked in the fiber (Fig. 3).
Figure: 3. Property of light refraction.
There are 2 types of fiber optic cable: multimode - transmits a beam of light, single-mode - thin to the limit of several wavelengths, acts almost like a waveguide, light travels in a straight line without reflection. Today's single-mode fiber can operate at 50 Gbps over distances of up to 100 km.
Three wavelength ranges are used in communication systems: 0.85, 1.30 and 1.55 microns, respectively.
The structure of a fiber optic cable is similar to that of a coaxial wire. The only difference is that there is no screening mesh in the former.
In the center of the fiber optic core is a glass core through which light propagates. In multimode fiber, the core diameter is 50 microns, which is about the thickness of a human hair. The core in single-mode fiber has a diameter of 8 to 10 microns. The core is covered with a glass layer with a lower refractive index than the core. It is designed to more reliably prevent light from escaping outside the core. The outer layer is a plastic shell that protects the glazing. Fiber optic conductors are usually bundled in bundles, protected by an outer jacket. Figure 4 shows a three-core cable.
Figure: 4. Three-core fiber optic cable.
In the event of a break, the connection of cable sections can be carried out in three ways:
- A special connector can be attached to the end of the cable, with which the cable is inserted into an optical outlet. The loss is 10-20% of the luminous intensity, but it makes it easy to change the system configuration.
Splicing - two neatly cut ends of the cable are laid next to each other and clamped with a special sleeve. Improved light transmission is achieved by aligning the cable ends. Loss - 10% of light power.
Fusion. There is practically no loss.
Table 1.
LED vs Semiconductor Laser Comparison Chart
The receiving end of the optical cable is a photodiode that generates an electrical pulse when light is incident on it.
Comparative characteristics of fiber optic cable and copper wire.
Optical fiber has several advantages:- High speed.
Less signal attenuation, fewer repeaters output (one per 50km, not 5)
Inert to external electromagnetic radiation, chemically neutral.
Lighter in weight. 1000 twisted copper pairs 1 km long weigh about 8000 kg. A pair of fiber optic cables weighs only 100 kg with more bandwidth
Low installation costs
- Complexity and competence during installation.
Fragility
More expensive than copper.
simplex transmission, a minimum of 2 cores are required between the networks.
Wireless connection
Electromagnetic spectrum
The movement of electrons generates electromagnetic waves that can propagate through space (even in a vacuum). The number of electromagnetic oscillations per second is called frequency, and is measured in hertz. The distance between two successive highs (or lows) is called the wavelength. This value is traditionally indicated by the Greek letter (lambda).If an antenna of a suitable size is included in the electrical circuit, then electromagnetic waves can be successfully received by the receiver at a certain distance. All wireless communication systems are based on this principle.
In a vacuum, all electromagnetic waves travel at the same speed, regardless of their frequency. This speed is called the speed of light, - 3 * 108 m / s. In copper or glass, the speed of light is about 2/3 of this value, in addition, it is slightly dependent on frequency.
Relationship between quantities, and:
If the frequency () is measured in MHz, and the wavelength () is in meters then.
The totality of all electromagnetic waves forms the so-called continuous spectrum of electromagnetic radiation (Fig. 5). Radio, microwave, infrared, and visible light can be used to transmit information using amplitude, frequency, or phase modulation of waves. Ultraviolet, X-ray and gamma rays would be even better because of their high frequencies, but they are difficult to generate and modulate, they do not penetrate buildings well and, in addition, they are dangerous to all life. The official names of the ranges are shown in Table 6.
Figure: 5. Electromagnetic spectrum and its application in communication.
Table 2.
Official band names by ITU
The amount of information that an electromagnetic wave can carry is related to the frequency range of the channel. Modern technology makes it possible to encode several bits per hertz at low frequencies. Under some conditions, this number can increase eightfold at high frequencies.
Knowing the width of the wavelength range, you can calculate the corresponding frequency range and data rate.
Example: For a 1.3 micron range fiber optic cable, then. Then at 8 bit / s it is possible to get a transfer rate of 240 Tbit / s.
Radio communication
Radio waves are easy to generate, travel long distances, pass through walls, bend around buildings, and travel in all directions. The property of radio waves depends on the frequency (Fig. 6). When operating at low frequencies, radio waves pass well through obstacles, but the signal strength in the air drops sharply as you move away from the transmitter. The ratio of power and distance from the source is expressed approximately like this: 1 / r2. At high frequencies, radio waves generally tend to travel in a purely straight line and bounce off obstacles. In addition, they are absorbed, for example, by rain. Radio signals of all frequencies are susceptible to interference from spark brush motors and other electrical equipment.Figure: 6. Waves of the VLF, LF, MF bands go around the roughness of the earth's surface (a), the waves of the HF and VHF bands are reflected from the ionosphere, absorbed by the earth (b).
Microwave communication
At frequencies above 100 MHz, radio waves travel almost in a straight line, so they can be focused into narrow beams. The concentration of energy in the form of a narrow beam using a parabolic antenna (like the well-known satellite television dish) leads to an improvement in the signal-to-noise ratio, however, for such a connection, the transmitting and receiving antennas must be fairly accurately directed towards each other.Unlike radio waves with lower frequencies, microwaves do not penetrate buildings well. Microwave radio communication has become so widely used in long distance telephony, cell phones, television broadcasting and other fields that the lack of spectrum bandwidth has begun to be felt.
This connection has a number of advantages over fiber optics. The main one is that there is no need to lay a cable, therefore, there is no need to pay for land lease on the signal path. It is enough to buy small plots of land every 50 km and install relay towers on them.
Infrared and millimeter waves
Infrared and millimeter-wave radiation without the use of a cable is widely used for communication over short distances (for example, remote controls). They are relatively directional, cheap and easy to install, but do not go through solid objects.Infrared communication is used in desktop computing systems (for example, to connect laptops to printers), but it does not play a significant role in telecommunications.
Communication satellites
The types of satellites used are geostationary (GEO), medium-altitude (MEO) and low-earth orbit (LEO) (Fig. 7).Figure: 7. Communication satellites and their properties: orbital altitude, delay, the number of satellites required to cover the entire surface of the globe.
Public switched telephone network
Telephone system structure
The structure of a typical medium-haul telephony route is shown in Figure 8.Figure: 8. Typical communication route with an average distance between subscribers.
Local lines: modems, ADSL, wireless
Since the computer works with a digital signal, and the local telephone line is an analog signal transmission, a modem device is used to perform digital to analog conversion and vice versa, and the process itself is called modulation / demodulation (Fig. 9).Figure: 9. Using a telephone line when transmitting a digital signal.
There are 3 modulation methods (Fig. 10):
- amplitude modulation - 2 different signal amplitudes are used (for 0 and 1),
frequency - several different signal frequencies are used (for 0 and 1),
phase - phase shifts are used when switching between logical units (0 and 1). Shear angles - 45, 135, 225, 180.
Figure: 10. Binary signal (s); amplitude modulation (b); frequency modulation (c); phase modulation.
All modern modems allow data transmission in both directions, this mode of operation is called duplex. An alternating connection is called half-duplex. A connection in which only one direction is transmitted is called simplex.
The maximum speed of modems that can be reached at the current moment is 56Kb / s. Standard V.90.
Digital subscriber lines. XDSL technology.
After the speed through modems reached its limit, the telephone companies began to look for a way out of this situation. Thus, a multitude of proposals appeared under the general name xDSL. xDSL (Digital Subscribe Line) - digital subscriber line, where instead of x there may be other letters. The best known technology from these offerings is ADSL (Asymmetric DSL).The reason for limiting the speed of modems was that they used the human speech transmission range for data transmission - 300Hz to 3400Hz. Together with the cutoff frequencies, the passband was not 3100 Hz, but 4000 Hz.
Although the spectrum of the local telephone line itself is 1.1Hz.
The first offer of ADSL technology used the entire spectrum of the local telephone line, which is divided into 3 bands:
- POTS - POTS band;
outgoing range;
incoming range.
An alternative method called Discrete MultiTone (DMT) modulation consists of dividing the entire 1.1 MHz local link spectrum into 256 independent 4312.5 Hz channels. Channel 0 is POTS. Channels 1 through 5 are not used to prevent the voice signal from interfering with the data signal. Of the remaining 250 channels, one is busy controlling transmission towards the provider, one towards the user, and all the others are available for transmission of user data (Fig. 11).
Figure: 11. ADSL operation using discrete multi-tone modulation.
The ADSL standard allows you to receive up to 8 Mb / s, and send up to 1 Mb / s. ADSL2 + - outgoing up to 24Mb / s, incoming up to 1.4 Mb / s.
A typical ADSL hardware configuration contains:
- DSLAM - DSL access multiplexer;
NID is a network interface device that separates the ownership of the telephone company and the subscriber.
Splitter - A splitter separating the POTS band and ADSL data.
Lines and seals
Saving resources plays an important role in the telephone system. The cost of laying and maintaining a high-throughput backbone and a low-quality line is almost the same (that is, the lion's share of this cost goes to digging trenches, and not to the copper or fiber-optic cable itself).For this reason, the telephone companies have jointly developed several schemes for carrying multiple conversations over a single physical cable. Multiplexing schemes (multiplexing) can be divided into two main categories FDM (Frequency Division Multiplexing) and TDM (Time Division Multiplexing) (Fig. 13).
In frequency division multiplexing, the frequency spectrum is divided between logical channels, and each user gets exclusive ownership of his sub-band. In time division multiplexing, users take turns (cyclically) to use the same channel, and each channel is given full bandwidth for a short period of time.
In fiber optic channels, a special version of frequency division is used. It is called Wavelength Division Multiplexing (WDM).
Figure: 13. An example of frequency multiplexing: initial signal spectra 1 (a), frequency-shifted spectra (b), compressed channel (c).
Commutation
From the point of view of the average telephone engineer, a telephone system consists of two parts: external equipment (local telephone lines and trunks, outside the switches) and internal equipment (switches) located at the telephone exchange.Any communication networks support some way of switching (communication) between their subscribers. It is practically impossible to provide each pair of interacting subscribers with their own nonswitched physical communication line, which they could monopoly "own" for a long time. Therefore, any network always uses a subscriber switching method that ensures the availability of the available physical channels simultaneously for several communication sessions between network subscribers.
In telephone systems, two different techniques are used: circuit switching and packet switching.
Channel switching
Circuit switching involves the formation of a continuous concatenated physical channel from sequentially connected individual channel sections for direct data transfer between nodes. In a circuit-switched network, before transmitting data, it is always necessary to perform the connection establishment procedure, during which the composite channel is created (Fig. 14).Packet switching
In packet switching, all messages transmitted by a network user are split at the source node into relatively small parts called packets. Each packet is provided with a header that specifies the address information required to deliver the packet to the destination node, as well as the package number that will be used by the destination node to assemble the message. Packets are transported across the network as independent information units. Network switches receive packets from end nodes and, based on address information, transmit them to each other, and ultimately to the destination node (Fig. 14).etc.................
2 Physical layer functions Bit representation by electrical / optical signals Bit coding Bit synchronization Bit synchronization / reception via physical communication channels Communication with the physical medium Transfer rate Range Signal levels, connectors In all network devices Hardware implementation (network adapters) Example: 10 BaseT - UTP cat 3, 100 ohm, 100m, 10Mbps, MII code, RJ-45
5 Data transmission equipment Transmitter Message - El. signal Encoder (compression, correction codes) Modulator Intermediate equipment Improving the quality of communication - (Amplifier) \u200b\u200bCreating a composite channel - (Switch) Channel compression - (Multiplexer) (PA may not be available in LAN)
6 Main characteristics of communication lines Throughput (Protocol) Reliability of data transmission (Protocol) Propagation delay Amplitude-frequency response (AFC) Bandwidth Attenuation Noise immunity 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 cables 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 beam reflection 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 beam reflection at the interface between two media n1\u003e n2 - (refractive index) n1 n2"> title="18 Fiber Optics Total internal beam reflection 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. - 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. rapid deployment ... Disadvantages: high level of interference (special means: codes, modulation ...), the complexity of using some ranges 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 techniques Special modulation and coding techniques for wireless С (Bit / s) \u003d Δ F (Hz) * log2 (1 + Ps / P N) Power reduction Noise immunity Stealth OFDM, FHSS (, Blue-Tooth), DSSS, CDMA
7. PHYSICAL LEVEL OF DATA TRANSFER
7.2. Discrete data transfer methods
When transmitting discrete data over communication channels, two main types of physical coding are used - based on a sinusoidal carrier signal and based on a sequence of rectangular pulses. The first method is also often 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 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 leads to a narrower spectrum at the same information transfer rate. However, the implementation of 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 .
Analog modulation is used to transmit discrete data over narrow bandwidth channels - the voice frequency channel (public telephone networks). This channel carries frequencies in the range of 300 to 3400 Hz, thus 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 physical coding method in which information is encoded by changing the amplitude, frequency, or phase of a sinusoidal carrier signal (Fig. 27).
When amplitude modulation (Fig. 27, b) one level of the carrier frequency sinusoid amplitude is selected for a logical unit, and another for a logical zero. This method is rarely used in its pure form in practice due to its low noise immunity, but is often used in combination with another type of modulation - phase modulation.
When frequency modulation (Fig. 27, c) the values \u200b\u200b0 and 1 of the original data are transmitted by sinusoids with different frequencies - f 0 and f 1,. This modulation method does not require complex circuitry in modems and is usually used in low speed modems operating at 300 or 1200 bps.
When phase modulation (Fig. 27, d) data values \u200b\u200b0 and 1 correspond to signals of the same frequency, but with 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.
Figure: 27. Various types modulation
The spectrum of the resulting modulated signal depends on the type and rate of modulation.
For potential coding, the spectrum is directly derived from the Fourier formulas for the periodic function. If discrete data is transmitted at a bit rate of N bits / 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 \u003d N / 2. The amplitudes of these harmonics decrease rather slowly - with the coefficients 1/3, 1/5, 1/7, ... from the amplitude of the harmonic f 0 (Fig. 28, a). As a result, the spectrum of a potential code requires a wide bandwidth for high-quality transmission. In addition, it should 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 signal of the potential code when transmitting 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 tone channel, the potential coding upper bound is reached for a data rate of 971 bps. As a result, the candidate codes on the tone channels are never used.
With amplitude modulation, the spectrum consists of a sinusoidal carrier frequency f with 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 with this coding method. At a low modulation frequency, the signal spectrum width will also be small (equal to 2f m ), so signals will not be distorted by the line if its bandwidth is greater than or equal to 2f m ... For a tone frequency channel, this modulation method is acceptable at a data rate not exceeding 3100/2 \u003d 1550 bps. If 4 amplitude levels are used to represent data, then the channel bandwidth increases to 3100 bit / s.
Figure: 28. Signal spectra in 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 with respect to the main carrier frequency, and their amplitudes rapidly decrease. Therefore, these modulations are also well suited for voice channel data transmission.
When digital coding of discrete information, potential and pulse codes are used. In potential codes, only the signal potential value is used to represent logical ones and zeros, and its differences 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 to transmit discrete information, it is necessary to choose a coding method that would simultaneously achieve several goals:
· had the smallest spectrum width of the resulting signal at the same bit rate;
· provided synchronization between transmitter and receiver;
· had the ability to recognize errors;
· had a low implementation cost.
A narrower spectrum of signals allows achieving higher data rates on the same line. The signal spectrum is often required to have no DC 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 in networks is more difficult to solve than when exchanging data between nearby devices, for example, between units inside a computer or between a computer and a printer. Therefore, in networks, so-called self-synchronizing codes are used, the signals of which carry for the transmitter an indication of at what point in time it is necessary to recognize the next bit (or several bits). Any sudden drop in the signal — called a front — can be a good indication for synchronizing the receiver with the transmitter.
When using sinusoids as a carrier signal, the resulting code has the property of self-synchronization, since a change in the carrier frequency amplitude enables the receiver to determine the moment when the input code appears.
The requirements for coding methods are mutually contradictory, therefore, each of the popular digital coding methods discussed below has its own advantages and disadvantages compared to others.
In fig. 29, a shows a potential coding method, also called coding no return to zero (Non Return to Zero, NRZ) ... The latter name reflects the fact that when a sequence of ones is transmitted, the signal does not return to zero during a cycle. 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 unable to determine the times when it is necessary to read the data from the input signal. 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 of clock frequencies can lead to an error in a whole cycle and, accordingly, reading an incorrect bit value.
Another serious drawback of the NRZ method is the presence of a low-frequency component, which approaches zero when transmitting long sequences of ones or zeros. Because of this, many communication channels that do not provide a direct galvanic connection between the receiver and the source do not support this type of coding. As a result, the NRZ code is not used in its pure form 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 start improving 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.
Figure: 29. Methods of discrete data coding
One of the modifications of the NRZ method is the method bipolar coding with alternate inversion (Bipolar Alternate Mark Inversion, AMI). This method (Fig. 29, b) 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.
The AMI code partially eliminates the DC and self-timing problems inherent in the NRZ code. This occurs when transmitting long sequences of ones. In these cases, the signal on the line is a sequence of bipolar pulses with the same spectrum as the NRZ code, which transmits alternating zeros and ones, that is, without a DC component and with a fundamental harmonic of N / 2 Hz (where N is the bit data 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 different bit combinations on the line, the use of the AMI code results in a narrower signal spectrum than for the NRZ code, and therefore higher line capacity. For example, when transmitting alternating ones and zeros, the fundamental f 0 has a frequency of N / 4 Hz. The AMI code also provides some capabilities for recognizing erroneous signals. Thus, a violation of the strict alternation of the polarity of the signals indicates a false pulse or the disappearance of the correct pulse from the line. This signal is called prohibited signal (signal violation).
The AMI code uses not two, but three signal levels on the line. The additional layer requires an increase in the transmitter power by about 3 dB 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.
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 (Not 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 shadow - are steadily recognized.
In 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 a pulse of one polarity, and zero is represented by another (Fig. 29, c). Each impulse lasts half a beat. This code has excellent self-synchronizing properties, but the constant component may be present, for example, when transmitting a long sequence of ones or zeros. In addition, its spectrum is wider than that of potential codes. Thus, when transmitting all zeros or ones, the frequency of the fundamental harmonic of the code will be equal to N Hz, which is 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.
IN local area networks until recently, the most common coding method was the so-called manchester code (Fig. 29, d). It is used in Ethernet and Token Ring technologies.
The Manchester code uses 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 coded by the slope from low to high signal level, and zero is coded by the reverse slope. At the beginning of each measure, an overhead signal can occur if several ones or zeros are to be represented in a row. Since the signal changes at least once per cycle of transmission 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 at best (when transmitting alternating ones and zeros) it is equal to N / 2 Hz, like 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 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.
In fig. 29, e shows a potential code with four signal levels for encoding data. This is a 2B1Q code, the name of which reflects its essence - every two bits (2B) are transmitted in one clock cycle by a signal that has four states (1Q). A pair of bits 00 corresponds to a potential of -2.5 V, a pair of bits 01 corresponds to a potential of -0.833 V, a pair 11 corresponds to a potential of +0.833 V, and a pair 10 corresponds to a potential of +2.5 V. This coding method requires additional measures to combat long sequences of identical pairs of bits, since in this case the signal turns 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 2B1Q code, you can transmit data over the same line two times faster 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.
