Communication line. Communication lines Physical data transmission medium

Baud rate(information rate or throughput, both English terms are used interchangeably) is the actual rate of the data stream passing through the network. This speed can be less than the suggested load speed, as data on the network can be corrupted or lost.

The capacity of a communication channel (capacity), also called bandwidth, represents the maximum possible data transfer rate over the channel.

The specificity of this characteristic is that it reflects not only the parameters of the physical transmission medium, but also the features of the selected method of transmitting discrete information over this medium.

For example, the capacity of a communication channel in an Ethernet on an optical fiber is 10 Mbps. This speed is the fastest possible for a combination of Ethernet and optical fiber technology. However, for the same optical fiber, it is possible to develop another data transmission technology that differs in the data coding method, clock frequency and other parameters, which will have a different capacity. Thus, Fast Ethernet technology provides data transmission over the same optical fiber with a maximum speed of 100 Mbit / s, and Gigabit Ethernet technology - 1000 Mbit / s. Transmitter communication device must operate at a speed equal to the bandwidth of the channel. This speed sometimescalled the bit rate of transmitter.

Bandwidth- this term can be misleading because it is used with two different meanings.

At first , with its help can characterize the transmission medium. In this case, it means the bandwidth that the line transfers without material misstatement. The origin of the term is clear from this definition.

Secondly , the term "bandwidth" is used synonymously with the term "communication channel capacity "... In the first case, the bandwidth is measured in hertz (Hz), in the second, in bits per second. It is necessary to distinguish the meanings of this term by context, although sometimes it is quite difficult. Of course, it would be better to use different terms for different characteristics, but there are traditions that are difficult to change. This double use of the term "bandwidth" has already entered many standards and books, so we will follow the established approach.

It should also be borne in mind that this term in its second meaning is even more common than capacity, so from the two synonyms we will use bandwidth.

Another group of characteristics of a communication channel is associated with the ability to transmit information over the channel to one or both sides.

When two computers interact, it is usually required to transfer information in both directions, from computer A to computer B and vice versa. Even in the case when it seems to the user that he only receives information (for example, downloads a music file from the Internet) or transmits (sends email), the exchange of information goes in two directions. There is simply a main stream of data that interests the user, and an auxiliary stream of the opposite direction, which form receipts of this data.

Physical communication channels are divided into several types depending on whether they can transmit information in both directions or not.

Duplex channelprovides simultaneous transmission of information in both directions. A duplex channel can consist of two physical media, each of which is used to transmit information in only one direction. A variant is possible when one medium serves for the simultaneous transmission of counter streams, in this case additional methods of separating each stream from the total signal are used.

Half duplex channelalso provides information transfer in both directions, but not simultaneously, but in turn. That is, during a certain period of time, information is transmitted in one direction, and during the next period - in the opposite direction.

Simplex channelallows information to be transmitted in only one direction. Often, a duplex link consists of two simplex links.

Communication lines

When building networks, communication lines are used in which various physical media are used: telephone and telegraph wires suspended in the air, laid underground and along the ocean floor, copper coaxial and fiber-optic cables, entangling all modern offices, copper twisted pairs, all penetrating radio waves

Consider the general characteristics of communication lines, independent of their physical nature, such as

Bandwidth,

throughput,

Immunity and

Reliability of transmission.

The width of the line transmission is a fundamental characteristic of a communication channel, since it determines the maximum possible information rate of the channel, whichcalled the channel bandwidth.

The Nyquist formula expresses this dependence for an ideal channel, and Shannon's formula takes into account the presence of noise in a real channel.

Classification of communication lines

When describing a technical system that transfers information between network nodes, several names can be found in the literature:

communication line,

compound channel,

channel,

Link.

Often these terms are used interchangeably, and in many cases this is not a problem. At the same time, there is also a specificity in their use.

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

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

CircuitIs the path between the two end nodes of the network. A spliced ​​link is formed by separate intermediate links and interconnects in switches. Often the epithet "composite" is omitted and the term "channel" is used to refer to both a composite channel and a channel between adjacent nodes, that is, within a link.

Communication line can be used synonymously for any of the other three terms.

Don't be too strict about the confusion of terminology. This is especially true for the differences in the terminology of traditional telephony and a newer area - computer networks. The convergence process only exacerbated the problem of terminology, since many of the mechanisms of these networks became common, but retained a couple (sometimes more) names from each area.

In addition, there are objective reasons for an ambiguous understanding of terms. In fig. 8.1 shows two options for a communication line. In the first case (Fig. 8.1, a), the line consists of a cable segment several tens of meters long and is a link.

In the second case (Fig. 8.1, b), the communication line is a composite channel deployed in a circuit-switched network. Such a network can be a primary network or a telephone network.

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

Primary networks are specially created in order to provide services of data transmission channels for computer and telephone networks, about which in such cases they say that they work "on top" of the primary networks and are superimposed networks.

Communication line characteristics

You and I need to understand such concepts as: harmonic, spectral decomposition (spectrum) of the signal,signal spectrum width, Fourier formulas, external interference, internalinterference, or interference, signal attenuation, linear attenuation, window
transparency, absolute power level, relative level
power, receiver sensitivity threshold, wave impedance,
line immunity, electrical connection, magnetic connection,
induced signal, near-end crosstalk, crosstalk
far-end interference, cable protection, transmission reliability
data, bit error rate, bandwidth, bandwidth
ability, physical, or linear, encoding, carrier signal,
carrier frequency, modulation, clock, baud.

Let's get started.

Spectral analysis of signals on communication lines

An important role in determining the parameters of communication lines is assigned to the spectral decomposition of the signal transmitted over this line. It is known from the theory of harmonic analysis that any periodic process can be represented as a sum of sinusoidal oscillations of different frequencies and different amplitudes (Fig. 8.3).

Each component of a sinusoid is also called a harmonic, and the set of all har-
Monique is called the spectral decomposition, or spectrum, of the original signal.

The width of the signal spectrum is the difference between the maximum and minimum frequencies of the set of sinusoids that add up to the original signal.

Non-periodic signals can be represented as an integral of sinusoidal signals with a continuous spectrum of frequencies. In particular, the spectral decomposition of an ideal pulse (unit power and zero duration) has components of the entire frequency spectrum, from -oo to + oo (Fig. 8.4).

The technique for finding the spectrum of any source signal is well known. For some signals that are described analytically (for example, for a sequence of rectangular pulses of the same duration and amplitude), the spectrum is easily calculated based on Fourier formulas.

For signals freeform found in practice, the spectrum can be found using special instruments - spectrum analyzers, which measure the spectrum of a real signal and display the amplitudes of the harmonic components on the screen, print them on a printer or transfer them for processing and storage to a computer.

Distortion of a sinusoid of any frequency by the transmitting line leads, ultimately, to distortion of the amplitude and shape of the transmitted signal of any kind. Distortion occurs when sinusoids of different frequencies are not equally distorted.

If this is an analog signal transmitting speech, then the timbre of the voice changes due to the distortion of overtones - side frequencies. When transmitting pulsed signals typical for computer networks, low-frequency and high-frequency harmonics are distorted, as a result, the pulse fronts lose their rectangular shape (Fig. 8.5) and the signals can be poorly recognized at the receiving end of the line.

The transmitted signals are distorted due to imperfect communication lines. An ideal transmission medium that does not interfere with the transmitted signal should at least have zero resistance, capacitance and inductance. However, in practice, copper wires, for example, always represent some combination of active resistance, capacitive and inductive loads distributed along the length (Fig. 8.6). As a result, sinusoids of different frequencies are transmitted by these lines in different ways.

In addition to signal distortions arising from not ideal physical parameters of the communication line, there are also external interference that contributes to the distortion of the waveform at the line output. This interference is created by various electric motors, electronic devices, atmosphericphenomena, etc. Despite the protective measures taken by the cable designers and the presence of amplifying and switching equipment, it is not possible to fully compensate for the influence of external interference. In addition to external interference in the cable, there are also internal interference - the so-called induction of one pair of conductors to another. As a result, the signals at the output of the communication line canhave a distorted shape (as shown in Figure 8.5).

Attenuation and Characteristic Impedance

The degree of distortion of sinusoidal signals by communication lines is estimated by characteristics such as attenuation and bandwidth. Attenuation shows how much the power of the reference sinusoidal signal at the output of a communication line decreases in relation to the signal power at the input of this line. Attenuation (A) is usually measured in decibels (dB) and is calculated using the following formula:

Here Рout is the signal power at the line output, Рin is the signal power at the line input. Since the attenuation depends on the length of the communication line, the following is used as a characteristic of the communication line:called linear attenuation, that is, attenuation on a communication line of a certain length. For LAN cables, this length is usually 100 m, since this value is the maximum cable length for many LAN technologies. For territorial communication lines, the linear attenuation is measured for a distance of 1 km.

Typically, attenuation is characterized by passive sections of the communication line, consisting of cables and cross sections, without amplifiers and regenerators.

Since the output signal power of the cable without intermediate amplifiers is less than the input signal power, the cable attenuation is always negative.

The degree of attenuation of the power of a sinusoidal signal depends on the frequency of the sinusoid, and this dependence is also used to characterize the communication line (Fig. 8.7).

Most often, when describing the parameters of a communication line, attenuation values ​​are given for only a few frequencies. This is due, on the one hand, to the desire to simplify measurements when checking the quality of the line. On the other hand, in practice, the fundamental frequency of the transmitted signal is often known in advance, that is, the frequency whose harmonic has the highest amplitude and power. Therefore, it is enough to know the attenuation at this frequency in order to approximately estimate the distortion of the signals transmitted over the line.

ATTENTION

As mentioned above, attenuation is always negative, but the minus sign is often omitted, and sometimes confusion arises. The statement that the quality of the communication line is the higher, the greater (taking into account the sign) the attenuation is completely correct. If we ignore the sign, that is, keep in mind the absolute value of the attenuation, then the attenuation of a better quality line is less. Let's give an example. For internal wiring in buildings, a Category 5 twisted pair cable is used. This cable, on which almost all LAN technologies work, is characterized by an attenuation of no less than -23.6 dB for a frequency of 100 MHz with a cable length of 100 m. b has an attenuation at a frequency of 100 MHz not less than -20.6 dB. We get that - 20.6> -23.6, but 20.6< 23,6.

In fig. 8.8 shows typical attenuation versus frequency for Category 5 and 6 unshielded twisted-pair cables.

Optical cable has significantly lower (in absolute value) attenuation values, usually in the range from -0.2 to -3 dB with a cable length of 1000 m, which means it is of better quality than twisted pair cable. Almost all optical fibers have a complex dependence of attenuation on wavelength, which has three so-called transparency windows. In fig. 8.9 shows the characteristic dependence of the attenuation for an optical fiber. It can be seen from the figure that the area of ​​effective use of modern fibers is limited to wavelengths of 850 nm, 1300 nm, and 1550 nm (35 THz, 23 THz, and 19.4 THz, respectively). The 1550 nm window provides the lowest loss, and therefore the maximum range with a fixed transmitter power and a fixed receiver sensitivity

As a characteristic of the signal power, the absolute and relative
relative power levels. The absolute power level is measured in
watts, the relative power level, like attenuation, is measured in deci-
belah. In this case, as a base value of power, relative to which
the signal power is measured, a value of 1 mW is taken. Thus,
the relative power level p is calculated using the following formula:

Here P is the absolute signal power in milliwatts, and dBm is a unit of measurement.
rhenium relative power level (decibels per mW). Relative
power values ​​are convenient to use when calculating the energy budget
that communication lines.

Extreme simplicity of calculation became possible due to the fact that as
the initial data were used the relative values ​​of the input power
signal and output signals. The value y used in the example is called
receiver sensitivity threshold and represents the minimum power
signal at the input of the receiver, at which it is able to correctly locate
know the discrete information contained in the signal. It is obvious that for
normal operation of the communication line, it is necessary that the minimum power
the transmitter signal, even weakened by the attenuation of the communication line, exceeded
receiver sensitivity threshold: x - A> y. Checking this condition is
is the essence of calculating the energy budget of the line.

An important parameter a copper communication line is its characteristic impedance,
representing the total (complex) resistance that meets
an electromagnetic wave of a certain frequency when propagating along one
a homogeneous chain. Characteristic impedance is measured in ohms and depends on such
parameters of the communication line, such as active resistance, linear inductance
and linear capacity, as well as on the frequency of the signal itself. Output impedance
the transmitter should be matched to the characteristic impedance of the line,
otherwise, the signal attenuation will be excessive.

Immunity and reliability

The immunity of a line, as the name implies, determines the ability of the line to withstand the effects of noise generated in the external environment or on the internal conductors of the cable itself. The immunity of a line depends on the type of physical medium used, as well as on the shielding and suppression means of the line itself. Radio lines are the least resistant to interference, cable lines have good stability, and fiber-optic lines, which are insensitive to external electromagnetic radiation, are excellent. Typically, to reduce interference from external electromagnetic fields, the conductors are shielded and / or twisted.

Electrical and magnetic coupling are parameters of a copper cable that are also the result of interference. The electrical connection is defined by the ratio of the induced current in the affected circuit to the voltage acting in the influencing circuit. Magnetic coupling is the ratio of the electromotive force induced in the affected circuit to the current in the influencing circuit. Electrical and magnetic coupling results in induced signals (pickups) in the affected circuit. There are several different parameters that characterize the resistance of a cable to interference.

Near End Cross Talk (NEXT) determines the stability of a cable when interference is caused by a signal generated by a transmitter connected to one of the adjacent pairs on the same end of the cable as the one connected to the affected cable. pair receiver (fig. 8.10). The NEXT exponent, expressed in decibels, is equal to 10 lg Pout / Pind> where Pout is the output signal power, Pind is the induced signal power.

The lower the NEXT value, the better cable... For example, for a Category 5 twisted pair cable, the NEXT should be less than -27 dB at 100 MHz.

Far End Cross Talk (FEXT) allows you to evaluate the immunity of a cable to interference when the transmitter and receiver are connected to different ends of the cable. Obviously, this indicator should be better than NEXT, since the signal arrives at the far end of the cable, attenuated by the attenuation of each pair.

The NEXT and FEXT values ​​are usually applied to a cable consisting of several twisted pairs, since in this case, the mutual interference of one pair to another can reach significant values. For a single coaxial cable (that is, consisting of one shielded core), this indicator does not make sense, and for a double coaxial cable it also does not apply due to the high degree of protection of each core. Optical fibers also do not create any noticeable mutual interference.

Due to the fact that in some new technologies data is transmitted simultaneously over several twisted pairs, recently crosstalk indicators with the PS prefix (PowerSUM - combined pickup), such as PS NEXT and PS FEXT, have also begun to be used. These indicators reflect the resistance of the cable to the total power of crosstalk on one of the cable pairs from all other transmitting pairs (Fig. 8.11).

Another practically important indicator is the cable protection (Attenuation / Crosstalk Ratio, ACR). Security is defined as the difference between the wanted signal and interference levels. The higher the value of the cable protection, the more, in accordance with the Shannon formula, with a potentially higher

speed can transmit data but this cable. In fig. 8.12 shows a typical characteristic of the dependence of the immunity of an unshielded twisted pair cable on the signal frequency.

The fidelity of data transmission characterizes the probability of distortion of each transmitted data bit. This is sometimes referred to as the Bit Error Rate (BER). The BER value for communication lines without additional means of protection against errors (for example, self-correcting codes or protocols with retransmission of distorted frames) is, as a rule, 10-4-10-6, in fiber-optic communication lines - 10 ~ 9. The value of the reliability of data transmission, for example 10-4, indicates that, on average, out of 10 000 bits, the value of one bit is distorted.

Frequently, the cutoff frequencies are considered to be the frequencies at which the output signal power is halved in relation to the input signal, which corresponds to an attenuation of -3 dB. As we will see later, the bandwidth has the greatest impact on the maximum possible data transfer rate over the communication line. The bandwidth depends on the type of line and its length. In fig. 8.13 shows the bandwidth of communication lines different types, as well as the most frequently used frequency ranges in communication technology

For example, since a physical layer protocol is always defined for digital lines, which sets the bit rate of data transmission, then the bandwidth for them is always known - 64 Kbit / s, 2 Mbit / s, etc.

In those cases, when it is only necessary to choose which of the many existing protocols to use on a given line, other characteristics of the line, such as bandwidth, crosstalk, noise immunity, etc., are very important.

Throughput, like data rate, is measured in bits per second (bps), and also in derived units such as kilobits per second (Kbps), etc.

The throughput of communication lines and communication network equipment is
It is measured in bits per second, not bytes per second. This is due to the fact thatdata in networks is transmitted sequentially, that is, bit by bit, and not in parallel, bytes, as it happens between devices inside a computer. Such units of measure,as kilobits, megabits or gigabits, in network technologies strictly correspond to powers of 10(that is, a kilobit is 1000 bits, and a megabit is 1 000 000 bits), as is customary in all
branches of science and technology, and not powers of two close to these numbers, as is customaryin programming, where the prefix "kilo" is 210 = 1024, and "mega" is 220 = 1,048,576.

The throughput of a communication line depends not only on its characteristics, such
both attenuation and bandwidth, but also from the spectrum of the transmitted signals.
If significant signal harmonics (that is, those harmonics whose amplitudes are
make the main contribution to the resulting signal) fall into the passband
line, then such a signal will be well transmitted by this communication line,
and the receiver will be able to correctly recognize the information sent by
the transmitter (Fig. 8.14, a). If significant harmonics go beyond the
the bandwidth of the communication line, the signal will significantly distort
Xia, and the receiver will make a mistake when recognizing information (Fig. 8.14, b).

Bits and baud

The choice of the way of presenting discrete information in the form of signals,
transmitted on a communication line is called physical, or linear, coding.

The spectrum of signals depends on the chosen coding method and, accordingly,
line capacity.

Thus, for one coding method, a line can have one
throughput, and for another - another. For example, a twisted pair cable
Rii 3 can transmit data with a bandwidth of 10 Mbps with a
sobe coding of the physical layer standard 10ВаБе-Т and 33 Mbit / s with a method
sobe coding standard 100Ваse-Т4.

In accordance with the main postulate of information theory, any discernible unpredictable change in the received signal carries information. Hence it follows thatsinusoid, in which the amplitude, phase and frequency remain unchanged, information is notcarries, since the change in the signal, although it occurs, is absolutely predictable. Similarly, pulses on the computer clock bus do not carry information,since their changes are also constant over time. But the impulses on the data bus cannot be predicted in advance, this makes them informational, they carry information
between individual blocks or devices of the computer.

In most coding methods, a change in any parameter of a periodic signal is used - the frequency, amplitude and phase of a sinusoid, or the sign of the potential of a sequence of pulses. A periodic signal, the parameters of which are subject to changes, is called the carrier signal, and its frequency, if the signal is sinusoidal, is called the carrier frequency. The process of changing the parameters of the carrier signal in accordance with the transmitted information is called modulation.

If the signal changes in such a way that only two of its states can be distinguished, then any change in it will correspond to the smallest unit of information - a bit. If the signal can have more than two distinguishable states, then any change in it will carry several bits of information.

The transmission of discrete information in telecommunication networks is timed, that is, the signal changes at a fixed time interval, called a cycle. The receiver of information considers that at the beginning of each cycle new information arrives at its input. In this case, regardless of whether the signal repeats the state of the previous cycle or if it has a state different from the previous one, the receiver receives new information from the transmitter. For example, if the clock cycle is 0.3 s, and the signal has two states and 1 is encoded with a potential of 5 volts, then the presence of a 5 volt signal at the input of the receiver for 3 seconds means receiving information represented by the binary number 1111111111.

The number of changes in the information parameter of the carrier periodic signal per second is measured in baud. One baud is equal to one change in the information parameter per second. For example, if the cycle of information transmission is 0.1 second, then the signal changes at a rate of 10 baud. Thus, the baud rate is entirely determined by the size of the cycle.

The information rate is measured in bits per second and is generally not the same as the baud rate. It can be either higher or lower than the speed

changes in the information parameter measured in baud. This relationship depends on the number of signal states. For example, if the signal has more than two distinguishable states, then with equal clock cycles and the corresponding coding method, the information rate in bits per second can be higher than the rate of change of the information signal in baud.

Let the information parameters be the phase and the amplitude of the sinusoid, and there are 4 phase states at 0, 90, 180 and 270 ° and two values ​​of the signal amplitude, then the information signal can have 8 distinguishable states. This means that any state of this signal carries information in 3 bits. In this case, a modem operating at a speed of 2400 baud (changing the information signal 2400 times per second) transmits information at a speed of 7200 bps, since with one change in the signal, 3 bits of information are transmitted.

If the signal has two states (that is, it carries information in 1 bit), then the information rate usually coincides with the number of baud. However, the opposite picture can also be observed, when the information rate is lower than the rate of change of the information signal in baud. This occurs when, for reliable recognition of user information by the receiver, each bit in the sequence is encoded with several changes in the information parameter of the carrier signal. For example, when a single bit value is encoded with a positive pulse and a zero bit value with a negative polarity pulse, the physical signal changes its state twice with each bit being transmitted. With this encoding, the line rate in bits per second is half that of baud.

The higher the frequency of the carrier periodic signal, the higher the modulation frequency can be and the higher the bandwidth of the communication line can be.

However, on the other hand, with an increase in the frequency of the periodic carrier signal, the width of the spectrum of this signal also increases.

The line transmits this spectrum of sinusoids with those distortions that are determined by its bandwidth. The greater the discrepancy between the bandwidth of the line and the bandwidth of the transmitted information signals, the more the signals are distorted and the more likely errors in the recognition of information by the receiving side, which means that the possible speed of information transmission turns out to be lower.

Bandwidth to bandwidth ratio

The relationship between the bandwidth of a line and its bandwidth, regardless of the adopted method of physical coding, was established by Claude Shannon:

C = F log 2 (1 + Pc / Psh) -

Here C is the line bandwidth in bits per second, F is the line bandwidth in hertz, Pc is the signal power, Psh is the noise power.

It follows from this relationship that there is no theoretical bandwidth limit for a fixed bandwidth line. However, in practice, there is such a limit. Indeed, it is possible to increase the line capacity by increasing the transmitter power or reducing the noise (interference) power in the communication line. Both of these components are very difficult to change. An increase in transmitter power leads to a significant increase in its size and cost. Reducing the noise level requires the use of special cables with good protective screens, which is very expensive, as well as noise reduction in the transmitter and intermediate equipment, which is not easy to achieve. In addition, the effect of the power of the useful signal and noise on the throughput is limited by the logarithmic dependence, which grows far less rapidly than the direct proportional one. So, with a fairly typical initial signal-to-noise power ratio, a 100-fold increase in transmitter power will only give a 15% increase in line throughput.

Essentially close to Shannon's formula is another ratio obtained by Nyquist, which also determines the maximum possible bandwidth of a communication line, but without taking into account the noise in the line:

C = 2Flog2 M.

Here M is the number of distinguishable states of the information parameter.

If the signal has two distinguishable states, then the bandwidth is equal to twice the bandwidth of the communication line (Fig. 8.15, a). If the transmitter uses more than two stable signal states to encode data, then the line capacity increases, since in one cycle of operation the transmitter transmits several bits of the original data, for example, 2 bits in the presence of four distinguishable signal states (Fig. 8.15, b).

Although the Nyquist formula does not explicitly take into account the presence of noise, indirectly
its influence is reflected in the choice of the number of states of the information signal
nala. The number of states should be increased to increase the throughput of the communication line, but in practice this is prevented by noise on the line. For example, the bandwidth of the line, the signal of which is shown in Fig. 8.15, b, can be doubled by using not 4, but 16 levels to encode the data. However, if the amplitude of the noise from time to time exceeds the difference between adjacent levels, then the receiver will not be able to steadily recognize the transmitted data. Therefore, the number of possible signal states is actually limited by the ratio of signal power to noise, and the Nyquist formula determines the maximum data transfer rate in the case when the number of states has already been selected taking into account the capabilities of stable recognition by the receiver.

Shielded and unshielded twisted pair

Twisted pair called a twisted pair of wires. This type of data transmission medium is very popular and forms the basis of a large number of both internal and external cables. A cable can consist of several twisted pairs (external cables sometimes contain up to several dozen such pairs).

Twisting the wires reduces the influence of external and mutual interference on the wanted signals transmitted over the cable.

The main features of the cable design are shown schematically in Fig. 8.16.

Twisted pair cables are symmetrical , that is, they consist of two structurally identical conductors. A balanced twisted pair cable can be either shielded and unshielded.

It is necessary to distinguish between electrical insulation of conductive cores, which is available in any cable, fromelectromagneticisolation. The first consists of a non-conductive dielectric layer - paper or a polymer, such as polyvinyl chloride or polystyrene. In the second case, in addition to electrical insulation, conductive cores are also placed inside an electromagnetic shield, which is most often used as a conductive copper braid.

Cable basedunshielded twisted pair,used for wiring

inside the building, divided in international standards into categories (from 1 to 7).

Category 1 cables apply where speed requirements are
are minimal. This is usually a cable for digital and analog voice transmission.
and low-speed (up to 20 Kbps) data transfer. Until 1983, it was
a new type of cable for telephone wiring.

Category 2 cables were first used by IBM to build
own cable system. The main requirement for cables of this category is
Rii - the ability to transmit signals with a spectrum of up to 1 MHz.

Category 3 cables were standardized in 1991. EIA-568 standard
determined the electrical characteristics of cables for frequencies in the range up to
16 MHz. Category 3 cables designed for both data transmission and
and for voice transmission, now form the basis of many cable systems
buildings.

Category 4 cables represent a slightly improved version of the
whites of category 3. Cables of category 4 are required to withstand tests for an hour.
to the transmission of a signal of 20 MHz and provide increased noise immunity
high speed and low signal loss. In practice, they are rarely used.

Category 5 cables have been specially designed to support high
high-speed protocols. Their characteristics are determined in the range up to
100 MHz. Majority high-speed technologies(FDDI, Fast Ethernet,
ATM and Gigabit Ethernet) are focused on the use of twisted pair cables
5. The category 5 cable replaced the category 3 cable, and today
all new cable systems large buildings are built on this type
cable (combined with fiber optic).

Cables take a special place categories 6 and 7, which the industry began to produce relatively recently. For Category 6 cable, specifications are specified up to 250 MHz, and for Category 7 cables up to 600 MHz. Category 7 cables must be shielded, both each pair and the entire cable as a whole. Category 6 cable can be either shielded or unshielded. The main purpose of these cables is to support high-speed protocols over cable lengths longer than Category 5 UTP cable.

All UTP cables, regardless of their category, are available in 4-pair design. Each of the four cable pairs has a specific color and pitch. Usually two pairs are for data transmission and two for voice transmission.

Fiber optic cable

Fiber optic cableconsists of thin (5-60 microns) flexible glass fibers (optical fibers) through which light signals propagate. This is the highest quality type of cable - it provides data transmission at a very high speed (up to 10 Gbit / s and higher) and, moreover, better than other types of transmission medium, it provides data protection from external interference (due to the nature of light propagation, such signals are easily shielded).

Each light guide consists of a central light conductor (core) - a glass fiber, and a glass cladding, which has a lower refractive index than the core. Spreading along the core, the light rays do not go beyond its limits, reflecting from the covering layer of the shell. Depending on the distribution of the refractive index and the size of the core diameter, there are:

multimode fiber with a step change in refractive index (Fig.8.17, a)\

multimode fiber with a smooth change in the refractive index (Fig. 8.17, b) \

single-mode fiber (Fig.8.17, v).

The term "mode" describes the mode of propagation of light rays in the core of the cable.

In a single mode cable(Single Mode Fiber, SMF) uses a center conductor of a very small diameter, commensurate with the wavelength of light - from 5 to 10 microns. In this case, practically all light rays propagate along the optical axis of the fiber without being reflected from the outer conductor. Manufacturing over

V multimode cables(Multi Mode Fiber, MMF) uses wider inner cores that are easier to manufacture. In multimode cables, multiple light beams exist simultaneously in the inner conductor, bouncing off the outer conductor at different angles. The angle of reflection of the beam is called fashion ray. In multimode cables with a gradual change in refractive index, the reflection mode of the rays is complex. The resulting interference degrades the quality of the transmitted signal, which leads to distortion of the transmitted pulses in the multimode optical fiber. For this reason specifications multimode cables are worse than singlemode cables.

As a result, multimode cables are used mainly for data transmission at speeds of no more than 1 Gbit / s over short distances (up to 300-2000 m), and single-mode cables are used for data transmission at ultra-high speeds of several tens of gigabits per second (and when using DWDM technology - up to several terabits per second) at distances of up to several tens and even hundreds of kilometers (long-distance communication).

The following are used as light sources in fiber-optic cables:

LEDs, or light emitting diodes (Light Emitted Diode, LED);

semiconductor lasers, or laser diodes.

For single-mode cables, only laser diodes are used, since with such a small diameter of the optical fiber, the light flux created by the LED cannot be directed into the fiber without large losses - it has an excessively wide radiation pattern, while the laser diode is narrow. Cheaper LED emitters are only used for multimode cables.

The cost of fiber-optic cables is not much higher than the cost of twisted-pair cables, but installation work with optical fiber is much more expensive due to the laboriousness of the operations and the high cost of the used installation equipment.

conclusions

Depending on the type of intermediate equipment, all communication lines are divided into analog and digital. In analog lines, intermediate equipment is designed to amplify analog signals. Analog lines use frequency multiplexing.

In digital communication lines, the transmitted signals have a finite number of states. In such lines, special intermediate equipment is used - regenerators, which improve the shape of the pulses and ensure their resynchronization, that is, restore their repetition period. The intermediate equipment for multiplexing and switching primary networks operates on the principle of time multiplexing of channels, when each low-speed channel is allocated a certain fraction of the time (time-slot, or quantum) of a high-speed channel.

Bandwidth defines the range of frequencies that are transmitted by the link with acceptable attenuation.

The throughput of a communication line depends on its internal parameters, in particular - the bandwidth, external parameters - the level of interference and the degree of attenuation of interference, as well as the adopted method of encoding discrete data.

Shannon's formula defines the maximum possible bandwidth of a communication line at fixed values ​​of the line bandwidth and the ratio of signal-to-noise power.

The Nyquist formula expresses the maximum possible bandwidth of a communication line in terms of the bandwidth and the number of states of the information signal.

Twisted pair cables are divided into unshielded (UTP) and shielded (STP) cables. UTP cables are easier to manufacture and install, but STP cables provide a higher level of security.

Fiber optic cables have excellent electromagnetic and mechanical characteristics, the disadvantage of which is the complexity and high cost of installation work.

1. How does a link differ from a composite communication channel?
   1. Can a compound channel be made up of links? And vice versa?
   2. Can a digital channel carry analog data?
   3. What type of communication line characteristics are: noise level, bandwidth, linear capacity?
   4. What measures can be taken to increase the information speed of a link:

O reduce the cable length;

O choose a cable with less resistance;

O choose a cable with a wider bandwidth;

Apply a coding method with a narrower spectrum.

1. Why is it not always possible to increase the channel capacity by increasing the number of states of the information signal?
   1. What mechanism is used to suppress interference in cables UTP?
   2. Which cable transmits signals of higher quality - with a higher parameter value NEXT or less?
   3. What is the spectrum width of an ideal pulse?
   4. Name the types of optical cable.
   5. What happens if a cable is replaced on a working network UTP with STP cable? Answer options:

The proportion of distorted frames in the network will decrease, since external interference will be suppressed more efficiently;

Oh nothing will change;

The proportion of distorted frames in the network will increase, since the output impedance of the transmitters does not match the impedance of the cable.

1. Why is it problematic to use fiber optic cable in a horizontal subsystem?
   1. Known quantities are:

Minimum transmitter power P out (dBm);

O catch-up attenuation of cable A (dB / km);

Receiver sensitivity threshold P in (dBm).

It is required to find the maximum possible length of the communication line at which signals are transmitted normally.

1. What would be the theoretical limit on the data rate in bits per second over a 20 kHz link bandwidth if the transmitter power is 0.01 mW and the noise power on the link is 0.0001 mW?
   1. Determine the bandwidth of a duplex communication line for each direction if you know that its bandwidth is 600 kHz, and the coding method uses 10 signal states.
   2. Calculate the signal propagation delay and the data transmission delay for the case of a 128 byte packet transmission (consider the signal propagation speed equal to the speed of light in a vacuum of 300,000 km / s):

О over a twisted pair cable 100 m long at a transmission speed of 100 Mbit / s;

О over a coaxial cable 2 km long at a transmission speed of 10 Mbps;

O via a satellite channel with a length of 72,000 km at a transmission rate of 128 Kbps.

1. Calculate the speed of the communication line if you know that the clock frequency of the transmitter is 125 MHz, and the signal has 5 states.
   1. Receiver and transmitter network adapter connected to adjacent cable pairs UTP. What is the power of the conducted interference at the input of the receiver, if the transmitter has a power of 30 dBm, and the indicator NEXT cable is -20 dB?
   2. Let it be known that the modem transmits data in full duplex mode at a speed of 33.6 kbps. How many states does its signal have if the bandwidth of the communication line is 3.43 kHz?

PAGE 20

In the receiving device, the secondary signals are converted back into message signals in the form of sound, optical or text information.

Etymology

The word "telecommunication" comes from the new lat. electricus and other Greek. ἤλεκτρον (electr, shiny metal; amber) and the verb "knit". The synonym is the word "telecommunication" (from French télécommunication), used in English-speaking countries. Word télécommunication, in turn, comes from the Greek tele-(τηλε-) - "distant" and from lat. communicatio - message, transmission (from Latin communico - I make it general), that is, the meaning of this word also includes non-electrical types of information transmission (using optical telegraph, sounds, fire on watchtowers, mail).

Telecommunication classification

Telecommunications is the object of study of the scientific discipline theory of electrical communications.

By the type of information transfer, all modern systems telecommunications are conventionally classified into those intended for the transmission of sound, video, text.

Depending on the purpose of the messages, the types of telecommunications can be qualified for the transmission of information of an individual and mass nature.

In terms of time parameters, the types of telecommunications can be operating in real time either carrying out delayed delivery messages.

The main primary signals of telecommunication are: telephone, sound broadcasting, facsimile, television, telegraph, data transmission.

Communication types

• Cable lines - electrical signals are used for transmission;
• Radio communication - radio waves are used for transmission;
  • DV-, SV-, HF- and VHF-communication without the use of repeaters
  • Satellite communications - communications using space repeater (s)
  • Radio relay communication - communication using terrestrial repeater (s)
  • Cellular communications - radio relay communications using a network of ground base stations

• Fiber optic communication - light waves are used for transmission.

Depending on the engineering method of organization, communication lines are divided into:

• satellite;
• air;
• terrestrial;
• underwater;
• underground.

• Analog communication is a continuous signal transmission.
• Digital communication is the transmission of information in discrete form (digital form). A digital signal is analog by its physical nature, but the information transmitted with its help is determined by a finite set of signal levels. Numerical methods are used to process a digital signal.

Signal

In general, the communication system includes:

• terminal equipment: terminal equipment, terminal device (terminal), terminal device, source and recipient of the message;
• signal conversion devices(OOI) at both ends of the line.

Terminal equipment provides primary processing of a message and signal, conversion of messages from the form in which they are provided by the source (speech, image, etc.) into a signal (on the side of the source, sender) and back (on the side of the receiver), amplification, etc. NS.

Signal conversion devices can protect the signal from distortion, shaping the channel (s), matching the group signal (signal of several channels) with the line on the source side, recovering the group signal from a mixture of the useful signal and interference, dividing it into individual channels, error detection and correction on the recipient's side. Modulation is used to form the group signal and match with the line.

The communication line may contain signal conditioning devices such as amplifiers and regenerators. The amplifier simply amplifies the signal along with the interference and transfers it further, it is used in analog transmission systems(ASP). Regenerator ("re-receiver") - performs signal recovery without interference and re-shaping of the linear signal, is used in digital transmission systems(DSP). Amplification / regeneration points are serviceable and non-serviceable (OUP, NUP, ORP and NRP, respectively).

In DSP, terminal equipment is called DTE (Data Terminal Equipment, DTE), MTP is called DCE ( data link termination equipment or line terminal equipment, DCE). For example, in computer networks, the role of the DTE is played by the computer, and the DCE is the modem.

Standardization

In the world of communications, standards are extremely important because communications equipment must be able to communicate with each other. There are several international organizations that publish communication standards. Among them:

• International Telecommunication Union (eng. International Telecommunication Union, ITU) is one of the UN agencies.
• (eng. Institute of Electrical and Electronics Engineers, IEEE).
• Special Commission for Internet Development (eng. Internet Engineering Task Force, IETF).

In addition, standards are often (usually de facto) determined by the leaders of the telecommunications equipment industry.