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The book "Computer Networks. Principles, Technologies, Protocols: Anniversary Edition »

imageThis publication is in a sense special - exactly 20 years have passed since the publication of the book. 20 years is a considerable period, during this time the children of our first readers grew up and, perhaps, became interested in computer networks. And, perhaps, they will have in their hands the 6th edition of the book “Computer Networks. Principles, technologies, protocols. ” This book is significantly different from the one their parents read. Much of what interested readers of the late 90s — for example, the 4-hub rule, matching IP and IPX networks, or comparing 100VG-AnyLAN and FDDI technologies — is not mentioned at all in recent editions. Over 20 years, many technologies have gone through the full cycle from a fashionable term and universal recognition to almost complete oblivion. Each new edition of the book in one way or another reflected the changing landscape of network technology.

This edition is not an exception - it is significantly revised, about a third of the material is either completely new information, or a substantially revised presentation of topics. For example, a new part of “Wireless Networks” appeared in the book; the part devoted to the primary network technologies SDH, OTN and DWDM is completely revised.

The publication is intended for students, graduate students and technical specialists who would like to gain basic knowledge about the principles of building computer networks, understand the features of traditional and promising technologies of local and global networks, and learn how to create large composite networks and manage such networks.

Sixth Edition Changes


First of all, a new part of Wireless Networks appeared in the book. It consists of three chapters.

In the first of them, the physical level of wireless communication lines is considered, which includes the specifics of the transmission medium, the range and nature of the propagation of electromagnetic waves, types of distortion, and methods of dealing with them. Since no wireless network nodes can do without an antenna, devices of this type are given considerable attention in this chapter - in particular, transmission methods using several antennas on the transmitting and receiving sides, the so-called MIMO technologies. This chapter discusses the spread spectrum coding techniques of FHSS, DSSS, CDMA, and OFDM, which were developed specifically for wireless transmission.

The content of the second chapter focuses on Wi-Fi wireless local area networks (IEEE 802.11), which in the fixed wireless Internet access sector have taken the same dominant position as Ethernet networks in local area networks. The chapter concluding this part is devoted to mobile cellular networks. This topic has not been studied in previous editions due to the fact that mobile networks were predominantly telephone. The complete transition of LTE (4G) mobile networks to the TCP / IP stack protocols, which were used to establish telephone calls and access the Internet, changed this situation. The chapter discusses the evolution of technologies of mobile networks of various generations, mobile versions of IPv4 and IPv6 protocols, the basic principles of building LTE networks; 5G network architecture overviewwho intend to incorporate the latest advances in computer networks and become the main type of access network for the Internet of things.

The IPv6 protocol description has been significantly revised and expanded - now a separate chapter is devoted to this protocol. The spread of IPv6 is growing steadily, and a deeper understanding of this protocol has become important to the modern network specialist.

In recent years, the concept of programmable computer networks has been established, therefore, sections have been added to the book that describe the technologies of software-defined networks SDN and virtualization of network functions NFV.

Completely redesigned part devoted to the technologies of primary networks SDH, OTN and DWDM.

And finally, the number of questions and tasks has significantly increased.

Excerpt. Wireless communication lines. Ranges of the electromagnetic spectrum


The characteristics of a wireless communication line — the distance between nodes, the area of ​​coverage, the speed of information transfer, etc. — largely depend on the frequency of the electromagnetic signal used. In fig. 21.2 shows the ranges of the electromagnetic spectrum. Summarizing, we can say that they and their corresponding wireless information transmission systems are divided into four groups.

  • 300 — . ITU ( ), (Extremely Low Frequency, ELF) (Extra High Frequency, EHF). 20 300 , , « ». , , AM- FM-, . , 2400, 9600 19 200 /.

image

  • 300 3000 . , , , , (Wireless Local Loop, WLL).
  • . . , .
  • ( ). .


First of all, let us recall several important physical phenomena associated with the propagation of waves in general and electromagnetic waves in particular. In fig. 21.3 it is shown that a signal, having encountered an obstacle, can propagate in accordance with three mechanisms: reflection, diffraction, and scattering. When a signal encounters an obstacle partially transparent to a given wavelength and at the same time having dimensions much greater than the wavelength, part of the signal energy is reflected from this obstacle. If the signal encounters an impenetrable obstacle (for example, a metal plate) of a much larger size than the wavelength, then diffraction occurs - the obstacle seems to be enveloped by the signal, which allows it to be received even without being in the line of sight. And finally, when faced with an obstacle whose dimensions are commensurate with the wavelength,the signal is scattered, propagating at different angles.

image

The ideal medium for the propagation of electromagnetic waves is vacuum, but in real life signals are often transmitted through the atmosphere, which is an unstable and inhomogeneous medium consisting of many layers with different conductive properties. The properties of a real transmission medium in combination with the frequency characteristics of the transmitted signals determine several basic methods of propagation of electromagnetic waves (Fig. 21.4).

Terrestrial or surface , wavesspread along the earth’s surface. Following a more or less terrain, they can travel long distances, up to several hundred kilometers, far beyond the line of the visible horizon. This method of wave propagation is characteristic of low frequency electromagnetic radiation - up to 2 MHz.

Electromagnetic waves of this frequency are scattered in the atmosphere in such a way that they do not penetrate the upper atmosphere. The most famous example of an earth wave is an AM radio signal from the long wavelength range. The main reason that waves follow the surface of the earth is diffraction. In this case, an impenetrable obstacle of a much larger size than the wavelength is the bulge of the earth. The ability of a wave to go around an obstacle depends on the ratio of the wavelength to the size of the obstacle; the smaller this ratio, the weaker the diffraction. Hence it is clear that for high-frequency electromagnetic signals, the diffraction effect can be neglected.

Ionospheric (spatial) wavescharacteristic for medium and high frequency signals from 2 to 30 MHz. The signals emitted by a ground-based antenna are reflected by the ionosphere (a less dense ionized upper atmosphere) to the ground, and therefore can propagate far beyond the visible horizon, to distances even greater than surface waves. With sufficient transmitter power, the radio waves of these ranges due to multiple reflection from the ionosphere can even go around the globe. Ionospheric waves are widely used in broadcasting and especially international broadcasting - for example, by companies such as the BBC Radio World Service.

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Direct waves , or lines of direct visibility , as their name implies, propagate only in a straight line, from the transmitter to the receiver. At the same time, the latter can be located both on earth and in space. This type of wave propagation is characteristic of electromagnetic signals with a frequency above 30 MHz - they can neither be reflected by the ionosphere nor envelope the convexity of the Earth. At frequencies above 4 GHz, they are in trouble: they begin to be absorbed by water, which means that not only rain, but also fog can cause a sharp deterioration in the quality of transmission of microwave systems. Infrared and visible light can only be transmitted along line of sight, as they do not pass through walls.

Tropospheric wavescan be generated by very high and ultra-high frequency radiation (30 MHz - 3 GHz). As mentioned above, electromagnetic signals from this range cannot be reflected by the ionosphere. However, they are able to propagate by refraction and scattering on inhomogeneities of the troposphere - the atmosphere layer closest to the earth. Tropospheric inhomogeneities are areas of space in which air at some points in time has a temperature, pressure and humidity that differ from the average values ​​for the environment. Tropospheric waves make it possible to transmit a signal, albeit a very weak one, over a distance of up to 1000 km.

The higher the carrier frequency, the higher the possible information transfer rate. The need for high-speed information transmission is prevailing, therefore, all modern systems of wireless information transmission operate in high-frequency ranges, starting from 800 MHz, despite the advantages that promise low-frequency ranges due to the propagation of a signal along the earth's surface or reflection from the ionosphere.

Anti-Signal Distortion in Wireless Lines


The rejection of wires and mobility leads to a high level of interference in wireless communication lines. If the bit error rate (BER) in the wired communication lines is equal, imagethen in wireless communication lines it reaches a value. imageIn urban conditions, in the frequency range of the useful signal, there is usually a large amount of interference, for example, from car ignition systems, from various household appliances.

As a result of diffraction, reflection and scattering of electromagnetic waves, which are ubiquitous in wireless communications in the city, the receiver can receive several replicas of the same signal that have passed to the receiver in different ways. This effect is called multipath signal propagation. At each reflection, the signal can change the phase, amplitude and angle of arrival at the receiver. The result of multipath propagation of the signal is often negative, since the signals can come in antiphase and suppress the main signal.

Since the propagation time of a signal along different paths is generally different, intersymbol interference can also be observed - a situation where, as a result of a delay, the signals encoding adjacent data bits reach the receiver during the time interval allotted for receiving one symbol. The signal obtained as a result of superposition of adjacent signals, the receiver may decode incorrectly.

Distortion due to multipath propagation leads to a weakening of the signal - this effect is called multipath fading (fading). It is known that when electromagnetic waves propagate in free space (without reflections), the signal power attenuation is proportional to the product of the square of the distance from the signal source by the square of the signal frequency. In cities, multipath fading leads to the fact that the attenuation of the signal becomes proportional not to the square of the distance, but to its cube or even to the fourth degree!

The problem of the high level of interference of wireless channels is solved in various ways. An important role is played by the broadband signal technologies discussed below. These technologies are based on the distribution of signal energy in a wide frequency range, so that narrow-band interference does not significantly affect the signal as a whole. To recognize a signal distorted due to its multipath propagation, various processing methods are used that compensate for intersymbol interference. One of them is adaptive equalizing signal (adaptive equalizing, Fig. 21.5).

The idea is to sum the signal measured at equal time intervals Δt during one clock cycle of the code symbol. Before summing, the signal values ​​are multiplied by their weight coefficient Ci. The value of the signal received after summing and called the aligned signal is considered the bit value of the transmitted code in this clock cycle.

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Weight selection is done adaptively using a pre-known binary code called a training sequence. The transmitter inserts this sequence after each block of user data of a certain length. The receiver applies the same alignment algorithm to the training sequence as to user data, compares the value of the received bit sequence with the expected training sequence; if they differ, then new values ​​of the weighting coefficients are calculated.

The use of self-correcting FEC codes also plays a large role. Radio communication has always been a pioneer of this technique - the frequency of occurrence of bit errors is much higher here than in wired data transmission. Another technique is the use of protocols with connection establishment and retransmissions of frames at the data link layer of the protocol stack. These protocols allow for faster error correction, as they work with lower timeout values ​​than transport-level corrective protocols such as TCP. Finally, they try to place signal transmitters (and receivers, if possible) on high towers (masts) to avoid multiple reflections.

Licensing


The problem of separation of the electromagnetic spectrum between consumers requires centralized regulation. Each country has a special state body, which (in accordance with ITU recommendations) issues licenses to telecom operators to use a certain part of the spectrum sufficient to transmit information using a certain technology. A license is issued for a certain territory, within which the operator uses the frequency range assigned to it exclusively.

There are also three frequency bands, 900 MHz, 2.4 GHz and 5 GHz, which are recommended by ITU as bands for international use without licensing. These ranges are allocated to general industrial wireless products, such as car door lock devices, scientific and medical devices. In accordance with the purpose of these ranges are called ISM-ranges (Industrial, Scientific, Medical - industry, science, medicine). The 900 MHz band is the most “populated” because low-frequency technology has always been cheaper. Today, the 2.4 GHz band is being actively mastered, for example, in IEEE 802.11 and Bluetooth technologies. 5G networks will operate in various frequency ranges, including in the high-frequency ranges 26-29 GHz.A prerequisite for the use of these ranges on a joint basis is to limit the maximum power of the transmitted signals to 1 Watt. This condition reduces the range of devices so that their signals do not interfere with other users who may use the same frequency range in other areas of the city.

»More information on the book can be found on the publisher’s website
» Table of Contents
» Excerpt

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