Unit 2 PDF - Data Communications

DEPARTMENT OF COMPUTER SCIENCE AND ENGINEERING XXXXXX Subject Code / Name : EID 205 / Data Communications /CSEN1071 SYLLABUS Module II Data transmission: Concepts and terminology, analog and digital data transmissi...

DEPARTMENT OF COMPUTER SCIENCE AND ENGINEERING XXXXXX Subject Code / Name : EID 205 / Data Communications /CSEN1071 SYLLABUS Module II Data transmission: Concepts and terminology, analog and digital data transmission, transmission impairments, channel capacity. Transmission Media: Guided and unguided. 1. DATA TRANSMISSION The successful transmission of data depends principally on two factors:  Quality of the signal being transmitted  Characteristics of the transmission medium. NOTE: To be transmitted, data must be transformed to electromagnetic signals. One of the major functions of physical layer is to move data in the form of electromagnetic signals across a transmission medium. 1.1 CONCEPTS AND TERMINOLOGY Transmission Terminology Data transmission occurs between transmitter and receiver over some transmission medium. Transmission media may be classified as guided or unguided. In both cases, communication is in the form of electromagnetic waves. With guided media, the waves are guided along a physical path; examples of guided media are twisted pair, coaxial cable, and optical fiber. Unguided media, also called wireless, provide a means for transmitting electromagnetic waves but do not guide them; examples are propagation through air, vacuum, and seawater. The term direct link is used to refer to the transmission path between two devices in which signals propagate directly from transmitter to receiver with no intermediate devices, other than amplifiers or repeaters used to increase signal strength. Note that this term can apply to both guided and unguided media. A guided transmission medium is point to point if it provides a direct link between two devices and those are the only two devices sharing the medium. In a multipoint guided configuration, more than two devices share the same medium. A transmission may be simplex, half duplex, or full duplex. Dept of CSE UNIT-II NOTES Data Communications   In simplex transmission, signals are transmitted in only one direction; one station is transmitter and the other is receiver.  In half-duplex operation, both stations may transmit, but only one at a time.  In full-duplex operation, both stations may transmit simultaneously; the medium is carrying signals in both directions at the same time. Frequency, Spectrum, and Bandwidth A signal is generated by the transmitter and transmitted over a medium. The signal is a function of time, but it can also be expressed as a function of frequency; that is, the signal consists of components of different frequencies. It turns out that the frequency domain view of a signal is more important to an understanding of data transmission than a time domain view. TIME AND FREQUENCY DOMAIN CONCEPTS ANALOG AND DIGITAL Both data and the signals can be either analog or digital in form. Analog and Digital Data The term analog data refers to information that is continuous; digital data refers to information that has discrete states. For example, an analog clock that has hour, minute, and second hands gives information in a continuous form; the movements of the hands are continuous. On the other hand, a digital clock that reports the hours and the minutes will change suddenly from 8:05 to 8:06. NOTE: Analog data, such as the sounds made by a human voice, take on continuous values. When someone speaks; an analog wave is created in the air. This can be captured by a microphone and converted to an analog signal or sampled and converted to a digital signal.  Digital data take on discrete values. For example, data are stored in computer memory in the form of Os and 1s. They can be converted to a digital signal or modulated into an analog signal for transmission across a medium. Analog and Digital Signals Like the data, signals can be either analog or digital.  An analog signal is one in which the signal intensity varies in a smooth fashion over time. In other words, there are no breaks or discontinuities in the signal.  A digital signal is one in which the signal intensity maintains a constant level for some period of time and then abruptly changes to another constant level. Values are often 1 and 0. TIME DOMAIN CONCEPTS The simplest way to show signals is by plotting them on a pair of perpendicular axes. The vertical axis represents the value or strength of a signal. The horizontal axis represents time. The time-domain plot shows changes in signal amplitude with respect to time (amplitude versus time plot). Dept of CSE UNIT-II NOTES Data Communications  Figure illustrates an analog signal and a digital signal. The curve representing the analog signal passes through an infinite number of points. The vertical lines of the digital signal demonstrate the sudden jump that the signal makes from value to value. Periodic and Non periodic Signals Both analog and digital signals can take one of two forms: periodic or non-periodic.  A periodic signal completes a pattern within a measurable time frame, called a period, and repeats that pattern i.e., in which the same signal pattern repeats over time. The completion of one full pattern is called a cycle.  A non-periodic signal will not repeat over time i.e., changes without exhibiting a pattern or cycle. Both analog and digital signals can be periodic or non-periodic.  Mathematically, a signal s(t) is defined to be periodic if and only if Where the constant T is the period of the signal (T is the smallest value that satisfies the equation). Otherwise, a signal is non periodic (also called aperiodic).  Periodic analog signals can be classified as simple or composite. A simple periodic analog signal, a sine wave, cannot be decomposed into simpler signals (fig a). A composite periodic analog signal is composed of multiple sine waves. (fig c:following eg. Contains two sine waves sin(2πft) and (1/3)sin(2π(3f)t) ) Dept of CSE UNIT-II NOTES Data Communications  The sine wave is the most fundamental form of periodic signal (fundamental periodic signal). A general sine wave can be represented by three parameters:  Peak amplitude (A)  Frequency (f)  Phase (⏀) Peak amplitude (A) The peak amplitude is the maximum value or strength of the signal over time i.e., highest intensity of the signal which is proportional to the energy it carries; typically, amplitude is measured in volts. Frequency (f) The frequency is the rate [in cycles per second, or Hertz (Hz)] at which the signal repeats i.e., it measures number of cycles the signal completes in 1second. An equivalent parameter is the period (T) of a signal, which is the amount of time it takes for one cycle; therefore, T=1/f. Phase (⏀) Phase is a measure of the relative position in time within a single period of a signal. i.e., phase describes the position of the waveform relative to time 0. Phase is measured in degrees or radians. Dept of CSE UNIT-II NOTES Data Communications  A phase shift of 360° corresponds to a shift of a complete period; a phase shift of 180° corresponds to a shift of one-half of a period; and a phase shift of 90° corresponds to a shift of one- quarter of a period The general sine wave can be written as below which is dependent on three parameters Amplitude (A), Frequency (f), Phase (⏀) Figure 3.3 shows the effect of varying each of the three parameters. In part (a) of the figure, the frequency is 1 Hz; thus the period is T=1 second. Amplitude=1volt, phase=0 degrees. Part (b) has the same frequency and phase but a peak amplitude of 0.5. In part (c) we have f=2, which is equivalent to T=0.5. Finally, part (d) shows the effect of a phase shift of π/4 radians, which is 45 degrees (2π radians = 360° = 1 period) The wavelength (λ) of a signal is the distance occupied by a single cycle,(i.e., a simple signal can travel in one period) or, put another way, the distance between two points of corresponding phase of two consecutive cycles. Wavelength depends on both frequency and the medium. Then the wavelength is related to the period as follows: Dept of CSE UNIT-II NOTES Data Communications  λ = Propogation Speed (v) * Time period (T) Where T=1/f and ν=c the speed of light in free space, which is approximately 3x108m/s. Therefore λ=c/f. FREQUENCY DOMAIN CONCEPTS To show the relationship between amplitude and frequency, we use frequency-domain plot. A frequency-domain plot is concerned with only the peak value (amplitude) and the frequency. Figure shows a signal in both the time and frequency domains. It is obvious that the frequency domain is easy to plot and conveys the information that one can find in a time domain plot. The advantage of the frequency domain is that we can immediately see the values of the frequency and peak amplitude. A complete sine wave is represented by one spike. The position of the spike shows the frequency; its height shows the peak amplitude. Composite Signal A single-frequency sine wave is not useful in data communications; we need to send a composite signal, a signal made of many simple sine waves. In practice, an electromagnetic signal will be made up of many frequencies. For example, the signal is shown in Figure 3.4c. The components of this signal are just sine waves of frequencies f(amplitude=1volt) and 3f(amplitude=1/3 volts); parts (a) and (b) of the figure show these individual components. Dept of CSE UNIT-II NOTES Data Communications  There are two interesting points that can be made about this figure: The second frequency is an integer multiple of the first frequency. When all of the frequency components of a signal are integer multiples of one frequency, the first frequency is referred to as the fundamental frequency. The period of the total signal is equal to the period of the fundamental frequency. The period of the component sin (2πft) is T=1/f, and the period of s (t) is also T, as can be seen from Figure. The above composite signal time domain plot is represented in frequency domain as follows: Dept of CSE UNIT-II NOTES Data Communications  We can say that for each signal, there is a time domain function S(t) that specifies the amplitude of the signal at each instant in time. Similarly, there is a frequency domain function S(f) that specifies the peak amplitude of the constituent frequencies of the signal. The spectrum of a signal is the range of frequencies that it contains. For the signal of Figure, the spectrum extends from f to 3f. The bandwidth of a signal is the width of the spectrum i.e., range of frequencies contained in a composite signal. NOTE: One final term to define is dc component. If a signal includes a component of zero frequency (i.e., horizontal line in time-domain plot), that component is a direct current (dc) or constant component. Composite signal can be periodic or nonperiodic. A periodic composite signal should be in the form of i.e., frequency component should be in odd multiples of fundamental frequency. i.e., f(fundamental frequency),3f, 5f, 7f and so on. Relationship between Data Rate and Bandwidth Bandwidth of a composite signal is width of the spectrum i.e., the difference between the highest frequency and the lowest frequencies contained in that signal. For above composite signal the bandwidth is 3f-1f=2f. If the composite signal, is made of signals with wide range of frequencies, more data can be sent with that signal, so data rate increases. i.e., if bandwidth increases data rate increases. The frequency components of the square wave with amplitudes A and –A can be expressed as follows: Bandwidth of the above composite signal, which is difference between highest and lowest frequencies is 5f-1f=4f. Dept of CSE UNIT-II NOTES Data Communications  Bandwidth of this signal is 7f-1f=6f. As we can see the range of frequencies is increasing for the composite signal the shape of the resulting waveform is reasonably close to that of the square wave.(but this is composite analog signal but not digital signal) NOTE: Composite analog signal is almost like a square wave, but have continuous amplitude values but does not have discrete values of amplitude like digital signals. Suppose that we are using a digital transmission system that is capable of transmitting signals with a bandwidth of 4 MHz. Let us attempt to transmit a sequence of alternating 1s and 0s as the square wave of Figure. What data rate can be achieved? Case I Let us approximate our square wave with the waveform of Figure 3.7a. Although this waveform is a “distorted” square wave, it is sufficiently close to the square wave that a receiver should be able to discriminate between a binary 0 and a binary 1.  If we let f=106 cycles/second=1MHz, then the bandwidth of the signal is 5X106 – 106=4MHz.  For f=1MHz, the period of the fundamental frequency is T = 1/106= 10-6 = 1 µs.  If we treat this waveform as a bit string of 1s and 0s, one bit occurs every 0.5 µs, for a data rate of 2X106=2Mbps. Thus for a bandwidth of 4MHz, a data rate of 2Mbps is achieved. Dept of CSE UNIT-II NOTES Data Communications Case II Let us approximate our square wave with the waveform of Figure 3.7a, but now with f =2MHz.  If we let f=2MHz(2x106 cycles/second), then the bandwidth of the signal is 5x2x106 – 2x106=8MHz.  Using the same line of reasoning as before, the bandwidth of the signal is (5x2x106) – (2x106) =8MHz.  But in this case T=1/f=0.5 µs. As a result, one bit occurs every 0.25 µs for a data rate of 4 Mbps. Thus, other things being equal, by doubling the bandwidth, we double the potential data rate. Case III  If we let f=2MHz(2x106 cycles/second), then the bandwidth of the signal is (3x2x106) – (2x106) =4MHz. Assume as in Case II that f =2MHz (2x106 cycles/second) and T=1/f=0.5 µs, so that one bit occurs every 0.25 µs for a data rate of 4 Mbps. The bandwidth of the signal is (3x2x106) – (2x106) =4MHz. Thus, a given bandwidth can support various data rates depending on the ability of the receiver to discern the difference between 0 and 1 in the presence of noise and other impairments. To summarize, Case I: Bandwidth = 4 MHz; data rate = 4 Mbps Case II: Bandwidth = 8 MHz; data rate = 4 Mbps Case III: Bandwidth = 4 MHz; data rate = 2 Mbps Limiting the bandwidth creates distortions, which makes the task of interpreting the received signal more difficult. The more limited the bandwidth, the greater the distortion, and the greater the potential for error by the receiver. One more illustration should serve to reinforce these concepts. Figure 3.8 shows a digital bit stream with a data rate of 2000 bits per second. With a bandwidth of 2500 Hz, or even 1700 Hz, the representation is quite good. Furthermore, we can generalize these results. If the data rate of the digital signal is W bps, then a very good representation can be achieved with a bandwidth of 2W Hz. Dept of CSE UNIT-II NOTES Data Communications  Thus, there is a direct relationship between data rate and bandwidth: The higher the data rate of a signal, the greater is its required effective bandwidth. Looked at the other way, the greater the bandwidth of a transmission system, the higher is the data rate that can be transmitted over that system. NOTE: Because bandwidth is a scarce resource, we would like to maximize the data rate that is achieved in a given bandwidth. 1.3 ANALOG AND DIGITAL DATA TRANSMISSION The terms analog and digital correspond, roughly, to continuous and discrete, respectively. These two terms are used frequently in data communications in at least three contexts: data, signaling, and transmission. Data - Entities that convey meaning, or information. Signals are electric or electromagnetic representations of data. Signaling is the physical propagation of the signal along a suitable medium. Transmission is the communication of data by the propagation and processing of signals. Dept of CSE UNIT-II NOTES Data Communications ANALOG AND DIGITAL DATA Analog data take on continuous values in some interval. For example, voice and video are continuously varying patterns of intensity. Most data collected by sensors, such as temperature and pressure, are continuous valued. Digital data take on discrete values; examples are text and integers. Analog data example: Audio It is in the form of acoustic sound waves, can be perceived directly by human beings. Below figure shows the acoustic spectrum for human speech and for music. Frequency components of typical speech may be found between approximately 100 Hz and 7 kHz. Although much of the energy in speech is concentrated at the lower frequencies, tests have shown that frequencies below 600 or 700 Hz add very little to the intelligibility of speech to the human ear. Analog data example: Video  Here it is easier to characterize the data in terms of the TV screen (destination) rather than the original scene (source) recorded by the TV camera.  To produce a picture on the screen, an electron beam scans across the surface of the screen from left to right and top to bottom.  For black-and-white television, the amount of illumination produced (on a scale from black to white) at any point is proportional to the intensity of the beam as it passes that point.  Thus at any instant in time the beam takes on an analog value of intensity to produce the desired brightness at that point on the screen. Dept of CSE UNIT-II NOTES Data Communications  Figure depicts the scanning process.  At the end of each scan line, the beam is swept rapidly back to the left (horizontal retrace).  When the beam reaches the bottom, it is swept rapidly back to the top (vertical retrace).  The beam is turned off (blanked out) during the retrace intervals.  To achieve adequate resolution, the beam produces a total of 483 horizontal lines at a rate of 30 complete scans of the screen per second. Tests have shown that this rate will produce a sensation of flicker rather than smooth motion.  To provide a flicker-free image without increasing the bandwidth requirement, a technique known as interlacing is used.  The odd numbered scan lines and the even numbered scan lines are scanned separately, with odd and even fields alternating on successive scans.  The odd field is the scan from A to B and the even field is the scan from C to D.  The beam reaches the middle of the screen’s lowest line after 241.5 lines. At this point, the beam is quickly repositioned at the top of the screen and recommences in the middle of the screen’s topmost visible line to produce an additional 241.5 lines interlaced with the original set.  Thus the screen is refreshed 60 times per second rather than 30, and flicker is avoided. NOTE: Appearance of unsteadiness in an image on a display screen, which occurs when video refresh rate is too low. Dept of CSE UNIT-II NOTES Data Communications Digital data example: Text  A familiar example of digital data is text or character strings.  A number of codes have been devised by which characters are represented by a sequence of bits. Perhaps the earliest common example of this is the Morse code.  Today, the most commonly used text code is the International Reference Alphabet (IRA). Each character in this code is represented by a unique 7-bit pattern; thus 128 different characters can be represented. This is a larger number than is necessary, and some of the patterns represent invisible control characters.  IRA-encoded characters are almost always stored and transmitted using 8 bits per character. The eighth bit is a parity bit used for error detection. This bit is set such that the total number of binary 1s in each octet is always odd (odd parity) or always even (even parity).  Thus a transmission error that changes a single bit, or any odd number of bits, can be detected. ANALOG AND DIGITAL SIGNALS Discuss various characteristics of Analog and Digital Signal [6Marks] In a communications system, data are propagated from one point to another by means of electromagnetic signals. Analog signal An analog signal is a continuously varying electromagnetic wave that may be propagated over a variety of media, depending on spectrum; examples are wire media, such as twisted pair and coaxial cable; fiber optic cable; and unguided media, such as atmosphere or space propagation. Digital signal A digital signal is a sequence of voltage pulses that may be transmitted over a wire medium; for example, a constant positive voltage level may represent binary 0 and a constant negative voltage level may represent binary 1. Bit Rate: Most digital signals are nonperiodic, and thus period and frequency are not appropriate characteristics. Another term-bit rate (instead of frequency)-is used to describe digital signals. The bit rate is the number of bits sent in 1second, expressed in bits per second (bps). Figure 3.16 shows the bit rate 8bps for signals. Dept of CSE UNIT-II NOTES Data Communications Bit Length: We discussed the concept of the wavelength for an analog signal: the distance one cycle occupies on the transmission medium. We can define something similar for a digital signal: the bit length. The bit length is the distance one bit occupies on the transmission medium. Bit length =propagation speed x bit duration NOTE: Analog signals use Broadband (use bandpass channel), which supports for FDM(frequency division multiplexing) and digital signals use baseband(use low pass channel),which supports for TDM(time division multiplexing) Advantages of digital signal  It is generally cheaper than analog signaling  It is less susceptible to noise interference  Easy to interpret the data of distorted digital signal at the receiver side, than the data of distorted analog signal.  Repeaters can be used to overcome attenuation, which regenerates signal with original strength without noise, where as for analog signal only amplifiers (which increase the strength of both signal and noise) can be used but not repeaters. Disadvantages of digital signal  It suffers more from attenuation than do analog signals Figure shows a sequence of voltage pulses, generated by a source using two voltage levels, and the received voltage some distance down a conducting medium. Because of the attenuation, or reduction, of signal strength at higher frequencies, the pulses become rounded and smaller. It should be clear that this attenuation can lead rather quickly to the loss of the information contained in the propagated signal. Examples for Analog signals: 1. Audio Signals The most familiar example of analog information is audio, or acoustic, information, which, in the form of sound waves, can be perceived directly by human beings. One form of acoustic information, of course, is human speech. This form of information is easily converted to an electromagnetic signal for transmission (Figure 3.12). In essence, all of the sound frequencies, whose amplitude is measured in terms of loudness, are converted into electromagnetic frequencies, whose amplitude is measured in volts. The telephone handset contains a simple mechanism for making such a conversion. Dept of CSE UNIT-II NOTES Data Communications  Conversion of Voice Input to Analog Signal The spectrum of speech is approximately 100 Hz to 7 kHz, although a much narrower bandwidth will produce acceptable voice reproduction. The standard spectrum for a voice channel is 300 to 3400 Hz. The telephone transmitter converts the incoming acoustic voice signal into an electromagnetic signal over the range 300 to 3400 Hz. 2. Video Signals  Another common example of analog data is video, as seen on a TV screen. Now consider video signal, produced by a TV camera. It is also example for analog data.  The US standard is 525 lines, with 42 lost during vertical retrace. Thus the horizontal scanning frequency is (525 lines)  (30 scan/s) = 15,750 lines per second, or 63.5 µs/line.  Of the 63.5 µs, about 11 µs are allowed for horizontal retrace, leaving a total of 52.5 µs per video line.  To estimate the bandwidth needed use max frequency when lines alternate black & white. The subjective resolution is about 70% of 525-42, or about 338 lines.  Want horizontal and vertical resolutions about the same, and ratio of width to height of a TV screen is 4 : 3, so the horizontal resolution computes to about 4/3  338 = 450 lines.  As a worst case, a scanning line would be made up of 450 elements alternating black and white, ie 450/2 = 225 cycles of the wave in 52.5 µs, for a maximum frequency of about 4.2 MHz. Examples for Digital signals: Binary data is generated by terminals, computers, and other data processing equipment and then converted into digital voltage pulses for transmission, as illustrated in Figure. A commonly used signal for such data uses two constant (dc) voltage levels, one level for binary 1 and one level for binary 0. Conversion of PC Input to Digital Signal Dept of CSE UNIT-II NOTES Data Communications Conversion of Data and Signals Analog signals used to represent analog data and digital signals used to represent digital data. Digital data can also be represented by analog signals by use of a modem (modulator/demodulator). The modem converts a series of binary (two-valued) voltage pulses into an analog signal by encoding the digital data onto a carrier frequency. At the other end of the line, another modem demodulates the signal to recover the original data. Analog data can be represented by digital signals. The device that performs this function for voice data is a codec (coder-decoder). In essence, the codec takes an analog signal that directly represents the voice data and approximates that signal by a bit stream. At the receiving end, the bit stream is used to reconstruct the analog data. Dept of CSE UNIT-II NOTES Data Communications ANALOG AND DIGITAL TRANSMISSION Both analog and digital signals may be transmitted on suitable transmission media. The way these signals are treated is a function of the transmission system. Analog transmission  It is a means of transmitting analog signals without regard to their content; the signals may represent analog data (e.g., voice) or digital data (e.g., binary data that pass through a modem).  To achieve longer distances, the analog transmission system includes amplifiers that boost the energy in the signal.  Unfortunately, the amplifier also boosts the noise components. With amplifiers cascaded to achieve long distances, the signal becomes more and more distorted. Digital transmission  It assumes a binary content to the signal. A digital signal can be transmitted only a limited distance before attenuation, noise, and other impairments endanger the integrity of the data.  To achieve greater distances, repeaters are used.  A repeater receives the digital signal, recovers the pattern of 1s and 0s, and retransmits a new signal. Thus the attenuation is overcome. Dept of CSE UNIT-II NOTES Data Communications  Both long-haul telecommunications facilities and intrabuilding services have moved to digital transmission and, where possible, digital signaling techniques. The most important reasons are as follows: Digital technology: The advent of large-scale integration (LSI) and very-large-scale integration (VLSI) technology has caused a continuing drop in the cost and size of digital circuitry. Analog equipment has not shown a similar drop. Data integrity: With the use of repeaters rather than amplifiers, the effects of noise and other signal impairments are not cumulative. Thus it is possible to transmit data longer distances and over lower quality lines by digital means while maintaining the integrity of the data. Capacity utilization: It has become economical to build transmission links of very high bandwidth, including satellite channels and optical fiber. A high degree of multiplexing is needed to utilize such capacity effectively, and this is more easily and cheaply achieved with digital (time division) rather than analog (frequency division) techniques. This is explored in Chapter 8. Security and privacy: Encryption techniques can be readily applied to digital data and to analog data that have been digitized. Integration: By treating both analog and digital data digitally, all signals have the same form and can be treated similarly. Thus economies of scale and convenience can be achieved by integrating voice, video, and digital data. 2.TRANSMISSION IMPAIRMENTS Explain different types of transmission impairments [8 Marks] With any communications system, the signal that is received may differ from the signal that is transmitted due to various transmission impairments. For analog signals, these impairments can degrade the signal quality. For digital signals, bit errors may be introduced, such that a binary 1 is transformed into a binary 0 or vice versa. The most significant impairments are Attenuation and attenuation distortion Delay distortion Noise Attenuation  The strength of a signal falls off with distance over any transmission medium. Attenuation means a loss of energy. When a signal, simple or composite, travels through a medium, it loses some of its energy in overcoming the resistance of the medium. To compensate for this loss, amplifiers are used to amplify the signal. Figure 3.26 shows the effect of attenuation and amplification. Dept of CSE UNIT-II NOTES Data Communications   For guided media, this reduction in strength, or attenuation, is generally exponential and thus is typically expressed as a constant number of decibels per unit distance.  For unguided media, attenuation is a more complex function of distance and the makeup of the atmosphere. Decibel:  To show that a signal has lost or gained strength, the unit of the decibel is used.  The decibel (dB) measures the relative strengths of two signals or one signal at two different points. Note that the decibel is negative if a signal is attenuated and positive if a signal is amplified.  Variables PI and P2 are the powers of a signal at points 1 and 2, respectively.  Because power is proportional to the square of the voltage, the formula in terms of voltage is dB =20 log 10 (V2IV1). Example 1: Suppose a signal travels through a transmission medium and its power is reduced to one-half. This means that P2 = ½ P1In this case, the attenuation (loss of power) can be calculated as A loss of 3 dB (-3 dB) is equivalent to losing one-half the power. Example 2: A signal travels through an amplifier, and its power is increased 10 times. This means that P2=10 P1. In this case, the amplification (gain of power) can be calculated as Attenuation introduces three considerations for the transmission engineer. 1) A received signal must have sufficient strength so that the electronic circuitry in the receiver can detect the signal 2) The signal must maintain a level sufficiently higher than noise to be received without error 3) Attenuation varies with frequency  The first and second problems are dealt with by attention to signal strength and the use of amplifiers or repeaters.  The third problem is particularly noticeable for analog signals. Because the attenuation varies as a function of frequency, the received signal is distorted, reducing intelligibility. To overcome this problem, techniques are available for equalizing attenuation across a band of frequencies. Another approach is to use amplifiers that amplify high frequencies more than lower frequencies. Dept of CSE UNIT-II NOTES Data Communications Delay Distortion Distortion means that the signal changes its form or shape.  Distortion can occur in a composite signal made of different frequencies. Each signal component has its own propagation speed (see the next section) through a medium and, therefore, its own delay in arriving at the final destination.  Differences in delay may create a difference in phase if the delay is not exactly the same as the period duration. In other words, signal components at the receiver have phases different from what they had at the sender. The shape of the composite signal is therefore not the same. Figure 3.28 shows the effect of distortion on a composite signal.  Delay distortion occurs because the velocity of propagation of a signal through a guided medium varies with frequency.  For a band limited signal, the velocity tends to be highest near the center frequency and fall off toward the two edges of the band. Thus various frequency components of a signal will arrive at the receiver at different times, resulting in phase shifts between the different frequencies.  Delay distortion is particularly critical for digital data. Consider that a sequence of bits is being transmitted, using either analog or digital signals. Because of delay distortion, some of the signal components of one bit position will spill over into other bit positions, causing intersymbol interference, which is a major limitation to maximum bit rate over a transmission channel.  Equalizing techniques can also be used for delay distortion. Noise  For any data transmission event, the received signal will consist of the transmitted signal, modified by the various distortions imposed by the transmission system, plus additional unwanted signals that are inserted somewhere between transmission and reception.  The latter, undesired signals are referred to as noise. Noise is the major limiting factor in communications system performance. Dept of CSE UNIT-II NOTES Data Communications   Noise may be divided into four categories: Thermal noise Intermodulation noise Crosstalk Impulse noise Thermal noise  It is due to thermal agitation of electrons. It is present in all electronic devices and transmission media and is a function of temperature.  Thermal noise is uniformly distributed across the bandwidths typically used in communications systems and hence is often referred to as white noise.  The amount of thermal noise to be found in a bandwidth of 1 Hz in any device or conductor is Where, The noise is assumed to be independent of frequency. Thus the thermal noise in watts present in a bandwidth of B Hertz can be expressed as Note: A Joule (J) is the International System (SI) unit of electrical, mechanical, and thermal energy. A Watt is the SI unit of power, equal to one Joule per second. The kelvin (K) is the SI unit of thermodynamic temperature. For a temperature in kelvins of T, the corresponding temperature in degrees Celsius is equal to T - 273.15. Dept of CSE UNIT-II NOTES Data Communications Intermodulation noise  When signals at different frequencies share the same transmission medium, the result may be intermodulation noise.  The effect of intermodulation noise is to produce signals at a frequency that is the sum or difference of the two original frequencies or multiples of those frequencies.  For example, the mixing of signals at frequencies f1 and f2 might produce energy at the frequency f1+f2. This derived signal could interfere with an intended signal at the frequency f1+f2.  Intermodulation noise is produced by nonlinearities in the transmitter, receiver, and/or intervening transmission medium. Crosstalk  Crosstalk has been experienced by anyone who, while using the telephone, has been able to hear another conversation; it is an unwanted coupling between signal paths.  It can occur by electrical coupling between nearby twisted pairs or, rarely, coax cable lines carrying multiple signals.  Crosstalk can also occur when microwave antennas pick up unwanted signals; although highly directional antennas are used, microwave energy does spread during propagation. Impulse noise  All of the types of noise discussed so far have reasonably predictable and relatively constant magnitudes. Thus it is possible to engineer a transmission system to cope with them.  Impulse noise, however, is noncontinuous, consisting of irregular pulses or noise spikes of short duration and of relatively high amplitude.  It is generated from a variety of causes, including external electromagnetic disturbances, such as lightning, and faults and flaws in the communications system.  Impulse noise is generally only a minor annoyance. Dept of CSE UNIT-II NOTES Data Communications  The above figure is an example of the effect of noise on a digital signal. Here the noise consists of a relatively modest level of thermal noise plus occasional spikes of impulse noise. The digital data can be recovered from the signal by sampling the received waveform once per bit time. As can be seen, the noise is occasionally sufficient to change a 1 to a 0 or a 0 to a 1. 2.1 CHANNEL CAPACITY The maximum rate at which data can be transmitted over a given communication path, or channel, under given conditions, is referred to as the channel capacity. There are four concepts here that we are trying to relate to one another. Data rate: The rate, in bits per second (bps), at which data can be communicated Bandwidth: The bandwidth of the transmitted signal as constrained by the transmitter and the nature of the transmission medium, expressed in cycles per second, or Hertz Noise: The average level of noise over the communications path Error rate: The rate at which errors occur, where an error is the reception of a 1 when a 0 was transmitted or the reception of a 0 when a 1 was transmitted. The problem we are addressing is this: Communications facilities are expensive and, in general, the greater the bandwidth of a facility, the greater the cost. Furthermore, all transmission channels of any practical interest are of limited bandwidth. The limitations arise from the physical properties of the transmission medium or from deliberate limitations at the transmitter on the bandwidth to prevent interference from other sources. For digital data, this means that we would like to get as high a data rate as possible at a particular limit of error rate for a given bandwidth. NYQUIST BANDWIDTH Define the channel capacity with Nyquist rate and Shannon Hartley rate [4 Marks]  In signal processing, the Nyquist rate, named after Harry Nyquist. In this case assumption of a channel is noise free; the limitation on data rate is simply the bandwidth of the signal.  For a noiseless channel, the Nyquist bit rate formula defines the theoretical maximum bit rate Bit Rate = 2 x bandwidth x log2 L Or Where, C is channel capacity or data rate, M is the number of levels & B is bandwidth.  In this formula, bandwidth is the bandwidth of the channel, L is the number of signal levels used to represent data, and Bit Rate is the bit rate in bits per second. Dept of CSE UNIT-II NOTES Data Communications  According to the formula, we might think that, given a specific bandwidth, we can have any bit rate we want by increasing the number of signa11eve1s. Although the idea is theoretically correct, practically there is a limit. When we increase the number of signal1eve1s, we impose a burden on the receiver. If the number of levels in a signal is just 2, the receiver can easily distinguish between a 0 and a 1. If the level of a signal is 64, the receiver must be very sophisticated to distinguish between 64 different levels. In other words, increasing the levels of a signal reduces the reliability of the system. Eg: 1 bit value (0 to 1) = 21 (M= 2 levels) 2 bit value (00 to 11) = 22(M= 4 levels) 3 bit value (000 to 111) = 23(M= 8 levels) EXAMPLE PROBLEM: Consider a voice channel being used, via modem, to transmit digital data. Assume a bandwidth of 3100 Hz. Then the Nyquist capacity, C, of the channel is 2B=6200bps. For M=8, a value used with some modems, C becomes 18,600 bps for a bandwidth of 3100 Hz. SHANNON CAPACITY  Nyquist’s formula indicates that, doubling the bandwidth doubles the data rate.  For a given level of noise we would expect that greater signal strength would improve the ability to receive data correctly in the presence of noise.  The key parameter involved in this reasoning is the signal-to-noise ratio (SNR, or S/N), which is the ratio of the power in a signal to the power contained in the noise that is present at a particular point in the transmission.  For convenience, this ratio is often reported in decibels:  This expresses the amount, in decibels, that the intended signal exceeds the noise level. A high SNR will mean a high-quality signal and a low number of required intermediate repeaters.  The signal-to-noise ratio is important in the transmission of digital data because it sets the upper bound on the achievable data rate. Shannon’s result is that the maximum channel capacity, in bits per second, obeys the equation ------------------------(Equation 1) where C is the capacity of the channel in bits per second and B is the bandwidth of the channel in Hertz.  The Shannon formula represents the theoretical maximum that can be achieved. In practice, however, only much lower rates are achieved.  One reason for this is that the formula assumes white noise (thermal noise). Impulse noises are not accounted for, nor are attenuation distortion or delay distortion. Dept of CSE UNIT-II NOTES Data Communications The Expression Eb/N0  Finally, we mention a parameter related to SNR that is more convenient for determining digital data rates and error rates and that is the standard quality measure for digital communication system performance.  The parameter is the ratio of signal energy per bit to noise power density per Hertz, Eb/N0.  Consider a signal, digital or analog, that contains binary digital data transmitted at a certain bit rate R. Recalling that 1 Watt= 1 J/s, the energy per bit in a signal is given by Eb=STb, where S is the signal power and Tb is the time required to send one bit.  The data rate R is just R = 1/Tb. Thus, Dept of CSE UNIT-II NOTES Data Communications  The ratio Eb/N0 is important because the bit error rate for digital data is a (decreasing) function of this ratio. Given a value of Eb/N0 needed to achieve a desired error rate, the parameters in the preceding formula may be selected. Note that as the bit rate R increases, the transmitted signal power, relative to noise, must increase to maintain the required Eb/N0. The advantage of Eb/N0 over SNR is that the latter quantity depends on the bandwidth. We can relate Eb/N0 to SNR as follows.We have The parameter N0 is the noise power density in Watts/Hertz. Hence, the noise in a signal with bandwidth B is N=N0B. Substituting, we have ---------------------------------(Equation 2) Another formulation of interest relates Eb/N0 to spectral efficiency. Shannon’s result (Equation C=B log2(1+SNR) can be rewritten as: Using Equation (2), and equating R with C, we have This is a useful formula that relates the achievable spectral efficiency C/B to Eb/N0. Dept of CSE UNIT-II NOTES Data Communications 2.2 TRANSMISSION MEDIA: GUIDED AND UNGUIDED TRANSMISSION MEDIA Explain various Transmission Medias in detail. [12 Marks] Transmission media (Transmission system) is one of the components of communication model. A transmission medium carries information from a source to a destination, is located below the physical layer and is directly controlled by the physical layer. Example: The transmission medium is usually free space, metallic (twisted pair or coaxial) cable, or fiber-optic cable.  The information is represented as signal, and is propagated through the medium (such as electrical or optical signal) from one device to other. CLASSIFICATION OF TRANSMISSION MEDIA Depending on how the data (signal) is transferred from source to destination Transmission media is basically classified into two types: 1. Guided or Wired Transmission Media: If the physical path (wire) is used to transmit data between the source and destination i.e., data in the form of electromagnetic waves are guided along a solid medium, such as copper twisted pair, copper coaxial cable and optical fibre, then it is known as guided media. 2. Unguided or Wireless Transmission Media: Unguided media or wireless transmission occurs using the atmosphere, outer space, or water i.e., no physical path is used to transfer the data between source and destination.  The characteristics and quality of a data transmission are determined both by the characteristics of the medium and the characteristics of the signal.  In the case of guided media, the medium itself is more important in determining the transmission characteristics.  For unguided media, the bandwidth of the signal produced by the transmitting antenna is more important than the medium in determining transmission characteristics. Dept of CSE UNIT-II NOTES Data Communications DESIGN FACTORS In considering the design of data transmission systems, key concerns are data rate and distance: the greater the data rate and distance the transmission media is considered as more efficient. A number of design factors that determine the data rate and distance are: Bandwidth: The greater the bandwidth of a signal, the higher the data rate that can be achieved. Transmission impairments: Impairments, such as attenuation, limit the distance. For guided media, twisted pair generally suffers more impairment than coaxial cable, which in turn suffers more than optical fibre. Interference: Interference is addition of unwanted signal to useful data signal which makes the signal, which is transmitting from source destination distorted.  Proper shielding of a guided medium can minimize this problem. Number of receivers: A guided medium can be used to construct a point to point link or a shared link with multiple attachments to improve transmission system utilization. But, in the latter case, each attachment introduces some attenuation and distortion on the line, limiting distance or data rate. WIRELESS PROPAGATION A signal radiated from an antenna travels along one of three routes: 1. Ground Wave 2. Sky Wave 3. Line Of Sight (LoS) Ground Wave Propagation Ground wave propagation more or less follows the contour of the earth and can propagate considerable distances, well over the visual horizon. This effect is found in frequencies up to about 2 MHz. Several factors account for the tendency of electromagnetic wave in this frequency band to follow the earth’s curvature. One factor is that the electromagnetic wave induces a current in the earth’s surface, the result of which is to slow the wavefront near the earth, causing the wavefront to tilt downward and hence follow the earth’s curvature. Another factor is diffraction, which is a phenomenon having to do with the behavior of electromagnetic waves in the presence of obstacles. Electromagnetic waves in this frequency range are scattered by the atmosphere in such a way that they do not penetrate the upper atmosphere. The best-known example of ground wave communication is AM radio. Sky Wave Propagation Sky wave propagation is used for amateur radio, CB radio, and international broadcasts such as BBC and Voice of America. With sky wave propagation, a signal from an earth-based antenna is reflected from the ionized layer of the upper atmosphere (ionosphere) back down to earth. Dept of CSE UNIT-II NOTES Data Communications  Although it appears the wave is reflected from the ionosphere as if the ionosphere were a hard reflecting surface, the effect is in fact caused by refraction. Refraction is described subsequently. A sky wave signal can travel through a number of hops, bouncing back and forth between the ionosphere and the earth’s surface.With this propagation mode, a signal can be picked up thousands of kilometers from the transmitter. Line-of-Sight Propagation Above 30 MHz, neither ground wave nor sky wave propagation modes operate and communication must be by line of sight (Figure 4.8c). For satellite communication, a signal above 30 MHz is not reflected by the ionosphere and therefore a signal can be transmitted between an earth station and a satellite overhead that is not beyond the horizon. For ground-based communication, the transmitting and receiving antennas must be within an effective line of sight of each other. The term effective is used because microwaves are bent or refracted by the atmosphere. The amount and even the direction of the bend depend on conditions, but generally microwaves are bent with the curvature of the earth and will therefore propagate farther than the optical line of sight. GUIDED MEDIA Discuss Guided media in detail. (Or) Define different guided media with their characteristics and limitations. [8 Marks] In guided media, the transmission medium is the physical path between transmitter and receiver. The three guided media commonly used for data transmission are twisted pair, coaxial cable, and optical fiber Dept of CSE UNIT-II NOTES Data Communications  Twisted-pair and coaxial cable use metallic (copper) conductors that accept and transport signals in the form of electric current(electrical signal). Optical fiber is a cable that accepts and transports signals in the form of light. TWISTED PAIR CABLE The least expensive and most widely used guided transmission medium is twisted pair. Physical Description: A twisted pair consists of two insulated copper wires arranged in a regular spiral pattern. A wire pair acts as a single communication link. Typically, a number of these pairs are bundled together into a cable by wrapping them in a tough protective sheath. Neighbouring pairs in a bundle typically have somewhat different twist lengths to reduce the crosstalk interference. The wires in a pair have thicknesses of from 0.4 to 0.9 mm. Applications It is the most commonly used medium in the telephone network and within buildings. In the telephone system, individual residential telephone sets are connected to the local telephone exchange, or “end office,” by twisted-pair wire. These are referred to as subscriber loops.  Twisted pair is also commonly used within a building for local area networks supporting personal computers. Data rates for such products are typically in the neighborhood of 100 Mbps. However for long-distance applications, twisted pair can be used at data rates of 4 Mbps or more.  Twisted pair is much less expensive than the other commonly used guided transmission media (coaxial cable, optical fiber) and is easier to work with.  Office buildings are prewired with excess unshielded twisted pair as this is the least expensive of all the transmission media commonly used for local area networks and is easy to work with and easy to install. Transmission Characteristics: Twisted pair may be used to transmit both analog and digital transmission.  The strength of a signal falls off with distance over any transmission medium known as attenuation. The attenuation for twisted pair is a very strong. For analog signals or digital signals, amplifiers & repeaters are required to overcome attenuation.  Compared to other commonly used guided transmission media (coaxial cable, optical fiber), twisted pair is limited in distance, bandwidth and data rate.  The medium is quite susceptible to interference i.e., unwanted external signal is added to the useful data signal. Dept of CSE UNIT-II NOTES Data Communications  Shielding the wire with metallic braid (sheathing) reduces interference and the use of different twist lengths in adjacent pairs reduces crosstalk. Unshielded and Shielded Twisted Pair: Twisted pair comes in two varieties: unshielded and shielded. Unshielded twisted pair (UTP) is ordinary telephone wire.  Unshielded twisted pair is subject to external electromagnetic interference, including interference from nearby twisted pair and from noise generated in the environment.  A way to improve the characteristics of this medium is to shield the twisted pair with a metallic braid or sheathing that reduces interference. This shielded twisted pair (STP) provides better performance at higher data rates. However, it is more expensive and more difficult to work with than unshielded twisted pair. Category 3 and Category 5 UTP EIA (Electronic Industry Association) recognizes three categories of UTP cabling: Category 3: UTP cables and associated connecting hardware whose transmission characteristics are specified up to 16 MHz Category 4: UTP cables and associated connecting hardware whose transmission characteristics are specified up to 20 MHz Category 5: UTP cables and associated connecting hardware whose transmission characteristics are specified up to 100 MHz Of these, it is Category 3 and Category 5 cable that have received the most attention for LAN applications. Category 3 found in abundance in most office buildings. Over limited distances, and with proper design, data rates of up to 16 Mbps should be achievable with Category 3. Category 5 is becoming standard for pre installation in new office buildings. Over limited distances, and with proper design, data rates of up to 100 Mbps are achievable with Category 5. A key difference between Category 3 and Category 5 cable is the number of twists in the cable per unit distance. Category 5 is much more tightly twisted, with a typical twist length of 0.6 to 0.85 cm, compared to 7.5 to 10 cm for Category 3. The tighter twisting of Category 5 is more expensive but provides much better performance than Category 3. COAXIAL CABLE Physical Description: Coaxial cable, like twisted pair, consists of two conductors, but is constructed differently to permit it to operate over a wider range of frequencies.  It consists of a hollow outer cylindrical conductor that surrounds a single inner wire conductor.  The inner conductor and the outer conductor are separated by the insulating material.  The outer conductor is covered with a jacket or shield. Dept of CSE UNIT-II NOTES Data Communications  A single coaxial cable has a diameter of from 1 to 2.5 cm. Coaxial cable can be used over longer distances and support more stations on a shared line than twisted pair. Applications Coaxial cable is a versatile transmission medium, used in a wide variety of applications.The most important of these are Television distribution Long-distance telephone transmission Local area networks  Coaxial cable is widely used as a means of distributing TV signals to individual homes—cable TV. Cable TV reaches almost as many homes and offices as the telephone. A cable TV system can carry dozens or even hundreds of TV channels at ranges up to a few tens of kilometers.  Coaxial cable has traditionally been an important part of the long-distance telephone network. Today, it faces increasing competition from optical fiber. Using frequency division multiplexing, a coaxial cable can carry over 10,000 voice channels simultaneously.  Coaxial cable is also commonly used for short-range connections between devices for local area networking. Transmission Characteristics Coaxial cable is used to transmit both analog and digital signals. Coaxial cable has frequency characteristics that are superior to those of twisted pair and can hence be used effectively at higher frequencies (i.e wider frequencies give higher bandwidth) and data rates.  Because of its shielded, concentric construction, coaxial cable is much less susceptible to interference and crosstalk than twisted pair.  The principal constraints on performance are attenuation, thermal noise, and intermodulation noise. For long-distance transmission, amplifiers & repeaters are needed for every few kilometers, with closer spacing required if higher frequencies are used & for higher data rates. Dept of CSE UNIT-II NOTES Data Communications OPTICAL FIBER Explain the characteristics of Optical Fiber cable. [4 Marks] (Or) What are the primary building blocks of the fiber optic cable? Explain. [6 Marks] Physical Description: An optical fibre is a thin (125micrometers), flexible medium capable of transferring information in the form of optical ray(light).  An optical fibre cable has a cylindrical shape and consists of three concentric sections: the core, the cladding, and the jacket (Figure 4.2c). The core is the innermost section, made of glass or plastic; Core is surrounded by its own cladding, a glass or plastic coating.The outermost layer, surrounding cladding, is the jacket. The jacket is composed of plastic and other material layered to protect against moisture, abrasion, crushing, and other environmental dangers. Various glasses and plastics can be used to make optical fibres.  Glass fibres are costly but provide good performance. Plastic fibre is less costly than glass fibre but performance is moderate. NOTE: Optical Fibre works on the principle of Total Internal Reflection (i.e., when light travels from denser to rarer medium following scenario occurs). If the angle of incidence I is less than the critical angle, the ray refracts and moves closer to the surface. If the angle of incidence is equal to the critical angle, the light bends along the interface. If the angle is greater than the critical angle, the ray reflects (makes a turn) and travels again in the denser substance. Note that the critical angle is a property of the substance, and its value differs from one substance to another. Dept of CSE UNIT-II NOTES Data Communications  Explain about construction of Optical fiber also list the advantages and disadvantage of Optical fiber cables. [6 Marks] Advantages Of optical Fibre: The following characteristics distinguish optical fiber from twisted pair or coaxial cable: Greater capacity: The bandwidth and data rate of optical fibre is immense. The performance is much higher when compared to the coaxial cable & twisted pair & travels for long distances. Smaller size and lighter weight: Optical fibres are thinner than coaxial cable or bundled twisted-pair cable. The corresponding reduction in weight and size helps for managing easily. Lower attenuation: Attenuation is significantly lower for optical fibre than for coaxial cable or twisted pair and is constant over a wide range. Electromagnetic isolation: Optical fibre systems are not affected by external electromagnetic fields. Thus the system is not vulnerable to interference. Even there is a high degree of security from eavesdropping i.e., the data sent by sender is seen only by the intended receiver. Greater repeater spacing: The data (optical signal) sent by optical fibre travels for long distances without attenuation. So, less number of repeaters are required for optical fibre i.e., repeater spacing is more than coaxial or twisted pair which reduces the cost of maintaining more repeaters. Applications: Optical fibres are used more in long-distance telecommunications, and its use in military applications is growing. The continuing improvements in performance and decline in prices, together with the inherent advantages of optical fiber, have made it increasingly attractive for local area networking. Five basic categories of application have become important for optical fibre: Long-haul trunks Metropolitan trunks Subscriber loops Local area networks  Long-haul fibre transmission is becoming increasingly common in the telephone network. Long-haul routes average about 1500 km in length and offer high capacity (typically 20,000 to 60,000 voice channels). Coaxial cable is rapidly being replaced by optical fibre in the telephone network.  Metropolitan trunking circuits have an average length of 12 km and may have as many as 100,000 voice channels in a trunk group, joining telephone exchanges in a metropolitan or city area.  Subscriber loop circuits are fibres that run directly from the central exchange to a subscriber. These facilities are beginning to replace twisted pair and coaxial cable links for the telephone networks.  A final important application of optical fibre is for local area networks. Standards have been developed and products introduced for optical fibre networks that have a total capacity of 10 Gbps. Dept of CSE UNIT-II NOTES Data Communications Transmission Characteristics Optical fiber transmits a signal-encoded beam of light by means of total internal reflection. Total internal reflection can occur in any medium that has a higher index of refraction (core) than the surrounding medium (cladding). Figure shows the principle of optical fibre transmission. Light from a source enters the cylindrical glass or plastic core. Rays which are incident at angles greater than critical angle are reflected and propagated along the fiber; other rays are absorbed by the surrounding material. Propagation Modes: Two modes multimode and single mode are supported for propagating light along the optical channels, each requiring the fibre with different physical characteristics.  Multimode: Multimode is so named because multiple beams from a light source move through the core in different paths. Multimode can be implemented in two forms: Step-index and graded index.  With Multimode Step-index transmission, multiple propagation paths exist, each with a different path length and so different time to traverse the fibre i.e., travel distance of signal is increased. This limits the rate at which data can be accurately received and sudden change in density of core and cladding also leads to distortion(i.e., the signal is changed from its original shape.) This type of fibre is best suited for transmission over very short distances.  When the fiber core radius is reduced, fewer angles will reflect. By reducing the radius of the core, only a single angle or mode can pass. This single-mode propagation provides superior performance for the following reason. Because there is a single transmission path with single- mode transmission, the distortion found in multimode cannot occur. Single-mode is typically used for long-distance applications, including telephone and cable television. But wider frequency range is not accepted as the radius of the core is reduced.  Finally, by varying the index of refraction of the core, a third type of transmission, known as graded-index multimode, is possible. This type is intermediate between the other two in characteristics.  The higher refractive index(the density) gradually reduces from the core to the cladding, which does not makes the light ray to travel in zig-zag path as there is no sudden change in density of core and cladding like step-index.  Light in the core curves helically because of the graded index, reducing its travel distance. The shortened path and higher speed allows light to arrive at a receiver at about the same time as the straight rays in the core axis. Graded-index fibers are often used in local area networks. Dept of CSE UNIT-II NOTES Data Communications  Two different types of light source are used in fiber optic systems: the light Emitting diode (LED) and the injection laser diode (ILD). Both are semiconductor devices that emit a beam of light when a voltage is applied. The LED is less costly, and has a longer operational life. The ILD, which operates on the laser principle, is more efficient and can sustain greater data rates. Disadvantages: 1. Scattering: Scattering refers to the change in direction of light rays after they strike small particles or impurities in the medium, scattering may also lead to data loss because of absorption of light by the medium. 2. Maintenance & Cost: Fibre-optical cable maintenance is difficult. The cable and the interfaces are relatively more expensive than those of other guided media. UNGUIDED OR WIRELESS TRANSMISSION MEDIA Unguided media transport electromagnetic waves without using a physical conductor. This type of communication is often referred to as wireless communication. Signals are normally broadcast through free space and thus are available to anyone who has a device capable of receiving them. Frequencies in the range of about 1 GHz to 40 GHz are referred to as microwave frequencies. Microwave is also used for satellite communications. Frequencies in the range of 30 MHz to 1 GHz are suitable for omni directional applications. We refer to this range as the radio range. Another important frequency range, for local applications, is the infrared portion of the spectrum. This covers, roughly, from 3x1011 to 2x1014Hz. Infrared is useful to local point-to-point and multipoint applications within confined areas, such as a single room. Dept of CSE UNIT-II NOTES Data Communications  For unguided media, transmission and reception are achieved by means of an antenna. Before looking at specific categories of wireless transmission, we provide a brief introduction to antennas. Antennas  An antenna can be defined as an electrical conductor or system of conductors used either for radiating electromagnetic energy or for collecting electromagnetic energy.  The simplest pattern is produced by an idealized antenna known as the isotropic antenna. An isotropic antenna is a point in space that radiates power in all directions equally. The actual radiation pattern for the isotropic antenna is a sphere with the antenna at the center. Parabolic Reflective Antenna An important type of antenna is the parabolic reflective antenna, which is used in terrestrial microwave and satellite applications. A parabola is the locus of all points equidistant from a fixed line and a fixed point not on the line. The fixed point is called the focus and the fixed line is called the directrix. If a parabola is revolved about its axis, the surface generated is called a paraboloid. A cross section through the paraboloid parallel to its axis forms a parabola and a cross section perpendicular to the axis forms a circle. Antenna Gain Antenna gain is a measure of the directionality of an antenna. Antenna gain is defined as the power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna). A concept related to that of antenna gain is the effective area of an antenna. The effective area of an antenna is related to the physical size of the antenna and to its shape. The relationship between antenna gain and effective area is Dept of CSE UNIT-II NOTES Data Communications  For example, the effective area of an ideal isotropic antenna is λ2/4π, with a power gain of 1; the effective area of a parabolic antenna with a face area of A is 0.56A, with a power gain of 7A/ λ2. Terrestrial Microwave Physical Description The most common type of microwave antenna is the parabolic “dish.”A typical size is about 3 m in diameter. The antenna is fixed rigidly and focuses a narrow beam to achieve line-of-sight transmission to the receiving antenna. Microwave antennas are usually located at substantial heights above ground level to extend the range between antennas and to be able to transmit over intervening obstacles. Applications  The primary use for terrestrial microwave systems is in long-haul telecommunications service, as an alternative to coaxial cable or optical fiber.  The microwave facility requires far fewer amplifiers or repeaters than coaxial cable over the same distance but requires line-of-sight transmission.  Microwave is commonly used for both voice and television transmission.  Another increasingly common use of microwave is for short point-to-point links between buildings. Transmission Characteristics Microwave transmission covers a substantial portion of the electromagnetic spectrum. Common frequencies used for transmission are in the range 1 to 40 GHz. The higher the frequency used, the higher the potential bandwidth and therefore the higher the potential data rate Table indicates bandwidth and data rate for some typical systems. As with any transmission system, a main source of loss is attenuation. For microwave (and radio frequencies), the loss can be expressed as Dept of CSE UNIT-II NOTES Data Communications  Where, d is the distance and λ is the wavelength, in the same units. Thus, loss varies as the square of the distance. Satellite Microwave Physical Description A communication satellite is, in effect, a microwave relay station. It is used to link two or more ground-based microwave transmitter/ receivers, known as earth stations, or ground stations. The satellite receives transmissions on one frequency band (uplink), amplifies or repeats the signal, and transmits it on another frequency (downlink). A single orbiting satellite will operate on a number of frequency bands, called transponder channels, or simply transponders. Figure depicts in a general way two common configurations for satellite communication. 1) The satellite is being used to provide a point-to-point link between two distant ground- based antennas. 2) The satellite provides communications between one ground-based transmitter and a number of ground-based receivers. For a communication satellite to function effectively, it is generally required that it remain stationary with respect to its position over the earth. Otherwise, it would not be within the line of sight of its earth stations at all times. To remain stationary, the satellite must have a period of rotation equal to the earth’s period of rotation. This match occurs at a height of 35,863 km at the equator. Dept of CSE UNIT-II NOTES Data Communications Applications The following are among the most important applications for satellites: Television distribution Long-distance telephone transmission Private business networks Global positioning Satellite transmission is also used for point-to-point trunks between telephone exchange offices in public telephone networks. There are a number of business data applications for satellite. The satellite provider can divide the total capacity into a number of channels and lease these channels to individual business users. A recent development is the very small aperture terminal (VSAT) system, which provides a low-cost alternative. Figure depicts a typical VSAT configuration. A number of subscriber stations are equipped with low-cost VSAT antennas. Using some discipline, these stations share a satellite transmission capacity for transmission to a hub station. The hub station can exchange messages with each of the subscribers and can relay messages between subscribers. A final application of satellites, which has become pervasive, is worthy of note. The Navstar Global Positioning System, or GPS for short, consists of three segments or components: A constellation of satellites (currently 27) orbiting about 20,000 km above the earth’s surface, which transmit ranging signals on two frequencies in the microwave part of the radio spectrum A control segment which maintains GPS through a system of ground monitor stations and satellite upload facilities The user receivers—both civil and military Dept of CSE UNIT-II NOTES Data Communications Transmission Characteristics The optimum frequency range for satellite transmission is in the range 1 to 10 GHz. Below 1 GHz, there is significant noise from natural sources, including galactic, solar, and atmospheric noise, and human made interference from various electronic devices. Above 10 GHz, the signal is severely attenuated by atmospheric absorption and precipitation. Broadcast Radio Physical Description The principal difference between broadcast radio and microwave is that the former is omni directional and the latter is directional. Thus broadcast radio does not require dish-shaped antennas, and the antennas need not be rigidly mounted to a precise alignment. Applications Radio is a general term used to encompass frequencies in the range of 3 kHz to 300 GHz. We are using the informal term broadcast radio to cover the VHF and part of the UHF band: 30 MHz to 1 GHz. This range covers FM radio and UHF and VHF television. This range is also used for a number of data networking applications. Transmission Characteristics The range 30 MHz to 1 GHz is an effective one for broadcast communications. Unlike the case for lower-frequency electromagnetic waves, the ionosphere is transparent to radio waves above 30 MHz. Thus transmission is limited to the line of sight, and distant transmitters will not interfere with each other due to reflection from the atmosphere. Infrared Infrared communications is achieved using transmitters/receivers (transceivers) that modulate non coherent infrared light. Transceivers must be within the line of sight of each other either directly or via reflection from a light-colored surface such as the ceiling of a room. One important difference between infrared and microwave transmission is that the former does not penetrate walls. Thus the security and interference problems encountered in microwave systems are not present. Furthermore, there is no frequency allocation issue with infrared, because no licensing is required. Dept of CSE UNIT-II NOTES Data Communications