Data Communications and Networks
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1 Data Communications and Networks Engr. Abdul Rahman Mahmood MS, MCP, QMR(ISO9001:2000) Usman Institute of Technology University Road, Karachi alphapeeler.sf.net/pubkeys/pkey.htm pk.linkedin.com/in/armahmood p p armahmood786@jabber.org alphapeeler@aim.com abdulmahmood sss alphasecure mahmood_cubix armahmood786@hotmail.com alphapeeler@icloud.com VC++, VB, ASP
2 Data Transmission Toto, I've got a feeling we're not in Kansas anymore. Judy Garland in The Wizard of Oz The successful transmission of data depends principally on two factors: the quality of the signal being transmitted and the characteristics of the transmission medium. The objective of this chapter and the next is to provide the reader with an intuitive feeling for the nature of these two factors.
3 Data Transmission Toto, I've Ive got a feeling we're not in Kansas anymore. Judy Garland in The Wizard of Oz The successful transmission of data depends principally on two factors: the quality of the signal bi being transmitted and the characteristics ti of the transmission medium. The objective of this chapter and the next is to provide the reader with an intuitive feeling for the nature of these two factors.
4 Transmission sso Terminology oogy data transmission occurs b/w transmitter & receiver via some medium guided medium eg. twisted pair, coaxial cable, optical fiber unguided / wireless medium eg. air, water, vacuum direct link no intermediate devices point to point direct link only 2 devices share link multi point more than two devices share the link simplex one direction. eg. television half duplex either direction, but only one way at a time, eg. police radio full duplex both directions at the same time, eg. telephone
5 Frequency, Spectrum and Bandwidth Time domain concepts analog signal; various in a smooth way over time digital signal; maintains a constant level then changes to another constant level periodic signal; pattern repeated over time aperiodic signal; pattern not repeated over time Analogue & Digital Signals
6 Periodic Signals Mathematically, a signal s(t) is defined to be periodic if and only if s(t + T) = s(t) -00 < t < +00 where the constant T is the period of the signal (T is the smallest value that satisfies the equation). Otherwise, a signal is aperiodic. Sine Wave peak amplitude (A): maximum strength of signal volts frequency (f): rate of change of signal Hertz (Hz) or cycles per second Period: time for one repetition (T) T = 1/f phase (φ): relative position in time
7 Varying Sine Waves s(t) = A sin(2πft +Φ)
8 Wavelength (λ) is distance occupied by one cycle between two points of corresponding phase in two consecutive cycles assuming signal velocity v have λ = vt or equivalently l λf = v especially when v=c c = 3*10 8 ms 1 (speed of light in free space)
9 Frequency Domain Concepts electromagnetic signal are made up of many frequencies When all of the frequency components of a signal are integer multiples of one frequency, the latter frequency is referred to as the fundamental frequency. Fourier analysis can shown that any signal is made up of component sine waves The period of the total t 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 next Figure.
10 Addition of Frequency Components (T=1/f) (c) is sum of f & 3f We can say that for each signal, there is a time domain function s(t) that atspecifies es 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
11 .
12 Frequency Domain Representations freq domain func of Fig 3.4c freq domain func of single square pulse
13 Spectrum & Bandwidth spectrum range of frequencies contained in signal absolute bandwidth width of spectrum effective bandwidth often just bandwidth narrow band of frequencies containing most energy DC Component component of zero frequency
14 frequency components of the square wave Consider Fig.3.2b 2bPositive i pulse = 0 and Negative pulse =1. waveform represents binary stream If T = 1/(2f f ); Then data rate = 2f bps What are the frequency components of this signal? To answer this question, consider again Figure 3.4. By adding together sine waves at frequencies f and 3f, we get a waveform that begins to resemble the original square wave. Let us continue this process by adding a sine wave of frequency 5f, as shown in Figure 3.7a, and then adding a sine wave of frequency 7f, as shown in Figure 3.7b.As we add additional odd multiples l of f, suitably scaled, the resulting waveform approaches that of a square wave more and more closely. Frequency components of the square wave with amplitudes A :
15
16 Data Rate and Bandwidth any transmission i system has a limited it band of frequencies this limits the data rate that can be carried (digital) square have infinite components and hence bandwidth but most energy in first few components If we attempt t to transmit this waveform as a signal over any medium, the transmission system will limit the bandwidth that can be transmitted For any given medium, Bandwidth Cost Bandwidth 1/distortion Bandwidth 1/error by the receiver Bandwidth data rate
17 Data Rate and Bandwidth ( I). 3.7(a): f=1mhz Bandwidth = (5 X 10^6) 10^6 = 4 MHz T=1/f = 10^ 6 = 1 µs [one bit occurs every 0.5 µ second; ve & +ve freq. in T] Data Rate = 1 / (T/2) bps = 1 / (1 µs /2) = 2 Mbps CASE II. 3.7(a) f =2MHz T=.5µs; Bandwidth = (5 X 2 X 10^6) 2 X 10^6 = 8 MHz T = 1/2f = 0.5 X 10^ 6 = 0.5 µs; [one bit btoccurs every eey µ second] Data Rate = 1 / (T/2) bps = 1 / (0.5µs /2) = 4 Mbps CASEIII. Fig 3.4c (4/ ) [sin(2 ft) + (1/3)sin(2 (3f)t)]; f =2MHz, T=.5µs; Bandwidth = (3 X 2 X 10^6) 2 X 10^6 = 4 MHz T = 1/2f = 0.5 X 10^ 6 = 0.5 µs; [one bit occurs every 0.25 µ second] Data Rate = 1 / (T/2) bps = 1 / (0.5µs /2) = 4 Mbps CASE I. 3.7(a) f =1MHz, T=1 µs; Bandwidth = 4MHz; data rate = 2Mbps CASE II. 3.7(a) f =2MHz, T=0.5µs; Bandwidth = 8MHz; data rate = 4Mbps CASE II. 3.4(c) f =2MHz, T=0.5µs; Bandwidth = 4MHz; data rate = 4Mbps Conclusion : higher the center frequency, the higher the potential bandwidth and therefore the higher the potential data rate.
18 Analog and Digital Data Transmission data as entities that convey meaning, or information signals electric or electromagnetic representations of data, physically propagates along medium signaling Signaling is the physical propagation of the signal along a suitable medium transmission Transmission communication of data by propagation and processing of signals In what follows, we try to make these abstract concepts clear by discussing the terms analog and digital as applied to data, signals, and transmission.
19 Acoustic Spectrum (Analog) coust c Spect u ( aog) Frequency components of speech (spectrum of speech) are between 100Hz - 7kHz; Frequencies below 600 or 700 Hz are hard to hear by human ear.
20 Video Interlaced Sg Signaling g 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. This rate produces a sensation of flicker rather than smooth motion. For flicker free image without increasing the bandwidth requirement, a technique known as interlacing is used.
21 Text etcode IRA, Attenuation Most popular example is Morse code. International Reference Alphabet (IRA). Each character in this code is represented td by a unique 7 bit pattern; 128 different characters IRA encoded characters are stored and transmitted using 8 bits/character. The eighth bit is a parity bit used for error detection. This bit is set such that the total number of binary 1 s 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. td Digital signaling are generally cheaper than analog signaling and is less susceptible to noise interference. The disadvantage is that digital signals suffer more from attenuation ti than do analog signals.
22 Audio Signals Frequency components of typical speech may be found between 100 Hz and 7 khz. easily converted into electromagnetic signals varying ay volume ou econverted to varying ay voltage otage Telephone handset limits frequency range for voice channel to Hz (within range 100Hz 7KHz)
23 Finding video signal bandwidth USA 483 lines per frame, at frames per sec have 525 lines but 42 lost during vertical retrace 525 lines x 30 scans per /sec = lines per sec 63.5µs per line [1/15750] 11µs for retrace, so = 52.5 µs per video line Find Max freq. if line alternates black & white No of lines = 70% of (525 42) = 338 If screen ratio is 4 : 3 Horizontal res. = 4/3 x 338 =450 Black &White lines Hence 450/2 = 225 cycles of wave in 52.5 µs [see above] T = 1 cycle is completed in = 52.5µs /225 = 0.23 µs f = 1/T = 1/ 0.23 µs = Hz ~ 4.2 MHz horizontal resolution is about 450 lines giving g225 cycles of wave in 52.5 µs; max frequency of 4.2MHz
24 Digital Data as generated by computers etc. has two dc components : 0 and 1 bandwidth depends on data rate
25 Analog aogsg Signals as
26 Digital gta Signals Sg as
27 Advantages & Disadvantages of Digital Signals cheaper less susceptible to noise but greater attenuation digital now preferred choice
28 Transmission Impairments signal received may differ from signal transmitted causing: analog degradation of signal quality digital bit errors most significant impairments are attenuation and attenuation distortion delay distortion noise
29 Attenuation signal strength falls with distance guided/unguided Attenuation depends on medium: It is exponential (guided); More complex (unguided). 3 considerations for Attenuation: 1. strong enough to be detected 2. sufficiently higher than noise to receive without error 3. Attenuation is increasing function of frequency [Analog] For 1 & 2: increase strength using amplifiers/repeaters, so equalize attenuation across band of frequencies used For 3 : use (1)loading coils (2)amplifiers for high frequencies For any other frequency f, the relative attenuation in decibels is A 1000 Hz tone of a given power level is applied to the input, and the power P 1000, is measured at the output.
30 Delay Distortion only occurs in guided media Occurs because propagation of a signal velocity varies with ih frequency hence various frequency components arrive at different times resulting in phase shifts particularly critical for digital data intersymbol interference : some of the signal components of one bit position will spill over into other bit positions, causing intersymbol interference.
31 Noise Noise: For any data transmission event, additional unwanted signals that are inserted somewhere between transmission and reception. (1) Thermal due to thermal agitation of electrons uniformly distributed d Also called white noise N o = kt(w/hz) N o = noise power density in watts per 1 Hz of bandwidth k = Boltzmann s constant = 1.38 * J/K T = temperature, in kelvins EXAMPLE 3.1 Room temperature is usually specified as T = 17 C, or 290 K. At this temperature, the thermal noise power density is N 0 = (1.38 X10 23 )X290 = 4 X10 21 W/Hz = 204 dbw/hz where dbw is the decibel watt, defined in Appendix 3A. Noise is assumed to be independent d of frequency. Thus the thermal noise in watts present in a bandwidth of B Hertz can be expressed as N = ktb or, in decibel watts : N = 10 log k + 10 log T + 10 log B N = dbw + 10 log T + 10 log B EXAMPLE 3.2 Given a receiver with an effective noise temperature of 294 K and a 10 MHz bandwidth, the thermal noise level at the receiver s output is N = dbw + 10 log(294) + 10 log 10 7 = = dbw
32 Noise ose (2)Intermodulation Noise: When signals at Effect of noise on a digital signal different frequencies share the same transmission medium; f1 + f2 (3)crosstalk: a signal from one line is picked up by another (4)impulse: irregular pulses or spikes eg. external electromagnetic interference short duration high amplitude a minor annoyance for analog signals but a major source of error in digital data a noise spike could corrupt many bits
33 Channel Capacity Channel Capacity is the max possible data rate on communication channel is a function of data rate in bits per second bandwidth in cycles per second or Hertz noise on comms link error rate of corrupted bits limitations are due to physical properties of medium we would like to make as efficient use as possible of a given bandwidth. The main constraint on achieving this efficiency is noise
34 Nyquist Bandwidth dt Consider noise free channels If rate of signal transmission is 2B then can carry signal with frequencies no greater than B Converse is also true: Given a bandwidth of B, the highest signal rate that t can be carried id is 2B. B for binary signals, 2B bps needs bandwidth B Hz can increase rate by using M signal levels (voltage) Nyquist Formula is: C = 2B log 2 M so increase rate by increasing signals EXAMPLE 3.3 Consider a voice channel being used, via modem, to transmit digital data. Assume B=3100 Hz, M=8. Then the Nyquist capacity, C, of the channel is C= 2Blog 2 M = 2X3100 log 2 8 = 6200 X 3 = 18,600 bps Online calculator :
35 Shannon Capacity Formula consider relation of data rate, noise & error rate faster data rate shortens each bit so bursts of noise affects more bits given noise level, higher rates means higher errors Shannon developed formula relating these to signal to noise ratio (in decibels) SNR db= 10 log 10 (signal/noise) Capacity C=B log 2 (1+SNR) theoretical maximum capacity get lower in practise
36 Homework #2 Submit answers to review questions/problems for Data Transmission of textbook. Rules: 1.Last date of submission i is 14 th of October Submit handwritten assignments only. 3.Assignments will not be accepted after due date. 4.Plagiarism, if detected, will result in zero marks! 5.Assignment must be submitted in a proper file cover, and must be labeled bld properly. 6.All answers in homework must also include questions numbered exactly as in the text book. 7.Cover : Student name, roll no, date of submission. 8.Print of this slide after cover page. 9.Selected homework will be converted to assignments
Data Communications and Networks
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