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Data Communications Theory Lecture-5

University of Palestine. Faculty of Information Technology. Data Communications Theory Lecture-5. Dr. Anwar Mousa. Communication Techniques. Major topics. This chapter presents Data encoding techniques: illustrates Analog encoding of digital data

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Data Communications Theory Lecture-5

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  1. University of Palestine Faculty of Information Technology Data Communications Theory Lecture-5 Dr. Anwar Mousa

  2. Communication Techniques

  3. Major topics • This chapter • presents Data encoding techniques: • illustrates Analog encoding of digital data • describes how analog data, such as voice, can be encoded by means of a codec so that it can be transmitted over digital facilities • explains the differences between asynchronous and synchronous transmission and when each technique is used • describes the process of error detection

  4. Data encoding techniques

  5. Encoding scheme • For digital data, • The mapping from binary digits to signal elements is the encoding scheme for transmission • encoding schemes are designed to minimize • errors in determining the start and end of each bit • errors in determining whether each bit is a 1 or a 0 • For analog data • encoding scheme is designed to enhance the quality, or fidelity, of transmission • the received analog data to be as close as possible to the transmitted data

  6. Analog encoding of digital data • Data encoding and decoding technique to represent digital data using the properties of analogwaves by using a modem • Modulation: the conversion of digital data to analog signal form • by using a constant-frequency signal known as a carrier signal • Demodulation: the conversion of analog signals back to digital data form

  7. Methods of modulation • Three basic forms of modulation of analog signals for digital data • Amplitude-shift keying (ASK) • Frequency-shift keying (FSK) • Phase-shift keying (PSK) • Quadrature Amplitude Modulation (QAM= combination of ASK &PSK) • These are the altering of the • amplitude, • frequency • phase of the carrier sine wave.

  8. Amplitude Shift Keying (ASK) • In radio transmission, known as amplitude modulation (AM) • The amplitude (or height) of the sine wavevaries to transmit the ones and zeros • Major disadvantage is that telephone lines are very susceptible to variations in transmission quality that can affect amplitude

  9. Amplitude Shift Keying (ASK) • ASK describes the technique where the carrier wave is multiplied by the digital signal . • Mathematically, the modulated carrier signal is:

  10. Amplitude Shift Keying (ASK)

  11. Amplitude Shift Keying (ASK)

  12. Frequency Shift Keying (FSK) • In radio transmission, known as frequency modulation (FM) • Frequency of the carrier wave varies in accordance with the signal to be sent • Signal transmitted at constant amplitude • More resistant to noise than ASK • Less attractive because it requires more analog bandwidth than ASK

  13. Frequency Shift Keying (FSK) • FSK describes the modulation of a carrier (or two carriers) by using a different frequency for a 1 or 0. • The resultant modulated signal may be regarded as the • sum of two amplitude modulated signals of different carrier frequency

  14. Frequency Shift Keying (FSK)

  15. Frequency Shift Keying (FSK)

  16. Frequency Shift Keying (FSK) • FSK is classified aswide-bandif the separation between the two carrier frequencies is larger than the bandwidth of the spectrums of f1(t) and f2(t) . • In this case the spectrum of the modulated signal appears as two separate ASK signals. • Narrow-bandFSK is the term used to describe an FSK signal whose carrier frequencies are separated by less than the width of the spectrum than ASK for the same modulation.

  17. Phase Shift Keying (PSK) • Frequency and amplitude of the carrier signal are kept constant • The carrier signal is shifted in phase according to the input data stream • Each phase can have a constant value, or value can be based on whether or not phase changes (differential keying)

  18. Phase Shift Keying (PSK) • PSK describes the modulation technique that alters the phase of the carrier. Mathematically: Binary phase-shift-keying, BPSKhas only two phases, 0 and taking the values -1 or 1 It is therefore a type of ASK with and its bandwidth is the same as that of ASK

  19. Phase Shift Keying (PSK) • Phase-shift-keying offers a simple way of increasing the number of levels in the transmission without increasing the bandwidth by introducing smaller phase shifts. Quadrature phase-shift-keying (QPSK) has four phases, M-ary PSK has M phases, For a given bit-rate, QPSK requires half the bandwidth of PSK and is widely used for this reason.

  20. Phase Shift Keying (PSK)

  21. PSK illustration 0 0 0 1 1

  22. Differential Phase Shift Keying (DPSK) 0 0 1 1 A 0 is represented by sending a signal burst of the same phase as the preceding signal burst sent. A 1 is represented by sending a signal burst of opposite phase to the preceding one.

  23. Signaling Rate • The number of times the signal parameter (amplitude, frequency, phase) is changed per second is called the signaling rate. • It is measured in baud. 1 baud = 1 change per second. • With binary modulations such as ASK, FSK and BPSK, the signaling rate equals the bit-rate. • With QPSK and M-ary PSK, the bit-rate may exceed the baud rate.

  24. bps vs. baud • In early modems only, baud=bps • Baud = # of signal changes per second • bps = bits per second • Today, each signal change can represent more thanone bit • through complex modulation of amplitude, frequency, and/or phase • Increases information-carrying capacity of a channel without increasing bandwidth • Increased combinations also lead to increased likelihood of errors

  25. Multilevel signaling • each signal element represents multiple bits • e.g., four different signals (voltages of 0, 1, 2, 3) are used, then one signal represents 00, second signal means 01, and so on • one signal represents two bits • With multilevel signaling, we must distinguish • data rate, in bps • modulation rate or signaling-elements/sec, in baud • a 2 baud line transmits 4 bits/sec in the example of above • baud rate may be larger than bit rate (see Manchester coding)

  26. Equipment -- Modems • The use of analog facilities for data transmission will be substantial for many years to come • the modem is one of the most widely used pieces of communications gear • modems are offered in several different formsfor use in different applications

  27. Voice-grade modems • They are designed for the transmission of digital data over ordinary telephone lines • modems make use of the same 4-kHz bandwidth available for voice signals • because they are used in pairs for communications and used often over the public telephone network, • allowing many different modems to be paired • standards are essential

  28. Modem specifications

  29. Digital encoding of analog information

  30. Pulse-code modulation • Voice data can be represented in digital form • the best-known technique for voice digitization is pulse-code modulation (PCM) • PCM is based on the sampling theorem • if a signal is sampled at regular intervals of time and at a rate higher than twice thesignificant signal frequency, the samples contain all the information of the original signal. • if voice data were limited to frequencies below 4000 Hz, 8000 samples/sec would be sufficient to characterize completely the voice signal • these are analog samples • to convert to digital, each of these analog samples must be assigned a binary code

  31. Converting samples to bits • Using quantizing technique • breaks wave into pieces, assigns a value in a particular range • Figure 16.5 shows an example • analog samples are taken at a rate of 2B • each analog sample is approximated by 16 different levels (4 bits) • if using 8-bit, 256 possible sample levels are achieved • 8000 samples/sec. x 8 bits/sample = 64 kbps is needed • More bits means greater detail, fewer bits means less detail

  32. Digital encoding of digital data

  33. Digital encoding of digital data • The most common and easiest way to transmit digital signals is to use two different voltage levels for the two binary digits • Typically, negative=1 and positive=0 • it is known as Nonreturn-to-Zero-Level (NRZ-L) • because signal never returns to zero, and the value during a bit transmission is a level voltage • is used for very short connections • between a personal computer and an external modem or a terminal and a nearby computer

  34. NRZI • A variation of NRZ is NRZI (NRZ, Invert on Ones) • a constant-voltage pulse for the duration of a bit time • the data themselves are encoded as the presence or absence of a signal transition at the beginning of the bit time • transition = 1, no transition = 0 • it is an example of differential encoding • it is more reliable to detect a change in polarity than it is to accurately detect a specific level

  35. Problems with NRZ • Difficult to determine where one bit endsand the next begins • In NRZ-L, long strings of ones and zeroes would appear as constant voltage pulses • Timing is critical • because any drift results in lack of synchronization and incorrect bit values being transmitted

  36. Biphase (Manchester & Differential Manchester) alternatives to NRZ • Require at least one pulse transition per bit time, and may even have two • Modulation rate is greater, so bandwidth requirements are higher • Advantages • Synchronization due to predictable transitions • Error detection based on absence of a transition

  37. Manchester code • Transition in the middle of each bit period • Transition provides clocking and data • Low-to-high = 1 , high-to-low = 0 • Used in Ethernet

  38. Differential Manchester • Mid-bit transition is only for clocking • Transition at beginning of bit period = 0 • Transition absent at beginning = 1 • Used in token-ring

  39. Using the Manchester encoding, two signal changes represents one bit, its baud rate is greater its bit rate.

  40. Asynchronous and synchronous transmission

  41. Why discuss this topic? • In order for the receiver to sample the incoming bits properly, it must know the arrival time and duration of each bit that it receives • e.g., the sender has a clock that governs a 1 Mbps • then, one bit will be transmitted every 1/10^6 = 1 μs • the receiver will sample the medium at the center of each bit time (0.5 μs) • if the receiver times its samples based on its own clock, then a problem may occur if both clocks are not precisely aligned • Assume the receiver’s clock is 1 % faster or slower than the transmitter’s clock • then the first sampling will be 0.01 of a bit time (0.01 μs) away from the center of the bit, after 50 samples, an error may occur

  42. If clocks are asynchronous 50 samples 0 1 1 % delay sample at the middle time of a pulse (50%)

  43. Asynchronous transmission • Asynchronous transmission is used for low data-rate transmission and stand-alone equipment. • The block length is only 8 data-bits, permitting different clocks with only approximate synchronism to be used. • Asynchronous transmission is used for transmissions up to 20Kb/s. • When the data rate is high, the addition of framing characters around each byte becomes very inefficient. • It is natural to wish to increase the block length, to maximise the data-rate. • However, as the block length becomes longer, so errors in receiver clock rate less easy to tolerate, because they are accumulative. .

  44. Asynchronous transmission • The strategy with this scheme is to avoid thetiming problem by not sending long, uninterrupted stream of bits • data are transmitted one character at a time • each character is 5 to 8 bits in length • timing or synchronization must only be maintained within each character • the receiver has the opportunity to resynchronize at the beginning of each new character

  45. Asynchronous transmission • The idle state of the line is equivalent to the signaling element for binary 1 • The beginning of a character is signaled by astart bit with a value of binary 0 • the least significant bit (b1) is transmitted first • the parity bit will be the last bit as a checking for the bits that have been sending • then a stop element (1, 1.5, or 2 bits long) with a value of binary 1

  46. Timing and overhead • The timing requirements for this scheme are modest • if receiver is 5% slower or faster than the transmitter, the sampling of the eighth character bit will be displaced by 45% and still be correctly sampled • if the example shown on the previous slide (c) has 6% time drift, the last sample is erroneous • It is simple and cheap but requires an overhead of 1 to 3 bits per character • 8 bits character + 1 start bit + 1 stop bit = 10 bits • 2 bits convey no information = 20% overhead

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