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Computer Networks

Learn why analog transmission is still used in computer networks, the process of modulation and demodulation, keying techniques, and the characteristics of different modulation schemes.

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Computer Networks

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  1. Computer Networks Chapter 5 – Analog Transmission

  2. Why Study Analog Transmission? • Before the ideas for digital transmission came in place, only the analog transmission was used • Usually the analog signals (voice) were transmitted • Computers were not invented, yet • Analog transmission is still used • The infrastructure cannot be changed in a short period of time • Even today digital signals sometimes need to be transmitted using analog transmission Computer Networks

  3. Digital-to-analog and Analog-to-analog Conversion • The process of digital-to-analog and analog-to-analog conversion is known as modulation. Digital-to-analog modulation Digital-to-analog Analog-to-analog modulation Analog-to-analog Computer Networks

  4. Modulation • Modulation is a process in which the high frequency sinusoidal signal (usually called carrier) changes some of its parameters according to the changes of the low frequency signal that carries information • The parameters that can be changed are: • Amplitude • Frequency • Phase Computer Networks

  5. Demodulation • Demodulation is a reverse process to modulation. • The modulated signal is simply multiplied by the carrier signal to get the original signal • Modems are devices that do both MODulatuion-DEModulation Computer Networks

  6. Keying – Modulation of Digital Signals • Modulation of digital signals is called keying • Depending on the parameter that is changed the keying can be: • Amplitude Shift Keying (ASK) – a change in amplitude • Frequency Shift Keying (FSK) – a change in frequency • Phase Shift Keying (PSK) – a change in phase • Quadrature Amplitude Modulation (QAM) – combination of ASK and PSK Computer Networks

  7. Modulation of a Digital Signal - Example Digital message Sinusoidal carrier Amplitude Frequency Phase Amplitude & Phase Carrier signal parameters to modify Computer Networks

  8. Bit Rate with Analog Transmission • In the PSTN (Public Switched Telephone Network), the available bandwidth for transmitting voice is fixed (300 – 3400 Hz) • The signal-to-noise ratio for the voice channel is known • Using the Shannon theorem the maximum bit rate can be calculated • This bit rate was increased by using modulation techniques in which multi-level pulses are carrying several bits/pulse Computer Networks

  9. Bit Rate vs. Baud Rate • If the parameter (amplitude, frequency or phase) has more than two values than there are more than two signal units in the modulated signal. • The number of signal units send per second is called Baud rate (Baudaut was a French scientist) • When the number of signal units is higher than two, then more then one bit can be sent with a single signal unit. Computer Networks

  10. Characteristics of ASK • The bandwidth of the modulated signal depends on the baud rate and the modulation factor • Susceptible to sudden gain changes • Inefficient • Although AM is used for radio (audio) transmissions, the resulting signal quality is too poor for computer use. Computer Networks

  11. Characteristics of FSK • The frequencies must be reasonably far apart in order the receiver to be able to do the detection. • The bandwidth depends on the baud rate and how far are the two frequencies • Less susceptible to error than ASK • Used for high frequency radio Computer Networks

  12. Characteristics of PSK • The simplest form of phase modulation uses phases of 180o for 0 and 0o for 1 or vice versa • PSK is less susceptible to noise and requires the same bandwidth as ASK • This makes it suitable for creating signal units with more different phases • With four phases, 2 bits can be carried with each signal units and bit rate = 2xbaud rate • With eight phases, 3 bits can be carried with each signal units and bit rate = 3xbaud rate Computer Networks

  13. Quadrature Amplitude Modulation (QAM) • This is more sophisticated modulation sheme used for high speed voice band telephone line modems. • The idea is to combine amplitude and phase modulation. This allows to increase the number of signal units and increase the bit rate. Computer Networks

  14. Signal Vector Representation Frequency of the carrier s(t) = Ac(t) cos (2pfct + f(t)) fixed!!! amplitude of the carrier Q S Amplitude Phase 0 degrees Computer Networks

  15. Constellations Two amplitudes, Same phase • Constellations show the number of signal units and the type of keying used S2 S1 Two phases Same amplitude S2 S1 Two amplitudes, Two phases S1 S2 Computer Networks

  16. Constellations - Examples 0111 0011 0100 0010 0101 0001 2A 0000 A 0110 1000 1110 1001 1101 1010 1100 1111 1011 16 signal units QAM 4 bits / unit Bit rate = 4xbaud rate 4 signal units PSK 2 bits/ unit Bit rate = 2xbaud rate 16 signal units 4 bits / unit QAM Bit rate = 4xbaud rate Computer Networks

  17. Bit Rate vs. Baud Rate - Examples • Compute the bit rate for a 1000 baud 16-QAM channel. Since 24 = 16, 4 bits are carried by each of the signals. Therefore the bit rate = 4 x 1000 = 4000 bps 2. Compute the baud rate for a 72000 bps 64-QAM signal. Since 216 = 64, there are six bits per signal elements. Therefore the baud rate = 72 000/6 = 12 000 baud Computer Networks

  18. Telephone Modems • Used to transmit digital data over analog voice lines • Only bandwidth of 2400 Hz is used • Some terminology used • DTE (Data Terminating Equipment) • Used to denote the computer • DCE (Data Communication Equipment) • Used to denote the modem • The modem modulates at the sender’s side and demodulates at the receiver’s site Computer Networks

  19. Modem Standards • V. 32 and V. 32bis • uses QAM and achievs 9600 and 14 400 bps. • V. 34 and V. 34bis • specifies a full-duplex data rate at 28.8 Kbps and 33.6 Kbps. These modems use sophisticated error control coding to provide reliable data transmission. In addition to that they use data compression to achieve higer speeds • The most recent, higher speed modems (56Kbps) would appear to have rate that is in violation with the Shannon limit • These modems exploit the fact that the PSTN for the most part uses digital tehniques • can be used only if one party is using digital signaling (usually the Internet provider) • They are asymmetrical (data from the Internet provider flow at 56 Kbps, while the data from PC flow at 33.6 Kbps.). • V90 and V92 use these features Computer Networks

  20. Modulation of Analog Signals • Modulation of analog signals is necessary in order to shift the frequency of the signal, usually to the higher frequency • The high frequency sine wave (the carrier) is used • The parameters of the carrier (amplitude, frequency or phase) are varied according to the low frequency signal • Used for radio transmission • Each station in a particular geographic area uses carrier with different frequency • Interference among stations is avoided and listeners can tune to the desired frequency Computer Networks

  21. Amplitude Modulation s(t) S(f) t f 0 fl Sm(f) sm(t) t fc fc + fl fc - fl f The frquency has been shifted The bandwidth is twice as large Computer Networks

  22. Frequency and Phase Modulation • The bandwidth of frequency modulated signal is 10 times the bandwidth of the low frequency signal • For stereo broadcast a bandwidth of 15 KHz is necessary • Radio stations need 10x15=150 KHz • The assigned bandwidth is 200 KHz • The additional bandwidth is called guard bands (its purpose is to protect stations’ bandwidth from overlapping) • Phase modulation is similar to FM Computer Networks

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