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Part II. Physical Layer and Media

Part II. Physical Layer and Media. Chapter 5. Analog Transmission. COMP 3270 Computer Networks Computing Science Thompson Rivers University. Learning Objectives.

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Part II. Physical Layer and Media

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  1. Part II. Physical Layer and Media Chapter 5. Analog Transmission COMP 3270 Computer Networks Computing Science Thompson Rivers University

  2. Learning Objectives • Given a bandwith and the number of signal elements to carry a bit, determine the bit rate, for various types of ASK, FSK, PSK and QAM. • Given a bit rate and a bandwith, design a QAM (or other conversion type) scheme to meet the given requirements. • List the advantages and disadvantages of AM and FM respectively.

  3. 1. DIGITAL-TO-ANALOG CONVERSION Digital-to-analog conversion is the process of changing one of the characteristics of an analog signal based on the information in digital data. Topics discussed in this section: • Aspects of Digital-to-Analog Conversion • Amplitude Shift Keying (ASK) • Frequency Shift Keying (FSK) • Phase Shift Keying (PSK) • Quadrature Amplitude Modulation (QAM)

  4. Aspects of DtA Conversion

  5. Types of digital-to-analog conversion

  6. Bit rate is the number of bits per second. Baud rate is the number of signal elements per second. In the analog transmission of digital data, the baud rate is generally less than or equal to the bit rate.

  7. Example: An analog signal carries 4 bits per signal element. If 1000 signal elements are sent per second, find the bit rate. Solution:

  8. Example: An analog signal has a bit rate of 8000 bps and a baud rate of 1000 baud. How many bits are carried by each signal element? How many different signal elements do we need? Solution:

  9. Amplitude Shift Keying (ASK)

  10. Binary amplitude shift keying (BASK) The minimum required bandwidth is equal to the baud rate: B = S. ☺ Advantages? - Simple ☺ Disadvantages? - Susceptible to noise - ☺ Idle state?

  11. Example: We have an available bandwidth of 100 kHz which spans from 200 to 300 kHz. What are the carrier frequency and the bit rate if we modulated our data by using ASK with d = 0? Solution:

  12. Example: In data communications, we normally use full-duplex links with communication in both directions. We need to divide the bandwidth into two with two carrier frequencies. The below figure shows the positions of two carrier frequencies and the bandwidths. The available bandwidth for each direction is now 50 kHz, which leaves us with a data rate of 50 kbps in each direction.

  13. Multilevel ASK More signal levels to modulate 2, 3, 4, or more bits at a time.

  14. Frequency Shift Keying (FSK)

  15. Binary frequency shift keying (BFSK) The minimum required bandwidth is equal to 2 times S. B = 2S. ☺ Advantages? -Strong to noise ☺ Disadvantages? - More bandwidth is required

  16. Example: We have an available bandwidth of 100 kHz which spans from 200 to 300 kHz. What should be the carrier frequency and the bit rate if we modulated our data by using FSK with d = 0? Solution:

  17. Phase Shift Keying (PSK) Binary phase shift keying (BPSK) The minimum required bandwidth is equal to the baud rate: B = S. 180 degree phase ☺ Advantages? - Strong to noise - Smaller bandwidth than FSK ☺ Disadvantages? - Difficult to manipulate

  18. Example: Find the bandwidth for a signal transmitting at 12 Mbps for QPSK (Quadrature PSK). Solution:

  19. 2-PSK, 4-PSK, 8-PSK, …

  20. Quadrature Amplitude Modulation (QAM) Quadrature amplitude modulation (QAM) is a combination of ASK and PSK.

  21. 2-QAM, 4-QAM, 8-QAM, 16-QAM, …

  22. The minimum bandwidth required for QAM is the same as that required for ASK and PSK. QAM has the same advantages as PSK over ASK. Most analog transmissions use QAM.

  23. Intermediate Summary • The idea of D-A • n-QAM • The minimum required bandwidth • Bit rate • Baud rate • Shannon capacity formula

  24. 2. ANALOG TO ANALOG Analog-to-analog conversion is the representation of analog information by an analog signal. One may ask why we need to modulate an analog signal; it is already analog. Modulation is needed if the medium is bandpass in nature or if only a bandpass channel is available to us. Topics discussed in this section: • Amplitude Modulation (AM) • Frequency Modulation (FM)

  25. Types of analog-to-analog modulation

  26. Amplitude Modulation (AM) ☺ Advantage? ☺ Disadvantage?

  27. Frequency Modulation ) ☺ Advantage? ☺ Disadvantage?

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