CSCI 465 Data Communications and Networks Lecture 7

1. CSCI 465Data Communications and NetworksLecture 7 Martin van Bommel CSCI 465 Data Communications and Networks

2. Data Communication Basics • Analog or Digital • Three Components • Data • Signal • Transmission CSCI 465 Data Communications and Networks

3. Analog Data Choices CSCI 465 Data Communications and Networks

4. Digital Data Choices CSCI 465 Data Communications and Networks

5. Transmission Choices • Analog transmission • only transmits analog signals, without regard for data content • attenuation overcome with amplifiers • Digital transmission • transmits analog or digital signals • uses repeaters rather than amplifiers CSCI 465 Data Communications and Networks

6. Data, Signals, and Transmission A Data D D Transmission System A A D Signal CSCI 465 Data Communications and Networks

7. Digital Transmission Advantages • The signal is exact • Signals can be checked for errors • Noise/interference are easily filtered out • A variety of services can be offered over one line • Higher bandwidth is possible with data compression CSCI 465 Data Communications and Networks

8. Analog Encoding of Digital Data • data encoding and decoding to represent data using the properties of analog waves • modulation: the conversion of digital signals to analog form • demodulation: the conversion of analog data signals back to digital form CSCI 465 Data Communications and Networks

9. Modem • an acronym for modulator-demodulator • uses a constant-frequency signal known as a carrier signal • converts a series of binary voltage pulses into an analog signal by modulating the carrier signal • the receiving modem translates the analog signal back into digital data CSCI 465 Data Communications and Networks

10. Methods of Modulation • amplitude modulation (AM) or amplitude shift keying (ASK) • frequency modulation (FM) or frequency shift keying (FSK) • phase modulation or phase shift keying (PSK) CSCI 465 Data Communications and Networks

11. Amplitude Shift Keying (ASK) • In radio transmission, known as amplitude modulation (AM) • the amplitude (or height) of the sine wave varies to transmit the ones and zeros • Up to 1200 bps over voice-grade lines • major disadvantage • telephone lines are very susceptible to variations in transmission quality that affect amplitude CSCI 465 Data Communications and Networks

12. ASK Illustration 1 0 0 1 CSCI 465 Data Communications and Networks

13. Frequency Shift Keying (FSK) • in radio transmission, known as frequency modulation (FM) • the frequency of the carrier wave varies in accordance with the signal to be sent • signal is transmitted at constant amplitude • more immune to noise than ASK • requires more analog bandwidth than ASK • still up to 1200 bps on voice-grade lines CSCI 465 Data Communications and Networks

14. FSK Illustration 1 1 0 1 CSCI 465 Data Communications and Networks

15. Full-Duplex FSK CS/IS 465: Data Communications and Networks

16. Phase Shift Keying (PSK) • also known as phase modulation (PM) • frequency and amplitude of the carrier signal are kept constant • the carrier 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) CSCI 465 Data Communications and Networks

17. PSK Illustration Two-level PSK 0 0 1 1 0 0 1 1 Differential-level PSK CSCI 465 Data Communications and Networks

18. Complex Modulations • Combining modulation techniques allows us to transmit multiple bit values per signal change (baud) • Increases information-carrying capacity of a channel without increasing bandwidth • Increased combinations also leads to increased likelihood of errors • Typically, combine amplitude and phase CSCI 465 Data Communications and Networks

19. Multilevel PSK • Can extend levels beyond one bit at a time • 9600 bps modem • 12 different phase angles • 4 of them use two different amplitude values (ASK) • 16 different signal types – 4 bits per signal type • 9600 bps modem / 4 bits per signal type • 2400 baud modem (signaling speed) CSCI 465 Data Communications and Networks

20. Quadrature Amplitude Modulation (QAM) • the most common method for quadbit transfer • combination of 8 different angles in phase modulation and two amplitudes of signal • provides 16 different signals, each of which can represent 4 bits CSCI 465 Data Communications and Networks

21. Quadrature Amplitude Modulation (QAM) • the most common method for quadbit transfer • combination of 8 different angles in phase modulation and two amplitudes of signal • provides 16 different signals, each of which can represent 4 bits CSCI 465 Data Communications and Networks

22. QAM Illustration 90 135 45 amplitude 1 180 0 amplitude 2 225 315 270

23. QAM Uses • CCITT V.22 bis modem • the "bis" is French for "duo" or "twice" • supports transmission of full-duplex 2400 bps synchronous or asynchronous data over a switched, 2-Wire, voice circuit • the modulation rate is 600 baud, with each baud representing four data bits (CCITT – Consultative Committee International for Telephony and Telegraphy) CSCI 465 Data Communications and Networks

24. 56kbps Modems • asymmetrical; can download at 56 kbps but upload at 33.6 kbps only • requires digital T-1 or ISDN PRI connection at central site or ISP • ITU Standard V.90 (Feb. 1998) • early examples • U.S. Robotics (56K x2) • Rockwell (56K flex) CSCI 465 Data Communications and Networks