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

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

3. Analog Data Choices 3 CSCI 465 Data Communications and Networks

4. Digital Data Choices 4 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 5 CSCI 465 Data Communications and Networks

6. Data, Signals, and Transmission 6 Data Signal TransmissionSystem A D D D A A 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 7 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 8 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 9 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) 10 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 11 CSCI 465 Data Communications and Networks

12. ASK Illustration 12 100 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 13 CSCI 465 Data Communications and Networks

14. FSK Illustration 14 1 1 01 CSCI 465 Data Communications and Networks

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

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) 16 CSCI 465 Data Communications and Networks

17. PSK Illustration 17 0011 CSCI 465 Data Communications and Networks 011 0 Two-level PSK Differential-level PSK

18. 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 19

20. 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. 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. 22 90 45 0 135 180 225 270 315 amplitude 1 amplitude 2 QAM Illustration

23. 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. 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