1. 1 K. Salah Module 3.1: Encoding and Modulating Conversion Schemes D to D Conversion A to A Conversion A to D Conversion D to A Conversion Final comments Why 56k modems, not more?

2. 2 K. Salah Different Conversion Schemes Some Jargon: –D to D encoding or conversion.  Encoding means representation of data into digital signal –A to D conversion or digitizing analog signal. –D to A conversion or modulating digital signal. –A to A conversion or modulating analog signal.

3. 3 K. Salah Digital to Digital Conversion Different D/D encoding –Cost  Voltage level  Number of levels  Number of signal changes –Speed  Synchronization  Detection –Reliability  noise  Bit errors

4. 4 K. Salah Examples of Polar Encoding

5. 5 K. Salah Analog to Analog Conversion Examples are radio, TV, and satellite communication

6. 6 K. Salah Amplitude Modulation

7. 7 K. Salah Frequency Modulation

8. 8 K. Salah Structure of Telephone System End office, known also as local central office Local loops (twisted pairs, analog signaling) Trunks (fiber optics or microwave, mostly digital) Intermediate switches

9. 9 K. Salah Need for A/D and D/A conversions Modem converts D/A signals Codec converts A/D signals

10. 10 K. Salah Analog to Digital Conversion (Codec) First Step: Pulse Amplitude Modulation (PAM) According to Nyquist theorem, the sampling rate must be at least two times the frequency to ensure the accurate reproduction of the original signal.

11. 11 K. Salah Second step: Quantized PAM Signal Quantizing Using Sign and Magnitude

12. 12 K. Salah Third Step: Pulse Code Modulation (PCM) Out of each sample, Telephone companies only uses the upper 7 bits. The lower bit (bit 0) is always assumed to be 0. For example, sampling values: +024  +024, +038  +038, +025  +024, +039  +038 This is not affecting sampling values much. In transmission, the most significant bit is used for control purposes, as we will see later. Remember this

13. 13 K. Salah From Analog to PCM digital signal

14. 14 K. Salah Digital to Analog Modulation (Modem)

15. 15 K. Salah Amplitude Shift Keying (ASK)

16. 16 K. Salah Frequency Shift Keying (FSK)

17. 17 K. Salah Phase Shift Keying (PSK)

18. 18 K. Salah Quadrature Amplitude Modulation (QAM) QAM means combining ASK and PSK in such a way we have the maximum contrast between each bit, dibit, tribit, quadbit, and so on.

19. 19 K. Salah Comments Bit rate is the number of bits per second. Baud rate is the number of signal units per second. The encoding/represenation of one baud is called symbol. Baud rate is less than or equal to the bit rate. ITU-T modem Baud RateBit RateModulation V.21300 FSK V.2260012004-PSK V.292400960016-QAM V.34240028800 or (33600)4096-QAM (data compression) Shanon formula and quantization limits data rate to 33.6kbps. How about 56K modems? 56K modems are achieved if we have an ISP that uses digital signaling. Uploading will always be 33.6k due to PCM quantization at the CO Codec. However, downloading can be 56k since there is no PCM quantization at the CO Codec. Quantization always limits the data rate. Why only 56K in downloading? Because of the way voice is digitized. CO switches use PCM and inverse PCM at sampling rate of 8000 samples and 7 bits per sample, which gives 8000 x 7 of data rate.

20. 20 K. Salah Why 56k?

21. 21 K. Salah Traditional Modems

22. 22 K. Salah 56K modems