1. Postacademic Interuniversity Course in Information Technology – Module C1p1 Contents Data Communications Applications –File & print serving –Mail –Domain Name Server –Telnet –File Transfer Protocol –World Wide Web Multi-media Applications –Voice –Images

2. Postacademic Interuniversity Course in Information Technology – Module C1p2 Digital Sound Sound= variation in air pressure Air pressure= continuous time function To digitize sound the pressure function is sampled periodically and the corresponding pressure memorized Any sound= sum of sine waves (pure sounds) Human ear can perceive pressure variations in the frequency range 25 Hz to 20000 Hz Perception proportional to logarithm of power

3. Postacademic Interuniversity Course in Information Technology – Module C1p3 Fourier Analysis* Any time dependant signal can be decomposed in a series of sine functions  T 5.2. .t sin 5 1 T 3.2. .t sin 3 1 T 2. .t 1 sinSquareWave * Students with inadequate mathematical background should skip this slide

4. Postacademic Interuniversity Course in Information Technology – Module C1p4 Sampled Signals F = 2.5 kHzSampling Frequency = 8 kHz Nyquist sampling theorem : Fs > 2 * F

5. Postacademic Interuniversity Course in Information Technology – Module C1p5 Quantization errors 0101 0100 0011 0010 0001 0000 1111 1110 1101 1100 1011

6. Postacademic Interuniversity Course in Information Technology – Module C1p6 Speech Redundancies Time domain redundancies –Nonuniform amplitude distributions –Sample-to sample correlation (~0.85) –Cycle-to-cycle correlation (= periodicity) –Pitch interval to pitch interval correlation –Inactivity factors (Speech pauses ~ 40%) Frequency domain redundancies –Non uniform long term spectral densities –Sound specific short term spectral densities

7. Postacademic Interuniversity Course in Information Technology – Module C1p7 Sound Communications Common Properties Throughput requirements (real time) –CD quality = 1.5 Mb/s –MP3 - CD quality= 128 Kb/s –Classical telephony = 64 Kb/s –GSM telephony= 13 Kb/s –Synthetic voice= 2.4 Kb/s Transmission errors almost harmless Full duplex communications –Connection oriented application –Very sensitive to delays One way communications –Connectionless or connection oriented –Sensitive to delay jitter if listening in real time

8. Postacademic Interuniversity Course in Information Technology – Module C1p8 Sound Communications Example : MP3 Music Server WWW and/or FTP

9. Postacademic Interuniversity Course in Information Technology – Module C1p9 Voice over the Internet The initial goal : free international calls Internet Sound enabled PC (full duplex) -To communicate, precise appointments need to be made,... by phone ?!? -Voice quality dependent from network delays -Acceptable if network not overloaded Sound enabled PC (full duplex)

10. Postacademic Interuniversity Course in Information Technology – Module C1p10 Internet Telephony Internet +Access through local PSTN -Quality dependant on network load Local PSTN Local PSTN Internet Telephone Gateways Low cost POTS emulation

11. Postacademic Interuniversity Course in Information Technology – Module C1p11 PABX & Intranet for traditional POTS and data Intranet PABX PSTN PABX needs to be IP compatible (H323, … )

12. Postacademic Interuniversity Course in Information Technology – Module C1p12 Intranet Telephony Innovative services welcome ! Intranet Gatekeeper (address translation) Public gateway PSTN Public gateway PSTN