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Contents • Data Communications Applications – File & print serving – Mail – Domain Name Server – Telnet – File Transfer Protocol – World Wide Web • Multi-media Applications – Voice – Images Postacademic Interuniversity Course in Information Technology – Module C1 p1 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 Postacademic Interuniversity Course in Information Technology – Module C1 p2 Fourier Analysis* Any time dependant signal can be decomposed in a series of sine functions SquareWave = 2. .t + 1 3.2. .t + 1 5.2. .t + 1 sin sin sin T 3 T 5 T * Students with inadequate mathematical background should skip this slide Postacademic Interuniversity Course in Information Technology – Module C1 p3 Sampled Signals F = 2.5 kHz Sampling Frequency = 8 kHz Nyquist sampling theorem : Fs > 2 * F Postacademic Interuniversity Course in Information Technology – Module C1 p4 Quantization errors 0101 0100 0011 0010 0001 0000 1111 1110 1101 1100 1011 Postacademic Interuniversity Course in Information Technology – Module C1 p5 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 Postacademic Interuniversity Course in Information Technology – Module C1 p6 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 Postacademic Interuniversity Course in Information Technology – Module C1 p7 Sound Communications Example : MP3 Music Server WWW and/or FTP Postacademic Interuniversity Course in Information Technology – Module C1 p8 Voice over the Internet The initial goal : free international calls Internet Sound enabled PC (full duplex) 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 Postacademic Interuniversity Course in Information Technology – Module C1 p9 Internet Telephony Local PSTN I n t e r n e t T e l e p h o n e PABX & Intranet for traditional POTS and data PSTN PABX Intranet PABX PSTN PABX needs to be IP compatible (H323, … ) Postacademic Interuniversity Course in Information Technology – Module C1 p11 Intranet Telephony Innovative services welcome ! PSTN Intranet Public gateway PSTN Public gateway Gatekeeper (address translation) Postacademic Interuniversity Course in Information Technology – Module C1 p12