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Modem

Modem. By Amir Taherin. Eng. M. Zamanian. Contents. Definition. 1. History. 2. V.21, V.22, V.22bis. 3. Modulation. 4. Contents. Amplitude shift keying (ASK). 5. Frequency shift keying (FSK). 6. Phase shift keying (PSK). 7. 8. More Complex Modulation. Contents.

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Modem

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  1. Modem By Amir Taherin Eng. M. Zamanian

  2. Contents Definition 1 History 2 V.21, V.22, V.22bis 3 Modulation 4

  3. Contents Amplitude shift keying (ASK) 5 Frequency shift keying (FSK) 6 Phase shift keying (PSK) 7 8 More Complex Modulation

  4. Contents Advanced Frequency Shift Keying 9 Advanced Phase Shift Keying 10 QuadraturePhase Shift Keying 11 Quadrature Amplitude Modulation 12

  5. Contents QAM Applied To Dialup 13 Modem Protocols 14 15 Modem and NGN 16 Preferences

  6. Definition Modem Modulation Demodulation Dem Mod

  7. 1982 Hayes, 103A 300-bit/s 1960-1962 Data-Phone 201A, 201B 1943 IBM, Punched card, 25 bit/s 1940 George Stibitz History

  8. V.21, V.22, V.22bis • In Bell telephony • Send 0  1070 Hz • Send 1  1270 Hz • Answarin modem • Send 0  2025 Hz • Send 1  2225 Hz • These frequencies were chosen carefully • Suffer minimum distortion on the phone system • Not harmonics of each other

  9. Modulation

  10. Amplitude shift keying (ASK) • Use different amplitude to represent 0 and 1. • Simple, low bandwidth • Sensitive to interference. • Multi-path propagation, noise or path loss heavily influence the amplitude. • A constant amplitude in wireless environment can not be guaranteed. • Used in wired optical communication. • A light pulse =1, no light =0.

  11. ASK Sample

  12. Frequency shift keying (FSK) • Binary FSK (BFSK) • One frequency for 0 and one frequency for 1. • Needs larger bandwidth • Avoid discontinuity • Discontinuity creates high frequencies as side effects. • Demodulation • Use two band-pass filters for 2 frequencies.

  13. FSK Sample

  14. Phase shift keying (PSK) • Use shift in phase to represent data. • Binary PSK (BPSK) • Shift the phase by 180. • Synchronization is important • More resistant to interference • More complex transmitters and receivers.

  15. PSK Sample

  16. More Complex Modulation • Low efficiency in ASK, FSK, PSK. • High bandwidth

  17. Advanced Frequency Shift Keying

  18. Advanced phase shift keying • Phase domain: use a vector (or a point) in the plane to represent the signal. • Length of the vector: amplitude • Angle: phase

  19. Advanced phase shift keying • BPSK (Binary Phase Shift Keying) • bit value 0: sine wave • bit value 1: inverted sine wave • very simple PSK • robust, used in satellite systems

  20. QuadraturePhase shift Keying • QPSK (Quadrature Phase Shift Keying) • 2 bits coded as one symbol • Symbol determines shift of sine wave • Needs less bandwidth compared to BPSK • More Complex • Transmitter and receiver are synchronized very often.

  21. Quadrature Phase shift Keying

  22. Quadrature Amplitude Modulation • Quadrature Amplitude Modulation(QAM): combines amplitude and phase modulation • It is possible to code n bits using one symbol • 2n discrete levels, n=2 identical to QPSK • Bit error rate increases with n, but less errors compared to comparable PSK schemes

  23. Quadrature Amplitude Modulation

  24. QAM Applied To Dialup • QAM is also used with dialup modems as a way to maximize the rate at which data can be sent • Most telephone connections transfer frequencies between 300 and 3000 Hz

  25. QAM for V.32 Dialup Modems

  26. QAM for V.32bis Dialup Modems

  27. Modem Protocols

  28. Modem Protocols

  29. Modem Protocols • V.34 • V.34+ • Speed up to 33.6 Kbit/s • V.42bis • For Error Control • BTLZ algorithm (British Telecom Lempel Ziv) • Alan Clark • V.90 • 56 Kbit/s Download • 33.6 Kbit/s Upload • Known as V. last

  30. Modems and NGN Service Media Gateway Controler Packetized Network Access

  31. Refrences • ITU-T (2006-09-29). "Data communication over the telephone network". ITU. Retrieved 2008-03-02. • http://3amsystems.com/wireline/hmo.htm • V.22, V.22bis & V.32 handshakes • http://www.itu.int/rec/T-REC-V/en • ITU-T Recommendations: V Series • http://en.wikipedia.org/wiki/Modem • Modem

  32. Question ? ? ? ? ? ? ?

  33. Thank You !

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