Modem Eng. M. Zamanian By Amir Taherin

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

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

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

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

Definition Modem Modulation Demodulation Mod Dem

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

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

Modulation

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.

ASK Sample

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.

FSK Sample

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.

PSK Sample

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

Advanced Frequency Shift Keying

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

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

Quadrature Phase 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.

Quadrature Phase shift Keying

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

Quadrature Amplitude Modulation

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

QAM for V.32 Dialup Modems

QAM for V.32bis Dialup Modems

Modem Protocols

Modem Protocols ITU Standard Name V.1 – V.8bis General Standards Interfaces and voice-band modems V.35 Wideband Modems V.41 – V.44 Error Control and Data Comparison V.61 – V.92 Simultaneous transmission of data and other signals V.110 – V.120 – V.150.1 Interworking with other networks V.230 Interface layer specifications for data communication Applies to V.250–V.299 Control procedures V.300 Digital circuit modems

Modem Protocols V.34 V.42bis V.90 V.34+ Speed up to 33.6 Kbit/s 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

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

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

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