Chapter 23. Modem Modulation
Here are the 3 basic types of modulation used in modems:
FSK - Frequency Shifted Keying
QPSK - Quadrature Phase Shifted Keying
QAM - Quadrature Amplitude Modulation
Modern modems use a combination of the above basic modulation techniques and compression to achieve high data transfer rates (14.4 Kbps and up).
FSK - Frequency Shift Keying
Frequency Shift Keying (or FSK) is the frequency modulation of a carrier that represents digital intelligence. For Simplex or Half Duplex operation, a single carrier (1170 Hz) is used - communication can only be transmitted in one direction at a time. A Mark or 1 is represented by 1270 Hz, and a Space or 0 is represented by 1070 Hz. The following diagram shows the Voice Channel with Simplex/Half Duplex FSK.
Simplex / Half Duplex FSK
Full Duplex FSK
For Full Duplex (i.e. simultaneous data communication in both directions), the upper bandwidth of the Voice Channel is utilized. Another carrier is added at 2125 Hz. A Mark or 1 is represented by 2225 Hz, and a Space or 0 is represented by 2025 Hz. The originating modem (the one which dials the phone number and starts the connection) uses the lower carrier (1170 Hz) and the answer modem (the one which answers the ringing phone line) uses the upper carrier (2125 Hz). This allocation of carriers is done automatically by the modem's hardware. The following diagram shows the Voice Channel with Full Duplex FSK.
Example of Originate's Frequency Modulated Carrier:
The originate modem transmits on the 1170 Hz carrier and receives on the 2125 Hz carrier. The answer modem receives on the 1170 Hz carrier and transmits on the 2125 Hz carrier. This way, both modems can be transmitting and receiving simultaneously!
The FSK modem described above is used for 300 baud modems only. The logical question is: "Why not use it for higher modems?". Higher data rates require more bandwidth. This would require that the Mark and Space frequencies for each band be moved farther apart (the originate and answer bands become wider). The two carriers would have to move farther apart from each other to prevent crosstalk (interference with each other). The limit for present phone lines is 1200 Baud Half Duplex (one way) used by Bell 202 compatible modems.
28b. QPSK - Quadrature Phase Shift Keying
Quadrature Phase Shift Keying employs shifting the phase of the carrier at a 600 baud rate plus an encoding technique. QPSK is used in Bell 212A-compatible modems and V.22 - both are 1200 bps Full Duplex standards. The originate modem transmits at 1200 Hz, and receives on 2400 Hz. The answer modem receives on 1200 Hz, and transmits on 2400 Hz.
The digital information is encoded using 4 (Quad) level differential PSK at 600 baud.
Remember that baud indicates how fast the analog signal is changing in the Voice Channel. The data is encoded as follows:
For every change in the baud rate (phase shift), we can decode 2 bits. This leads to the following:
2 bits x 600 baud = 1200 bps
Example of Carrier Phase Modulation:
QAM - Quadrature Amplitude Modulation
Quadrature Amplitude Modulation refers to QPSK with Amplitude Modulation. Basically, it is a mix of phase modulation and amplitude modulation. QAM phase modulates the carrier and also modulates the amplitude of the carrier.
Phase Modulated and Amplitude Modulated Carrier:
There are two types, 8-QAM and 16-QAM. 8-QAM encodes 3 bits of data (23=8) for every baud and 16-QAM encodes 4 bits of data (24=16) for every baud. Both are used in the V.32 standard for 9600 bps modem (a milestone for communications!). 8-QAM transfers 4800 bps and 16-QAM transfers 9600 bps. The baud rate used with QAM is 2400 baud half-duplex.
16-QAM has 12 phase angles, 4 of which have 2 amplitude values! 16-QAM changes phase with every baud change.
16-QAM Phasor Diagram
Higher transfer rates use much more complex QAM methods. For example, V.32bis (14.4 kbps) uses a 64 point constellation to transfer 6 bits per baud. Compare that to the above 16 point constellation!
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