Introduction to Data Communications
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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:

DIBIT Phase Shift
00 +90
01 0
10 180
11 270

For every change in the baud rate (phase shift), we can decode 2 bits! This leads to:

2 bits x 600 baud = 1200 bps

Example of Carrier Phase Modulation:

Introduction to Data Communications
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