How V.34 and V.
32 Work
Analog modems use two methods of transferring data, those are Phase Modulation (PM) and Amplitude
Modulation (AM). Quadrature Amplitude Modulation (QAM) is the combination of both PM and AM.
Phase Modulation (PM)
PM is a process where two sinusoidal waveforms are compared to each other. The relation in time from
one sine wave in comparison to the second sine wave at the equivalent point is called the Phase Angle.
As is shown both waveforms A and B are going in the same direction at the same time, this is called a
Zero Phase Angle or Zero Phase Shift. Sinusoidal (Sine) Wave starts at 0o and complete the cycle at
360 and then repeat starting at 0. This means that a 0 and 360 Phase Shift are equal.
0 Phase Shift
With a phase shift of 180, waveform B starts at the mid point of Waveform A. With a 180 phase shift
when waveform A is positive then waveform B is negative and the opposite is true.
180 Phase Shift
With two separate phase states we are able to represent a signal bit of information. A single bit of
information can ether be a "1" or a "0" and if we assign this to the two phase states, the phase shifts will
represent the status of the single bit of data. For example, we can assign a 0o phase shift to represent a
"1" data state and a 180 phase shift for a "0" phase shift for the transmission of information.
With the addition of the 90 and 270 phase shift states, as shown in the following graphs, we are able to
identify a total of four unique phase states (0, 90, 180 and 270). With four unique phase states we are
able to transmit at twice the rate of the two phase state option.
1
90 Phase Shift
270 Phase Shift
Four unique phase states can be graphically represented in an X-Y plot as the following.
The above graph shows the four possible phase options in which data can be represented. These four
phase states correspond to the total possibilities that two bits of data can have ( 00, 01, 10, & 11). If we
assign 0 to data state 00, and 90 to data state 01 and so on, a graphical representation will look like the
following.
Amplitude Modulation (AM)
The other segment of QAM is to represent data in the amplitude of the waveform. Varying the
amplitude of the transmit data is called Amplitude Modulation (AM), different amplitudes can be
represent by different logic states. The "1" and "0" shown are shown as examples.
2
Quadrature Amplitude Modulation (QAM)
With QAM combining both Phase Modulation (PM) and Amplitude Modulation (AM) information we
can transmitted in both the phase shift of PM and in the signal magnitude of AM at the same time. The
graphical representation of QAM is called a constellation, because of all the different transmitted points
that make it up. The following constellation shows what it would look like for 3-bits of information per
point. Each point can also be called a symbol.
As more different phase shifts and magnitudes levels are used the more bits of information can be
incorporated into each point or symbol. The problem arises when the constellation points are so close
together so that it is impossible for the receiving end to distinguish from one point to the next due to
noise on the line. The maximum data transfer rate considering the least amount of realistic noise on the
telephone lines is 35Kbps according to Shannon’s theory. However on any given line the noise and
distortion encountered may inhibiting you to only achieving a fraction to the theoretical maximum
speeds. Speed reductions are achieved by reducing the number of symbols in the constellation so to
allow for more decision making space between each symbol.
