10.9
DIGITAL TRANSMISSION ON AN ANALOG CHANNEL
275
Table 10.3
Phase Encoding for the CCITT Rec.
V.29 Modem
Q
2
Q
3
Q
4
Phase Change (
)
0
0
1
0
0
0
0
45
0
1
0
90
0
1
1
135
1
1
1
180
1
1
0
225
1
0
0
270
1
0
1
315
bits of information. The first three bits derive from 8-ary PSK and the fourth bit derives
from using two equivalent amplitude levels. Table 10.3 gives the phase encoding for the
V.29 modem and Table 10.4 shows its amplitude phase relationships. The four bits are
denominated
Q
1
,
Q
2
,
Q
3
, and
Q
4
. Figure 10.17 shows the signal space diagram of the
V.29 modem. There are 16 points in the diagram representing the 16 quadbit possibilities.
Other modems of the CCITT V series are designed to operate at 14,400 bps, 28,800
bps, 33.6 kbps, and up to 56 kbps over the standard voice channel. The actual transmission
rates that are achievable also have a lot to do with how far out (e.g., distance) from the
local switch the modem is attached on the subscriber loop and the condition of the loop.
Their waveforms become extremely complex.
Table 10.4
AmplitudePhase Relationships for the
CCITT Rec. V.29 Modem
Absolute Phase (
)
Q
1
Relative Signal
Element Amplitude
0, 90, 180, 270
0
3
1
5
45, 135, 225, 315
0
2
1
3
2
Figure 10.17
Signal space diagram for the CCITT Rec. V.29 modem when operating at 9600 bps
(Ref. 12).
Summary :
V.29 Modem Q 2 Q 3 Q 4 Phase Change ( ) 0 0 1 0 0 0 0 45 0 1 0 90 0 1 1 135 1 1 1 180 1 1 0 225 1 0 0 270 1 0 1 315 bits of information.
Tags :
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