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G3 FACSIMILE COMMUNICATIONS
ably. An increase of the pathmetric to be stored means that hardware
equivalent to that value will be required, and in terms of economy this can-
not be said to be efficient. For this reason, the scale of the hardware can be
reduced by configuring a circuit in such a way that the calculation of the
pathmetric reduces fixed values at a certain point in time. This concludes
discussion on trellis coding/Vitterbi decoding. It is considered that coding/
decoding schemes such as these will be researched into all the more in the
future.
5.5.3 Path signals
The modulation system of V.33 (same in V.17, too) is a QAM modulation
system that uses a trellis coding-based ECM system. 128 points are
mapped onto a signal spatial diagram (constellation) as discussed in the
previous item. Otherwise, the main features of V.33 path signals are:
1. 12000 bps is stipulated as the fallback speed,
2. 1800 Hz is stipulated as the carrier frequency.
At initial stages when
exclusively for telephone lines having a transmission speed exceeding
9600 bps, compatibility with V.29 was studied as V.33 was positioned as
an extension of V.29, and work for standardization as a MODEM having
fallback speeds of 12000 bps and 9600 bps was advanced. However, other
work groups in ITU-T were already studying the transmission speeds of
already recommended MODEMs with a view to reducing that speed as
much as possible, and so the fallback speed was determined at only 12000
bps in keeping with that spirit. This fact means that compatibility with
V.29 MODEMs has not been taken into consideration. Essentially, fallback
is performed when it is difficult to secure a transmission speed of 14400
bps in a state where something has caused line quality to drop. In the case
of a V.33 MODEM, it has actually been proven that equivalent perfor-
mance to that of a V.29 MODEM can be obtained by trellis coding technol-
ogy, for example, by causing a fallback to 12000 bps; and when the speed
of 9600 bps in the V.29 system is to be secured, it is anticipated that the
speed of 12000 bps of the V.33 system will be ensured. On the premise that
a V.33 MODEM does not take compatibility with a V.29 MODEM into
consideration, this fact means that a fallback speed of 9600 bps is not
required. Due to this background, the fallback speed of a V.33 MODEM
was determined to be only 12000 bps. When a fallback is made to 12000
bps, coding is performed excluding Q6n shown in the figure, and mapping
shown in Fig. 5-58 is performed on the signal spatial diagram.
5–70
ITU-T
deliberated V.33 as a MODEM standardized
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