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5.4
High-speed MODEMs above 14400 bps
Since 9600 bps MODEMs recommended as V.29 in 1976 were adopted in
facsimiles, MODEM technology has advanced all the more to the point
that facsimiles capable of transmitting and receiving at transmission
speeds exceeding 9600 bps are in the process of becoming mainstream.
The standards for these MODEMs have already been recommended by
ITU-T, and have gradually come to be installed even in low-price range
products.
Of these MODEMs, let's describe an outline of the 14400 bps MODEM
recommended in V.17 (V.33) and the 33600 bps MODEM recommended
in V.34.
5.5
V.17 (V.33) MODEM Technology
V.17 (V.33) MODEMs recommended by ITU-T are capable of transmitting
data at a high speed of 14400 bps. Until this recommendation was reached,
several 14400 bps MODEMs using existing technology were put onto the
market. PARADYNE Inc. first put a MODEM onto the market in 1980 and
was followed by CODEX Inc. the following year in 1981, who announced
a 14400 bps MODEM using multi-value QAM technology that had a 64-
point signal constellation. These products improved transmission speeds
by using spatial diagram patterns that were resistant to factors for impaired
quality such as noise during communications. Then, in 1984, MODEMs
using a modulation system called the "error correction coding QAM
mehod" started to be developed, and a series of high-function MODEMs
Fig. 5-48 Target of QAM Method and Spatial Pattern
G3 FACSIMILE COMMUNICATIONS
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