Fax communications over PSTN
nFaxes
follow standard PSTN modem communications recommendations or recommendation V.17 (02/91) (- Wire modem for facsimile applications with rates up to 14
400 bit/s)
nFaxes
are divided into groups:
nGroup 1 (´68):
Analog scanning, 2400 bits/s
nGroup 2 (´76):
Analog scanning, 4800 bits/s
nGroup 3 (´80):
Digital scanning, 14400 bits/s
nGroup 4 (´84):
Digital scanning, 64 kbit/s (ISDN)
nExample
of tasks of group 3 transmitting fax
Scanning
Coding
Compression
Modem (D/A)
Modified
Huffman
Huffman
QAM, V.27ter/
V.29
V.29
A4/US letter,
1144 lines
1144 lines
Gray scales
by dithering
by dithering