Digital Signal Processing Reference
In-Depth Information
Upload
(32 carrier)
ISDN
Download (256 carrier)
40 kHz
f
1,1 MHz
Upload
(32 carrier)
ISDN
Download (256 carrier)
Influence of the cable attenuation
f
40 kHz
1,1 MHz
Illustration 279:
ADSL as a special - asymmetrical - DMT-procedure
The system is, however, adapted to the properties of the transmission path by automati-
cally measuring the signal-noise distance. With regular Cu-wires transmission loss at
around 1 MHz increases drastically. Therefore the possible states of the carriers in that
range are reduced so that only 4-QAM instead of e.g. 32-QAM is made use of. If there are
strong narrowband interferences in that range - e.g. caused by broadcasting - the carriers
are “switched off”.
Transmission loss increases proportionally to the length of a
cable. Therefore the transmission rate in ADSL is proportional to
the length of the line. Manufacturer information on ADSL usually
refers to lengths of 2 to 3 km.
