Networking Reference
In-Depth Information
The European ETSI and the American ANSI have set down
their standards on the basis of a modulation called DMT (“discrete
In DMT, the spectrum is split into subbands 4.3125 kHz in size.
The subcarriers are modulated in QAM, with variable
constellations dependent on the transmission quality, from 4 to
In the ADSL standard, the telephone pair is the support of 256
subchannels, each 4.3125 kHz in size, hence an overall bandwidth
of 1,104 kHz.
Subchannel 1 is reserved for the analog telephone channel.
Subchannels 2 to 6 are reserved for telephone channel
signalling and act as guard bands with the ADSL subchannels.
The 250 remaining subchannels are used to transport the
ADSL stream, over a useable bandwidth which stretches from
25 kHz to 1.1 MHz. The guard band between the two subchannels
is 300 Hz.
Carrier signals can be modulated from 0 to 15 bits/s per Hertz,
which allows a bit rate of 64 kbps for each of the transmission
DMT modulation allocates the data in such a way as to
optimize the bit rate of each channel by adapting the transmission
to the characteristics of the telephone line.
The number of bits per Hertz of each carrier signal depends on
its position in the spectrum.
The higher the frequency, the more significant attenuation
becomes, necessitating the use of a more robust modulation with
less bits per Hertz.
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