Figure 1. FDM
ADSL: Technology & Modulation
ADSL exploits the fact that all telephony signals are below 4 KHz in frequency although a typical copper pair line can transmit usable signals up to 1MHz. ADSL uses the rest of the full copper pair line frequency spectrum, from above the voice frequencies up to 1.1MHz.
ADSL uses the DMT modulation, rather than its earlier competitor CAP modulation. If the line is of good quality, using the DMT modulation, up to 15 bits per signal can be encoded on each carrier frequency.
The CAP modulation has its own advantages over DMT like lower cost, lower latency and rate adaptively, but ADSL prefers the use the DMT modulation because it considered more reliable and sophisticated technology, and capable of more speed than CAP. DMT has some more important advantages over CAP like Impulse noise handling, reducing RF interference and adapting to impairments.
In ADSL modems, creating multiple channels is done by dividing the available bandwidth of a telephone line. One way to this is by Frequency Division Multiplexing (FDM).
FDM transmits multiple signals simultaneously over a single transmission path. Each signal travels within its own unique frequency range (carrier), which is modulated by the data (text, voice, video, etc.). FDM assigns one band for upstream data and another band for downstream data. In this way, ADSL splits off a 4 kHz region for POTS at the end of the band.
Figure 1. FDM
The ADSL modem will combine the data stream from the computer, divide it
into the available aggregate channels, interleave, attach an ECC and then
finally modulate the data on the copper wires to be received by the demodulator
at the other end.
The receiving end will perform error checking, correct the errors or request
for re-transmit if not possible. Interleaved blocks will be kept in the buffers
until the entire blocks can be reconstructed from subsequent
frames.
