On 15/01/2011 23:32, wrote:

Several symbols (carriers) are usually used simultaneusly

'Symbols' are not 'carriers'.

A symbol (one ore more bits) is the basic unit of data transmitted.
Symbols are transmitted in time sequence to send a data stream.
http://en.wikipedia.org/wiki/Symbol_%28data%29

The number of carriers (one or many) is quite separate. Multi carrier
FDM systems like DMT and (C)OFDM are used so that a fast stream can be
broken into many slower ones, each with a correspondingly slower symbol
rate. This, within limits, can make a system much more tolerant to
multipath reflections occurring in the transmission channel.

--
Andy