During a lecture my teacher told me that the UPSTREAM in ADSL is very much affected from near end crosstalk due to DOWNSTREAM, while DOWNSTREAM is not affected by this phenomena. This lead to the decision to dedicate lower frequency to the UPSTREAM and higher frequency to the DOWNSTREAM. Is this correct? In case it is, how is the DOWNSTREAM not affected by near end crosstalk effect?
Most of the noise is physically generated where the ISP's multiplexors split up the signal onto individual wires to each consumer.
The upstream data signal is generated by the consumer and has been attenuated by the cable when it reaches that point in the network.
For downstream data, the cable attenuates both the downstream signal and the noise which reaches the consumer.
So the signal to noise ratio is better for downstream data and the bandwidth can be higher.