Why are some variations of xDSL asymmetric?
[top] It is primarily due to near-end crosstalk (NEXT). The large bundle of wire at the CO is heavily susceptible to crosstalk when the data is traversing from the far end (the end user). At the far end, there are fewer problems with NEXT so bandwidth is greater from the CO to the user. High bit rates, or in this case, higher frequencies suffer a greater amount of attenuation. The reason that the upstream speed in ADSL is generally much less than the downstream rate is due to this fact. When the high frequencies have attenuated at the CO end, they are very susceptible to all the other signals in the binder group due to EMI. In the downstream direction, the high frequencies still attenuate, but at the customer end, they have a better chance of avoiding crosstalk since most subscribers will not have large bundles of cables running into their premises.
