The A in ADSL means asymmetric DSL. This recognises that for home use people tend to be more consumers of data than producers, so a slower upstream (upload) speed can traded off for a faster downstream (download) speed. In addition, the ADSL standard allows for analogue phone calls to be squeezed into the low end of the frequency spectrum. ADSL therefore needs a microfilter to be inserted into the phone socket.
There are three basic forms of DSL, and they are:-
ADSL - as described above
SDSL - Upload and download speeds are equal
RADSL - Upload speed stays constant, but download speeds may increase and decrease, depending on Network traffic, numbers of people connected at the same time, etc.