The data transfer capacities for both ADSL and SDSL depend on the a number
of factors, however, the distance from the telephone exchange
to the computer is the most important. The shorter the
distance, the higher the bit rate.
Another factor that impacts throughput capacity is the nature of the
"twisted pair" telephone wiring that connects the telephone exchange to the
home or business where the computer is located. The telephone
infrastructure in Europe in general supports higher capacities than in North
America.
Although ADSL in theory can support data transfer rates of
1.5 to 9 Mbps (megabits per second) when
receiving data, it can only send data at speeds of 16 to 640
Kbps. By comparison, SDSL can support data transfer rates ranging from
1 to 3 Mbps in both directions.
The following table shows theoretical minimum and maximum transfer rates
for ADSL and SDSL in bits/second: