the channel can never transmit much more than 13 Mbps, no matter how many or
how few signal levels are used and no matter how often or how infrequently samples
are taken. In practice, ADSL is specified up to 12 Mbps, though users often
see lower rates. This data rate is actually very good, with over 60 years of communications
techniques having greatly reduced the gap between the Shannon capacity
and the capacity of real systems.