Is multirate signal processing involved in ADSL?

Telecom companies offer different data rates for home/office purposes depending on the money they pay.

• How are the telecom people able to control the flow of data of different cables?
• Say supposing a cable can be used till $20\textrm{ MBps}$. But he paid only for $5\textrm{ MBps}$ scheme, why is the user not able to steal the remaining bandwidth?
• Is there a multi rate theory behind this?
• Or it is a simple filtering?
• May I be pedantic and point out that your two speeds are the wrong way round? Also ADSL only goes up to about 20 Mbps anyway. – Simon B Feb 12 '15 at 13:04
• ok i've edited my question.thank you @SimonB – phanitej Feb 13 '15 at 4:57