A technology that allows data to be sent over existing copper telephone lines ( POTS ). ADSL supports data rates from 1.5Mbps to 9Mbps when receiving data (known as the downstream rate) and from 16Kbps to 640Kbps when sending data (known as the upstream rate). ADSL requires a special modem, termed the ATU-R (ADSL Termination Unit Remote). ADSL2+ is a more recent incarnation of ADSL and has been deployed widely. The difference between the two is a matter of data rate, with the later version offering downstream rates of up to 24Mbps and upstream rates of up to 1Mbps (theoretical). This is achieved using improved physical layer techniques in conjunction with a reduction in the permissible length of the last mile.