The earliest variation of DSL to be widely used. HDSL is applied for wideband digital transmission within corporate sites and between telephone companies and customers. The main characteristic of HDSL is that it is symmetrical; i.e., an equal amount of bandwidth is available in both directions. For this reason, the maximum data rate for HDSL is lower than for ADSL. HDSL can carry as much on a single wire of twisted-pair wire as can be carried on a T1 line in North America or an E1 line in Europe (2,320 kbit/s).