Leased lines have been used extensively to carry corporate data since the 1970s. Mainframe networks of this era used analogue leased lines of a very low datarate (2400 baud) over the Public Switched Telephone Network (PSTN) with modems at either end. During the 1990s, many corporations rented digital leased lines from telecom providers to construct their own data networks. These lines were typically of E0 (64-kbit/s) or T1/E1 (1.544/2.048-Mbit/s) datarates; however, for many customers the cost was prohibitive. A typical company might require data communication over a spread geographical distance (perhaps 1000 km); although it is easy to provision a circuit over such a distance, it would be very expensive. The telecom providers responded by offering data overlay networks.