 Several approaches were used, but all of them began as solutions to one fundamental problem with phone lines.While phone lines theoretically have the bandwidth to handle data encoded at speeds above 33k, by the time faster speeds were desirable, telephone companies had switched almost entirely to all-digital networks, which introduced problems beyond simple audio bandwidth constraints.In 1990s phone company networks, as soon as a phone line reached a local central office, a "line card" converted the analog signal from the subscriber to a digital one, and vice versa.