While voices are recognizable with the standard telephone frequency range, that range of frequencies is likely to be inadequate for other types of sounds-for instance, music sounds lousy over the telephone. To allow for gradual rolloffs of high and low frequencies, the telephone companies allow an analog telephone channel a bandwidth of 4,000Hz to work with. At the central office, the odds are that the analog signal will be digitized in order to be switched across the telephone network. Aside from Giblet County, AK, and Rat Fork, WY, the U.S. telephone network that interconnects central offices uses digital signaling. Although many urban business telephone users have digital services direct to their PBXs or data communication devices, and ISDN lines are digital, the local loop is sometimes referred to as the last mile, because residences, generally saddled with analog-only transmission facilities, are rarely capable of bandwidth greater than 4,000Hz.
Modems convert digital signals from a computer into analog signals in the telephone frequency range.
There is a hard upper limit to the capacity of a channel with a given bandwidth. If you lease a T1 line, you get a DS1 channel. The second important distinction to make about telephone lines is whether they are dedicated circuits you lease or switched services you buy. While frame relay services are switched through a cloud frame by frame, they are almost invariably sold as permanent virtual circuits, where the customer specifies the end points. For purposes of designing a network layout, frame relay links have more in common with dedicated lines than with switched lines, but the cost can be substantially lower for an equivalent capacity.