Line in use detection

Abstract

An apparatus and method for ascertaining states of a telephone line. A communication device, such as a modem, is coupled to a telephone line that also has other communication device or devices coupled to the line. The state of the line is determined before the modem goes off-hook. An energy level of a capacitor that is coupled to the telephone line is ascertained by a data pump of the modem. Should the energy level of the capacitor exceed a threshold, then the telephone line is in use and the modem does not go off-hook. If the energy level is at or below the threshold level, then the modem goes off-hook. In another implementation, an energy profile is established for the other communication devices when they transition from an on hook state to an off hook state and vise versa. The energy profile can be detected by the data pump to ascertain the state of the telephone line or the state of the communication device. The disclosed technique provides an elegant way of ascertaining the state of the telephone line without the addition of hardware. By ascertaining the state of the telephone line, the modem can avoid interfering with the other communication devices on the telephone line.