TDM is a technique for transmitting two or more signals over the same telephone line, radio channel, or other medium. Each signal is sent as a series of pulses or packets, which are interleaved with those of the other signal or signals and transmitted as a continuous stream. It is used when the bit rate of the transmission medium exceeds that of the signal to be transmitted. This form of signal multiplexing was developed in telecommunications for telegraphy systems in the late 19th century, but found its most common application in digital telephony in the second half of the 20th century.