Semiconductor
The T1/E1 protocol is implemented as a "line interface unit" in silicon. The semiconductor chip contains a decoder/encoder, loop backs, jitter attenuators, receivers, and drivers. Additionally, there are usually multiple interfaces and they are labeled as dual, quad, octal, etc., depending upon the number.
The transceiver chip's primary purpose is to retrieve information from the "line", i.e., the conductive line that transverses distance, by receiving the pulses and converting the signal which has been subjected to noise, jitter, and other interference, to a clean digital pulse on the other interface of the chip.
Read more about this topic: Digital Signal 1