E&M interface

An ear & mouth or receive & transmit (E&M) interface uses a RJ-48 telephone cable to connect a PBX. The PBX sends signals on the M (mouth) line and receives signals on the E (ear) line. The voice router receives M signals from the PBX and sends E signals to the PBX. An E&M interface can only be connected to another E&M interface.

When E&M is applied in VoIP communication, two or four voice wires can be used. There are two or four signaling wires as well. Therefore, 4-wire analog E&M actually has six wires. In two-wire mode, voice is transmitted over each wire in each direction simultaneously. In four wire mode, voice is transmitted over each wire pair in each direction simultaneously.