E&M is a type of signaling traditionally used in the telecomunications industry. Indicates different channels that corresponds to the Ear (receive) and Mouth (Transmit) on a communication link. There are different E&M standards, with 8, 6, 4 and even 2 wires. In 8 wire E&M, the control and payload channels are separate for both receive and transmit. In 6 wire E&M, control is separated, but the payload is bothway, using traditional echo-cancelation techniques. In 4 wire E&M, 2 wires is used for receive and 2 wires for transmit, incorporating control and payload in each channel. The 2 wire E&M is technically same thing with a normal CO trunk.

E & M signaling: In telephony, an arrangement that uses separate leads, called the "E" lead and "M" lead, for signaling and supervisory purposes. Note 1: The near end signals the far end by applying -48 vdc to the "M" lead, which results in a ground being applied to the far end's "E" lead. When -48 vdc is applied to the far end "M" lead, the near-end "E" lead is grounded. Note 2: The "E" originally stood for "ear," i.e. , when the near-end "E" lead was grounded, the far end was calling and "wanted your ear." The "M" originally stood for "mouth," because when the near-end wanted to call (i.e. , speak to) the far end, -48 vdc was applied to that lead.

References

• Glossary of telecommunications terms

telephony