E and M signaling
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E&M is a type of signaling traditionally used in the North American telecommunications industry. 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-cancellation 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.
[edit] References
note: A middle 20th century mnemonic has the letters stand for Ear (receive) and Mouth (Transmit) on a communication link. This gave rise to a misconception that these were the orginal meanings of hte letters. On the original schematics, it was simply the alphabetic designtion.
