E and M signaling

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E&M is a type of supervisory line signaling that uses separate leads, called the "E" lead and "M" lead, traditionally used in the North American telecommunications industry. There are different E&M standards, with 8, 6, 4 and even 2 wires.

1 Number of wires
2 Alternative numbering
3 Notes
4 References

[edit] Number of wires

In 8 wire E&M, the most common kind in the 20th century, control and payload channels are separate for both receive and transmit. Payload was a four wire circuit for speech, carried on leads T, R, T1 and R1. Signalling is carried by E, M, SB and SG. The latter two carry not signals but reference potentials for the signals, but are considered part of the E&M set.
In 6 wire E&M, control is separated, but the payload is two wire circuit bothway, using traditional echo-cancellation techniques.
In 4 wire E&M, 2 wires is used for receive and 2 wires for transmit, incorporating simplex control and differential payload in each channel.
The 2 wire E&M is technically same thing with a normal CO trunk.

[edit] Alternative numbering

Connections can also be designated by the number of wires used for voice transmission, also called payload. In this scheme:

2 wire E&M means 2 wires (one pair) for signalling E&M and 2 wires (one pair) for Audio. (2 pairs total)
4-wires E&M is 1 pair for one direction of voice, 1 pair for the other direction of voice, and one pair for signalling. (3 pairs total)

[edit] Notes

Note 1: The near end sends a signal to 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" traditionally stood for "ear," i.e., when the near-end "E" lead was grounded, the far end was calling and "wanted your ear." The "M" traditionally 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. This middle 20th century tradition gave rise to a misconception that these were the original meanings of the letters. On the original schematics, it was simply the alphabetic designation.