E and M signaling

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.

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 (tip), R (ring), T1 (tip1) and R1 (ring1). Signaling is carried by E, M, SG (signal ground) and SB (signal battery). The latter two carry not signals but reference potentials for the signals, but are considered part of the E&M set. Of the many types of E&M standards, the diagram on right illustrates "Type IV E&M".
*In 6 wire E&M, two interpretations exist:
**(a) SG and SB wires are omitted, instead, the common building ground is used;
**(b) T1 and R1 wires are omitted, the payload is two wire circuit, using traditional hybrid coil techniques.
*In 4 wire E&M, 2 wires are 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.

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 signaling E&M and 2 wires (one pair) for Audio. (2 pairs total)
*4-wires E&M is 1 pair for the transmit direction of voice, 1 pair for the receive direction of voice, and one pair for signaling. (3 pairs total)

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: Various alternative explanations for the term "E&M" have arisen:
#"E" stands for "ear," i.e., when the near-end "E" lead was grounded, the far end was calling and "wanted your ear." Whereas "M" is commonly called "mouth," because when the near-end wanted to call (i.e., speak to) the far end, -48 vdc was applied to that lead. Probably this is a mnemonic turned folk etymology.
#In yet another story, the proposed labels were R and T for reception and transmission of signaling. However R and T were already used as labels for the tip and ring wire pair carrying voice signals. Thus a letter within each of the word "recEive" and "transMit" was chosen. This explanation is probably a backronym.
#The most likely explanation is that E&M comes from "earth" and "magneto" from the very earliest days of telephony. An actual magneto (coil) was used to apply -48 volts to the M lead through mechanical relay switches, while the E lead is normally held to ground (earth) unless acknowledging the signaling from the M lead.

References

* [http://www.its.bldrdoc.gov/fs-1037/fs-1037c.htm Glossary of telecommunications terms]

* [http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/voicdata.htm Earth and Magneto]