E and M signaling

E and M signaling is a type of supervisory line signaling that uses DC signals on separate leads, called the "E" lead and "M" lead, traditionally used in the telecommunications industry between telephone switches. Various mnemonic names have been used to memorize these letters, such as "Ear" and "Mouth", the most common variation.
E&M was originally developed for signaling between PABXs in different geographic locations over an analog private circuit. The protocol was later extended for use on digital carrier system with Channel Associated Signaling (CAS).
Signaling units and trunk circuits.
The E&M standards were initially developed by Bell Labs and extended by national PTT administrations. The standard defines two sides of the interface, the trunk circuit and the signaling unit. The trunk circuit is normally the side to which the PABX is connected. The signaling unit is the special modem that converts the DC signaling protocol into tones that could be transmitted over a four-wire link back to the PTT exchange.
The signaling unit and trunk circuit communicate their status over the E and M leads, using a combination of battery and earth (also known as ground) levels. The battery signal level used in the standard is nominally −48 VDC. All E&M installations require that the positive terminal of the battery is connected to a reliable shared earth circuit. The maximum distance between the signaling unit and the trunk interface is determined by the resistance of the wire, but is normally less than 100 meters for adequate noise immunity.
Variants.
The group of E&M signaling includes several variations.
"Type I" is the most common standard in North America and Japan, and signals an outgoing call from the PBX to the signaling unit with Battery on the M ("Mouth") lead. Incoming calls from the CO are signaled by an earth (ground) signal on the E ("Ear") lead. The interface is vulnerable to poor earthing at either end, and interference from external electrical noise which act on inferior voice signal quality. 
"Types II to IV" are variants which attempt to overcome the main limitation of Type I, which is the reference to earth at each end of the circuit. Types II-IV use the Signal Battery (SB) and Signal Ground (SG) lead in conjunction with the E&M wires. This improves noise immunity as the Signal Ground does not carry the same heavy currents as normal ground connections and provides a low resistance return path for signaling. However, if the main ground connection fails all earth current may flow via the interface, causing signaling failure, hum and in extreme cases destruction of equipment.
"Type V" is the most common variant in use outside United States. In contrast to Type I, both ends of the connection indicate a call by grounding the relevant lead. This means that it is easy to interconnect two PABXs "back-to-back" by crossing over the E&M leads and transmit and receive pairs.
"SSDC5" is commonly used in the United Kingdom and unlike Type V the on- and off-hook state, are backwards to allow for fail-safe operation. If the line breaks, the interface defaults to busy.
Number of wires.
E&M defines eight wires:
"4-wire E&M" uses a 4-wire (2-pair) transmission path for the voice signal.
"2-wire E&M" uses a single pair for both transmit and receive voice signal. This is much inferior to 4-wire E&M as the 2-wire interface uses hybrid transformers which reduce signal quality and can introduce echo.
Address signaling.
The mechanisms described so far only allow circuit seizure – on-hook and off-hook – to be signaled. In order to allow dialing over the interface, "start" signaling mechanisms are defined. This allows the other end to know when to send the dialed digits, which are transmitted by pulse (loop disconnect) or multi-frequency tones. E&M defines three methods of "start" signaling:
Origin of "E&M".
The choice of letters for the E and M leads was fortuitous, unrelated to any names or meanings. However, various names have been associated with the letters "E" and "M":