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 to allow PABXs in different geographic locations to communicate over an analog private circuit. Some digital interfaces such as Channel Associated Signaling also use versions of E&M signaling. E&M is considered an obsolete technology for new installations, which generally use Basic Rate (BRI) or Primary Rate (PRI) digital interfaces.

The E&M standards were initially developed by Bell Labs and extended by national PTT administrations. The standard defines two sides to the interface:

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). The Battery signal used in the standard is nominally −48VDC. All E&M installations require that the +ve terminal of the Battery is connected to a shared reliable Earth.
The maximum distance between the Signaling Unit and the Trunk interface is determined by the resistance of the wire, but will normally be less than 100m for adequate noise immunity.

The group of E&M signaling includes the following variations:

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.

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:

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