E and M signaling

E&M (earth & magneto, or ear & mouth) 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.

8 Wires of E and M signaling (Type IV E&M)

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.

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 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.

Variants

There are six main variants of E&M.

Number of wires

E&M defines 8 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"

Various alternative explanations for the term "E&M" have arisen:

  1. "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.
  2. In 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.
  3. Yet another explanation is that these were sequential designations on the wiring list of the original design. The original design, however, has not been offered in evidence.
  4. The most likely explanation is that E&M comes from "earth" and "magneto" from the very earliest days of telephony. An actual magneto 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.
  1. "E" stands for Exchange as the E wire it switched between −48Vdc and 0Vdc at the Exchange.
  2. "M" is for Modulator it allows a contact closure to occur and in-turn switches an OOB signal modulator to the Carrier line.

<Telecom Australia Standard 187>

See also

References

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