E&M 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

Telephone exchange

In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...

.
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

Channel Associated Signaling

Channel Associated Signaling , also known as per-trunk signaling , is a form of digital communication signaling. As with most telecommunication signaling methods, it uses routing information to direct the payload of voice or data to its destination. With CAS signaling, this routing information is...

 also use versions of E&M signaling. E&M is considered an obsolete technology for new installations, which generally use Basic Rate (BRI)

Basic rate interface

Basic Rate Interface is an Integrated Services Digital Network configuration intended primarily for use in subscriber lines similar to those that have long been used for plain old telephone service...

 or Primary Rate (PRI)

Primary rate interface

The Primary Rate Interface is a standardized telecommunications service level within the Integrated Services Digital Network specification for carrying multiple DS0 voice and data transmissions between a network and a user....

 digital interfaces.

Signaling units and trunk circuits

The E&M standards were initially developed by Bell Labs

Bell Labs

Bell Laboratories is the research and development subsidiary of the French-owned Alcatel-Lucent and previously of the American Telephone & Telegraph Company , half-owned through its Western Electric manufacturing subsidiary.Bell Laboratories operates its...

 and extended by national PTT administrations. The standard defines two sides to the interface:

• Trunk Circuit - this is normally the PABX
• Signaling Unit - this used to refer to the special modem that converted the DC signaling systems into tones that could be transmitted over a 4-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). 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.

• Type I is the most common standard in North America and Japan, and signals an outgoing call to the signaling unit with Battery on the M ("Mouth") lead. Incoming calls 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
• 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 line breaks interface defaults to busy.

Number of wires

E&M defines 8 wires:

• E: Originally "Earth", now called "Ear"
• M: Originally "Magneto", now called "Mouth"
• SG and SB: Signal Ground and Signal Battery
• T and R: Tip and Ring - the ground and battery of the receive voice pair
• T1 and R1: the ground and battery of the transmit voice pair

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

On-hook

In telephony, the term on-hook has the following meanings:# The condition that exists when a telephone or other user instrument is not in use, i.e., when idle waiting for a call. Note: on-hook originally referred to the storage of an idle telephone receiver, i.e., separate earpiece, on a switchhook...

 and off-hook

Off-hook

In telephony, the term off-hook has the following meanings:# The condition that exists when a telephone or other user instrument is in use, i.e., during dialing or communicating. Note: off-hook originally referred to the condition that prevailed when telephones had a separate earpiece , which hung...

 - 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

Multi-frequency

In telephony, multi-frequency signaling is an outdated, in-band signaling technique. Numbers were represented in a two-out-of-five code for transmission from a multi-frequency sender, to be received by a multi-frequency receiver in a distant telephone exchange...

 tones. E&M defines three methods of "start" signaling:

• Wink Start - when the originator goes off hook, the other end transmits a short (140-290ms) off-hook signal and returns to on-hook. The originator detects the wink and then sends the dialed digits. The other end goes permanently off-hook (seized) when the call is answered.
• Delay Start - the originator goes off hook, waits a pre-defined delay, and then checks for on-hook from the other end before sending the digits.
• Immediate Start - The originator goes off hook, waits 150ms and then sends the dialed digits.

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
   Etymology
   Etymology is the study of the history of words, their origins, and how their form and meaning have changed over time.For languages with a long written history, etymologists make use of texts in these languages and texts about the languages to gather knowledge about how words were used during...
   
   , the proposed labels were R and T for reception and transmission of signaling. However R and T
   Tip and ring
   "Tip" and "Ring" are common terms in the telephone service industry referring to the two wires or sides of an ordinary telephone line. Tip is the ground side and Ring is the battery side of a phone circuit. In the UK these are referred to as the 'A' and 'B' wires...
   
    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
   Backronym
   A backronym or bacronym is a phrase constructed purposely, such that an acronym can be formed to a specific desired word. Backronyms may be invented with serious or humorous intent, or may be a type of false or folk etymology....
   
   .
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
   Magneto
   A magneto is a type of electrical generator.Magneto may also refer to:* Magneto , permanent magnetic alternating current rotary generator* ignition magneto, magnetos on internal combustion engines...
   
   " 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.