Signaling units and trunk circuits
The E&M standards were initially developed byVariants
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: * E: It is often called ''ear'' * M: It is often called ''mouth'' or ''magneto'' * SG and SB: signal ground and signal battery * T and R: Tip and Ring – the ground and battery of the receiver voice pair * T1 and R1: the ground and battery of the transmitter 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 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) orOrigin of "E&M"
The choice of letters for the E and M leads was fortuitous, unrelated to any names or meanings.Bell Telephone Laboratories, G.E. Schindler (ed.), ''A History of Engineering and Science in the Bell System—Switching Technology (1925-1975)'', 1982, p.52 However, various names have been associated with the letters ''E'' and ''M'': *''E'' 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. *''E'' for ''earth'' and '' magneto'' for ''M'', 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. *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. *Yet another explanation is that these were sequential designations on the wiring list of the original design.See also
* Pulse link repeater * Signaling (telecommunications)References
{{DEFAULTSORT:E And M Signaling Telephony signals