One of the most common interfaces for PBXs and switches is E&M. In addition to the voice path, there are two signaling circuits, one to send and one to receive. Depending on your source, the abbreviation stands for:
·Earth and Magneto, early terms for ground and battery.
·E(ar) and M(outh), indicating what the switch or PBX equipment does on each lead when talking on the multi-wire interface to the FRAD or transmission line ;
·connector block positions that were marked alphabetically, E and M, rather than numbered, used years ago to install this type interface. The positions were codified in a standard ‘practice’ or instruction set for telephone company craftsmen, thus preserving the name.
Note that E&M leads may consist of either 2 wires (plus ground lead) or 4 wires (two balanced circuits). However these are not the wires referred to when calling an E&M interface ’2-wire’ or ’4-wire’:
E&M interfaces are characterized as 2-wire or ^-wire by the nature of the voice path, not the signaling leads. That is, voice can be carried on either a single loop of one pair of wires or on a separate wire pair transmitting in each direction. E&M modules are available to support both 2-wire and 4-wire voice paths. There are defined five different types of E&M signaling circuits, written as Roman numerals I to V. The number of physical wires used for signaling depends on the Type.
As noted for Type IV E&M, below, two CO\VFRAD may be hooked together directly (back to back) if the E lead on one CO\VFRAD is connected to the M lead on the other (the voice path must be connected as well).
Types I and III need an auxiliary circuit between them, to provide battery as well as cross-connect the E’s to M’s. Types II, IV, and V are easily connected together if the sB, M, sG, and E of one side are wired to the sG, E, sB, and M leads of the other, in that order. Ignore the sG and sB on Type V
Off the shelf interface converters are available to connect an E&M with practically any other interface, including another E&M. Some types and mixed types may have to be kluged together by adding a battery, ground, relay, etc.
The signal on the E or M lead is any of several conditions, again depending on the Type of E&M interface. The conditions called ground, battery, open, and closed. Typically, only two of the conditions are possible on any specific lead, one for each possible state of that lead: idle or asserted.
Note that signaling and hook switches in the diagrams that follow are irked N.O. (normally open) or N.C. (normally closed). In this section, normal means on-hook or idle, the state in which phones spend most of their lives. When a device goes off hook. the N.O. contacts close; the N.C. contacts open.
The voice Frame Relay Access Device (VFRAD) is always assumed in this section on E&M to be transmission equipment, like the channel bank, and not the voice switch. It is the switch that has a mouth and ear. Therefore the E lead carries a signal from the CO\VFRAD to the PBX, and the M lead allows the PBX to signal the CO\VFRAD.
In cases where there is another E&M interface at the far end of the transmission line, any E&M signaling system conveys the M signal on one end to the E lead on the other. In this way there is a full duplex signaling path between the two switches or PBXs.
In the traditional telco world of channel banks and T-1 lines, the two states of the M lead (idle or asserted) are the only signaling messages. They translate into the two signaling states of the A bit: 0 = idle, 1 = asserted.
When the E&M interface is carried on frame relay, the FRADs on each end that are part of the signaling path have the same lead designation (E or \I) as the attached PBX:
(E on PBX) <E on CO\VFRAD) <frame relay net] <M on CO\VFRAD) <M on PBX) (M on PBX> (M on CO\VFRAD> [frame relay net> (E on CO\VFRAD> (E on PBX)
E&M is used in local loops. But rather than run so many wires (up to 8 for one voice circuit) over any great distance, the phone company uses signaling conversion devices to convey the state of the M lead on each end to the E lead at the other end. A CO\VFRAD performs this function on a frame relay network.
On analog circuits, the state of the M lead is converted to a tone signal carried on the voice path. The tone doesn’t interfere with callers because the presence of the tone indicates the line is idle: the tone must be absent if the circuit is in use (‘seized’ or ‘off hook’).
Often a type of E&M interface at one side is allowed to interact with some other interface, like loop start. These applications may use proprietary features.
The E&M interface is defined from the switch to transmission equipment, with the assumption that there is another switch at the other side of the transmission trunk. For the PBX to seize the trunk, it asserts the M signal when the E lead is not asserted (E asserted means the trunk is busy, or seized from the other end). When the far switch responds, by asserting its M lead, the originating switch sees that response signal on its E lead.
If dialing information is to be transmitted between the switches, the called end may indicate its readiness to receive digits in one of two ways:
·’Wink start’ ROM: The called end, when ready to receive dialing information, sends back a ‘wink’. This is a signal of at least 140 milliseconds, asserted on the far-end M lead, that appears on the originating end’s E lead. The Called Switch also may return audible dial tone on the voice path at the same time. The ANSI standard for PBXs (EIA-464) allows dial tone on the voice path to be substituted for the wink signal, if the other PBX will accept it; a configuration issue. The calling end then dials, either by toggling the M lead or by transmitting DTMF tones on the voice path.
·Delay dial trunk: when the called end sees the E lead active, it immediately (<300 ms) asserts its M lead (goes ‘off hook’) until ready to receive dialed digits. When ready, it returns to ‘on hook’ and may or may not supply dial tone. The calling end then dials, using pulse or tone. When the called switch is not busy and can respond quickly, both wink-•start and delay-dial will behave similarly and may be difficult to tell apart. A busy switch will be ready to receive dialing at a certain time, regardless of e trunk signaling type. That is, a ‘Delay’ pulse will be longer than a ‘Wink’ pulse but both will end at the same time.
A third trunk type doesn’t offer acknowledgement to a seizure: —
·’Immediate start’ trunk: the originating end starts dialing a short, \ed time after seizing the trunk. Dial tone from the called switch is optional.
In all cases the called PBX will assert its M lead when the called extension answers. The calling PBX sees this on its E lead.
There is no hard rule against any of these three trunk types having an interface with any of the five types of E&M. However, due to the delay across a frame-based network, it would be difficult, perhaps impossible, for two switches to use delay-dial on any E&M interface. Delay-dial requires a quick response to each seizure, in a time that may be less than the round-trip propagation delay across the frame relay backbone.
Originally, the M lead was toggled between two states (asserted and not asserted) to transmit dial pulses (rotary dialing). By mapping pulse dialing into M lead states, and that into signaling bits, it is possible to transport pulse dialing under the FR Forum Implementation Agreement for VoFR, using the ‘dialed digits’ subframe payload type.
Signaling between E&M interfaces may involve fairly critical timing of events The length of a wink is closely controlled to distinguish it from a disconnect (hang-up) followed immediately by another call request. Replicating dial pulses may have to preserve the ratio of on to off, which varies by country. In the US, the break (open loop circuit, on-hook state) is held close to 61% of the pulse time; in other countries the make: break ratio can be closer to 1:1.
Timing for these older, analog interfaces is the reason the FRF VoFR implementation agreement includes an emulation of ABCD signaling bits. In moving to the future, with ISDN and digital interfaces, the signaling will be based on messages like those in Q.931/Q.933 and Q.SIG from the ITU.
Once the near end has seized a trunk (and been acknowledged by the far end if the trunk type is wink start or delay dial) the originating end will transmit dialing. While it is still possible to toggle the M lead, switches today most often use DTMF (dual tone multi-frequency), the push-button dialing signals. DTMF is much faster than rotary dialing. Between central office switches, there is also a telco system of multi-tone signaling that is not included in the first version of the VoFR IA.
After the called station (telephone) answers (goes ‘off hook’) the called switch asserts the M signal lead, which appears on the E lead at the calling end. Both E and M remain asserted while the call is in progress. When either switch detects an ‘on hook’ from its station, it drops its M signal (causing E to drop at the other end). The second switch, if its phone is not hung up, should give it dial tone and behave as if a call were originating.
Note that asserting a signal lead might result in ending a signaling tone on the voice path, as happens with interoffice analog E&M signaling.
Type I: 2 E&M wires, ground
The PBX supplies battery voltage for both E and M in Type I signaling. Battery on the E lead is presented through a current detector. The positive side of the battery is grounded. The CO\VFRAD asserts the E signal by grounding the E lead at its end, drawing a current that is detected by the PBX. The transmission equipment (CO\VFRAD) terminates the M lead through a current detector tied to ground. The PBX asserts M by switching its end of the M lead from ground to battery, delivering a current that is detected in the CO\VFRAD. Type I is common in the US. It is convenient for transmission equip ment, like CO\VFRAD because the PBX supplies battery for both E and M The circuitry in the CO\VFRAD is passive.
Type II: 4 E&M wires
Because the PBX supplies battery on both leads to all trunks in Type I, the unbalanced ground current may become large and cause interference. Type II avoids this problem by making both E and M leads balanced. That is:
·The CO\VFRAD does not ground the E lead locally, but at the PBX through a separate lead, sG (signal ground).
·The battery source for the M lead is in the CO\VFRAD, delivered to the PBX on the sB lead (signal battery); the PBX closes the loop to assert M (there is no grounding of M at idle).
Type II is convenient because it is almost symmetrical—except for the location of the detector in the transmission interface being on the grounded leg, where it is on the battery leg in the switch. Still, two Type II devices of the same kind (two switches, or two CO\VFRAD may be connected together directly (locally) by crossing E to M and M to E (with their signal grounds). However, Type II also may be inconvenient because it is symmetrical. Now both sides have to supply battery. This is not a problem if there are two real switches connected by 6 or 8 copper wires (2- or 4-wire voice path). However, a VoFR device would need a suitable voltage source (at least -21 V), which is more than the 12 V commonly found in electronics equipment.
Type III: 4 E&M wires
The E lead on Type III is unbalanced, the same as Type I. M signaling is the same as Type I (switch from ground to battery to assert) except that both are provided by the CO\VFRAD (via sG and sB leads).
Type IV: 4 E&M wires
The switch side of Type IV is identical to the Type II, but the transmission side has the detector in the battery leg of the M circuit. Both sides are identical.
Type V: 2 E&M wires, ground
The E lead of Type V is the same as Type I. For the M lead, the CO\VFRAD supplies battery through a current detector. The PBX grounds M to assert a signal, drawing a current that is detected by the CO\VFRAD. The E lead is symmetrical with the M lead. Thus they may be crossed between similar equipment types to create a back-to-back connection. Type V is popular in Europe and the UK.
This introduction is meant to teach you about the functions and technology of a Central Office.
Analog to Digital Bandwidth
The Telephone Network
A Topology of Connection
Network Hierarchy (pre 1984)
Network Hierarchy (post 1984)
North American Numbering Plan
The Subscriber Extension
Local Access and Transport Areas
Wiring Connections: Hooking Things Up
Types of Communication
Lines Vs. Trunks
Foreign Exchange Signal