When analog Microwave Radio was deployed to transmit FDM channels a method to pass basic call setup signals of going off hook and back on hook needed to the other side of the trunk was needed. A common method of Out-of-band signaling is using a signaling tone frequency outside the 300Hz to 3,400Hz VF passband (but still within the 4kHz channel slot). A common example of this is the use of a 3,825Hz signaling tone.
Figure 17 E& M type 1
It was used with Signaling-E- & M-leads with two sides; the trunk side and the signaling side. E&M stands for Earth (Ear) and M for Magnetic (Mouth) with the trunk side typically connected to the Switch and the signaling side connected to the Microwave modem card.
Figure 17 shows that the Trunk side has an M lead relay when it is not energized its dry contact defaults to ground indicating an on hook condition. Conversely on the signaling side the E Lead is open during an on hook state. To emphasize the Trunk side is a line card configured as a Trunk side card the blue dashed line is where the wires cross-connected to the other card configured as a signaling side card. Note the Transmit is connected to the Receive wire pairs of each other but the E & M leads are connected straight through.
When E lead is energized it connects the ground to the M lead relay which creates a path for battery current to flow through the M lead relay bringing to an off hook position. So in an off hook position the M Lead provides battery the E Lead makes a ground connection.
Figure 18 E & M Type 2
If you consider that wire cross-connects could be up to several hundred feet between the transport side (which would be the signaling side) in a Central office or other wiring center to the Voice switch. RFI and other noise can interfere with the one wire signal system. To overcome that balancing the signaling solves that problem. Type 2 E&M shown in Figure 18 works about the same way but it uses 8 wires creating a balanced 4 wire signaling method that is less susceptible to noise. With Type 2 M-Lead is paired with the SB lead (Signal Battery) the E-Lead is paired with the SG (Signal Ground).
In the On hook state both the M-lead and the E Lead are Open, if the E-Lead relay is energized that is completed for current to flow through the M-Lead relay coil closing the contact to the off hook position, likewise the E-lead is connected to ground.
So in the On-Hook condition is M-lead is signal battery and the E-Lead is Signal Ground
Figure 19 Type E&M over Microwave
The analog microwave FDM channels would typically be made up of two cards one is the baseband card that accepts the 250-4000 k voice and then the mux-modem card that modulates the baseband audio to a radio frequency slot. The mux-modem connections that relate to signaling are the E- & M-leads. The E-lead connection is dedicated for receive signaling use, while the M-lead is for transmit.
Typically, the mux modem transmits signaling to the far-end when a voltage is applied to its M-lead connection. The far-end modem detects the signaling and activates its E-lead. Usually, the mux modem is equipped with an E-lead relay that supplies external supervisory equipment with a dry set of relay contacts.
In Figure 19 are two PBX’s with a 4 wire E&M Tie line one at location 1 and the other at location 4.
• PBX 1 has a trunk side E&M card that is cross-connected to a Signal side E&M card installed in a Microwave radio shelf.
• PBX 1 E&M tie line goes off hook which completes the ground going to the current detector in the Microwave signal side E&M card.
• This energizes the M-lead back to the PBX and the mux-modem signals an E-lead off hook through the Microwave at location 3. Which responds by generating an E-Lead ground over the cross-connect to PBX 4 E&M Card
Type 3 E & M Signaling
E & M Type 3 Signaling is one of two variations on Type 2 signaling, it still uses the 2 pair balanced signaling with this variation.
On Hook condition: E-Lead is open and M-Lead is set to ground that is connected to the SG lead of the signaling side.
Off Hook condition: On Trunk side M-Lead is connected to the SB lead which is connected to battery on the signaling side. The E-Lead on the signaling side is connected to SG lead that is connected to Trunk side circuit ground.
Type 3 had rare application.
Type 4 E & M Signaling
Type 4 is also has the same balanced pairs for signaling,
On Hook condition: Both E and M Leads are open
Off Hook condition: On Trunk side M-Lead is connected to the SB Lead that is connected to ground on the signaling side. The E-Lead on the signaling side is connected to the SG lead that is connected to the ground on the Trunk side.
Type 5 E & M Signaling
Is used outside of North America and is like Type 1 in that it also uses just and E and M Lead wires.
On Hook condition: Both E and M Leads are open
Off Hook condition: On Trunk side M Lead is grounded. On the signaling side the E-lead is also grounded. In each case the ground connection completes the path battery current to flow through the Current detector of each line card.
More will be covered on E&M signaling as digital application