TELEPHONY FUNDAMENTALS

E&M Signaling

E&M is a common trunk-signaling technique used on telephony switches and PBXs. The signaling and voice trunks in E&M are separated. In E&M, voice is transmitted over either two or four-wire circuits, with six methods for signaling. E&M signaling methods are referred to as Types I, II, III, IV, and V; they also are known by the British Telecom (BT) standard, SSDC5.

The remainder of this section focuses on four-wire E&M Types I through V. E&M lead conditions for off-hook and on-hook for Types I through V are summarized in Table 3-1.

Table 3-1. E&M Signaling

Type	M Lead		E Lead
	Off-Hook	On-Hook	Off-Hook	On-Hook
I	Battery	Ground	Ground	Open
II	Battery	Open	Ground	Open
III	Loop current	Ground	Ground	Open
IV	Ground	Open	Ground	Open
V	Ground	Open	Ground	Open

Type I

With the Type I interface, the trunk equipment generates the E signal to the PBX by grounding the E lead (shown in Figure 3-1). The PBX detects the E signal by sensing the increase in current through a resistive load. Similarly, the PBX generates the M signal by sourcing a current to the trunk equipment, which detects it through a resistive load. The numbers 7, 2, 6, and 3 are the pinouts used on an RJ-48c connector.

Type II

E&M Type II has two additional leads over Type I: signal battery (SB) and signal ground (SG). In this method, the E lead is paired up with the SG lead, and the M lead is paired up with the SB lead. An on-hook at the PBX end is indicated when the E and M leads are open. Alternatively, an off-hook is indicated when the E lead is grounded and the M lead is providing battery (see Figure 3-2).

Type III

E&M Type III is used mostly in older telephone company switching centers. Figure 3-3 shows the Type III setup.

Figure 3-3. E&M Type III

Type IV

E&M Type IV is similar to E&M Type II; however, from the PBX side, an on-hook occurs when the E and M leads are open, and an off-hook occurs when both leads are at ground (see Figure 3-4).

Figure 3-4. E&M Type IV

Type V

Under E&M Type V, both the PBX and the switching endpoint supply battery (see Figure 3-5). At the PBX, battery is supplied on the E lead, and at the endpoint it is supplied on the M lead. Type V is the most common method of E&M signaling outside North America.

Figure 3-5. E&M Type V

JOSE LEONARDO MONCADA TORRES

C.I 18878408

CRF

http://codeidol.com/telecommunications/voiceip/Basic-Telephony-Signaling/EM-Signaling/