EM 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

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