AT&T 518-600-016 manual Signaling

E&M SIGNALING

Most signaling systems, other than loop signaling, are separate from the trunk equipment. They are normally located between the trunk equipment and the line facility. E&M signaling systems derive their name from the historical designations of the signaling leads on schematics covering these systems. By convention, the "E" stands for ear and the "M" stands for mouth (rEceive and transMit). The E&M lead signaling interface consists of two leads between the switching equipment (central office) and the signaling equipment (PBX). The “M lead carries signals from the switching equipment to the signaling equipment. The “E” lead carries return signals from the signaling equipment to the switching equipment. The E&M interface is designated as TL31M, and the 50- pin connector is designated as RJ2GX. At the RJ2GX interface, the PBX is the switching equipment side, and the network side (toward the central office) is the signaling equipment side. An example of E&M signaling is shown in Figure 1-11.

Theory of Operation 1-45