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E&M – Earth & Mouth is a type of line signaling that uses separate leads, called the "E" lead and "M" lead.There are NEAR & FAR ends.The near end sends a signal to the far end by applying -48 vdc to the "M" lead, which results in a ground being applied to the far end's "E" lead ie the near-end wanted to call the far end. When -48 vdc is applied to the far end "M" lead, the near-end "E" lead is grounded. When the near-end "E" lead is grounded, the far end is calling and wanted your ear.

• 2 wire E&M is technically same with a normal CO trunk.2 wire E&M means 2 wires ie one pair for signaling E&M and 2 wires ie one pair for Voice.-Total of 2 pairs
  

• In 4 wire E&M, 2 wires ie 1 pair is used for receiving voice and 2 wires ie 1 pair for transmitting voice and one pair for signalling.-Total 3 pairs