E&M Type I signaling uses two leads, E and M, which share a common ground path. This signaling type is commonly used with electromechanical switching systems. Most electronic systems do not use it due to the noise induced by the common ground path.

The Type II interface is designed for use with electronic switching systems. It uses four wires, a pair for the E lead and a pair for the M lead. The signaling system is fully looped with the CU and SE having independent batteries and grounds. The SE controls the E lead and the CU controls the M lead.

The E&M Type V interface is widely used outside North America. It can be used in SSDC5A type circuits on the IDNX. This interface is an unbalanced version of E&M Type IV signaling from the trunk circuit to the signaling facility is over the M lead; signaling in the reverse direction is over the E lead. Local ground is used for off-hook instead of the ground obtained from the SB or SG lead. This makes this interface a two-wire type.

Integrated Services Data Network (ISDN)

ISDN, an international communications standard for sending voice, video, and data over digital telephone lines or normal telephone wires. ISDN supports data transfer rates of 64 Kbps (64,000 bits per second).


Each protocol is parameterized, giving developers the ability to adapt the protocol to multiple target environments worldwide. The MFC-R2 protocol states and events are identical for all countries, but certain parameters, such as tone duration, vary.