E&M Signaling
E&M is another signaling technique used mainly between PBXs or other network-tonetwork
telephony switches (Lucent 5 Electronic Switching System [5ESS], Nortel DMS-
100, and so on). E&M signaling supports tie-line type facilities or signals between voice
switches. Instead of superimposing both voice and signaling on the same wire, E&M uses
separate paths, or leads, for each.
There are six distinct physical configurations for the signaling part of the interface. They
are Types I–V and Signaling System Direct Current No.5 (SSDC5). They use different
methods to signal on-hook or off-hook status, as shown Table 3-4. Cisco voice implementation
supports E&M Types I, II, III, and V.
Table 3-4 E&M Signaling Types
Type M-Lead Off-Hook M-Lead On-Hook E-Lead Off-Hook E-Lead 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
SSDC5 Earth On Earth Off Earth On Earth Off
The following list details the characteristics of each E&M signaling type introduced in
Table 3-4:
■ Type I: Type I signaling is the most common E&M signaling method used in North
America. One wire is the E lead. The second wire is the M lead, and the remaining
two pairs of wires serve as the audio path. In this arrangement, the PBX supplies
power, or battery, for both E and M leads. In the idle (on-hook) state, both the E and
M leads are open. The PBX indicates an off-hook by connecting the M lead to the
battery. The line side indicates an off-hook by connecting the E lead to ground.
■ Type II: Type II signaling is typically used in sensitive environments because it produces
very little interference. This type uses four wires for signaling. One wire is the
E lead. Another wire is the M lead, and the two other wires are signal ground (SG)
and signal battery (SB). In Type II, SG and SB are the return paths for the E lead and
M lead, respectively. The PBX side indicates an off-hook by connecting the M lead
to the SB lead. The line side indicates an off-hook by connecting the E lead to SG
lead.
■ Type III: Type III signaling is not commonly used. Type III also uses four wires for
signaling. In the idle state (on-hook), the E lead is open and the M lead is connected
to the SG lead, which is grounded. The PBX side indicates an off-hook by moving
the M lead from the SG lead to the SB lead. The line side indicates an off-hook by
grounding the E lead.
■ Type IV: Type IV also uses four wires for signaling. In the idle state (on-hook), the E
and M leads are both open. The PBX side indicates an off-hook by connecting the M
lead to the SB lead, which is grounded on the line side. The line side indicates an offhook
by connecting the E lead to the SG lead, which is grounded on the PBX side.
■ Type V: Type V is the most common E&M signaling form used outside of North
America. Type V is similar to Type I because two wires are used for signaling (one
wire is the E lead and the other wire is the M lead). In the idle (on-hook) state, both
the E and M leads are open as in the preceding diagram. The PBX indicates an offhook
by grounding the M lead. The line side indicates an off-hook by grounding the
E lead.
■ SSDC5: Similar to Type V, SSDC5 differs in that on- and off-hook states are backward
to allow for fail-safe operation. If the line breaks, the interface defaults to offhook
(busy). SSDC5 is most often found in England.
Note E&M Type IV is not supported on Cisco voice gateways. However, Type IV operates
similarly to Type II except for the M-lead operation. On Type IV, the M-lead states are
open/ground, compared to Type II, which is open/battery. Type IV can interface with
Type II. To use Type IV you can set the E&M voice port to Type II and perform the necessary
M-lead rewiring.