E&M Signaling Types

E&M (Ear & Mouth) signaling is typically used for trunks. Originally these were 8-wire circuits which interconnected different Central Offices. Using digital E&M signalling, there are only two physical states: busy (off-hook) and idle (on-hook).

Since E&M is a trunking interface, to allow for dialed digit information to be passed between the two endpoints, there are three flavors of E&M that are used:

E&M Immediate

In Immediate Start, the originating switch goes off-hook and sends any digits to the far end (terminating) switch. After it has received the digits (and the call is completed), the terminating switch goes off-hook for the duration of the call. The originating switch is off-hook for the entire call. Either end can terminate the call by going on-hook. The only drawback to this form of signaling is that there is a possibility that the terminating switch was not ready to accept digits from the originating switch. To solve this, Wink Start was created.

E&M Wink

(aka "Feature Group B")

In Wink Start, the terminating switch responds to an off-hook from the originating switch with a short wink. This wink tells the originating switch that the terminating switch is ready to receive digits. After it has received the digits (and the call is completed), the terminating switch goes off-hook for the duration of the call. The remainder of the call is processed just as in Immediate Start.

E&M Wink-Wink

(aka "Feature Group D")

With Wink-Wink, the terminating switch responds to an off-hook from the originating switch with a short wink just as in a normal Wink Start call. This tells the originating switch that the terminating switch is ready to receive digits. After receiving all of the digits, the terminating switch then sends another wink that tells the originating switch that the terminating side has collected all of the dialed digits. Once the call is completed, the terminating switch goes off-hook for the duration of the call. The remainder of the call is processed just as in Immediate Start.

 

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