There are three types of signaling mainly used on trunk side. It includes Start Dial signaling, Ground start signaling and E & M signaling. These are used in analog telephony as supervisory signaling.
Start Dial Signaling
Three types of signaling fall under Start Dial category of signaling. It includes wink start, immediate start and Tone start.
Wink start - Here originating trunk goes off hook first, remote switch transmit off hook pulse, and originating switch transmits dial digits.
Immediate Start - Here originating switch places trunk off hook and originating switch transmits dial digits.
Tone start - Originating switch places trunk off hook, receiving switch generates dial tone and dial digits are transmitted.
Ground Start Signaling
- It switches trunk lines and momentary grounds ring lead. PBXs actually use Ground Start with off-hook detection via grounded wire.
- It is modified Local loop signaling.
- It eliminates potential for two switched to gain control of the trunk
- It provides mechanism for current detection
E & M signaling
- E & M signaling is the most commonly used
- Separate paths for voice and signaling available
- M lead is used as transmit signaling(outbound)
- E lead is used as receive signaling (inbound)
- In this E&M signaling originating switch requests trunk by raising M lead, which other side of switch responds by raising the E Lead.
State E Lead M Lead
On-hook Open Ground
Off-hook Ground Battery Voltage