E&M start mode
An E&M interface supports E&M signaling and divides each voice connection into trunk circuit side and signaling unit side (similar to DCE and DTE).
An E&M interface provides on-hook/off-hook signals with minimum interference. Instead of using dial tones, E&M interfaces use one of the following main start dial supervision signaling protocols:
Immediate start—As shown in Figure 9, the calling side goes off-hook, waits for a certain period of time, and sends the dialed digits regardless of whether the called side is ready or not. If the called side receives the digits, it rings to alert the called party of the call. If the called party picks up the phone, the call is connected.
Figure 9: Immediate start mode
Delay start—As shown in Figure 10, the calling side goes off-hook to seize the trunk and the called side (PBX) goes off-hook to respond to the seizure. When the called side (PBX) is ready, it goes on-hook. The off-hook interval is the delay dial signal. Then, the calling side starts sending the address information and the PBX at the called side routes the call to its destination. When the called party answers, the called side goes off-hook and both sides remain off-hook for the duration of the call.
Figure 10: Delay start mode
Wink start—As shown in Figure 11, the calling side goes off-hook to seize the trunk. The called side (PBX) remains on-hook until it receives the connection signal from the calling side. Once the called side is ready, it sends a wink signal. Once the calling side receives the wink, it sends the address information, and the call is routed to its destination. When the called party answers, the called side goes off-hook and both sides remain off-hood for the duration of the call.
Figure 11: Wink start mode
