Compelled signaling is a method of conveying numerical and status information between two pieces of equipment. It is often used on E-1 trunks for dialing numbers, getting DDI digits and so on. Single and dial tones may be used, depending on the specifics of the protocol. To send one item of information, such as one -digit, both the sender and the receiver of the information play one tone.
The sequence of events is like this
1. The sender begins transmitting a tone.
2. The receiver detects the tone. In response, the receiver begins transmitting a response tone.
3. When the sender detects the response, it turns of the original tone.
4. When the receiver detects the end of the original tone, it stops sending the response tone.
It sounds complicated, and it is. R2/MF signaling is based on the compelled signaling model, and comes in many variations in different regions.