The following subsections detail the trunkside signaling that takes place after an off-hook condition, and prior to output of digits over the trunk circuit to the remote switch.
- Wink Start - Immediate Start - Tone Start - Delay Dial
Wink Start operation is the most commonly used trunk signaling type. In Wink Start operation, the Originating trunk circuit goes off-hook. The remote switch detects the off-hook signal, transmits an off-hook pulse that is between 140 and 290 mS in length, and returns to the idle (on-hook) state.
The Originating trunk detects the Wink Back, waits for at least 210 mS and outputs digits to the remote switch. When the call is answered, the terminating switch will indicate this by an off-hook condition.
With Immediate Start operation, the Originating trunk circuits goes off-hook, and maintains this condition for at least 150 mS. Then the Originating trunk will output digits.
This operation requires typically requires dedicated Signaling senders and receivers, although it is possible to operate using common receiving equipment that operates in a "fast link", or "bylink" mode.
Although typically not used in the PSTN, it is used occassionally throughout the PBX community.
In Tone Start mode, the Originating trunk circuit goes off-hook. Upon detecting the off-hook condition, the Terminating switch returns another dial tone. At this point the Originating switch may automatically output digits (automatic mode), or the End User may output digits (cut-through mode).
THIS TYPE OF SIGNALING SHOULD NOT BE USED ON TRUNKS EQUIPPED WITH ECHO CANCELERS, SINCE INITIAL DTMF OR MF DIGITS OUTPUT CAN BE GARBLED (no impact if Pulse Dialing is used).
In this mode of operation, the Originating switch and trunk circuit will go off-hook. After a timing interval (75 to 300 mS), the Originating switch will look at the status of the incoming signaling from the Remote switch. If the supervision is on-hook, the Originating office will output digits to the Remote switch. If the supervision if off-hook, the Originating office will wait for the supervision to return to an on-hook state, then output digits. There is no minimum time to wait for a delay-dial pulse from the Remote switch. In fact, if the remote switch was in an on-hook state, there is no waiting at all prior to output of digits. If the Remote switch was not ready to receive digits, improper call completions can result.
Delay Dial systems can operate with "integrity". In this mode, the Originating switch will not output digits until a Delay Dial (start-dial) pulse has been received from the Remote switch. In Delay Dial systems WITHOUT INTEGRITY, the Originating switch will wait for a predefined amount of time, then output digits.
The off-hook condition must be present for a minimum of 140 mS. The Off-Hook to On-Hook transistion must not occur for at least 210 mS after the Off-Hook condition.