Understanding and Troubleshooting Analog E&M Start Dial Supervision Signaling
Introduction
This document discusses analog recEive and transMit (E&M) Start Dial Supervision signaling. Start Dial Supervision is the line protocol that defines how the equipment seizes the E&M trunk and passes the address signaling information (sends dual tone multifrequency (DTMF) digits). The three main start dial supervision protocols used on E&M circuits are Immediate Start, Wink Start, and Delay Dial.
Wink Start Signaling
Wink is the most commonly used protocol. This is the Wink Start operation process:
1. Originating side seizes the trunk by going off-hook.
2. Terminating side remains idle (on-hook) until the digit collection equipment is attached.
3. Once the terminating side is ready, it sends a wink. A wink is an on-hook to off-hook to on-hook transition. This transition period ranges from 100 to 350 ms.
4. Once the origination side receives the wink, (which is interpreted as an indication to proceed), it sends the address (digits) information.
5. The call is then routed to its destination.
6. When the distant end answers, the terminating side signals answer supervision towards the originating side by going off-hook.
7. Both ends remain off-hook for the duration of the call.
8. Either end can disconnect the call by going on-hook.
The main reason for Wink Start (over Immediate Start) is to ensure that the side that receives the DTMF digits is ready to receive them. For PBX and central office (CO) products, the DTMF receivers are a shared resource and there may be less of them than there are total lines and trunks. Another reason is the glare reduction. Glare occurs when both ends of the trunk attempt to seize the trunk at the same time.
Immediate Start Signaling
Immediate Start signaling is the most basic protocol. The originating side goes off-hook, waits for a finite period of time (200 ms, for example), then sends the dial digits without regard to the far end (refer to the diagram).
The Immediate Start signaling method is less reliable than Wink Start. In Immediate Start, there is no wink from the end that receives the call to signify that it is ready to accept digits. In some situations, the PBX may be under heavy load and not able to switch a DTMF receiver in place quickly enough to receive the digits from the Cisco product. In that case, the call fails to complete because the Cisco product sends the DTMF digits before the PBX is ready to accept them. Therefore, for maximum reliability, Wink Start is preferred over Immediate Start.
Delay Dial Signaling
The Delay Dial operation process is shown here (refer to the diagram):
1. The originating side seizes the trunk by going off-hook.
2. The terminating side responds to the seizure by going off-hook.
3. The terminating side remains off-hook until it is ready to receive address information.
4. When the terminating side is ready, it goes on-hook. The off-hook interval is the delay dial signal.
5. The originating side starts sending address information.
6. The call is routed to its destination.
7. When the distant end answers, the terminating side signals answer supervision towards the originating side by going off-hook.
8. Both ends remain off-hook for the duration of the call.
9. Either end can disconnect the call by going on-hook.
Delay Dial is created because there are still problems in the field with Wink Start. There is equipment in the field that sends a wink, but it was not ready to receive digits the very instant after it sent the wink.
This document discusses analog recEive and transMit (E&M) Start Dial Supervision signaling. Start Dial Supervision is the line protocol that defines how the equipment seizes the E&M trunk and passes the address signaling information (sends dual tone multifrequency (DTMF) digits). The three main start dial supervision protocols used on E&M circuits are Immediate Start, Wink Start, and Delay Dial.
Wink Start Signaling
Wink is the most commonly used protocol. This is the Wink Start operation process:
1. Originating side seizes the trunk by going off-hook.
2. Terminating side remains idle (on-hook) until the digit collection equipment is attached.
3. Once the terminating side is ready, it sends a wink. A wink is an on-hook to off-hook to on-hook transition. This transition period ranges from 100 to 350 ms.
4. Once the origination side receives the wink, (which is interpreted as an indication to proceed), it sends the address (digits) information.
5. The call is then routed to its destination.
6. When the distant end answers, the terminating side signals answer supervision towards the originating side by going off-hook.
7. Both ends remain off-hook for the duration of the call.
8. Either end can disconnect the call by going on-hook.
The main reason for Wink Start (over Immediate Start) is to ensure that the side that receives the DTMF digits is ready to receive them. For PBX and central office (CO) products, the DTMF receivers are a shared resource and there may be less of them than there are total lines and trunks. Another reason is the glare reduction. Glare occurs when both ends of the trunk attempt to seize the trunk at the same time.
Immediate Start Signaling
Immediate Start signaling is the most basic protocol. The originating side goes off-hook, waits for a finite period of time (200 ms, for example), then sends the dial digits without regard to the far end (refer to the diagram).
The Immediate Start signaling method is less reliable than Wink Start. In Immediate Start, there is no wink from the end that receives the call to signify that it is ready to accept digits. In some situations, the PBX may be under heavy load and not able to switch a DTMF receiver in place quickly enough to receive the digits from the Cisco product. In that case, the call fails to complete because the Cisco product sends the DTMF digits before the PBX is ready to accept them. Therefore, for maximum reliability, Wink Start is preferred over Immediate Start.
Delay Dial Signaling
The Delay Dial operation process is shown here (refer to the diagram):
1. The originating side seizes the trunk by going off-hook.
2. The terminating side responds to the seizure by going off-hook.
3. The terminating side remains off-hook until it is ready to receive address information.
4. When the terminating side is ready, it goes on-hook. The off-hook interval is the delay dial signal.
5. The originating side starts sending address information.
6. The call is routed to its destination.
7. When the distant end answers, the terminating side signals answer supervision towards the originating side by going off-hook.
8. Both ends remain off-hook for the duration of the call.
9. Either end can disconnect the call by going on-hook.
Delay Dial is created because there are still problems in the field with Wink Start. There is equipment in the field that sends a wink, but it was not ready to receive digits the very instant after it sent the wink.
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