How does the DNIS delay option of the TSU voice FXS module work?
Q: How can I use the DNIS delay option of the TSU voice FXS cards to signal an off-hook condition back to the telco after a simulated delay needed to allow my called device time to answer? For example, my called device will be a Fax Server.
A: The TSU's network port is connect to the telco's T1 line. The FXS card uses its analog 2-wire ports to connect to (in this example) the fax server. When a call comes in from telco, the FXS card then sends ring voltage to the fax server. The fax server responds by answering the call. The FXS card winks back to telco that the server is ready for DNIS. At the same time the FXS card winks, it starts counting down the time configured under the DNIS delay option. The fax server receives the DNIS tones and uses them to route the call to the appropriate fax machine. This procedure takes a few seconds to route and for the fax machine to answer. When the DNIS delay time has expired, the FXS card sends an off hook condition to the telco to finish connecting the call.
Here is an example of the signaling bits for a basic DNIS delay call flow:
| RX | TX | State |
| 00 | 00 | IDLE |
| 11 | 00 | Incoming call - send ring voltage out FXS port to fax server |
| 11 | 11 | When the fax server goes off hook a WINK is sent for 200 ms indicating that the end equipment is ready for DNIS digits to be sent from the Telco |
| 11 | 00 | A 00 is sent to end the WINK and the DNIS Timer starts |
| 11 | 11 | After the DNIS Timer expires a 11 is sent to signal an off hook condition, and persists for the duration of the call |
NOTE: The DNIS Timer is based on the DNIS Delay setting on the FXS port configuration.
