Here is a listing of the basic T.30 message timers for G3 faxing. These timers are used in error recovery and to make sure that the fax transaction never gets stuck in a hung state. In calling scenarios involving international connections or satellite hops, fax calls may experience a delay that is high enough to cause certain timers to expire.
T.30 Protocol Timers
| Timer | Value | Description |
|---|
| T0 | 60 ± 5 sec | Amount of time a calling fax machine waits for an answer from the terminating side. Usually a CED or DIS stops this timer. |
| T1 | 35 ± 5 sec | Amount of time a fax device attempts to identify the other fax device. This timer is active during the DIS/DCS negotiation. |
| T2 | 6 ± 1 sec | Amount of time a fax device waits to receive a command. This timer also detects the loss of command/response synchronization. |
| T3 | 10 ± 5 sec | Amount of time a fax device alerts an operator after a procedural interrupt. |
| T4 | 3 sec ± 15% | Amount of time a fax device waits for a response to a sent message. |
| T5 | 60 ± 5 sec | Amount of time a transmitting fax device waits for a busy condition on the receiving fax device to clear. This timer is only used during ECM. |
The most common timer expiry scenario seen with G3 fax calls is with the T4 timer. This is because this timer is the most susceptible to network delays. After the terminating fax machine plays the CED tone, it sends the DIS message with it capabilities to the originating fax machine. If the terminating fax does not receive a response to the DIS message (typically a DCS) within the T4 period (i.e. 3 seconds), then the terminating fax machine will retransmit the DIS message. Typically a fax device will retransmit a message 3 times before disconnecting the call.
