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Important G3 Timers

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
T060 ± 5 secAmount of time a calling fax machine waits for an answer from the  terminating side. Usually a CED or DIS stops this timer.
T135 ± 5 secAmount of time a fax device attempts to identify the other fax device. This  timer is active during the DIS/DCS negotiation.
T26 ± 1 secAmount of time a fax device waits to receive a command. This timer also  detects the loss of command/response synchronization.
T310 ± 5 secAmount of time a fax device alerts an operator after a procedural interrupt.
T43 sec ± 15%Amount of time a fax device waits for a response to a sent message.
T560 ± 5 secAmount 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.

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‎06-11-2009 12:14 AM
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