# Timer overview

Values are assigned to various ISDN timers for different country variants. These timers control the behavior of network signaling layer 3 in the ISDN protocol stack.

These values are included as timer_val_t structures in the ISDN_PROTOCOL_PARMS_Q931CC and ISDN_PROTOCOL_PARMS_LAPD structures referenced by isdnStartProtocol.

The following table summarizes the meanings of the timers. The items shown in all capital letters are the names of Q.931 messages. For example, ALERT refers to the alert message indicating that an incoming call has arrived.

Timer | Description | Started... | Reset... | On expiration...
--- | --- | --- | --- | ---
T300 | Internal timer. Determines the time the stack has to respond to an incoming SETUP or DISCONNECT. For SETUP, if none of the in_calls_behaviour bits are set, the application must initiate this response. | After a SETUP or DISCONNECT is received. | When the first message in response to the SETUP or DISCONNECT is sent. | The call is cleared.
T301 | ALERT message timing. | After an ALERT is received. | When a CONN is received. | The call is cleared.
T302 | Overlap receiving timer. | After a SETUP_ACK is sent. | When an INFO is received. | The call is cleared.
T303 | Setup message timing. | After a SETUP is sent. | When a CALL_PROC, ALERT, SETUP_ACK, or REL_COM is received. | The call is cleared with a REL_COM.
T304 | Control of overlapped sending state. | After an INFO is sent. | When an INFO is received. | A DISC is sent.
T305 | Disconnection control. | After a DISC is sent. | When a REL or REL_COM is received. | The link is placed in maintenance state and CRVs are released.
T306 | Call is in disconnect indication. | After a DISC with progress indicator is received. | When a REL or DISC is received. | A REL is sent on the line (ACU_CLEAR_CO is sent to the application).
T307 | Internal timer. | After SUSPEND_ACK is sent. | When a RESUME_ACK is received. | The call is cleared (ACU_CLEAR_CO is sent to the application).
T308 | Release message control. | After a REL is sent. | When a REL or REL_COM is received. | The link is placed in maintenance state and CRVs are released.
T309 | Allows the data link to be dropped without losing calls. | After a data link release message is sent. | When a data link establish message is received. | All calls are cleared locally.
T310 | Used to govern the behavior of CALL_PROC. | After a CALL_PROC is received. | When an ALERT, CONN, DISC, or PROG is received. | A DISC is sent.
T313 | Controls the behavior of CONN message. | After a CONN is sent. | When CONN_ACK is received. | A DISC is sent.
T314 | Future usage for segmented messages. | After a message segment is received. | After last message segment is received. | The incomplete message is discarded.
T316 | RESTART procedure timing. | After a RESTART is sent. | ---- | A RESTART is sent and the timer is restarted.
T317 | Controls the internal clearing of CRVs after a RESTART. | After a RESTART is received. | ---- | A maintenance indication sent to the application.
T318 | Used when the state of the call is suspend request. | When a RESUME message is sent. | When a RESUME_ACK or RESUME_REJ is received. | The call is cleared (ACU_CLEAR_CO is sent to the application).
T319 | Used when the state of the call is suspend request. | When a SUSPEND message is sent. | When a SUSPEND_ACK, SUSPEND_REJ is received. | ACU_CLEAR_CO is sent to the application.
T320 | Internal timer. | When a DL establishment indication or confirmation is received. | When a call request packet or DL_RELEASE is received. | DL_RELEASE request is sent to the line.
T321 | Controls the internal timing of backup D channels. | ---- | ---- | ----
T322 | Controls the behavior of STATUS_ENQUIRY. | After a STATUS_ENQUIRY is sent. | When a STAT, DISC, REL, or REL_COM is received. | STATUS_ENQUIRY is submitted.