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Call Answer Timeout and 183 Periodic Retransmission functionality

Feature F-6484 adds two new fields to the SIP Advanced Settings object. The first field added is the Call Answer Timeout. Prior to the introduction of this feature, when making an Early-Media SIP call, the IMG 2020 allowed up to 3 minutes for the call to get established. If the call was not accepted by the remote gateway within that time frame, the call was canceled. The new functionality allows the user the ability to configure the Call Answer Timeout anywhere from 3 minutes to 60 minutes and during this timeout period, the call will not be canceled unless separate factors either internal or external are used.

Because of the longer Call Answer Timeout value described above, the call can take longer to complete. The destination gateway on the B leg of the call can start periodically retransmitting the 183 Session Progress messages back to the IMG 2020. On the A leg of the call, the IMG 2020 can also start retransmitting 183 Session Progress messages once every minute. In this case,the IMG 2020 may end up sending twice the amount of 183 Session Progress messages back to the gateway on the incoming or A Leg of the call. One set of 183 Session Progress messages are sent from the response from the gateway on the B-Leg of the call and a second set of 183 Session Progress messages are sent from the IMG 2020 when the Retransmit timer in the IMG 2020 expires.

To solve the issue of too many retransmissions of the 183 Session Progress message, the second field labeled 183 Periodic Retransmission has been added to either enable or disable the retransmission of the 183 Session Progress messages on the A Leg of the call. In this situation only the 183 Session Progress responses from the external SIP gateway on the B Leg of the call will be transmitted out the A Leg of the call.

Refer to the information below. 

Call Answer Timeout Call Flows

The first call flow diagrams below displays the call being cancelled because the Call Answer Timeout expired waiting for early media such as tones being entered. The second call flow displays the Call Answer Timeout value being configured to a longer value so the Early Media can be entered.

 

As displayed in the call flow diagram above, the Call Answer Timeout value which is set to 3 minutes expires before the call can be connected (Receipt of the 200 OK).

As displayed in the second call flow diagram above, the Call Answer Timeout value which was previously set to 3 minutes has been modified to wait for 10 minutes. In this scenario the Call Answer Timeout value is long enough to accept and early media.

Call Answer Timeout Configuration

The Call Answer Timeout Value is configured in the SIP Advanced Settings object which is configured under the SIP Profile object. The SIP Profile with the Call Answer Timeout value modified must be configured on the outgoing or B Leg of the call.

  • Right click on the Profiles object and select New SIP Profiles. A SIP Profiles object will get created. The SIP Profiles object is a parent or container object and no configuration is required. Refer to the SIP Profiles topic for more information on this object.

  • Right click on the SIP Profiles object and select New SIP Profile. The first profile that gets created is a Default SIP Profile and cannot be modified. Disregard this profile.

  • Right click on the SIP Profiles object and select New SIP Profile again. In the SIP Profile object that appears, either enter a name that identifies this SIP Profile or accept the default name already entered. Refer to the SIP Profile - SGP topic for more information on this object.

  • Right click on the SIP Profile object and select New SIP Advanced Settings. Highlight the value in the Call Answer Timeout field and enter a new value. In the screen capture below, 10 minutes was entered.

Refer to the topic for more information.

  • Once the Call Answer Timeout has been configured, the SIP Profile can be selected in the drop down menu of the Profiles field in the External Gateway object on the outgoing or B Leg of the call.

For information on configuring external gateways and channels groups in SIP, refer to the or .

Configurable Periodic 183 Session Progress

As described in the overview above, once the Call Answer Timeout is lengthened, the IMG 2020 could possibly transmit double the amount of 183 Session Progress messages back to the A Leg. In the call flow below, the following could occur.

  • The gateway on the B Leg responds to the INVITE message and transmits an initial 183 w/SDP message to the IMG 2020. Since the Call Timeout Value described above has been lengthened, the call may not be answered for some time. In this instance, the gateway in Network B could send retransmitted 183 messages back to the IMG 2020 as long as the call has not been answered. The 183 retransmitted Session Progress messages will typically come about one every minute or so.

  • The 183 Session Progress message is received on the B Leg of the IMG 2020. The 183 Session Progress message is interworked and transmitted to Network A. At this point the retransmit timer on the SIP A leg starts.

  • The Retransmit Timer expires and a second retransmitted 183 Session Progress message is transmitted out the A leg.

  • In the meantime the gateway in Network B transmits a second 183 Session Progress message as well. The IMG 2020 receives the 183 Session Progress and inter-works it to the A leg and transmits this 183 Session Progress message as well. At this time two 183 Session Progress messages have been sent to the gateway in Network A.

  • This continues as long as the call has not connected (200 OK) and double the number of 183 Session Progress messages are sent to the gateway in Network A.

To stop the extra 183 Session Progress messages from being sent, the 183 Periodic Retransmission field was added to disable the SIP A leg from sending retransmitted 183 Session Progress messages. Refer to the Call Flow diagram below.

  • In the call scenario above, the 183 Periodic Retransmission field is set to Disabled. In this scenario, the IMG 2020 does not transmit any of the subsequent retransmitted 183 Session Progress messages to the A Leg. Only the 183 Session Progress messages that are interworked from the B Leg are transmitted. 

183 Periodic Retransmission Configuration

The 183 Periodic Retransmission functionality is configured in the SIP Advanced Settings object which is configured under the SIP Profile object. The SIP Profile with the 183 Periodic Retransmission functionality disabled must be configured on the incoming or A Leg of the call.

  • Right click on the Profiles object and select New SIP Profiles. A SIP Profiles object will get created. The SIP Profiles object is a parent or container object and no configuration is required. Refer to the topic for more information on this object.

  • Right click on the SIP Profiles object and select New SIP Profile. The first profile that gets created is a Default SIP Profile and cannot be modified. Disregard this profile.

  • Right click on the SIP Profiles object and select New SIP Profile again. In the SIP Profile object that appears, either enter a name that identifies this SIP Profile or accept the default name already entered. Refer to the topic for more information on this object.

  • Right click on the SIP Profile object and select New SIP Advanced Settings. Select Disabled from the drop down menu of the 183 Periodic Retransmission field. Refer to the screen capture below.

Refer to the SIP Profile - Advanced Settings topic for more information.

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