In any SIP call scenario, two entities are involoved in the call flow: UAC and UAS. In this exercise, PSTN makes a call to the Lync client over the SBC Edge, where the SBC Edge acts as a UAC and the Lync/SBA acts as a UAS. In our example we are assuming that a SBC Edge is integrated in the topology shown below.

This article describes the steps necessary to configure a SBC Edge to employ the SIP Timer Functionality in the depicted topology:

Topology

Configuration

The configuration process comprises of two overall steps:

SIP Profile - Session Timer

  1. In the WebUI, click the Settings tab.
  2. In the left navigation pane, go to SIP > SIP Profiles
  3. Select the relevant SIP Profile.

  4. Configure the Session Timer as shown in the figure below.
    (info) Important: The Offered Session Timer value must be equal to or greater than the Minimum Acceptable Value.

    Configure Session Timer

Signaling Group

  1. In the WebUI, click the Settings tab.
  2. In the left navigation pane, go to Signaling Groups > (relevant SIP Signaling Group).
  3. Select the SIP Profile configured above from the SIP Profile selection list.

  4. Click Apply.

    Apply Configured SIP Profile

     

Call Flow

After having applied the configuration changes, in the call flow, the SBC Edge supports the UPDATE and INVITE  methods for changing the media capabilities. Session Timer (RFC 4028) uses the INVITE or UPDATE as a mechanism for a periodic refresh of SIP sessions.

Currently the SBC Edge supports RTCP-based dead call detection. Session timers provide an alternate methodology for the detection of dead calls using SIP Signaling. If the SIP peer fails to respond to an UPDATE or INVITE message, this enhancement cleans up the call by sending a BYE which will enable the call stateful SIP proxies (Lync/SBA in this example) to clean up the resources associated with the call. Additionally, the relevant DISCONNECT is sent to the PSTN link by the SBC Edge.

You can verify this using the Log Exchange utility — the following diagram depicts a SBC Edge to Lync/SBA call with the Session Timer enabled and in effect:

SBC 1000/2000 to Lync/SBA Call