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In any SIP call scenario, two entities are involoved involved in the call flow: UAC and UAS. In this exercise, PSTN makes a call to the Lync Skype client over the

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, where the 

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acts as a UAC and the LyncSkype/SBA acts as a UAS. In our example we are assuming that a a

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is integrated in the topology shown below.

...

Currently the 

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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 (LyncSkype/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

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.

You can verify this using the Log Exchange utility — the following diagram depicts a 

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to LyncSkype/SBA call with the Session Timer enabled and in effect:

SBC 1000/2000 to LyncSkype/SBA Call

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