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The following steps describe the sequence of messages exchanged between the user
agents:

• The following messages are exchanged while establishing a session:

1. UA1 sends an INVITE request message to invite UA2 to participate in the
session.
2. UA2 responds with a 100 - Trying response message to indicate that the
request has been received.
3. UA2 sends the 180-Ringing response to indicate that UA2 is ringing.
4. UA2 sends the 200-OK response to inform that the user at UA2 has picked
up the call.
5. UA1 sends the ACK request to confirm receipt of the 200-OK response.

• The following messages are exchanged while terminating a session:
6. When the user at UA1 ends the call, a BYE request message is sent to UA2.
7. UA2 responds with the 200-OK message and informs the user at UA2 that
the session has ended.

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Posted by sagitraz

1. Initiate a call - A UAC initiates a call by sending a transaction to a SIP Proxy Server. The first transaction asks the Proxy Server if the User Agent Client can join the network and start a call. The Proxy Server checks the UAC’s credentials and capabilities
2. Invite the called party - If the UAC checks out, Proxy invites the requested UAS to join the call
3. Complete the media connection - If both User Agents have common codecs and other capabilities, they can talk directly to each other. They are expected to notify the Proxy when their call is terminated

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Posted by anders