1. Packetizer ® Copyright © 2008 SIP Call Flows Simple? Let’s Place a Video Call to Find Out. 1

2. www.packetizer.com Packetizer ® Initiating the Video Call… 2 UE1P-CSCF 1S-CSCF 1 I-CSCF 2 S-CSCF 2 P-CSCF 2 UE2 INVITE 100 Trying INVITE 100 Trying INVITE 100 Trying INVITE 100 Trying INVITE 100 Trying INVITE 100 Trying

3. www.packetizer.com Packetizer ® Phone is ringing… 3 UE1P-CSCF 1S-CSCF 1 I-CSCF 2 S-CSCF 2 P-CSCF 2 UE2 183 Ringing

4. www.packetizer.com Packetizer ® User Answers! 4 UE1P-CSCF 1S-CSCF 1 I-CSCF 2 S-CSCF 2 P-CSCF 2 UE2 200 OK ACK 30 Messages on the wire!

5. www.packetizer.com Packetizer ® Oh! Where’s the Video? Believe it or not, after all of this, if the proposed video codec is not supported by the called endpoint, you will hear audio only. Likewise, if the video is supported, but not the audio, you’ll have video only! So, how do we propose a different video codec? 5

6. www.packetizer.com Packetizer ® Send More Messages! 6 UE1P-CSCF 1S-CSCF 1 I-CSCF 2 S-CSCF 2 P-CSCF 2 UE2 INVITE 100 Trying INVITE 100 Trying INVITE 100 Trying INVITE 100 Trying INVITE 100 Trying INVITE 100 Trying 200 OK ACK 24 more messages, for a total of 54 messages!!