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TITRE Adaptive Packet Video Streaming Over IP Networks - LaBRI

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6.1.1.4 S<strong>IP</strong> Communication Model<br />

S<strong>IP</strong> is based on the request-response paradigm. To initiate a session, the UAC sends a request<br />

(called an INVITE) addressed to the person the caller want to talk to. The addresses are similar to<br />

mailto URL (sip:user@server). The message is not send directly to the called party but rather to the<br />

proxy. Then, the proxy delivers the message to the called party. The called party sends a response,<br />

accepting or rejecting the invitation, which is forwarded back to the caller in reverse order like<br />

undelivered mail in Internet.<br />

Figure 6-3: A S<strong>IP</strong> call-setup signaling<br />

Bellow is an example of S<strong>IP</strong> scenario:<br />

1 User A (a@caller.com) calls User B (b@example.com). User A send INVITE message to the<br />

proxy of example.com (INVITE sip: b@example.com), then the proxy of example.com tell<br />

User A, that it is trying to connect to b@example.com<br />

2 The proxy locates in which terminal (PC) the User B is logged currently. This task is<br />

performed by a REGISTER message send by User B when he turned on his S<strong>IP</strong> user agent<br />

client. This would allow that User B is actually at foo.example.com (sip: b@foo.example.com).<br />

The bindings registered are periodically refreshed.<br />

3 The proxy send INVITE message to UAC of User B (INVITE sip:b@foo.example.com), this<br />

later replies the proxy with ringing informational message. The proxy informs the caller that is<br />

ringing at User B.<br />

4 When B accepts the invitation an acknowledgement message (ACK) is sent, and the session is<br />

established, Media can then flow between User A and User B. Others ways, User B can rejects<br />

the invitation. Figure 6-3 shows an example of a basic configuration between 2 UAs using S<strong>IP</strong><br />

protocol.<br />

6.1.2 ISO/IEC 14496-6 (MPEG-4 DMIF)<br />

MPEG-4 DMIF is the control plane of MPEG-4 Delivery layer that allows applications to<br />

transparently access and view multimedia streams whether the source of the stream is located on an<br />

interactive remote end-system, the stream is available on broadcast media or is located on stored<br />

media [144].<br />

143

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