Chapter 9 Voice
Pulse Code Modulation
Pulse Code Modulation (PCM) measures analog signal amplitudes at regular time intervals and converts them into bits.
SIP Call Progression
The following figure displays the basic steps in the setup and tear down of a SIP call. A calls B.
Table 64 SIP Call Progression
A | B |
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1. INVITE
2. Ringing
3. OK
4. ACK
5.Dialogue (voice traffic)
6. BYE
7. OK
1A sends a SIP INVITE request to B. This message is an invitation for B to participate in a SIP telephone call.
2B sends a response indicating that the telephone is ringing.
3B sends an OK response after the call is answered.
4A then sends an ACK message to acknowledge that B has answered the call.
5Now A and B exchange voice media (talk).
6After talking, A hangs up and sends a BYE request.
7B replies with an OK response confirming receipt of the BYE request and the call is terminated.
SIP Call Progression Through Proxy Servers
Usually, the SIP UAC sets up a phone call by sending a request to the SIP proxy server. Then, the proxy server looks up the destination to which the call should be forwarded (according to the URI requested by the SIP UAC). The request may be forwarded to more than one proxy server before arriving at its destination.
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