Chapter 21 Voice
188
1
User Agent 1
sends a SIP INVITE request to
Proxy 1
. This message is an invitation to
User Agent 2
to participate in a SIP telephone call.
Proxy 1
sends a response indicating that it is trying to
complete the request.
2
Proxy 1
sends a SIP INVITE request to
Proxy 2
.
Proxy 2
sends a response indicating that it is trying
to complete the request.
3
Proxy 2
sends a SIP INVITE request to
User Agent 2
.
4
User Agent 2
sends a response back to
Proxy 2
indicating that the phone is ringing. The response is
relayed back to
User Agent 1
via
Proxy 1
.
5
User Agent 2
sends an OK response to
Proxy 2
after the call is answered. This is also relayed back
to
User Agent 1
via
Proxy 1
.
6
User Agent 1
and
User Agent 2
exchange RTP packets containing voice data directly, without
involving the proxies.
7
When
User Agent 2
hangs up, he sends a BYE request.
8
User Agent 1
replies with an OK response confirming receipt of the BYE request, and the call is
terminated.
Voice Coding
A codec (coder/decoder) codes analog voice signals into digital signals and decodes the digital
signals back into analog voice signals. The Router supports the following codecs.
• G.711 is a Pulse Code Modulation (PCM) waveform codec. PCM measures analog signal
amplitudes at regular time intervals and converts them into digital samples. G.711 provides very
good sound quality but requires 64 kbps of bandwidth.
• G.726 is an Adaptive Differential PCM (ADPCM) waveform codec that uses a lower bitrate than
standard PCM conversion. ADPCM converts analog audio into digital signals based on the
difference between each audio sample and a prediction based on previous samples. The more
similar the audio sample is to the prediction, the less space needed to describe it. G.726 operates
at 16, 24, 32 or 40 kbps.
• G.729 is an Analysis-by-Synthesis (AbS) hybrid waveform codec that uses a filter based on
information about how the human vocal tract produces sounds. G.729 provides good sound
quality and reduces the required bandwidth to 8 kbps.
BYE
200 OK
Table 105
SIP Call Progression
UA 1
PROXY 1
PROXY 2
UA 2
Summary of Contents for Movistar GPT-2541GNAC
Page 47: ...Chapter 4 LAN 47 Figure 25 LAN Setup...
Page 99: ...Chapter 11 DNS 99 Figure 62 DNS Server...
Page 144: ...Chapter 20 Wireless 144 Figure 92 Wireless Security...
Page 166: ...Chapter 21 Voice 166...
Page 196: ...Chapter 21 Voice 196...