Chapter 9 Voice
P-2812HNU-51c User’s Guide
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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 P-2812HNU-51c
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.
