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User Agent 2 sends a response back to Proxy 2 indicating that the phone is ringing. The response is
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relayed back to User Agent 1 via Proxy 1.
User Agent 2 sends an OK response to Proxy 2 after the call is answered. This is also relayed back to User
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Agent 1 via Proxy 1.
User Agent 1 and User Agent 2 exchange RTP packets containing voice data directly, without involving
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the proxies.
When User Agent 2 hangs up, he sends a BYE request.
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User Agent 1 replies with an OK response confirming receipt of the BYE request, and the call is
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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 Zyxel Device 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.
Voice Activity Detection/Silence Suppression
Voice Activity Detection (VAD) detects whether or not speech is present. This lets the Zyxel Device
reduce the bandwidth that a call uses by not transmitting "silent packets" when you are not speaking.
Comfort Noise Generation
When using VAD, the Zyxel Device generates comfort noise when the other party is not speaking. The
comfort noise lets you know that the line is still connected as total silence could easily be mistaken for a
lost connection.
Echo Cancellation
G.168 is an ITU-T standard for eliminating the echo caused by the sound of your voice reverberating in
the telephone receiver while you talk.
MWI (Message Waiting Indication)
Enable Message Waiting Indication (MWI) enables your phone to give you a message–waiting
(beeping) dial tone when you have a voice message(s). Your VoIP service provider must have a
messaging system that sends message waiting status SIP packets as defined in RFC 3842.
Chapter 23 Voice
VMG/XMG Series User's Guide
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