ZyXEL Communications LTE5366 Series User Manual page 131

Table 58 SIP Call Progression
UA 1
ACK
RTP
200 OK
User Agent 1 sends a SIP INVITE request to Proxy 1. This message is an invitation to User Agent 2 to
1
participate in a SIP telephone call. Proxy 1 sends a response indicating that it is trying to complete the
request.
Proxy 1 sends a SIP INVITE request to Proxy 2. Proxy 2 sends a response indicating that it is trying to
2
complete the request.
Proxy 2 sends a SIP INVITE request to User Agent 2.
3
User Agent 2 sends a response back to Proxy 2 indicating that the phone is ringing. The response is
4
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
5
Agent 1 via Proxy 1.
User Agent 1 and User Agent 2 exchange RTP packets containing voice data directly, without involving
6
the proxies.
When User Agent 2 hangs up, he sends a BYE request.
7
User Agent 1 replies with an OK response confirming receipt of the BYE request, and the call is
8
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 LTE5366 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.
Chapter 18 Voice over 3G
PROXY 1
180 Ringing
200 OK
LTE5366 Series User's Guide
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PROXY 2
180 Ringing
200 OK
UA 2
180 Ringing
200 OK
RTP
BYE