User`s guide

Chapter 12 Voice
P-2602HWUDL-D User’s Guide
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The following table shows the SIP call progression.
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.
12.2.7 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.
Table 54 SIP Call Progression
USER
AGENT 1
PROXY 1 PROXY 2
USER
AGENT 2
Invite
Invite
100 Trying Invite
100 Trying 180 Ringing
180 Ringing 200 OK
180 Ringing 200 OK
200 OK
ACK
RTP RTP
BYE
200 OK