User guide

Three important things that SIP does are :
1. Dealing with authentication
2. Negotiating the quality
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of the phone call
3. Handling the IP addresses and port numbers to use when sending the
“voice conversations”
4.4.2 Proxy Servers
Although two SIP devices (IP phones) can communicate directly, SIP makes use of some additional
elements called proxy servers to facilitate the establishment of a phone call. With Internet telephony
you can physically move your “phone number” to any location in the world. The “phone numbers” are
not tied to a given physical location. One of the functionalities of a SIP Proxy Server is to act as an
intermediary that knows how to find a certain phone number in the network. A SIP Proxy Server learns
where users are located by a process known as “registration”.
Image 2: The registering process between clients and a SIP proxy server. Signalling (SIP) and voice (RTP)
travel via different paths.
4.4.3 Real Time Protocol and NAT
In the Internet, SIP voice calls are streams of small packets that are carried by another protocol known
as RTP.
The Real-time Transport Protocol (RTP) is the carrier of the actual voice
content itself. RTP defines a standardized packet format for delivering audio
and video over the Internet. As a normal conversation involves two
communication parties, an IP based conversation also involves two different
and separated RTP streams.
15. A big difference between traditional telephony and IP telephony is that the quality of the phone conversation can be
negotiated.
Page 13 TRICALCAR | www.wilac.net/tricalcar – Version: February 2008