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How does SIP work and what is its use in VOIP?

Do I have to have it or can I use a direct plug-in to the computer to use VOIP?

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Xavier : all this info is easily found on google. –  harrymc Sep 29 '09 at 21:11
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@harrymc. Thanks. So, if I understand your comment, you think I shouldn't have asked this question here. I'm interested in why you think that. Will you elucidate please? –  Xavierjazz Sep 30 '09 at 3:04
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You find more on-depth info and articles on, for example, wikipedia, and will gain more understanding, than from the short answers you'll get here. You're trying to start a dialog with a power user, but this is a forum, not chat. –  harrymc Sep 30 '09 at 5:55
    
I Understand. Thanks –  Xavierjazz Sep 30 '09 at 13:08
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@harrymc, I believe that the question has value since articles on the internet may not reveal the answer to this question easily. –  Vass Oct 14 '11 at 14:59

4 Answers 4

up vote 3 down vote accepted

Right, SIP is an application protocol that runs on top of TCP.

There are many different ways to transfer voice over TCP/IP, however SIP is probably the most used and understood in the industry with the maximum compatibility.

If you just want to make calls from your computer, many come with the phone dialer application (not sure if it is included any more in Vista/7), but you should be able to find one online. This uses a standard modem and had nothing to do wit SIP, it is purely POTS (Plane old telephone system).

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But I will be able to use the dialer with a voip application? No phone necessary? –  Xavierjazz Sep 30 '09 at 0:19
    
@Xavierjazz yes, no phone necessary. No voip need phones at all. It's implicit when you say "it's TCP/IP". –  Cawas Apr 17 '10 at 14:59
    
Quibble: SIP is transport protocol independent: it runs equally well over UDP, TCP, TLS, DTLS, SCTP, ... –  Frank Shearar Aug 18 '10 at 8:25

As Wil said, SIP is an application layer protocol. SIP is used as signaling protocol in VoIP, ie to manage sessions(voice, video or file transfer) between participants. The actual media is transported using different protocols based on the need, voice packets are transferred as RTP once the SIP signalling is successfully done.

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A SIP trunk is a direct connection between your organization and an Internet telephony service provider. SIP trunk enables you to extend VoIP telephony beyond your organization’s firewall without the need for an IP-PSTN gateway. This simpler configuration is easier and less expensive to design, operate, maintain, and upgrade. Here is a suggested [blog]: http://www.dovecommunications.com/modern-voip-features-explained-sip-trunking/ where you can read in more details about SIP trunking.

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Try http://gizmo5.com which is a VOIP client that suports SIP-based internet calling. You would be able to make calls through your PC with a headset.

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