I feel there are contradictions between the phrase below :
http://tldp.org/HOWTO/VoIP-HOWTO-4.html

Quote:

VoIP doesn't use(I feel this is not the right, because VoIP can use TCP) TCP because it is too heavy for real time applications, so instead a UDP (datagram) is used.

And

http://www.jdsu.com/test_and_measurement/technical_resources/product_documents/whitepaper/voipterm_wp_acc_tm_ae_1205.pdf
VoIP utilizes IP as its basic transport method. VoIP utilizes both the TCP and UDP protocols over IP.


http://arstechnica.com/guides/tweaks/voip.ars
Also, SIP can use either TCP or UDP; most providers utilize the UDP method.


http://www.searchvoip.com.au/topics/article.asp?DocID=1230489&NodeID=299014
Why is UDP used more in VoIP than TCP?
If UDP is unreliable, why is it used more in VoIP rather than TCP?
The biggest reason, right off the bat, is that all routers and switches on the Internet have the ability to understand it, and route it fast and with great efficiency. As TCP is a connection-oriented protocol -- it requires a handshake and session between sender and receiver. UDP can route packets based on a variety of factors.

Any comment

Regards

OK here goes, this realtes to SIP (but most are the same).

TCP is a reliable connection. This doesn't mean it is better. What it means is if some packet fail to arrive, they get resent, so lot's of handshaking take place. This makes it slower and more bandwidth intensive.
In Voip it's used for things like, call set up, tear down, transfers, hold, MWI's etc etc.
UDP does no handshaking and retransmits, therefore is less intensive and faster. In voice, it's used for your real time traffic (voice and video).
So why use UDP?

Well if a couple of voice packets don't arrive, what is the point of sending a messge back, then the device retransmitting and second or two later, it would arrive in the wrong order or the other packets would wait until it arrives before carrying on. This would cause problems with stuttering and delays.
With UDP it fires a constant stream of data, presuming some will get lost on the way. This is where all the cleaver technologies come (algorythms, compressions, CODECS etc), to try and make up for this loss.

As a quick guide>
TCP - Reliable, but slow
UDP - Unreliable but quick

Hope this helps

Only the truly stupid believe they know everything.
Stu.. 2004

Thanks StuReeves for that