TCP port 1720


I am fairly new to the VoIP technology. I went through one document which told me that VoIP uses UDP as transport layer protocol.

But another document which contradicts this, mentioned that TCP port

1720 is used for call control signaling in VOIP.

I am a little confused over this. IS TCP used as a transport protocol in VoIP or not ???

Thanx in advance..

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There's no contradiction at all as long as you know the different parts of a call. There are basically two parts, call signalling and media streaming. The signalling part is where your ip phone/PC gets it's marching orders from the call-control agent, and this is usually TCP as it is more reliable than UDP and will retransmit dropped packets. The media streaming part is the actual voice packets that make up the conversation and this part uses Real-Time Protocol which is UDP. If you get a dropped packet, you would actually be worse off using TCP because it would retransmit the packet and it would arrive out of sequence and voice quality would suffer.

Hope that clears things up a bit.

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mahajanl napisa³(a):


sometimes is UDP sometimes is TCP as u wish :)


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Actualy at least four:

  1. Call-level signalling (H.225) over TCP usualy.
  2. Communication control (H.245) (usualy using the same or separate TCP transport channel).
  3. Media streams (RTP protocol) - at least forward and reverse UDP streams.
  4. Media control streams (RTCP protocol) - optional f/r UDP streams.

.... which _uses_ UDP as transport level protocol. Actualy can use any non-reliable (in general) protocol. Uses RTCP for quality control. Both RTP/RTCP is RFC-based and very simple.

TCP port 1720 is used as well-known port to receive new call signalling requests/channels (H.225 messages encapsulated into Q.931 messages encapsulated into TPKT shell and all of this over TCP ).

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