For voice data, session layer use SIP, H.323, MGCP, H.248, etc protocols.

Do these protocols add any header to the voice data payload?

When voice data is transmitted over the Internet, does session layer add any header to the packet?

  • Why do you assume those protocols have anything to do with VoIP payload data? Some of them have absolutely nothing to do with the actual VoIP data payload; they are signalling and control protocols to do things like set up and tear down calls, and they don't transport the VoIP data payload at all.
    – Ron Maupin
    Oct 18 '15 at 3:34

The mentioned protocols are signalling and control protocols, the voice data isn't transported using them. Its main feature is manage the connection and hangup of multimedia sessions.

In the case of VoIP using SIP, most of the systems rely on RTP to transport voice data.

RTP is the appropiate transport for real-time data such as audio or video. RTP does not control resource reservation and can't guarantee quality-of-service.

RTP adds up to 128 bits to the data, where the most important fields added are a sequence number and a timestamp. Using both of them the receiver knows if the package is getting in order, later, etc.

  • do all voip software use RTP?
    – user15062
    Oct 21 '15 at 9:42
  • Most commercial and open source systems (Cisco, Avaya, Grandstream, Asterisk, etc.) use RTP and its sister protocol RTCP because they look for interoperability with other systems. However closed systems as Skype, Viber or Whatsapp use propietary protocols.
    – jcbermu
    Oct 21 '15 at 9:48
  • What about applications like Google+Hangout, Yahoo Messenger, etc.?
    – user15062
    Oct 21 '15 at 9:54
  • Most of them use propietary implementations based on XMPP as Jingle.
    – jcbermu
    Oct 21 '15 at 9:57

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