VoIP protocols - Telecommunications Blog

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Wednesday, April 18, 2012

VoIP protocols

Real-Time Transport Protocol (RTP)

RTP is used for transmitting audio and video packets between communicating computers. RTP is defined in RFC 3550 and "profiles", which specify how to packetize various audio and video codecs used in an RTP session, are defined in RFC 3551.

 RTP also addresses issues like packet order and provides mechanisms to help address delay and jitter. RTP was improved upon with the result being called Secure RTP which provides for encryption, authentication, and integrity of the audio and video packets transmitted between communicating devices.


Developed by Study Group 16 of the ITU-T, H.323 is an umbrella standard encompassing
many subcomponent standards and annexes for transmitting multimedia (voice, video and data) across packet based networks. The two main subcomponent protocols are H.225 (call control), & H.245 (bearer control and capabilities exchange). H.225 consists of two main parts Q.931 (basic call control as used in ISDN networks) and RAS (Registration, Admission & Status).

H.323 also references other standards such as Real Time Protocol (RTP), G.xxx audio codecs, H.26x video codecs and T.120 real time data conferencing protocol.


Developed by IETF, SIP is a mechanism to initiate, terminate & modify sessions in an IP
network. It uses a client / server architecture and the protocol is request-response based. It
enables personal mobility by tracking down users and delivering calls to an endpoint. It is a
lightweight, text-based protocol and reuses much of the construct of other internet protocols
such as HTTP and SMTP. SIP does not know about the underlying details of a session and relies on IETF protocol Session Description Protocol (SDP) to describe the session. It also interworks with other IETF protocols such as Megacop, RTP, RTSP, RSVP and SAP.

Both H.323 and SIP can be referred to as "intelligent endpoint protocols". What this means is that all of the intelligence required to locate the remote endpoint and to establish media streams between the local and remote device is an integral part of the protocol.

MGCP & Megacop/H.248 

There is another class of protocols which is complementary to H.323 and SIP referred to as "device control protocols". Those protocols are H.248 and MGCP. 

MGCP (Media Gateway Control Protocol) & Megacop/H.248 are relatively low level, master/slave protocols used between Call Agents and MGs. MGCP is not officially a standard but does exist as an informational RFC (RFC 3435). Megacop (IETF RFC 3015) & H.248 (ITU Rec.) are the same protocol developed by an IETF and ITU collaboration. It is derived from, and draws heavily from MGCP but adds several new enhancements.

Some service providers provide users with devices that implement H.248 or MGCP (or comparable protocols). In the core of the network, some device serving as the MGC provides the H.323 or SIP logic necessary to properly terminate VoIP calls around the world.

VoIP OSI Protocol Stack

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I will explain QOS at next article .


  1. Very interesting blog on VOIP. Thanks for sharing these interesting facts.

  2. This is a good source of info about VoIP. Thanks for the post. VoIP is a good mean of communications for cheaper price. Good thing that a lot of people are choosing this service.