VoIP Protocols

There are a number of different VoIP protocols that are used by many different products or services. One of the most popular communications protocols for hosted VoIP solutions is the Session Initiation Protocol (SIP). Although non-hosted, on-premise business phone systems can also use SIP as the communications protocol, it is more common for other protocols or proprietary protocols to be used. An example of a proprietary protocol is the Cisco SCCP or Skinny protocol that is predominantly used for some series of Cisco IP phones that are for use with the Cisco call-manager phone system solution.

The most common VoIP protocols are listed as follows:

VoIP Protocol Summary
Authenticated Identity Body (AIB) This is a method of sharing authenticated identity with parties in the network and uses a special MIME format. Works in conjunction with SIP S/MIME. Discussed in RFC 3893
Call Admission Control (CAC) A type of quality of service and congestion control for voice, CAC ensures that there is no oversubscription of voice calls on the VoIP network. It prevents any potential for excessive voice calls and protects existing voice traffic.
Distributed Universal Number Discovery Somewhat self explanatory protocol that enables directory information to be collected.
H.248 (Gateway Control Protocol) Also known as Megaco Protocol. This is a master/slave protocol and allows a media gateway controller to control media gateways. Uses H.323 and SIP to communicate between controllers. Discussed in detail in RFC 3525.
H.323 Commonly used by telecom equipment manufacturers and service providers. ITU recommendation that addresses call signaling and control. H.323 is a master/slave protocol.
Inter-Asterisk Exchange (IAX) Asterisk protocol but also used by many other equipment manufacturers as a communications protocol between servers. IAX has now been replaced by IAX2. Discussed in detail in RFC 5456.
Media Gateway Control Protocol  (MGCP) VoIP signaling and call control protocol that enables communications with the PSTN (public switched telephone network). Master/Slave protocol. The core RFC for MGP is RFC 3435 and there are also a number of related RFCs.
Network Voice Protocol Early version of Voice over Internet Protocol.
Open Settlement Protocol This is a client server protocol that is used by ISP's for the exchange of authorization and accounting information related to VoIP.
Real-time Transport Protocol (RTP) Protocol that manages real-time communications over telephony networks. Discussed in detail in RFC 3550.
Registration, Admission and Status (RAS) Manages communications between H.323 endpoints and gatekeepers.
SIP (Session Initiation Protocol) Signaling protocol for voice and video communications. Peer to peer protocol.
SIP Requests Special commands that are sent as requests by the SIP protocol to peers.
SIP Response Codes Codes that are used by the SIP protocol to respond to SIP requests
Simple Gateway Control Protocol Predecessor to MGCP protocol.
Skinny Call Control Protocol Cisco proprietary protocol used to communicate between Cisco endpoints (IP phones) and the Cisco call-manager phone system.
Skype Protocol Peer to peer protocol used by Skype.
T.37 Enables the sending of fax messages via email by encapsulating the fax in an email message. Can also work with a fax gateway to operate with a regular fax machine. Commonly referred to as Internet Fax or Online Fax.
T.38 Enables faxes to be transmitted over a packet network.

If you are aware of any VoIP protocols that are commonly used and have not been included in the table above, please contact us to submit a request for the addition to be made. For additional information on other VoIP terminology, please visit our VoIP dictionary.

Provider Comparison

If you are not ready to get comparison price quotes, we recommend that you review the following comparison table and attempt to narrow down which provider may be a good fit for your business. Now that you know all about VoIP protocols, you can ask some very educated questions.

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Published by WhichVoIP

Related Articles for Further Reading:

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