VoIP Technology and Standards
Media and Control Signals
A phone call on an ordinary digital phone network and on a VoIP network is made up of media signals and control signals. The voice conversation is the media stream.
Dial tones and ringing tones, for example, are an indication that call control processes are occurring.
The different VoIP protocols use very different technologies, though they have the same aim. VoIP protocols handle these call control (or gateway) control and media functions:
- Call Control (signaling): Responsible for:
- setting up the call
- finding the peer
- negotiating coding protocols
- making the connection
- ending the call
- Gateway Control: Responsible for control signals between VoIP gateways, rather than between endpoint phones. These gateways negotiate VoIP traffic on behalf of the phones.
- Media: The voice or video payload. VoIP networks and ordinary phone networks use RTP/RTCP for the media. RTP carries the actual media and RTCP carries status and control information.
Control signals open fixed (known) ports and dynamic ports. The parties on a call then use control signals to negotiate dynamically assigned ports that each side opens to receive the RTP/RTCP media stream.
Supported SIP RFCs and Standards
The Security Gateway supports these SIP RFCs and standards:
- RFC 3261 SIP: Session Initiation Protocol
- RFC 3372 Session Initiation Protocol for Telephones (SIP-T)
- RFC 3311 UPDATE message
- RFC 2976 INFO message
- RFC 3515 REFER message
- RFC 3265 SIP Events
- RFC 3262 Reliability of Provisional Responses
- RFC 3428 MESSAGE message
- RFC 4566 SDP Session Description Protocol
- RFC 3264 An Offer-Answer Model with Session Description Protocol
- RFC 3265 Specific Event Notification
- RFC 3840 Indicating User Agent Capabilities in SIP
- RFC 3263 Locating SIP Servers
- RFC 3581 An Extension to the SIP for Symmetric Response Routing
- RFC 3892 SIP Referred-By Mechanism
- RFC 5194 Framework for Real-Time Text over IP Using SIP
- RFC 3326 The Reason Header Field for SIP
Supported MGCP RFCs and Standards
The Security Gateway supports these MGCP RFCs and standards:
- RFC-2705.
- RFC-3435 (version 1.0)
- ITU TGCP specification J.171.
Supported H.323 Protocols and Standards
Media in H.323 uses the RTP/RTCP and/or T.120 protocols.
Signaling is handled by these H.323 protocols:
- RAS manages registration, admission, and status. RAS uses a fixed port: UDP 1719.
- Q.931 manages call setup and termination. Q.931 uses a fixed port: TCP 1720.
- H.245 negotiates channel usage and capabilities. H.245 uses a dynamically assigned port.
As an H.323 call is processed by a gatekeeper, these protocols are used in sequence and then the media passes. To end a call, the signaling protocols are used in reverse order.
When an endpoint connects to a gateway, it does not use RAS. Otherwise, the protocol sequence for a gateway is the same as for a Gatekeeper.
The Security Gateway also supports H.245 tunneling and Fast Connect, an H.323 capability. That ensures that audio is available when the phone is answered. This feature is active by default, and is always available.
These H.323 ITU standards are supported:
- H.323 Versions 2, 3, and 4
- H.225 Versions 2, 3, and 4
- H.245 Versions 3, 5, and 7
