Voice protocols

The VoIP gateway can transfer voice or fax over the IP network by using different protocols. The basic voice protocols that routers support are H.323 and SIP. The fax protocol is T.38.

1. H.323

   H.323 is a standard protocol established by ITU-T. The H.323 protocol stack, implemented at the application layer, mainly describes terminals, devices, and services used for multimedia communication without QoS guarantee over an IP network. An H.323 network usually consists of VoIP gateway, an optional GK, an MCU, and terminals. According to the ITU-T specifications, the GK should provide H.323 terminals, a gateway, or MCU in LANs or WANs with the following functions:

   • Address translation

   • Access permission

   • Bandwidth control and management

   • Area management and security check

   • Call control signaling and call management

   • Routing control and accounting

   The GK not only controls the call service, but also functions as the central control point within its management area. The GK implements the control function by exchanging information with the VoIP gateway. If there is any GK, the router will be under the control of the GK. To implement the control function of the GK, perform related configurations on the router. For more information, see "Configuring H.323."

2. SIP

   SIP is the core protocol of the IETF multimedia data and control architecture. SIP is used for signaling control and communication with a softswitch platform in the IP network. A SIP network consists of a user agent (SIP endpoint), proxy server, registration server, location server, and redirect server. Here, the proxy server, registration server, location server, and redirect server are functional entities. In practice, multiple functional entities can be integrated into one physical entity.

   • In a complete SIP system, all SIP endpoints serve as user agents and should register with the registration server to inform of their locations, session capabilities, and call policies. The registration server then sends the registration information to the location server for storage.

   • SIP endpoints use the proxy server to set up calls. SIP endpoints send signaling messages to the proxy server, and then the proxy server forwards them to the next hop. In this process, multiple proxy servers might be involved. Eventually, channels are established to transfer the upper layer voice service.

   • Unlike the proxy server, the SIP redirect server will not forward the received session request messages. Instead it will inform the originating SIP endpoints of the addresses of the terminating SIP endpoints by returning reply messages. The originating SIP endpoints directly re-originate a session request message to the terminating SIP endpoints. The terminating SIP endpoints also directly return a reply message to the originating SIP endpoints.

   As a SIP endpoint, the voice router needs to exchange information with the servers to accomplish functions such as registration. For more information, see "Configuring SIP."

3. Fax protocol

   FoIP complies with ITU-T T.30 and T.4 on PSTN and T.38 on the IP network.

   • T.30 defines the procedures necessary for document transmission between facsimile terminals on PSTN. It gives detailed descriptions and stipulations on the communication process, signal format, control signaling, and error correction of Group 3 facsimile terminals on the general switched telephone network.

   • T.4 is a standard protocol used for document transmission between Group 3 facsimile terminals. It standardizes image coding, signaling modulation, rate, transmission time, error correction, and document transmission of Group 3 facsimile terminals.

   • T.38 describes the technical features necessary to transfer facsimile document in real time between Group 3 facsimile terminals over the Internet or other networks by using IP protocols. It gives descriptions and stipulations on communication mode, message format, error correction, and part of communication processes.

   Before applying the fax service, configure the technical protocols and physical characteristics. For more information, see "Configuring fax over IP."