SGCP extensions for Remote Access Servers

SGCP is used to remote control Voice over IP gateways. But voice over IP gateways have a lot in common with Internet access servers. In one case, the server will pass the audio signal to a compression program implemented on a DSP. In the other case, the gateway will pass the audio signal to a demodulation program implemented in a DSP. In theory, the same DSP could handle different programs. We could thus very much envisage devices that are at the same time VoIP gateways and access servers, as envisaged on the following picture:

When a computer modem dials the access code of an ISP, the call is forwarded to the gateway, and the corresponding SS7 message is received by the call agent. The call agent can deduced from the called number, for example 1-800-Internet, that this call is bound to an Internet service -- presumably, there is some form of contract between the Internet service and the owner of the gateway. At that point, the call agent could use SGCP to instruct the gateway that a modem call is being received on an incoming trunk. But, for that, we need to extend SGCP.

The extensions are not defined yet. We know that the call agent will first have to instruct the gateway to "turn on PPP" on the selected trunk. It should then wait for specific events, such as the reception of PPP login and password information. The information should be notified to the call agent, which would send it to the selected Radius server -- or to whatever means of authentication is selected by the service provider. Depending on the result of the authentication, the call agent could either terminate the call, restart the authentication, or send a modify request to the gateway, to activate the IP address. In practice, we will have to extend SGCP in the following ways:

• Define an SDP media format to specify a modem or PPP media type,
• Define new events and signals for PPP connections.