Explain how a VOIP call is set-up and carried over an IP network using the SIP protocol and a SIP proxy server?

Explain how a VOIP call is set-up and carried over an IP network using the SIP protocol and a SIP proxy server. Contrast this to the operation of voice communication over a traditional PSTN or mobile network and explain how a device in a VOIP system can communicate with a device on a traditional network.

What is a VPN?

VPN is (Virtual Private Network) were first used by companies to enable their employees to securely access internal systems such as email remotely (e.g. from home or while on business trips). Today they are increasingly being used for personal use by individuals to protect their privacy while online in public places (e.g. when using the wi-fi connection in a cafe) or in a country where the internet is censored / blocked (e.g. China, Saudi Arabia��).