6 NGN services
6.4 VoIP VPN

Combination of two technologies: the voice over IP and virtual private networks provides VoIP VPN technology which offers a delivery of secure voice. As was already mentioned, the VoIP transmits a human voice as a digital data stream.

Then it is quite easy to provide voice encryption via VPN tunnels just by applying standard data-encryption algorithms which are inherently available in protocols used to implement VPN tunnels.

Application of the Voice over IP through VPN brings another benefit, however. It is difficult to pass the SIP protocol through a firewall because it uses random port numbers for connections setup. A VPN is good solution how to avoid firewall problems when configuring remote VoIP clients.

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Fig. 9 - Secure Voice over VPN Tunnels [17]