VPNs refer to encrypted tunnels that carry packetized data between remote sites. VPNs can use private lines or use the Internet through one or more Internet Service Providers (ISPs). VPNs are implemented in both dedicated hardware and software but can also be integrated as an application to existing hardware and software packages. A common example of an integrated package is a firewall product that can provide a barrier against unauthorized intrusion as well as perform the security features that are needed for a VPN session.

The encryption process can take from less than 1 ms to 1 s or more, at each end. VPNs can represent a significant source of delay and therefore, have a negative impact on voice performance. Also, because most VPN traffic runs over the Internet and there is little control over QoS parameters for traffic crossing the Internet, voice quality might suffer due to excessive packet loss, delay, and jitter. You can negotiate a service-level agreement with the VPN provider to guarantee an acceptable level of service. Before implementing IP telephony with a VPN, you should test their VPN network over time to ensure that it consistently meets the Avaya requirements.