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Many definitions exist for Virtual Private Networks (VPNs). 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 millisecond (ms) to 1 second or more, at each 
end. Obviously, VPNs can represent a significant source of delay, and therefore have a negative 
affect on voice performance. Avaya VPN products encrypt traffic with less than 1ms of delay, 
and thus are appropriate for IP Telephony. Also, because most VPN traffic runs over the Internet 
and there is little control over QoS parameters for traffic crossing the Internet, voice quality may 
suffer due to excessive packet loss, delay, and jitter. Users might be able to negotiate a 
service-level agreement with the VPN provider to guarantee an acceptable level of service. 
Before implementing IP Telephony with a VPN, users should test their VPN network over time to 
ensure that it consistently meets the requirements that are specified in the Avaya IP Voice 
Quality Network Requirements Document Summary. 

For increasing numbers of enterprises, the VPN carries not only data, but voice 
communications. Though voice communication over IP networks (IP Telephony) creates new 
quality of service (QoS) and other challenges for network managers, there are compelling 
reasons for moving forward with convergence over maintaining a traditional voice and data 
infrastructure: 

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A converged infrastructure makes it easier to deploy eBusiness applications, such as 
customer care applications, that integrate voice, data, and video.

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Enterprises can reduce network costs by combining disparate network infrastructures, and 
eliminating duplicate facilities.

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A converged infrastructure can increase the efficiencies of the IT organization.

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Long distance charges can be reduced by sending voice over IP networks.

Voice over IP VPN is emerging as a viable way to achieve these advantages. The emergence of 
public and virtual private IP services promises to make it easier for customers, suppliers, and 
businesses to use data networks to carry voice services. As with any powerful new technology, 
however, VPNs require skilled management to achieve top performance. The highest network 
performance becomes imperative when the VPN network must deliver high-quality voice 
communication. Not all IP networks can meet these quality requirements today. For instance, 
the public Internet is a transport option for voice communication only when reduced voice 
performance is acceptable, and global reach has the highest priority. When high voice quality is 
a requirement, ISPs and Network Service Providers (NSPs) can provide other VPN connections 
that meet required Service Level Agreements (SLAs).