Second-generation VoIP networks have achieved a fair degree of commercial success. Companies are using the technology to save money on trunking costs and for functional enhancements such as mobility, presence-related functions and unified messaging. VoIP networks are beginning to support roaming from Ethernet-based phones to cellular service. Large companies are using VoIP for contact center support, particularly for hosted automatic call distribution capabilities and virtual contact centers (where agents are distributed throughout a large geographic area). Carriers are deploying VoIP services to generate new revenue to replace the revenue lost from traditional TDM services.

Yet two fundamental problems preclude the unconstrained scalability of VoIP to a large-population base along with guaranteed "industrial-grade" service levels. The first problem is lack of de facto intrinsic QoS in many IP networks, both at the carrier and enterprise levels. The second problem relates to end-to-end integrity of the VoIP signaling and bearer paths. It is difficult to carry VoIP packets across firewalls, not only because of protocol considerations, but also because of network address translation (NAT) issues. Security concerns, such as eavesdropping and hacking, are another potential problem. Next-generation VoIP networks based on IPv6 are now on the drawing board to address these issues, specifically scalability and commercial-grade reliability. These networks are based on IPv6.

The large voice carriers could claim that with TDM, anyone in the world could call anyone else, any time, anywhere, and get a good telephonic connection. Currently, this is an unachievable goal for the VoIP industry. IPv6 offers the potential for achieving the scalability, reachability, end-to-end interworking, QoS and commercial-grade robustness necessary if VoIP is to replace the TDM infrastructure worldwide. Specifically, IPv6 deals with the QoS and NAT problems mentioned above.

NAT-based accommodation is a short-term solution to this anticipated VoIP growth; a better solution is needed. NAT techniques make the Internet, applications and devices more complex, which implies a cost overhead. IPv6's benefits include expanded addressing; streamlined header format and flow identification; autoconfiguration ("plug-and-play"); and more robust mobility, multicast, QoS and security support.