21.1.1 Brief History of VPN

The driving force behind the development of VPN technologies has been the desire for economically efficient private networks to connect the geographically distributed locations of many organizations in such a way that the organizations can have control over the services provided over their private networks. Mirroring the evolution of communications networks, VPN has evolved from pure voice services to data services to integrated voice and data services.

In regard to voice services, VPN had its origin in the telephone volume discount outbound calling plan used by corporate customers for calls between different branches. Then it evolved into the leased line service, where a leased telephone line connects two branch locations of a corporation. These are point-to-point lines that can be in two-wire or four-wire configurations. Eventually leased lines were used to connect PBXs at various locations. These dedicated lines between the distributed locations of organizations effectively created private networks. Furthermore, sophisticated numbering and dialing plans have been developed for such private networks.

The next major development in private-network voice services was the centrex service. It is the responsibility of each organization using a leased-line private network to manage the services and maintain the PBX...