AT&T employed their well known offering called Common Channel Switching Arrangement (CCSA) to design and maintain voice networks for all enterprises during the period before 1975. CCSA allowed the sharing of AT&T's switches among several enterprises and use of private transmission lines (in the form of access lines connecting CPE's to switches and backbone trunk bundles). The recurring monthly charges for switching was determined by the number of terminations. The monthly cost of transmission lines was mainly determined by the number and length of voice-grade lines connecting those CPE's and shared switches. An enterprise had no say in the design of their networks. Users assumed that AT&T's expertise (as expounded in the Bell Laboratories publications and reported in the Transactions of International Conferences on Teletraffic Theory) was always used in designing their corporate voice network. But not so! Please read on.

The need to design an economical enterprise voice network arose in 1968 when Carterfone decision allowed the deregulation of customer premise equipment (CPE) and in 1973 when Collins Radio Company (which later became a part of Rockwell International in 1972) developed the first commercial digital voice switch. The digital voice switch later became an integral part of corporate voice and Automatic Call Distribution (ACD or Incoming Call Management) networks. The need for designing an economical enterprise voice network really became pronounced with the availability of digital T1 leased lines and when other vendors, notably Northern Telecom (now called Nortel) also began offering digital voice switches.

The interconnected STAR topology is primarily used to synthesize a modern voice network. Gone are the days of shared switches and voice-grade  lines in a voice network. Significant savings were obtained by optimally choosing the number and location of switching nodes, and the mix of transmission lines (analog voice grade or 56-Kbps, T1or T3 for digitally encoded voice) to satisfy the need of an enterprise in a cost effective manner. These design parameters define the network topology - the soul of the network. No vendor or inter-exchange carrier (IEC) should determine the number and location of the switches. Only the network design effort and the corporate traffic flows should determine the network topology in its entirety. Things have  changed since 1975. Many enterprises of today don't own their voice networks. They choose instead virtual switched offerings from long distance carriers. But with the advent of new technologies (e.g. ATM and IP) the need for cost effective INTEGRATED (voice+data+video) enterprise networks will force them to reconsider the design of optimum enterprise networks again. The multi-level interconnected star topology characterizes most enterprise integrated broadband/multimedia networks.

Please refer to Sharma's latest Textbook or papers for a more detailed appreciation of voice network design algorithms. For a short history of network design for data applications please go to A Short History of Enterprise Data Network Design.