Digital Telephony Invades PSTN Territory

When digital  networks were implemented back  in the  1960s, the telephone carrier companies began using  a technique that permitted them to accept analog telephone calls coming into their switching facilities and  convert those signals into digital  form for transmission on their shiny new networks. They  had  not  yet made the  leap  into packetizing telephone calls,  which is what  we have  today with VoIP. At the time,  they  thought it best to keep  the  circuit-switched telephone carrier network physically separate from the  evolving packet-switched computer network.

The phone companies were able  to make other improvements to circuit- switched telephone services. After their circuit-switched carrier network received the  caller’s  telephony signals, they  were able  to convert the signals into digital  form, as necessary. They  discovered that digital signals allowed them to aggregate many  more  calls onto a given circuit and  through a given switch than they  could before. This enabled them to streamline how circuit-  switched telephone calls could be made.

One innovation was the  addition of area  codes, which help  to process calls over  a circuit-switched network. The entire telephone number, including area code, identifies the  number of circuits and  the  location of the  switching devices for a given phone on the  PSTN network. For example, consider a call originating in Mountain View, California (area code 415) to a person in Pittsburgh, Pennsylvania (area code 412). The call is switched out  over  three physically distinct circuit switches — 415 to 412 — to set  up and  carry  the  call. Figure 2-2 illustrates the  routing of such a circuit-switched call.

If these two locations are on the  same computer network, and  both are using  VoIP, the  call could be carried over  the  company’s computer network in packet form. This process is known  as on-net VoIP telephony. None of the  packets would  touch the  PSTN. There would  be no toll, regulatory, or metered charges for this  long-distance telephone call. Figure 2-3 illustrates the  routing of such a packet-switched VoIP call.