The PSTN CSI

The public switched telephone network (PSTN) is the  oldest CSI, actually beginning with the work of Alexander Graham  Bell in the late 1800s.

Over the years, carriers have been installing, expanding, improving, and inter- connecting various parts of the  PSTN. Compared to all other carrier service infrastructures, the  PSTN is the  largest.

It was under the  PSTN CSI, on a POTS transport, that VoIP first started — and  it wasn’t  pretty.  The inherent bandwidth limitations and circuit-switched protocols required by POTS impose clear limitations when  it comes to the  packet-switched requirements of VoIP. These limits were discovered by acci- dent in the  first VoIP call made by a pair  of Internet hobbyists in 1995.

The earliest VoIP experiments were not  pretty by today’s standards. But by the  late 1990s, VoIP was being  viewed by dot  com startups, the carriers, and even  several telecommunications equipment manufacturers as having  great  potential for the  future of packetized telephony.

Even though VoIP can work over  the  PSTN for a single  call, it’s not  a viable solution for large companies that need to make multiple calls at the  same time.  Quality  of service (QoS) quickly  comes into play, and  dedicated lines start becoming the  minimum level at which sufficient QoS can be achieved. The PSTN CSI doesn’t provide dedicated lines,  so it doesn’t provide a suitable solution for robust VoIP.

Poor QoS in the PSTN CSI is caused by the inherent bandwidth limits of POTS, the circuit-switched protocols used on the PSTN, and the fact that  the number of switching hops in a POTS call add  too  much overhead into each and  every  packet in the  transmission. This overhead, more  than anything else,  is the  major culprit.

That  said,  if you are an individual (not a company), and  you want  to run only a single  line over  VoIP, you can get satisfactory QoS using  a regular broad-  band connection such as DSL. Better still, this  makes VoIP quite affordable because the  cost of DSL is relatively low these days. However, using  such a transport for VoIP is not  reasonable for larger  businesses that need better QoS for larger  numbers of calls.