PSTN and Switching-Routing

I have noticed over the last 20 years a tendency by some people to consider PSTN to mean the Public Telephone Switched Network. It should come as no surprise to people who know me, that I take a different position.

Let us take a step back  to the first 100 hundred years of the telephone. Independent of who has the right to be called the inventor of the telephone, Alexander Graham Bell certainly brought the telephone to the public.

Each call across theose telephone networks would consist of a set-up phase, a connected phase and a clear-down phase. Typically, at least in the days of the manual operator, the calling party would identify the called party by one of two means:

  1. A description of the end subscriber, such as Mr. Sherlock Holmes, 221B Baker Street, London
  2. The identity of the end point on the telephone network, such as London, Whitehall, 1212 i.e. the telephone exchange and the “local number” on that exchange. (Whitehall 1212 was, from about 1934 till the 1960s, the  London Metropolitan Police station known as Scotland Yard.)

In the first case the operator would do a check in the telephone directory to convert the subscriber identity to an end point identity. Once the end point identity was known the operator would select a transport path that would extend the call in the appropriate direction. This decision, based on the context of the call (target destination ), took in to account the resources available to the operator, and the decision was and still is called routing.

Every telephone call over the PSTN is routed.  Sorry, but I see no room for discussion on this.

Once the call set-up has reached the called party, and the called party answers the call, then the call set-up phase ends and the call connection phase starts. In the call connection phase there is an identifiable semi-permanent route (often called a path) between the caller and called party. That route/path took the form of a galvanic connection between the parties (in the analogue telephone system).

In the implementation of the telephone service that started in the mid 1980s the galvanic connection was replaced by a sequence of logical transformations on digital signals in time and space

It was common, even in the analogue telephone days, to refer to the infrastructure that handled the routing decision and that interconnected the transport paths as a telephone switch.

Now we must step back to the 1970’s and 1980’s. In the early 1970’s the basics of the Internetworking Protocol were established by Bob Kahn and Vincent Cerf. The resulting telecommunications networks, based on the Internetworking Protocol, led to the establishment of the Internet as we know it today:

a set of interconnected infrastructure that has a shared end-point addressing scheme and a shared understanding of the structure and interpretation of data packets that the infrastructure transports.

Now we enter the period in which the fledgling Internet caught the attention of financially orientated concerns.
One of the topics that was turned in to an issue, was whether companies who provided public access to the Internet were subject to the regulations normally applicable to providers of telecommunications services.

One argument against the applicability of the telecommunications regulations to the Internet hinged on questionable logic:

  • The PSTN is subject to regulations.
  • PSTN “stands” for the Public Switched Telephone Network.
  • The Internet is routed, and has nothing to do with “switched”.
  • The Internet should therefore not be subject to telecommunications regulations.

I think that an appropriate phrase to some up this logic, and its conclusion, is horse feathers.

Hmmm, this was  wonderful application of the now common  alternative facts, and yes, it was spearheaded by companies based in the USA.

It was of course, totally ignored by such non-regulation preferring companies, that the Internet in the last 20 or so years makes extensive (i.e. massive) use of  switching in the MPLS cores and in the Ethernet access networks.

I have said it before, and I will say it again.

Wishy washy words lead to wishy washy thinking”.

If you want to muddy the waters of your opposition, start using wishy washy words, obfuscate the issue.

To quote that author whom I have already referred to:

Terry Pratchett – ‘A lie can run round the world before the truth has got its boots on.’

If you really want to understand something, then consider this: the tools of comprehension are words and sentences. If you use the wrong tools do not be surprised if the results are sub-optimal.

Leave a Reply

Your email address will not be published. Required fields are marked *