User guide

4.2 PSTN
PSTN stands for public switched telephone network, “the telephone network
of the telephone networks” or most commonly known as “the telephone
network.”PSTN stands for public switched telephone network, “the telephone
network of the telephone networks” or most commonly known as “the
telephone network.”
In the same way that the Internet is the global IP network, the PSTN is the amalgamation of all circuit-
switched telephone networks in the world. An important difference between the PSTN and the Internet
is the meaning of “flow of information”. In telephony (PSTN) a flow of information is one complete
“phone call” while in the Internet a single data packet is a flow of information by itself. The PSTN and
the Internet are conceptually very different and represent two very different (technical and political)
philosophies. If a phone call is placed over the PSTN, a dedicated channel (circuit) of 64 Kbps needs to
be reserved. If the phone call is placed over the Internet it coexists with many other services
simultaneously. Although this difference might seem irrelevant at first sight, it has huge implications for
the deployment of ICTs in developed and developing regions. In the traditional model, a “copper cable”
provides access to the PSTN and offers one single type of service: an analogue voice channel. If the
same cable is used to reach a packet-switched network, such as the Internet, any type of IP-based
service can be provided.
The PSTN has historically been governed by technical standards created by the ITU
13
while the Internet
is governed by IETF
14
standards. Both networks, the PSTN and the Internet, use addresses to route
their flows of information. While in the PSTN, telephone numbers are used to switch calls between
telephone exchanges. IP addresses are used to switch packets between Internet routers.
4.3 Signalling in traditional telephony
Telephone exchanges are the “routers” of the PSTN. A Foreign Exchange
Office (FXO) is any device that, from the point of view of a telephone
exchange, acts like a regular telephone.
An FXO should be able to accept ring signals, go on-hook and off-hook, and send and receive voice
signals. Imagine an FXO as a “telephone” or anything that rings (like a fax machine or a modem).
13. The ITU is the is the International Telecommunications Union, an organization responsible for standardization, allocation of
radio spectrum and and organizing interconnection arrangements between different countries to allow international phone
calls. It is part of the United Nations system with a formal membership structure.
14. The IETF is the Internet Engineering Task Force, an organization responsible for developing and promoting Internet
standards. It is an open, all-volunteer standards organization, with no formal membership or membership requirements.
Page 10 TRICALCAR | www.wilac.net/tricalcar – Version: February 2008