Page 173 - CISSO_Prep_ Guide

Over the years, the way of communicating across networks has
changed dramatically. Early communications were based on the
public switched telephone network (PSTN), and messages were
transmitted using circuit switching over voice-grade cabling.
The [central] offices of the telecommunications providers in
every part of each city and town were linked together through
cabling, and the switches and multiplexing equipment in each
office established the circuits used to carry communications
traffic. The early forms of transmission were analog, which had
limited bandwidth (based on the range of the human voice).

When computers first started communicating over the PSTN, a
modem was required to convert the digital signal of the
computer into an analog signal that could be transmitted over
the existing networks. These were all circuit switching in that a
circuit between the two endpoints was established at the
beginning of the communications session according to the best
route available at the time. This circuit remained established as
long as the two parties remained connected. When either party
hung up, the circuit would be dropped. This form of
communication worked very well for voice since it had a
consistent level of latency (delay) and a fairly stable level of
quality. But the voice was very tolerant of noise, whereas data is
not. Data is easily confused by noise on the line, not knowing if
the noise is legitimate data or not. Because of this, early
communications for data had to include extensive error-
correcting.

Instead of a modem, many companies then moved to a leased
line. A line (circuit) that the company purchased from a
telecommunications carrier that was dedicated to the company's
own private use - hence the term private line was born to