Page 910 - (ISC)² CISSP Certified Information Systems Security Professional Official Study Guide

Switching Technologies

When two systems (individual computers or LANs) are connected over
multiple intermediary networks, the task of transmitting data packets
from one to the other is a complex process. To simplify this task,

switching technologies were developed. The first switching technology
was circuit switching.

Circuit Switching

Circuit switching was originally developed to manage telephone calls

over the public switched telephone network. In circuit switching, a
dedicated physical pathway is created between the two communicating
parties. Once a call is established, the links between the two parties
remain the same throughout the conversation. This provides for fixed
or known transmission times, a uniform level of quality, and little or
no loss of signal or communication interruptions. Circuit-switching

systems employ permanent, physical connections. However, the term
permanent applies only to each communication session. The path is
permanent throughout a single conversation. Once the path is
disconnected, if the two parties communicate again, a different path
may be assembled. During a single conversation, the same physical or
electronic path is used throughout the communication and is used
only for that one communication. Circuit switching grants exclusive

use of a communication path to the current communication partners.
Only after a session has been closed can a pathway be reused by
another communication.

Real-World Circuit Switching

There is very little real-world circuit switching in the modern world
(or at least in the past 10 to 15 years or so). Packet switching,

discussed next, has become ubiquitous for data and voice
transmissions. Decades ago we could often point to the plain old
telephone service (POTS)—also called public switched telephone
network (PSTN)—as a prime example of circuit switching, but with