2 cissp ® Official Study Guide Eighth Edition

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Chapter 12 
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Secure Communications and Network Attacks
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 communi-
cation interruptions. Circuit-switching systems employ permanent, physical connections. 
However, the term 
permanent
applies only to each communication session. The path is per-
manent 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 the advent of digital switching and VoIP systems, 
those days are long gone. That’s not to say that circuit switching is nonexistent in today’s 
world; it is just not being used for data transmission. Instead, you can still find circuit 
switching in rail yards, irrigation systems, and even electrical distribution systems.