Why is a constant bit rate (CBR) telephone system classified as circuit-switching technology?

A constant bit rate (CBR) telephone system is classified as circuit-switching technology primarily because it involves the establishment of a dedicated communication path or circuit for the entire duration of the call. This approach means that the necessary bandwidth is reserved before the communication begins, ensuring a consistent and stable connection for the transmission of signals.

When converting the analog audio signal into a digital DS0 circuit, the system sets up a fixed bandwidth that is constantly available for that particular communication session. This dedicated circuit allows for real-time transmission of voice without the interruptions that could occur with other technologies that do not guarantee a constant bit rate, such as packet switching.

In contrast, the other options do not correctly address the fundamental nature of circuit-switching technology. For instance, transmission mediums such as fiber optics (as mentioned in one choice) do not inherently define the switching method used; circuit-switching can operate over various types of transmission mediums, not limited to fiber optics. The reliance solely on digital signals or the method of transmitting data in packets are characteristic of other types of communication systems, such as digital and packet-switched networks, which do not provide the same reliability in terms of constant bit rate and dedicated circuit quality of service provided by circuit-switching technologies.