What is an X.25 network, whether public or private, typically built upon?

An X.25 network is typically built upon the leased-line facilities of the public telephone network. This is because X.25 is a packet-switched network protocol that was historically used for connecting devices over long distances. It operates over communication lines that are leased from telecommunication companies, making it reliant on the existing infrastructure of the public telephone network.

The leased-line aspect refers specifically to dedicated communication paths that ensure a stable connection and consistent data transmission, which is crucial for the performance of X.25 networks. This approach allows for reliable communication in environments where data must be transmitted efficiently and without delays.

In contrast, while optical fiber connections can be part of broader networking solutions, they are not exclusive to X.25 networks. Furthermore, referring to packet-switched networks exclusively does not capture the fundamental requirement of relying on leased-line facilities. Lastly, the use of the cable modem standard is not relevant to X.25 technology, as it is designed for broadband connections rather than the specifics of X.25 protocol operations.