Which of the following is widely used inside the telephone system for…

Answer: ATM

Why this is correct:

• ATM uses small fixed-size cells (53 bytes), which enables efficient hardware switching and predictable latency.

• ATM supports quality of service (QoS) and can carry voice, video, and data streams with different service guarantees—important for telephone carrier backbones.

• ATM scales to high speeds and was widely adopted in telco long‑haul and backbone networks for aggregated traffic.

Why the other choices are not the best answer:

• ISDN is a subscriber-access technology for connecting end users to the network, not a carrier long-haul backbone technology.

• Frame Relay is a packet-switched WAN technology commonly used for enterprise links; it lacks the same cell-based design and strict QoS capabilities historically used in telco backbones.

• ISTN is not a standard technology for long-haul telephone transport and appears to be a distractor.