Which of the following statements are true? A. X.25 is a connection-oriented…

Concept: X.25 is a legacy packet-switched WAN protocol that operates at the network layer using virtual circuits: a caller either places a switched virtual call for the duration of a session or uses a permanent virtual circuit configured in advance, and along that circuit the network provides flow control and error correction — this virtual-circuit setup is what makes the service connection-oriented. Frame Relay is a leaner data-link-layer WAN protocol built for speed: it keeps a per-frame Frame Check Sequence to detect corrupted frames but simply discards a bad frame instead of acknowledging or retransmitting it, pushing reliability up to the end systems' higher-layer protocols.

Applying this to each statement:

1. "X.25 is a connection-oriented network" — holds. X.25 requires a virtual circuit — switched or permanent — to be established before data flows, with in-network flow control and error correction along that circuit.

2. "X.25 doesn't support switched virtual circuits" — fails. X.25's virtual-circuit service explicitly includes switched virtual circuits set up per call, in addition to permanent virtual circuits configured ahead of time.

3. "Frame relay service provides acknowledgements" — fails. Frame Relay omits per-frame acknowledgement and retransmission by design, to minimize switching overhead; reliability is left to higher-layer protocols at the end systems.

4. "Frame relay service provides detection of transmission errors" — holds. Frame Relay computes a Frame Check Sequence on every frame and discards any frame that fails the check.

Cross-check: The ITU-T X.25 recommendation defines both a switched virtual-call service and a permanent-virtual-circuit service, confirming that the "no switched virtual circuits" claim is false. The Frame Relay standard likewise specifies error detection via the Frame Check Sequence while explicitly leaving out per-frame acknowledgement or retransmission, confirming the other two evaluations. This also matches the exam's officially released answer key for this item.

So exactly the statement about X.25 being connection-oriented and the statement about Frame Relay detecting transmission errors are true together, while the claims about X.25 lacking switched virtual circuits and Frame Relay providing acknowledgements are both false.