Which statement about synchronous modems is true?

• A. Synchronous modems require a clock pulse to ensure the receiver and transmitter operate at the same clocking speed.
• B. Synchronous modems do not require a clocking signal.
• C. Asynchronous modems use a continuous clock signal to sync data.
• D. Asynchronous modems transmit with no timing concerns and always send a fixed-rate stream.

Answer: (A)

Timing alignment is essential in synchronous transmission: the receiver must sample each bit exactly when the transmitter is sending it. To achieve this, both ends share a clock reference, so a clock pulse is used to keep them in lockstep. This clocking ensures the two devices operate at the same timing rate, preventing bit boundaries from drifting and causing errors.

In asynchronous transmission, timing is managed differently—each character is framed with start and stop bits, and the receiver uses its own local clock to time sampling rather than relying on a continuous clock on the line. Because of that framing method, the idea that timing concerns vanish or that a continuous clock is used for asynchronous is incorrect. Likewise, asynchronous transmissions aren’t a guaranteed fixed-rate stream without timing considerations; they still depend on proper character timing.

So the statement that synchronous modems require a clock pulse to ensure the transmitter and receiver are synchronized is the correct one.