Why are synchronous modems typically more expensive than asynchronous modems in data communication systems?

Introduction / Context:
Synchronous and asynchronous modems differ in how they align sender and receiver timing. Cost differences arise from the additional circuitry and signal processing needed to maintain bit/byte alignment without start/stop overhead.

Given Data / Assumptions:

• Synchronous links send a continuous bit stream without per-character start/stop bits.
• Receivers must align to the sender's bit timing accurately.
• Asynchronous links embed timing via start/stop bits, reducing timing complexity.

Concept / Approach:

In synchronous transmission, timing is recovered from the incoming signal using a clock recovery circuit (e.g., PLL). This hardware (and sometimes DSP logic) adds complexity and cost. Asynchronous modems avoid this by delimiting characters with start/stop bits at known intervals.

Step-by-Step Solution:

Verification / Alternative check:

Standards for synchronous links (e.g., HDLC/PPP framing at the data link layer) assume continuous streams with external or embedded clocking, reinforcing the need for clock extraction hardware.

Why Other Options Are Wrong:

Large volume of data: Volume is an outcome, not the reason for higher cost.

Start/stop bits: That is asynchronous behavior, not synchronous.

Larger bandwidth: Not inherently required; synchronous vs asynchronous is about timing, not necessarily bandwidth.

None of the above: Incorrect because clock recovery is the reason.

Common Pitfalls:

Assuming synchronous always implies higher data rate; cost stems from timing circuitry, not merely speed.

Final Answer:

They contain clock recovery circuits