In a telemetry channel, how do raw data rate, payload rate, and overhead relate?

• A. Raw data rate equals payload rate.
• B. Overhead is optional and does not affect total rate.
• C. Effective data rate = payload rate / (payload + overhead).
• D. Payload rate is always larger than raw data rate.

Answer: (C)

The key idea is that the total rate you transmit is made up of the payload plus overhead. The raw data rate is the sum of the payload rate and the overhead rate: R = P + O. The portion of that raw rate that actually carries useful information is the payload relative to the total, which is P/R. Since R = P + O, this becomes P/(P + O). This is the same as the statement “effective data rate = payload rate / (payload + overhead).”

So the channel’s efficiency depends on how large the overhead is relative to the payload. If payload is 2 Mbps and overhead is 0.5 Mbps, the raw rate is 2.5 Mbps and the effective rate is 2/2.5 = 0.8 (80%).

Options that raw data rate equals payload rate would only be true if overhead were zero, which isn’t typical. Overhead isn’t optional—it contributes to the total rate. And payload rate isn’t necessarily larger than the raw data rate; the raw rate already includes overhead.