A sender, connected to a receiver with a 70 Mbps channel, transmits at full channel speed, but is idle 20% of the time because it has to wait for acknowledgements from the receiver. What is the channel utilization (efficiency)?

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BlackTom AI · Accepted Answer

Re-expressing the problem in my own words helps set the stage: a sender is linked to a receiver over a 70 Mbps channel, it transmits at full channel speed, but it is idle for 20% of the time while waiting for acknowledgments. We want the channel utilization, i.e., the fraction of time the channel is actively used for useful transmission.

Key idea: channel utilization equals the fraction of time the channel is not idle due to waiting for acknowledgments. If the sender is idle 20% of the time, it is actively using the channel 80% of the time. In other words, utilization = 1 - idle_time_fraction = 1 - 0.20 = 0.80.

Consider potential misunderstandings: if someone thinks utilization should be based on data throughput or on the effective payload portion of each transmission, it would be tempting to factor in protocol overhead or round-trip timing. However, the question specifies the sender’s idle time due to waiting for ACKs and asks for the channel utilization, which directly corresponds to the fraction of time the channel is in use for transmitting rather than idling.

Another way to phrase it: with a 20% idle window, the channel spends 20% of the time not carrying data, leaving 80% of the time producing data transmission, so the channel utilization equals 80%. If one misreads and assumes the channel is only used when a data frame is being transmitted without considering waiting periods, that would ignore the time the channel is allocated but not actively transmitting, leading to an incorrect, higher estimate.

Additionally, note the given data point of the 70 Mbps channel speed is the theoretical maximum link speed; the utilization focuses on the time fraction of the link being actively used, not the raw data rate alone. The reduction from 100% to 80% purely reflects the idle time introduced by the acknowledgment-waiting delay, independent of the absolute data rate.

Summarizing in plain terms: since the sender is idle 20% of the time, the channel is utilized for the remaining 80% of the time, giving a channel utilization of 80%.

CMPT371 D100 Midterm

A sender, connected to a receiver with a 70 Mbps channel, transmits at full channel speed, but is idle 20% of the time because it has to wait for acknowledgements from the receiver. What is the channel utilization (efficiency)?

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