A network administrator notices throughput lower than the bandwidth when measuring backbone transfer for a mission-critical application. Which single factor most directly explains this difference?

• A. The sophistication of the encapsulation method applied to the data
• B. The amount of traffic that is currently crossing the network
• C. The bandwidth of the WAN connection to the Internet
• D. The reliability of the gigabit Ethernet backbone

Answer: (B)

Throughput is the actual amount of data that gets delivered over a link in a given time, while bandwidth is the maximum capacity of that link. When throughput is lower than the backbone’s bandwidth, the most direct cause is how much traffic is currently crossing the network — the utilization. If the link is carrying a lot of traffic, it becomes congested, packets queue in buffers, delays grow, and protocols like TCP reduce their sending rate to avoid loss, so the observed data rate falls short of the link’s theoretical maximum.

Encapsulation overhead can reduce efficiency a bit, but it doesn’t explain a broad gap caused by active congestion. The WAN connection bandwidth to the Internet isn’t the limiting factor for measuring backbone transfer inside the network. Reliability of the gigabit backbone matters mainly when there are noticeable error-induced retransmissions, which is less common; the primary, direct explanation in typical scenarios is high utilization.