Why does TCP performance degrade over high-loss wireless links?

• A. Higher throughput with loss
• B. Loss triggers congestion control to reduce throughput; high RTT variance and retransmission delays worsen performance.
• C. Wireless has no effect on TCP
• D. Congestion control increases throughput in wireless

Answer: (B)

The main idea is that TCP ramps the sending rate down whenever it detects loss, treating loss as a sign of network congestion. On high-loss wireless links, many losses come from the wireless channel itself (bit errors, fading, interference) rather than from congestion, so TCP’s reaction backfires: it reduces its congestion window and slows down the sender even though the network isn’t actually congested. This implicit back-off lowers throughput just when the channel might still be capable of delivering data.

In addition, wireless links exhibit large RTT variance (jitter) and frequent retransmission delays. The variable round-trip times make TCP’s estimates of how long to wait for acknowledgments unreliable, which often leads to longer retransmission timeouts or unnecessary slowdowns. The memoryless restarts and backoffs these timeouts trigger further interrupt the data flow, compounding the loss- and delay-induced throttling. Retransmissions themselves occupy bandwidth and stall the pipeline, delaying new segments from being sent and acknowledged.

So, the degradation comes from TCP interpreting non-congestive losses as congestion and aggressively reducing throughput, worsened by unstable RTTs and extra delays from retransmissions. The other statements don’t reflect why TCP performance actually deteriorates on high-loss wireless links.