The physical link: multiple-choice review
Multiple-choice synthesis across the physical-link unit — latency floors, Shannon ceilings, bufferbloat, RoCE fabrics, and the frontier.
Six questions that cut across the whole unit. Each one mirrors a call you make in a real incident — distinguishing physics from a bug, picking the right lever, reading the failure mode — not a definition to recite.
Confirm you can connect propagation floors, the Shannon ceiling, bandwidth-delay product, bufferbloat versus long-RTT congestion control, and the datacentre fabric — the synthesis the individual lessons built toward.
A team upgrades its NYC→Sydney link from 10 Gbps to 100 Gbps and is surprised that page-load latency does not improve. What is the correct read?
An RF team wants more Wi-Fi capacity. They can either double the channel width or push from 1024-QAM to 4096-QAM. Which moves capacity more, and why?
A household runs a video call that stutters whenever a cloud backup uploads, yet the speed test reads 300/40 Mbps green. Idle ping is 18 ms; under upload it climbs to 420 ms. Diagnosis and first fix?
The same loss-based CUBIC sender runs well over a Starlink LEO link (~50 ms RTT) but starves over a GEO link (~600 ms RTT). Why, and what is the right response on GEO?
A 64-GPU training job runs 3× slow. Each leaf has 2×400G uplinks but 8×200G server NICs, and PFC PAUSE frames appear on every uplink. What is happening?
A high-frequency-trading firm wants the lowest possible NYC↔Chicago round-trip latency. Which physical-layer investment actually helps?
The through-line of the unit is one separation: the propagation floor (distance ÷ signal speed) is physics you design around, and everything above it is engineering you can fix. Shannon caps a link’s bits/s (bandwidth linear, SNR logarithmic); BDP says how many bytes must be in flight to fill it; bufferbloat and long-RTT starvation are two different congestion failures with two different fixes (AQM versus BBR); the datacentre fabric trades the same bandwidth and loss budgets at hyperscale; and only a shorter path — or hollow-core fibre — ever lowers the floor.
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