Exercise 2: Unplugging the Cable — A Physical-Layer Failure vs. a Transport-Layer Failure — Possible Solution
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What happens at the Physical layer:

Per the chapter's own worked example, "Physical is the actual radio
signal between your laptop and the router" -- in the ethernet cable
version of that example, Physical is the actual electrical signal
traveling through the cable itself. Unplugging the cable mid-download
destroys the Physical layer instantly and completely: there is no
longer any medium at all for a signal to travel across. Every single
layer built on top of that connection -- Data Link's framing, Network's
IP routing, Transport's TCP session, all the way up to the
Application-layer HTTP request in progress -- loses its underlying
foundation simultaneously and immediately, since none of those higher
layers can function without a working physical medium beneath them.

Why this is different from a Transport-layer failure:

A Transport-layer failure is a completely different kind of problem --
the Physical, Data Link, and Network layers could all be working
perfectly (the cable is plugged in, signals are traveling, packets are
being routed correctly), while the TCP connection itself experiences a
problem specific to its own layer -- for example, a TCP timeout because
the server actually stopped responding to that specific connection, or
a connection reset. In that scenario, the lower layers are healthy and
uninvolved in the failure; only the layer-4 connection state itself is
broken.

The practical distinction: unplugging the cable is a failure that
starts at the bottom of the stack and takes every layer above it down
with it, since each higher layer depends on the ones below actually
working. A Transport-layer failure can happen in complete isolation,
with the lower layers (physical connectivity, routing) working
perfectly fine the entire time -- which is exactly the kind of
distinction the chapter's own "knowing which layer the problem lives
in" framing (from net1-1) is meant to help someone diagnose.

WHY THIS WORKS AS AN ANSWER
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This explains why a Physical-layer failure cascades upward through
every dependent layer simultaneously, while a Transport-layer failure
can occur in isolation with the lower layers completely healthy,
directly illustrating why "which layer" matters for real diagnosis.