FE Electrical & Computer Practice Question Solution included after investigation
CASE E-039 · FE

The 1500-Byte Packet That Did Not Arrive in 12 Microseconds

Serialization took only 12 microseconds. The fiber path added another 500 microseconds.

▶ INTERACTIVE CASE FILE • 5–10 MIN • CLICK TO INVESTIGATE

Inspect, commit, prove, fix, and sign off.

Case E-039 engineering failure visual
HOW THIS WORKS

Think like
a real engineer.

Follow the investigation process used in the field — in five guided steps.

1

Inspect

Review the problem, diagram, and evidence.

2

Commit

Choose your hypothesis.

3

Prove

Run calculations and test your idea.

4

Fix

Select and validate a safe correction.

5

Sign Off

See the full debrief and key takeaways.

Case Brief

SAME PRINCIPLES. HIGHER STANDARDS.
PROBLEM STATEMENT

A 1500-byte packet crosses a 100 km fiber link operating at 1 Gbit/s. A network note predicts about 12 microseconds of link delay using packet size divided by bit rate. The first bit cannot reach the far end that quickly. Calculate transmission and propagation delay separately and estimate their sum.

Computer Networks Electrical & Computer Design Verification Root Cause Analysis Safe Correction
E-039
INVESTIGATION WORKSPACE WORK THROUGH THE CASE — YOUR CHOICES MATTER.
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DEBRIEF (LOCKED)

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Find the root cause, confirm the fix, and see how this connects to the exam.

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🔒Exam takeawayComplete the case
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CASE CLOSED · ENGINEERING VERDICT

What actually failed — and what should change.

What failed

The predicted arrival time used serialization delay as total link delay.

What was missed

Physical propagation across 100 km of fiber.

Corrected model

Compute L/R and d/v separately, then combine the applicable delay components.

QUICK ANSWER

Transmission delay is packet length divided by link rate, L/R. Propagation delay is distance divided by signal speed, d/v. For 1500 bytes at 1 Gbit/s, transmission is 12 µs; for 100 km fiber at 2×10^8 m/s, propagation is 500 µs.

ENGINEERING TAKEAWAY

Transmission delay is L/R. Propagation delay is distance/propagation speed. A basic one-link latency estimate includes both.

CASE QUESTIONS

Questions this case should settle.

What is the transmission delay of a 1500-byte packet at 1 Gbit/s?

12 microseconds.

What is the propagation delay across 100 km at 2×10^8 m/s?

500 microseconds.

NEXT FE PRACTICE QUESTION

The Router That Ignored the Shorter-Looking Route

Computer Networks · 5–10 min

Continue Practice →
WHAT YOU’LL LEARN

Real failures. Lasting skills.

Each case is designed to build the judgment, analysis, and confidence you need for engineering exams — and beyond.

  • Apply core engineering concepts to real-world problems
  • Practice structured troubleshooting and analysis
  • Strengthen exam-ready thinking through realistic scenarios
COMMON QUESTIONS
Do I need prior knowledge to use these cases?

Basic subject familiarity helps, but every case is designed to teach through the investigation itself.

How long does a case take?

Most cases are designed for a focused 5–10 minute investigation.

Are the cases aligned with engineering exams?

Each case is mapped to a verified exam, subject, topic, and misconception before publication.

What happens after I complete a case?

The sealed debrief unlocks with the root cause, corrected reasoning, fix, and takeaway.