← ACARA Digital Technologies Years 7–8

Signal weakening over distance in cables

The weakening of a signal as it travels along a cable. Copper cables have a length limit because of it, while fibre can go much further. Repeaters boost the signal.

Part of: ACARA Digital Technologies Years 7–8 — Wired and Wireless Networks — Module 15, WA Computer Science ATAR Year 11 — Network Communications — Module 10

Understanding Signal weakening over distance in cables in ACARA Digital Technologies Years 7–8

What it does: Ethernet copper cable is limited to about 100 metres before attenuation makes the signal too weak to read. Light in fibre fades far more slowly than electricity in copper, so one fibre run can reach many kilometres. A repeater receives a fading signal and sends it on at full strength, extending how far it can go. Signals fade with distance. This is why copper cables have a length limit. Attenuation means the signal gets weaker. Amplification is the opposite, making it stronger, which is what a repeater does.

Common mistakes: Copper cables lose signal strength quickly, so their limit is only about 100 metres, not 100 kilometres. It should be copper_limit ← 100 metres. Attenuation is the signal getting weaker as it travels along a cable. It does not grow. It should be attenuation = signal weakens.

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This module's key terms are also in a printable crossword (Modules 54–68) and word search (Modules 13–68).

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Learning
Duo
What happens to a signal as it travels further along a cable?
It gets weakerIt gets stronger
Matches
A 300 m copper cable run works fine with no repeater because signals never weaken.
↓
Signals do weaken. Over 300 m, copper attenuation makes the signal too weak, so a repeater or fibre is needed.
Applying
Meanings
Term
attenuation
Meaning
The weakening of a signal as it travels along a cable. Copper cables have a length limit because of it, while fibre can go much further. Repeaters boost the signal.
Practice
cable_length <- 150
Testing
Gaps
= weaker signal over distance
Check Answer
Spots
1copper_limit ← 100 kilometres
2fibre_limit ← 40000 metres

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Quick Questions

1. What happens to a signal as it travels further along a cable?
A. It gets weaker ✓
B. It gets stronger
This weakening is called attenuation. The longer the cable, the more the signal fades.
2. Why do copper network cables like Ethernet have a maximum length?
A. The signal weakens over distance ✓
B. The copper runs out of electricity
Copper signals fade as they travel. Past the length limit, the signal is too weak to be read correctly.
3. Which cable can carry a signal much further before it becomes too weak?
A. Copper cable
B. Fibre optic cable ✓
Fibre sends light, which fades much more slowly than electrical signals in copper, so it can reach much further.
4. What does a repeater do?
A. Slows a signal down to make it safer
B. Boosts a weak signal so it can travel further ✓
A repeater receives a fading signal and sends it on at full strength, extending how far it can go.
5. A school needs to link two buildings 500 metres apart. Which is the better choice?
A. Fibre cable, or copper with repeaters ✓
B. One long copper cable with no repeaters
One copper cable that long would be too weak. Fibre or repeaters solve the problem.

Match the Pairs

attenuation → The weakening of a signal as it travels along a cable
repeater → A device that boosts a weakening signal so it can travel further
Copper cables have a length limit because the signal fades over distance → Ethernet cable: maximum 100 m
Fibre cables lose less signal, so they can carry data much further than copper → Fibre cable: many kilometres
A 300 m copper cable run works fine with no repeater because signals never weaken. → Signals do weaken. Over 300 m, copper attenuation makes the signal too weak, so a repeater or fibre is needed.
A repeater makes the cable shorter so the signal does not weaken. → A repeater does not shorten the cable. It boosts the weakened signal so it can keep travelling.

Key Term

attenuation
The weakening of a signal as it travels along a cable. Copper cables have a length limit because of it, while fibre can go much further. Repeaters boost the signal.

Fill in the Blanks

1. attenuation = weaker signal over distance
Hint: Type the term for a signal getting weaker as it travels along a cable.
2. IF length > max THEN ADD repeater
Hint: Type the device that boosts a weak signal so it can travel further.
3. IF cableLength > copperLimit THEN
Hint: Type the comparison operator that checks if the cable is longer than the limit.
4. copperLimit < fibreLimit
Hint: Type the operator showing copper's length limit is shorter than fibre's.
5. strength ← strength - loss
Hint: Type the arithmetic operator that lowers the signal strength by the loss.

Spot the Error

Question 1
A copper_limit ← 100 kilometres
B fibre_limit ← 40000 metres
C DISPLAY copper_limit
Line A — Copper cables lose signal strength quickly, so their limit is only about 100 metres, not 100 kilometres. It should be copper_limit ← 100 metres.
Question 2
A attenuation = signal grows
B cause ← distance travelled
C fix ← add a repeater
Line A — Attenuation is the signal getting weaker as it travels along a cable. It does not grow. It should be attenuation = signal weakens.
Question 3
A signal ← 100
B distance ← 90
C loss ← distance * 1
D signal ← signal + loss
E DISPLAY signal
Line D — Attenuation means the signal gets weaker over distance, so the loss must be subtracted. It should be signal ← signal - loss.
Question 4
A IF cable = fibre THEN
B limit ← 50 metres
C ELSE
D limit ← 100 metres
E ENDIF
Line B — Fibre has a much longer limit than copper. The fibre branch should use a large limit such as 40000 metres. A tiny limit like 50 metres is shorter than the copper limit.
Question 5
A length ← 150
B IF length > 100 THEN
C REMOVE repeater
D ENDIF
Line C — When a copper cable is longer than its limit, a repeater is added to boost the signal. Removing one would leave the signal weaker. It should be ADD repeater.

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