← IGCSE Computer Science Paper 1

Why packets can take different routes

Packets from one message can take different routes because each router picks the best available path at that moment, avoiding busy or broken links. Routes may take different lengths of time.

Part of: IGCSE Computer Science 0478 — 2.1 Data Transmission Types and Methods — Module 18, ACARA Digital Technologies Years 7–8 — Wired and Wireless Networks — Module 48, ACARA Digital Technologies Years 9–10 — Networked Systems and Access Control — Module 42

Understanding Why packets can take different routes in IGCSE Computer Science Paper 1

What it does: If a link has failed, the router picks a working path, which is why packets can be sent along different routes. Routers decide the path for each packet as it arrives, using the conditions on the network at that moment. In packet switching each packet is routed on its own, so packets from one message may travel by different paths.

Common mistakes: A router picks the best available path at that moment, not the worst. Line 2 should be: router picks best path. The router should avoid a broken link and use link B. The condition should be IF link A is broken, otherwise the ELSE branch would use a broken link.

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This module's key terms are also in a printable crossword (Modules 121–130) and word search (Modules 121–141).

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Learning

Duo

Why can two packets from the same message take different routes?

Each router picks the best available path at that momentThe sender gives every packet a fixed route in advance

Matches

All packets in a message must follow the same route

↓

Packets from one message can take different routes, because each router chooses the best available path at that moment.

Applying

Meanings

Term

different routes

Meaning

Packets from one message can take different routes because each router picks the best available path at that moment, avoiding busy or broken links. Routes may take different lengths of time.

Practice

FOR packetNumber ← 1 TO 4

Testing

Gaps

IF routeBusy = THEN

Check Answer

Spots

1FOR each packet in message

2 router picks worst path

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Quick Questions

1. Why can two packets from the same message take different routes?

A. Each router picks the best available path at that moment ✓

B. The sender gives every packet a fixed route in advance

Routers decide the path for each packet as it arrives, using the conditions on the network at that moment.

2. A router finds that one route is very busy (congested). What does it do with the next packet?

A. Always sends it along the busy route anyway

B. Sends it along a less busy route ✓

Routers avoid congested routes where they can, which helps the packets arrive faster.

3. A cable on one route is broken. What happens to the packets that are still to be sent?

A. Routers send them along a different working route ✓

B. The whole message is lost and must be sent again

A broken link is avoided. Packets are sent by another path, so the message can still get through.

4. True or false: every packet in a message must travel along the same route.

A. True - all packets must follow the first packet

B. False - packets can take different routes ✓

In packet switching each packet is routed on its own, so packets from one message may travel by different paths.

5. Packet 1 takes a short route and packet 2 takes a longer route. What is likely to be true?

A. The two packets may take different lengths of time to arrive ✓

B. Both packets always arrive at exactly the same time

Different routes can take different lengths of time, which is why packets may arrive out of order.

Match the Pairs

route = best_available_path() → A router picks the best path open at that moment for each packet

if link_busy: choose_other_route() → A congested route is avoided and the packet is sent another way

A broken link is avoided, so the router sends the packet along a working route instead → if link_broken: use_working_route()

Different routes can take different lengths of time, so packets may arrive at different times → arrival_time = sum(route_delays)

All packets in a message must follow the same route → Packets from one message can take different routes, because each router chooses the best available path at that moment.

A router keeps sending packets down a busy route → A router avoids busy or congested routes and picks a better available path, so the packets are not stuck in a queue.

Key Term

different routes

Packets from one message can take different routes because each router picks the best available path at that moment, avoiding busy or broken links. Routes may take different lengths of time.

Fill in the Blanks

1. IF routeBusy = TRUE THEN route ← "B" ENDIF

Hint: Type the Boolean value that means the route is congested, so the router avoids it.

2. IF linkBroken = TRUE THEN route ← "C" ENDIF

Hint: Type the comparison operator that checks whether the link is broken, so the packet is sent another way.

3. IF timeA < timeB THEN route ← "A" ENDIF

Hint: Type the comparison operator that picks route A when it takes less time than route B.

4. IF linkBroken = TRUE THEN route ← "B" ENDIF

Hint: Type the keyword that follows the IF condition and begins the action to take when the link is broken.

5. IF routeBusy = TRUE THEN route ← "B" ENDIF

Hint: Type the keyword that closes the IF statement after the router has chosen a different route.

Spot the Error

Question 1

A FOR each packet in message

B router picks worst path

C IF path is busy

D choose another path

Line B — A router picks the best available path at that moment, not the worst. Line 2 should be: router picks best path.

Question 2

A router receives packet

B IF link A is working

C send via link B

D ELSE send via link A

Line B — The router should avoid a broken link and use link B. The condition should be IF link A is broken, otherwise the ELSE branch would use a broken link.

Question 3

A route A: 3 routers, 40 ms

B route B: 5 routers, 90 ms

C packet 1 on A arrives last

D packet 2 on B arrives later

Line C — Route A takes less time (40 ms against 90 ms), so the packet on A arrives first. Different routes take different lengths of time.

Question 4

A message split into packets

B packet 1 goes via route A

C packet 2 goes via route B

D all packets must use route A

Line D — Packets from the same message can take different routes. They do not all have to use the same one. The line should say routes may differ per packet.

Question 5

A route A is congested

B router checks route B

C router sends packet via A

D packet avoids the busy route

Line C — Route A is congested (busy), so the router should avoid it and send the packet via route B.

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