← WA Computer Science ATAR Unit 1

Purpose of layers in a network model

Splits network communication into layers that each do one job and talk only to the layers above and below, so vendors interoperate, layers change alone and faults are easier to find.

Part of: WA Computer Science ATAR Year 11 — Network Communications — Module 24

Understanding Purpose of layers in a network model in WA Computer Science ATAR Unit 1

What it does: Because layers only talk to their neighbours through set interfaces, one layer can be replaced while the layers above stay the same. Separation of concerns means each layer has a small, focused job, so it can be designed, tested and improved independently. A layered model limits communication to neighbouring layers, which keeps each layer's job separate and simple. Agreed layer standards give interoperability between vendors, so equipment from different makers can communicate.

Common mistakes: Each layer does one job. The application layer serves web pages and must not also do routing, which belongs to the network layer. It should be app = "serves web pages". The core idea of a layered model is separation of concerns: each layer has exactly one job. jobs_per_layer should be 1, not 6.

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This module's key terms are also in a printable crossword (Modules 315–470) and word search (Modules 100–470).

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Learning
Duo
Which statement describes how a layered network model splits up communication?
One layer does every job so the data only has to be handled onceEach layer does one job and only talks to the layers directly above and below it
Matches
Application layer sets the voltage on the cable
↓
This gives a layer a job that belongs to a different layer. Voltage and signals are handled by the lowest layer, not the application layer.
Applying
Meanings
Term
network layer model
Meaning
Splits network communication into layers that each do one job and talk only to the layers above and below, so vendors interoperate, layers change alone and faults are easier to find.
Practice
FOR each layer IN network_model
Testing
Gaps
Each layer does job
Check Answer
Spots
2net = "routes packets"
3app = "pages and routing"

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Quick Questions

1. Which statement describes how a layered network model splits up communication?
A. Each layer does one job and only talks to the layers directly above and below it ✓
B. One layer does every job so the data only has to be handled once
A layered model separates concerns: each layer has a single job and uses only the layer below it and serves the layer above it.
2. A company makes network cards and another company makes routers. Why can they work together?
A. Both companies must buy all their parts from the same vendor
B. Both follow the same layer rules, so their products are interoperable ✓
Standard layers define what each layer must do and how it talks to its neighbours, so different vendors' products can work together.
3. A new wireless standard is invented. What happens to the layers above it, such as the application layer?
A. They keep working unchanged because only the lower layer is replaced ✓
B. They must all be rewritten to match the new standard
Because layers only talk to their neighbours through a fixed interface, one layer can be changed without affecting the others.
4. A student can load a school website on a wired connection but not on Wi-Fi. How do layers help the technician?
A. The fault can be narrowed down to one layer, such as the physical layer ✓
B. The technician must check every part of the whole network at once
Layers make troubleshooting easier because a fault can be isolated to the layer responsible for that job.
5. Which is a benefit of the separation of concerns in a network model?
A. Every layer must be built by the same team at the same time
B. Each layer can be designed and tested on its own ✓
Separation of concerns means each layer has a small, focused job, so it can be designed, tested and improved independently.

Match the Pairs

Layer 4: Transport -> Layer 3: Network → Each layer only passes data to the layer directly above or below it, never skipping across the model
Router (vendor A) <-> Router (vendor B) → Devices from different vendors can work together because they follow the same layer rules
Changing one layer does not affect the others, so Wi-Fi can be swapped for Ethernet without changing the web browser → Link layer: Wi-Fi -> Ethernet Application layer: browser unchanged
Layers make troubleshooting easier because a fault can be tracked down one layer at a time → ping 8.8.8.8 works, website will not load -> check upper layers
Application layer sets the voltage on the cable → This gives a layer a job that belongs to a different layer. Voltage and signals are handled by the lowest layer, not the application layer.
Changing the Transport layer means rewriting every other layer → This is wrong because layers are independent. Changing one layer should not force changes in the other layers.

Key Term

network layer model
Splits network communication into layers that each do one job and talk only to the layers above and below, so vendors interoperate, layers change alone and faults are easier to find.

Fill in the Blanks

1. Each layer does one job
Hint: Type how many jobs each layer has, written as a word
2. Layer N talks to layers N+1 and N-1
Hint: Type the expression for the layer directly below layer N
3. IF both follow the layer standard THEN devices interoperate
Hint: Type the verb for devices from different vendors working together
4. layer3 = newVersion layers 1, 2 and 4 stay unchanged
Hint: Type the word describing the state of the other layers after one layer is replaced
5. FOR EACH layer IN stack test(layer) IF fault THEN fix(layer)
Hint: Type the keyword that links the loop variable to the collection being checked one layer at a time

Spot the Error

Question 1
A link = "sends frames"
B net = "routes packets"
C app = "pages and routing"
Line C — Each layer does one job. The application layer serves web pages and must not also do routing, which belongs to the network layer. It should be app = "serves web pages".
Question 2
A jobs_per_layer = 6
B app = "web requests"
C net = "routes packets"
Line A — The core idea of a layered model is separation of concerns: each layer has exactly one job. jobs_per_layer should be 1, not 6.
Question 3
A def transport_send(data):
B return link_send(data)
C print(transport_send("hi"))
Line B — A layer talks only to the layer directly below it. The transport layer must hand data to the network layer with net_send(data), not skip down to the link layer.
Question 4
A cisco_net = "IP"
B juniper_net = "private format"
C print(cisco_net == juniper_net)
Line B — Interoperability works because vendors follow the same layer standard. If one vendor used a private format the comparison would print False and the devices could not communicate. It should be juniper_net = "IP".
Question 5
A link = "Wi-Fi"
B net = "IP"
C app = "HTTP"
D net = "Ethernet"
Line D — Swapping Wi-Fi for Ethernet should change only the link layer, leaving the other layers untouched. Ethernet is a link layer technology, so the line should be link = "Ethernet".

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