Purpose of layers in a network model

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Quick Questions

Shade in the A or B circle for each question.

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

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

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

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

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

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1Layer 4: Transport -> Layer 3: Network		AEach layer only passes data to the layer directly above or below it, never skipping across the model
2Router (vendor A) <-> Router (vendor B)		BLink layer: Wi-Fi -> Ethernet Application layer: browser unchanged
3Changing one layer does not affect the others, so Wi-Fi can be swapped for Ethernet without changing the web browser		CDevices from different vendors can work together because they follow the same layer rules
4Layers make troubleshooting easier because a fault can be tracked down one layer at a time		Dping 8.8.8.8 works, website will not load -> check upper layers
5Application layer sets the voltage on the cable		EThis is wrong because layers are independent. Changing one layer should not force changes in the other layers.
6Changing the Transport layer means rewriting every other layer		FThis gives a layer a job that belongs to a different layer. Voltage and signals are handled by the lowest layer, not the application layer.

Definitions

network layer model

Fill in the Blanks

Complete each line of code by filling in the missing parts.

Each layer does ___ job

Layer N talks to layers N+1 and N__

IF both follow the layer standard THEN devices ____________

layer3 = newVersion layers 1, 2 and 4 stay _________

FOR EACH layer __ stack test(layer) IF fault THEN fix(layer)

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A link = "sends frames"

B net = "routes packets"

C app = "pages and routing"

Question 2

A jobs_per_layer = 6

B app = "web requests"

C net = "routes packets"

Question 3

A def transport_send(data):

B return link_send(data)

C print(transport_send("hi"))

Question 4

A cisco_net = "IP"

B juniper_net = "private format"

C print(cisco_net == juniper_net)

Question 5

A link = "Wi-Fi"

B net = "IP"

C app = "HTTP"

D net = "Ethernet"

Answer Key

Purpose of layers in a network model

Section A: Quick Questions

A — Each layer does one job and only talks to the layers directly above and below it

B — Both follow the same layer rules, so their products are interoperable

A — They keep working unchanged because only the lower layer is replaced

A — The fault can be narrowed down to one layer, such as the physical layer

B — Each layer can be designed and tested on its own

Section B: 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.

Section C: Definitions

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.

Section D: Fill in the Blanks

Each layer does one job

Layer N talks to layers N+1 and N-1

IF both follow the layer standard THEN
    devices interoperate

layer3 = newVersion
layers 1, 2 and 4 stay unchanged

FOR EACH layer IN stack
    test(layer)
    IF fault THEN fix(layer)

Section E: Spot the Error

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".

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.

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.

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".

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".