OCR J277 Mark Scheme - Network Layers

OCR	GCSE (9-1) Computer Science Mark Scheme J277/01: Unit 1.3 Network Layers

Question	Answer	Marks	Guidance
1a	A division of network functionality into self-contained sections.	1
1b	Layer 3: Transport (1) Layer 1: Link (1)	2	Examiner's Note: J277 specifies the 4-layer model: Application, Transport, Internet, Link. Accept: "Network Access" for Link Layer. Reject: "Network Layer" (This is OSI model terminology, though often confused. Stick to J277 terms: Internet Layer).
2	HTTP -> Application Layer (1) TCP -> Transport Layer (1) Ethernet -> Link Layer (1)	3	If a student draws multiple lines from one box, award 0 for that item.
3	Benefit 1: Decomposition / Simplification It breaks network communication into smaller, manageable pieces (1)... so developers can concentrate on just one area without needing to understand the whole system (1). Benefit 2: Interoperability / Hardware Independence Layers are self-contained (1)... so changes in one layer (e.g., upgrading hardware) do not affect other layers (e.g., the software still works) (1).	4	Examiner's Note: "Self-contained" is a key term. Example for Benefit 2: "You can switch from Wired to Wi-Fi (Link layer) and your Browser (Application layer) doesn't need to change."
4a	Encapsulation: Extra information / Headers are added to the data. (1) Detail: e.g. IP addresses are added at the Internet layer / MAC addresses at the Link layer. (1)	2	Key Term: Encapsulation (though not strictly required if the description is good). Reject: "The data gets bigger" (Too vague without saying what is added).
4b	Decapsulation: The headers / extra information is removed. (1) Flow: The original data is passed up to the application. (1)	2	Examiner's Note: Must imply the reverse of part (a).
5	Any 4 logical points: Layers are independent of each other. The Link Layer only deals with physical connections / MAC addresses. It does not care about the data payload / content being carried. Because of protocols/standards, data passed down from the Application layer will always be in a format the Link layer understands. This allows hardware from different manufacturers to work with software from different developers (Interoperability).	4	Examiner's Note: This question tests the ultimate goal of layering: different manufacturers working together. Award marks for terms: Independent, Standard, Protocol, Interoperability.