Why do we carry out error detection and/or correction at Layer 2, given that it is carried out at Layers 3 and 4 as well?
Try an answer before revealing the guidance below.
Key Concepts
- Hop-by-hop vs end-to-end
- Local retransmission
- Error-prone links
- CRC strength
Answer Approach
- Compare where each layer detects an error: per link or end to end.
- Consider how far a corrupted frame would otherwise travel.
- Consider how fast a local retransmission can fix it.
Full Answer
Answer status: Draft answer (unofficial). Revision notes, not an official marking scheme.
Layer 2 detects errors hop by hop, right at the link where they happen. A corrupted frame can be dropped (and on lossy links such as wireless, retransmitted locally and quickly) instead of being carried across the rest of the path and only recovered by a slow end-to-end retransmission. Link-layer CRCs are also stronger than the simple checksums at Layers 3 and 4, while the higher-layer checks still catch errors introduced inside routers or on links without error control.
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