09. OSI and TCP/IP layers - revision mindmap
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Purpose of layers
- Defined responsibilities
- Uses service below; provides service above
- Protocol communication between corresponding entities
- Standard interfaces enable interoperability and modular change
OSI seven layers
- Application: network services
- Presentation and Session: representation and dialogue
- Transport: process delivery; Network: addressing/routing
- Data link: frames; Physical: signals
School TCP/IP five layers
- Application combines OSI upper three
- Transport and Network/Internet
- Data link and Physical remain separate
- Four-layer references combine the lower pair; name the model
Encapsulation
- Application message
- TCP segment with ports/sequence information
- IP packet with host/network addressing
- Link frame with local addressing/trailer → signals
Journey through a router
- Remove incoming frame
- Read/process IP information and choose route
- Construct outgoing frame
- Ordinary routing preserves endpoint IPs; link addresses change
Worked example & exam traps
- Web data → TCP segment → IP packet → link frame → signals.
- Router removes old frame and builds a frame for the next link.
- Without NAT, endpoint IPs remain; local link addresses change.
- Avoid: remote server MAC needed locally; all transport protocols reliable
Sources and reading
This map condenses Chapter 09 and its source trail. Use the self-learning reading guide for the corresponding VJC pages and A notes. It does not add topics to the stated promo scope.