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1. Communication layer service description. What service it provides. What are the goals.
1.1 From service requirements (functionalities) list. Use cases are useful here.
1.2 Define used N-1 layer.
2. Find out communicating entities and their associations.
- Thinking communication needed per requirement (use case) and presenting it in UML sequence diagram identifies needed entities.
2.1 Create layer model architecture: N layer entities and used N-1 layer and user N+1 (in abstract form)
3. Define interfaces with message names for entities. Sequence diagram here is a good information source again.
4. Define state machines for entities to establish message exchange order and define the lifespan of entities.
5. For interfaces define abstract messages.
6. For abstract PDU message (messages that are really transferred, horizontal) define concrete message encoding rules.
Found definitions are moved to specification. Note, scenarios, especially sequence diagrams are not definitions, but could be added to appendix part of specification for their usefulness in understanding the communication.