OSI Layer Data Unit Terms | Generated by AI
Question: In the OSI Reference Model (OSI/RM), what is the term for the group of bits passed between entities at different layers?
Answer:
The correct answer is A. 数据单元 (Data Unit).
Detailed explanation in English:
In the OSI Reference Model, communication between layers uses specific terminology:
-
Data Unit (数据单元): General term for the block of bits (data + possibly control information) that is passed between entities in adjacent layers (i.e., from layer N to layer N-1 or vice versa).
→ This is the broadest and correct term for the question. -
Protocol Data Unit (PDU) (协议数据单元): The specific name for the data unit when it is exchanged horizontally between peer entities at the same layer on different systems, according to the rules (protocol) of that layer.
-
Service Data Unit (SDU) (服务数据单元): The pure user data that a layer N entity receives from the layer above (layer N+1) before it adds its own header/trailer.
Key distinction in OSI terminology:
| Direction / Relationship | Term used | Meaning |
|---|---|---|
| Between adjacent layers (N ↔ N-1) | Data Unit (数据单元) | General term for bits passed up/down the stack |
| Between peer entities (same layer, different systems) | Protocol Data Unit (PDU) | Data unit exchanged according to layer protocol |
| From upper layer to current layer | Service Data Unit (SDU) | User data delivered via service interface |
The question specifically asks about bits passed between entities of each layer (各层的实体之间), which refers to the vertical communication between adjacent layers inside one system — therefore the correct general term is 数据单元 (Data Unit).
Why C. 服务原语 (Service Primitive) is wrong:
Service primitives (request, indication, response, confirm) are the abstract interactions at the service boundary (between a service user and service provider), not the actual bit groups being transferred.
Why D. 服务访问点 (Service Access Point - SAP) is wrong:
SAP is the logical interface point between layers (address-like identifier), not the data itself.
Why B. 接口 (Interface) is wrong:
Interface is the boundary or set of rules, not the data block being passed.
Summary – correct mapping:
- Bits passed between layers (adjacent) → Data Unit
- Bits passed between peer protocol entities → PDU
- Bits passed between service user and service provider → SDU (from upper to lower) or encapsulated PDU (from lower to upper)
Therefore the correct answer for this question is A. 数据单元.
References:
- None needed (standard OSI reference model terminology)