A Protocol Data Unit (PDU) is like a container of signals, while a frame is the physical package that carries the PDU. The distinction lies in their relationship and how they're used in communication protocols.
PDU (Protocol Data Unit)
A PDU can be understood as a logical unit of data that carries the actual information or signals. Think of it as the content inside an envelope. Key characteristics include:
- Signal Container: A PDU is designed to hold signals that need to be transmitted.
- Multiple Frames: A single PDU can potentially span across multiple frames. This means that the logical data (PDU) might be too large for one physical transmission unit (frame) and may need to be split.
- Content: PDUs contain the actual data that an application is trying to send or receive.
Frame
A frame is the actual physical transmission unit, like the envelope that carries the content. Here’s a breakdown of its features:
- Physical Container: A frame is the tangible container that is transmitted over a communication medium (like a wire or wireless channel).
- Contains PDUs: A frame carries the PDUs.
- One or More PDUs: A frame can contain one or more PDUs, or a portion of a PDU that is too large to fit in a single frame.
- Transmission: Frames are the actual units transmitted over a physical medium.
Relationship Between PDUs and Frames
The relationship between PDUs and frames can be summarized as:
| Feature | PDU | Frame |
|---|---|---|
| Concept | Logical data unit | Physical transmission unit |
| Function | Holds signals/data | Carries data over the medium |
| Size | Variable, can span frames | Fixed or variable based on the protocol |
| Relationship | Can be contained within multiple frames | Contains one or more PDUs |
Examples:
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Multiple Frames for One PDU: Imagine sending a large image. The image data (PDU) might be so large that it needs to be broken down and transmitted in several frames.
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One Frame for One PDU: The reference states that in NI-XNET for CAN and LIN protocols, there's typically a one-to-one relationship between frames and PDUs. So one frame carries one complete PDU.
Key Takeaway
The crucial difference is that PDUs are about what data is being communicated, and frames are about how that data is physically transmitted. A PDU is the message content, and a frame is the transmission envelope.
