Context in The OSI Model
PDUs are relevant in relation to each of the first 4 layers of the OSI model as follows:
- The Layer 1 (Physical Layer) PDU is the bit or, more generally, symbol (can also been seen as "stream")
- The Layer 2 (Data Link Layer) PDU is the frame
- The Layer 3 (Network Layer) PDU is the packet
- The Layer 4 (Transport Layer) PDU is the segment for TCP, or the datagram for UDP
Given a context pertaining to a specific OSI layer, PDU is sometimes used as a synonym for its representation at that layer.
Read more about this topic: Protocol Data Unit
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