Identify PDU in each layer of OSI?

In the OSI model, the Protocol Data Unit (PDU) varies by layer. At the Application layer (Layer 7), the PDU is called a message. In the Transport layer (Layer 4), it is referred to as a segment (or datagram in connectionless protocols). The Network layer (Layer 3) uses packets, while the Data Link layer (Layer 2) uses frames. Finally, at the Physical layer (Layer 1), the PDU is a bit.