What is the PDU of the Data Link Layer?

• A. bits
• B. packets
• C. frames
• D. segments

Answer: (C)

PDUs are the data units a layer passes to the layer below after encapsulation. For the Data Link Layer, the PDU is a frame. This layer is responsible for moving data across a single link, so it takes the Network Layer packet (the SDU from above) and encapsulates it with a header and trailer to form a frame. The header carries addressing and control information (like source and destination MAC addresses and the frame type), while the trailer provides error detection (such as a CRC). This framing is what gets transmitted over the physical medium and then reassembled at the other end.

Bits are the PDU of the Physical Layer, packets are the PDU of the Network Layer, and segments are the PDU of the Transport Layer. So the frame is the appropriate term for the Data Link Layer.