A receiving host computes the checksum on a frame and determines that the frame is damaged. The frame is then discarded. At which OSI layer did this happen?

Question

Options

• Asession
• Btransport
• Cnetwork
• Ddata link
• Ephysical

• A
  3% (1)
• B
  6% (2)
• C
  3% (1)
• D
  87% (27)

Explanation

Data Link (Layer 2) is responsible for framing, physical addressing, and error detection via checksums (such as CRC - Cyclic Redundancy Check). When a frame arrives and its computed checksum doesn't match the expected value, the Data Link layer identifies it as corrupted and discards it - exactly what the question describes.

Why the distractors are wrong:

• Physical (E): Handles raw bit transmission; it has no concept of frames or checksums - it just moves electrical/optical signals.
• Network (C): Deals with packets and logical addressing (IP); error checking at this layer operates on packets, not frames.
• Transport (B): Handles end-to-end delivery of segments (TCP/UDP); it works above the frame level.
• Session (A): Manages communication sessions between applications - far too high to be involved in frame-level error detection.

Memory tip: Think of "Data Link = Damaged frames Dropped." The word frame is your key signal - frames are the PDU (Protocol Data Unit) of Layer 2, so any question mentioning frame-level checksum, CRC, or MAC addresses points to Data Link.

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