OSI Protocol Layers: Identifying the Incorrect Layer–Function Pair
To determine which pair is incorrect, map each OSI layer (and the common sub-layer role of MAC) to the standard networking function it provides:
- Network layer → Routing: Routing is a classic network-layer responsibility (e.g., deciding paths across networks).
- Data Link Layer → Bit synchronization: Synchronization of bits/frames on a link is typically associated with the data-link layer’s framing and physical-link coordination.
- Transport layer → End-to-end process communication: The transport layer provides end-to-end delivery between processes/hosts (e.g., segmentation, reassembly, reliability/flow control).
- Medium Access Control (MAC) sub-layer → Channel sharing: MAC controls who transmits over a shared medium (channel access).
At face value, this looks like a “trick question” where the only questionable pair is (ii): Data Link Layer and Bit synchronization. In many canonical textbook mappings, bit-level synchronization is attributed to the Physical layer (layer 1), while the data-link layer is more about framing, link access, and error detection/correction over a hop.
So the incorrect pair is: (ii) Data Link Layer and Bit synchronization.
A concise way to justify:
- Physical layer handles raw bit transmission characteristics and timing/synchronization.
- Data link layer organizes bits into frames and manages link-level delivery (framing, MAC addressing on a hop, and error detection).
Exam-tip: OSI “synchronization” wording
If an option explicitly says “bit synchronization,” many syllabi expect this to be Layer 1 (Physical), not Layer 2 (Data Link). If it said “frame synchronization,” Layer 2 becomes more plausible.
How each option aligns with OSI responsibilities
Below is a direct alignment (and the likely misalignment) using the usual OSI role descriptions.
Key terms you should know for this question:
- Routing
- Framing
- End-to-end process communication
- MAC
Step-by-step elimination to find the incorrect pair
- 1Step 1
Network layer is responsible for inter-network forwarding decisions (routing/path selection). Therefore (i) is correct.
- 2Step 2
Transport provides end-to-end communication between processes, including segmentation and (optionally) reliability/flow control. Therefore (iii) is correct.
- 3Step 3
MAC regulates access to a shared medium (who can transmit when). Therefore (iv) is correct.
- 4Step 4
Bit-level timing/synchronization is generally treated as a Physical-layer concern (raw bit transmission). Data Link is primarily framing and link-level delivery. Therefore (ii) is the incorrect pair.
Where synchronization and delivery happen in the OSI stack (conceptual flow)
Physical transmission
Layer 1Convert frames into raw signal; manage bit-level timing/synchronization."
Framing & hop delivery
Layer 2Add frame boundaries, do link-level error detection, deliver across one hop."
Routing across networks
Layer 3Choose routes and forward packets between networks."
End-to-end transport delivery
Layer 4Deliver data to the correct application/process end-to-end."
Common misconceptions
Knowledge Check
Which pair is INCORRECT?