Question

On which of the following OSI layer is checksum not operable ?

• A. Session layer
• B. Transport layer
• C. Network layer
• D. Data link layer

Hint:

Think of the OSI layers' jobs: Session layer is the "dialogue manager". Transport and below are the "delivery services", and delivery services need to check if the package (data) is damaged.

Answer 1

The Correct Option is A

Step 1: Review the functions of the OSI layers regarding error detection.

(D) Data Link Layer (Layer 2): Uses a Frame Check Sequence (FCS), often a Cyclic Redundancy Check (CRC), to detect errors in frames.
(C) Network Layer (Layer 3): The IPv4 header includes a checksum to check for errors in the header itself.
(B) Transport Layer (Layer 4): Both TCP and UDP use checksums to verify the integrity of the header and the data payload.
(A) Session Layer (Layer 5): Its primary role is to manage, maintain, and terminate sessions (dialogues) between applications. It is not directly concerned with bit-level error detection of the data payload; that is handled by the layers below it.

Step 2: Conclude which layer does not use a checksum. Error detection via checksums is a key function of the Data Link, Network, and Transport layers. The Session layer does not operate a checksum.