Layering and X.25 basics: identify the one incorrect statement about X.25 layers, OSI layer names, and error handling responsibilities.
-
AThe CCITT Recommendation X.25 specifies three layers of communication: physical, link, and network.
-
BThe second layer of communication is the data-link layer.
-
CErrors in the physical layer can be detected by the data-link layer.
-
DThe fourth layer, in reference to the OSI model, is the session layer.
-
ENone of the above
Answer
Correct Answer: The fourth layer, in reference to the OSI model, is the session layer.
Explanation
Introduction: Networking exams often test whether you can keep straight the relationships among the OSI model, X.25’s three-layer view (Physical, Data Link, Packet/Network), and which layer performs which function (like error detection). This question asks you to pick the single incorrect statement from a set that otherwise sounds plausible.
Given Data / Assumptions:
- X.25 defines a three-layer architecture: Physical (X.21), Data Link (LAPB), and Network (PLP).
- OSI layers from bottom to top are: Physical (1), Data Link (2), Network (3), Transport (4), Session (5), Presentation (6), Application (7).
- Typical link-layer frames include error-detection fields (e.g., CRC) to catch physical-layer errors.
Concept / Approach: Check each statement against canonical definitions. Confirm that X.25 indeed references three layers; verify that layer 2 is Data Link; remember that the OSI 4th layer is Transport, not Session; and recall that data-link error detection is explicitly designed to catch bit-level corruption introduced on the physical medium.
Step-by-Step Solution:
1) Option A: Correct — X.25 uses a three-layer stack up to the network level.2) Option B: Correct — layer 2 is Data Link.3) Option C: Correct — CRC/FCS in Data Link detects physical-layer bit errors.4) Option D: Incorrect — in OSI, layer 4 is Transport, while Session is layer 5.Verification / Alternative check: Any standard OSI table will show Transport at layer 4 and Session at layer 5, confirming that calling the fourth layer “Session” is wrong.
Why Other Options Are Wrong (as the incorrect choice):
- A, B, C are accurate statements and align with textbook definitions.
- E (None of the above) would claim there is no incorrect option, which is false because D is incorrect.
Common Pitfalls: Mixing up Session and Transport layer names; assuming that “None of the above” is a safe pick when one statement is clearly wrong.
Final Answer: The fourth OSI layer is Transport, not Session, so the incorrect statement is “The fourth layer … is the session layer.”