In the classic X.25 protocol stack, how many OSI layers does the standard encompass directly (considering the physical, data link, and packet layers)?

Introduction / Context:
X.25 is an early packet-switched networking standard that predated today’s IP-based Internet. It defined how user equipment connects and exchanges packets over public data networks. Understanding where X.25 sits in the OSI model clarifies its scope and how it relates to higher-layer services.

Given Data / Assumptions:

• X.25 comprises: physical interface, link-layer control, and packet layer protocol.
• OSI layering reference is assumed.
• We count the layers that X.25 directly specifies.

Concept / Approach:
X.25 spans three OSI layers: Layer 1 (physical interface, often via X.21 or V-series), Layer 2 (LAPB — Link Access Procedure, Balanced), and Layer 3 (Packet Layer Protocol for virtual circuits). It does not define transport, session, presentation, or application functions.

Step-by-Step Solution:
Map X.25 physical attachment to OSI Layer 1.Map LAPB link control to OSI Layer 2.Map packet layer virtual circuits to OSI Layer 3.Total layers covered: three.

Verification / Alternative check:
Classic diagrams show X.25’s packet layer offering a network service to higher layers while relying on LAPB and a carrier physical interface below.

Why Other Options Are Wrong:
two: Omits one of the layers X.25 specifies.

seven or six: Overstate X.25’s scope beyond the network service boundary.

None of the above: Incorrect because “three” is correct.

Common Pitfalls:
Confusing X.25 end-to-end application services with its network-service role; transport and above are out of scope for the standard.

Final Answer:
three