According to CCITT (now ITU-T) Recommendation X.25, the public packet-switched interface is defined in terms of which three communication layers?

Introduction / Context:
X.25 is a historic standard for public packet-switched networks. It predates widespread Internet use and describes how user equipment connects to a carrier’s packet network. Understanding which layers X.25 specifies clarifies its position within layered architectures.

Given Data / Assumptions:

• We are referring to the original CCITT X.25 recommendations.
• The focus is on the user–network interface over public packet services.

Concept / Approach:
X.25 defines three layers: the physical layer (electrical/connector characteristics), the data link layer (LAPB for reliable link control), and the network layer (packet layer protocol for virtual circuits). Higher OSI layers (session, presentation, application) are outside X.25’s scope.

Step-by-Step Solution:

Verification / Alternative check:
Documentation shows X.21/X.21bis for physical, LAPB (link) as Q.922-like reliability, and the packet layer protocol at network level for virtual calls and data transfer.

Why Other Options Are Wrong:

• Application/presentation/session: too high in the OSI stack.
• Session/transport/network: includes layers not defined by X.25.
• Data link/network/transport: transport is not part of X.25 at the UNI.
• None: incorrect because the correct trio is well known.

Common Pitfalls:
Assuming every legacy standard maps one-to-one to OSI seven layers; forgetting LAPB’s role as the reliable link underpinning the packet layer in X.25 networks.

Final Answer:
Physical, data link, and network