In telecommunications terminology, what does the acronym ISDN expand to in full?

Introduction / Context:
Many networking acronyms compress long technical names into short forms. Correctly expanding them matters in exams and in the field because the wording often hints at the technology’s goals. ISDN is a classic circuit-switched digital access technology that aimed to carry voice, data, and signaling in an integrated manner over the public telephone network.

Given Data / Assumptions:

• The acronym is ISDN and the domain is telecom/access networks.
• ISDN historically provided Basic Rate Interface (BRI) and Primary Rate Interface (PRI).
• The question tests expansion accuracy, not deep technical operation.

Concept / Approach:
ISDN emphasizes “Integrated Services” (voice + data + signaling) delivered over a “Digital Network.” The canonical expansion found in standards and textbooks is “Integrated Services Digital Network.” Any variation that substitutes “Indian,” “Intelligent,” or “Data” in place of “Digital” and “Services” is incorrect for this acronym.

Step-by-Step Solution:

Verification / Alternative check:
Technical references for BRI (2B + D channels) and PRI (e.g., 23B + D for T1, 30B + D for E1) clearly label ISDN as Integrated Services Digital Network, validating the choice beyond rote memorization.

Why Other Options Are Wrong:

• Indian Standard Digital Network: incorrect geography-specific wording; not the standard term.
• Intelligent Services Digital Network: nonstandard phrasing; “Integrated” is correct.
• Integrated Services Data Network: wrong substitution; the accepted expansion uses “Digital,” not “Data.”
• None of the above: incorrect because the proper expansion is provided in option (b).

Common Pitfalls:
Mixing up “Digital” with “Data,” or assuming “I” is “Indian” due to regional syllabi; forgetting that the purpose of ISDN was to integrate multiple services over one digital access line.

Final Answer:
Integrated Services Digital Network