In digital communication terminology, the abbreviation ISDN stands for which of the following?

Introduction / Context:
ISDN is an important term from the evolution of digital telecommunication networks. It refers to a set of standards that integrate voice, data and other services over traditional telephone networks using digital signals. Although many newer technologies have since appeared, ISDN still features in networking and communication theory courses, and its full form is often asked as a direct question. Knowing the correct expansion helps you connect historical and modern communication systems.

Given Data / Assumptions:

• The abbreviation under discussion is ISDN.
• The context is digital network and telecommunication services.
• Options provide several expansions, including International, Indian, Integrated and Internal Service Digital Network plus another variant.
• We assume standard usage as defined by telecommunication standards bodies.

Concept / Approach:
ISDN stands for Integrated Services Digital Network. The word integrated indicates that multiple services, such as voice, data and video, are combined over a single digital network. Services stresses that these include different types of communication services rather than just one. Digital refers to the use of digital signalling instead of older analogue methods, and network refers to the telecommunication infrastructure itself. Alternatives like International Service Digital Network or Indian Service Digital Network do not match official standards and are not used in technical literature. Therefore, the correct expansion is Integrated Services Digital Network.

Step-by-Step Solution:

Verification / Alternative check:
Telecommunications standards documents and networking textbooks consistently expand ISDN as Integrated Services Digital Network. They explain how ISDN provides bearer services and supplementary services for voice and data over B channels and D channels. No authoritative source uses International Service Digital Network or Indian Service Digital Network as official meanings. Examination preparation guides for network certifications and competitive exams also list ISDN with the same expansion. This strong consensus confirms that Integrated Services Digital Network is the correct answer.

Why Other Options Are Wrong:

• International Service Digital Network: Sounds plausible but is not the recognised expansion of ISDN.
• Indian Service Digital Network: Incorrect; ISDN is a global standard, not a country specific network name.
• Internal Service Digital Network: Not used in official telecommunication terminology.
• Internet Services Distribution Network: A made up phrase that does not correspond to ISDN in any standard.

Common Pitfalls:
Because ISDN is a network that can operate internationally, some learners mistakenly think the I stands for International. Others guess Indian in local exam contexts. To avoid these errors, focus on the key concept of integrating multiple services. Whenever you see ISDN, remember the phrase Integrated Services Digital Network and the idea of combining voice and data over digital lines. This mental link ensures that you choose the correct expansion even if other options look attractive.

Final Answer:
In digital communication, ISDN stands for Integrated Services Digital Network.