Digital vs. analog communications: which of the following services is an example of a digital (not analog) communication service?

Introduction / Context:
Many legacy telecommunication offerings were designed for analog voice circuits, while others were purpose-built for data using digital signaling. Being able to distinguish a digital data service from analog voice-oriented services is a common networking fundamentals question.

Given Data / Assumptions:

• Services listed are traditional telecom offerings.
• We must pick the one that is specifically a digital communication service.
• No calculations are required; this is a concept/definition item.

Concept / Approach:
DDS stands for Digital Data Service, a carrier-provided, circuit-like service delivering fixed-rate digital connectivity (for example, for terminals, POS devices, or router serial links). In contrast, DDD (Direct Distance Dialing) and WATS (Wide Area Telephone Service) refer to analog voice calling and billing plans. The distractor DDT is not a standard telecom service in this context. Therefore, DDS is the only clearly digital service listed.

Step-by-Step Solution:

Verification / Alternative check:
Historical carrier catalogs classify DDS as a digital leased service, whereas DDD and WATS are voice dialing/billing constructs. Modern equivalents would be digital leased lines or Ethernet private lines.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming any service beginning with a ‘‘D’’ implies digital; in telecom history many ‘‘D’’ acronyms did not denote digital signaling.

Final Answer:
DDS.