In classic telecom offerings, which of the following is an example of a medium-speed, switched communications service appropriate for data connections?

Introduction:
Before modern broadband and packet-switched services became ubiquitous, businesses often leased or dialed into switched services that provided modest data rates sufficient for terminals, transaction systems, and early computer communications. Identifying representative offerings helps contextualize legacy network design.

Given Data / Assumptions:

• We are concerned with medium-speed and switched services—not dedicated high-speed private lines.
• The terms reflect historical product names from common carriers.
• The question seeks a specific example recognized in exam-oriented literature.

Concept / Approach:
Dataphone 50 is a known Bell/AT&T family designation for switched data services offering mid-range speeds (e.g., tens of kilobits per second in some variants) suitable for interactive terminals and host access. While DDD (Direct Distance Dialing) is a public switched voice service rather than a specialized data service, and “Series 1000” is not a widely cited switched data product in standard networking curricula, “Dataphone 50” aligns with the intended category.

Step-by-Step Solution:
Clarify the scope: switched, carrier-provided, medium-speed data capability.Evaluate options: Dataphone 50 is specifically a carrier data service brand.Exclude DDD as voice-oriented and Series 1000 as not standard in this context.Select Dataphone 50 as the best-fit example.

Verification / Alternative check:
Telecom references list Dataphone-branded services as dial-up or switched offerings distinct from leased private lines, often used with modems or DSUs for enterprise connectivity.

Why Other Options Are Wrong:

• Series 1000 (A): not a canonical switched data service in typical exam catalogs.
• DDD (C): refers to switched voice dialing, not a data-specific service.
• All of the above (D): incorrect because the set includes non-data or unclear entries.
• None of the above (E): invalid because Dataphone 50 fits.

Common Pitfalls:
Assuming any switched telephone offering is a data service; data-grade services had specific branding, conditioning, and performance guarantees.

Final Answer:
Dataphone 50