Telecom basics: Integrated Services Digital Network (ISDN) uses which signal technology for bearer channels?

Introduction / Context:
ISDN was designed to carry voice and data over the public network using a digital end-to-end architecture. Knowing whether the bearer channels are digital or analog is fundamental for configuring terminal adapters and PBX interfaces.

Given Data / Assumptions:

• ISDN Basic Rate Interface (BRI) and Primary Rate Interface (PRI) are considered.
• Bearer channels (B channels) carry user information.
• Signaling occurs on a D channel, also digital.

Concept / Approach:

ISDN provides digital signaling and digital bearer channels. Even voice is encoded digitally on B channels. Although ISDN lines may traverse analog segments deeper in the carrier plant historically, the service interface to the customer is digital. Therefore, the accurate choice is “only digital.”

Step-by-Step Solution:

Verification / Alternative check:

Standards (e.g., ITU-T I.430/I.431) define the electrical and framing properties of ISDN as digital interfaces, confirming the answer.

Why Other Options Are Wrong:

• Digital and analog / only analog / neither: Contradict the digital nature of ISDN interfaces.
• Mostly digital: Hedging phrase; ISDN is defined as digital at the interface.

Common Pitfalls:

Confusing ISDN with analog POTS lines or with DSL variants; assuming “digital phone service” must be VoIP—ISDN predates IP telephony and is circuit-switched digital.

Final Answer:

only digital