In wide-area data communication, computers around the world can exchange data over public telephone/cable lines or wireless links. Which special accessory on the user’s side enables a digital computer to communicate over such analog carrier networks?

Introduction / Context:
Before always-on broadband and fiber became ubiquitous, most wide-area computer communications relied on carrier networks such as telephone lines or hybrid cable systems. A personal computer generates digital signals, whereas these carrier networks historically transported analog waveforms. A device was required to convert between the two worlds so that distant computers could “talk.”

Given Data / Assumptions:

• The end system is a digital computer.
• The long-haul link is an analog carrier (classic PSTN/cable) or uses similar modulation concepts.
• Goal: enable bidirectional exchange of data between distant endpoints.

Concept / Approach:
A modem (modulator/demodulator) converts outgoing digital bits into analog signals suitable for transmission and converts incoming analog signals back into digital form. Even with modern digital access links, the conceptual role remains: adapt the computer’s digital interface to the service provider’s access technology (DSL/cable/fixed wireless/fiber ONT act as evolutions of this function).

Step-by-Step Solution:

Verification / Alternative check:
Observe dial-up or cable/DSL setups: the customer-premises equipment negotiates line parameters (training/handshake), establishes symbol rates and error control, and exposes a digital interface (serial/Ethernet) to the computer or router.

Why Other Options Are Wrong:

• Keyboard: human input device, not a communication adapter.
• Scanner: digitizes images, not line signaling.
• Fax machine: sends images of pages via analog modulation but is not a general-purpose computer data interface.
• None of the above: incorrect because a modem fills the exact role.

Common Pitfalls:
Confusing “modem” with “router”—a router forwards IP packets between networks; a modem adapts signals to the access medium. Many ISP devices bundle both functions, but the modulation/demodulation role remains essential.

Final Answer:
Modem