Which networking device is used to connect two systems when the systems may use different protocols, performing necessary translation at higher layers?

Introduction / Context:
Interconnecting heterogeneous networks often requires more than simple electrical or frame-level forwarding. When two systems speak different protocols, a device that operates at higher layers must translate or mediate between them so applications can communicate transparently.

Given Data / Assumptions:

• Two systems or networks may use different upper-layer protocols.
• The goal is to enable communication despite these differences.
• We compare common devices: repeater, hub, bridge, and gateway.

Concept / Approach:
A gateway traditionally refers to a protocol-conversion device operating at or above the network layer, often up to the application layer, translating between dissimilar protocol suites. By contrast, repeaters and hubs work at the physical layer, and bridges (or switches) at the data link layer—they forward frames but do not translate higher-layer protocols.

Step-by-Step Solution:

Verification / Alternative check:
Examples include email gateways converting between formats or VoIP gateways translating signaling and media between networks; these operate above simple frame forwarding.

Why Other Options Are Wrong:

• Hub/Repeater: Only regenerate or distribute signals; no protocol awareness.
• Bridge: Forwards frames based on MAC addresses; no higher-layer translation.

Common Pitfalls:
Using modern 'default gateway' (router) terminology loosely; while routers forward between IP networks, 'gateway' in classical exam contexts emphasizes protocol conversion.

Final Answer:
gateway