Which device is primarily associated with application-layer conversion of protocols?

The device primarily associated with application-layer conversion of protocols is a gateway. A gateway functions as a "gate" between two networks, often performing protocol conversions to facilitate communication between different network architectures or applications. This capability is essential when the protocols used by the communicating devices are incompatible.

For instance, if one device uses HTTP for web traffic and another uses FTP, the gateway can translate between these application-layer protocols, allowing both devices to successfully communicate despite their differences. Gateways operate at various layers of the OSI model but are particularly important at the application layer where different protocols need to interact.

In contrast, routers primarily operate at Layer 3 (Network Layer) and are responsible for directing data packets between networks based on IP addresses, but do not generally perform protocol translations at the application layer. Switches operate at Layer 2 (Data Link Layer), facilitating communication within the same network by switching frames, but they do not handle protocol conversion. Repeaters work at Layer 1 (Physical Layer) and are used merely to regenerate and amplify signals, not to convert between protocols. This distinction clarifies why a gateway is the correct answer for this question regarding application-layer protocol conversion.