CCNA Practice Exam 2026 – Complete Study Resource

The purpose of a Channel Service Unit/Data Service Unit (CSU/DSU) is to interface between a digital data line (typically a leased line) and a router. Specifically, it converts the digital signals from the router into a format that can be transmitted over the leased line to the service provider's network. This conversion is crucial because leased lines often require specific signal types to ensure proper communication over the telecommunications infrastructure.

In this context, the role of the CSU/DSU goes beyond mere connectivity; it also incorporates functions such as signal conditioning, line monitoring, and sometimes diagnostic testing. These capabilities ensure the reliability of data transmission over longer distances.

Other options present scenarios that do not capture the primary role of a CSU/DSU, such as connecting two routers directly, performing error correction on analog signals (since a CSU/DSU works with digital signals), or providing routing capabilities, which is not its function.