What function does the NIU at a customer's premises serve?

The Network Interface Unit (NIU) at a customer's premises primarily functions as a demarcation point between the service provider's network and the customer's internal network. In this role, it is responsible for converting signals from the service provider into a form that can be used by the customer's equipment. Specifically, if the NIU converts RF (radio frequency) signals into a telephony format, it is facilitating the transition from the coaxial cables used for data transport into the analog signals or VoIP required for telephony services.

This conversion process allows the customer to utilize telephony services based on the incoming data from the service provider. The NIU acts as a vital link that ensures seamless communication and correct signal interpretation, which is essential for maintaining the integrity and functionality of the services provided.

While other options discuss functions related to internet access, distributing television signals, or controlling network traffic, those functions may be components of a broader system that the NIU is part of. However, the key role of the NIU is specifically tied to converting RF signals, making that the most accurate answer.