The SIU provides the interface between the shelf-mounted T-SMART Diagnostic Channel Service Units (CSUs) and the operator's control terminal or automated Network Management System (NMS).

The SIU provides the electrical interface between shelves, so that up to 184 units in several shelves can be addressed by one controlling terminal or NMS. Multiple shelves (up to 16) can be chained together in one site using the IN and OUT ports.