The techniques introduced here provide a single, passive radio link control entity with a unified interface. A sending network device that wants to initiate a new transmission over the network first enters a probing mode in which it sends data packets at a priority level that is lower than the target priority level and at a transmission rate up to a peak probing transmission rate that may be the same or slightly higher than the desired transmission rate. The employee may be located within a retail store or elsewhere, such as a customer service telephone extension. Ninety second of manual activation enables about five minutes of illumination. One of a data read or a data write is executed without accessing the contentual metadata of the extended cache stored within the extended cache. A cover in the form of a plate is removably mounted on the front part of the housing, on fastening elements provided for mounting an additional bloc.