The reader, in RFID. It interrogates the tag to determine it's ID. The interrogation is an interaction between the reader and the tag. The tags are asked for their first bit as a 1. If more than one tag responds, then the reader (or interrogator) asks if the second bit is a 1. Again if more than one tag responds, then the tag continues until it finds a single tag that responds or not responds. If no tag responds, then it asks if the bit was a 0. Then it continues until the whole tag ID is complete.