Prior to a reader-to-tag or tag-to-reader communication, the reader must activate a field and transmit energy to the tag. After a certain time, when the tag has collected sufficient energy and stored it in a buffer capacitor, an interrupt is generated and the tag starts operating. Operating in this instance means modulating, or transmitting data. To enable sending commands and/or data to the tag, the reader field is interrupted for defined field gap times. The command pattern always starts with a long field gap (which is called start gap), followed by pulse packages of several clock cycles, and always interrupted by field gaps. The result is a serial bit stream with gaps and signal periods. Long signal periods indicate a logical 1, shorter ones a 0. This is the write mode principle. The Read mode is different; the reader always keeps the field on and is constantly transmitting energy. When the tag is transmitting data, it draws more energy from the field. This causes an amplitude modulation on the reader field. This is named ASK modulation (amplitude shift keying). A reader is able to detect ASK modulation and to decode the transmitted information.