ncy of IP services and Ethernet. reverse channel A simultaneous data path in the reverse direction over a half-duplex facility. Normally the reverse channel is used for positive/negative acknowledgments of previously received data blocks. RF (radio frequency) A frequency that is much higher than the audio frequencies but below the infrared frequencies. RF is usually above 20KHz. RFI (radio frequency interference) EMI that causes unwanted signals (interference or noise) in radio communications. RFID (radio frequency identification) At a simple level, a technology involves that emit radio signals and devices called readers that pick up the signal. RFID is a method of remotely storing and retrieving data by using RFID tags and readers. RFID can be used to iden- tify and track any movable item or asset. RFID is an alternative to bar coding. Its advantages include data capacity, read/write capability, and a lack of line-of-sight requirements. Glossary RFID encoder RFID encoder A read/write RFID reader. RFID encoders can be used to read information from an RFID tag and to program information into a blank RFID tag. RFID reader A device that processes the data from RFID tags according to the application requirements. RFID readers are used to query RFID tags in order to obtain identification, loca- tion, and other information about the object the tag is embedded in. RFID tag A small object that can be attached to or incorporated into a product, an animal, or a person and then read by an RFID reader. An RFID tag is a tiny microchip composed of a proces- sor, memory, and a radio transmitter that is mounted onto a substrate or an enclosure. Also referred to as an RFID transponder. Rijndael algorithm The algorithm used for AES. The algorithm was designed by two Belgian cryptologists, Vincent Rijmen and Joan Daemen. ring (1) A ring-shaped contact of a plug, usually positioned between but insulated from the tip and sleeve. (2) An audible alerting signal on a telephone line. (3) A network topology in which stations are connected