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RFID is a generic term for technologies which use RF waves to identify, track, or categorize any object. One of the research areas in RFID systems is a tag anti-collision protocol; how to reduce identification time with a given number of tags in the field of an RFID reader. There are two types of tag anti-collision protocols for RFID systems: tree based algorithms and slotted aloha based algorithms. Since the tree based tag anti-collision protocols achieve 100% read rate, we consider how to improve the performances of the tree based RFID tag anti- collision protocols. This paper proposes bi-slotted tree based tag anti-collision protocols, bi-slotted query tree algorithm (BSQTA) and bi-slotted collision tracking tree algorithm (BSCTTA), which reduce both prefix overhead and iteration overhead by time divided responses depending on whether the collided bit is '0' or '1'. According to the simulation results, the bi-slotted tree based RFID tag anti-collision protocols require less time consumption for tag identification than the present tag anti-collision protocols.