Radio Frequency Identification (RFID) technology uses radio waves to wirelessly identify objects. The RFID system has many components. Among them, the most important are tags, readers and antenna. The tag and reader have many subcomponents for their operation. The object is recognized by the reader in terms of signal transmission to the tag [4]. The RFID system has the following features such as long life, less maintenance, no need for line of sight, contactless technology, working in harsh environments and tag duplication is difficult [10]. A tag collision occurs if more than one tag attempts to communicate with a reader at the same time. The algorithm used to resolve tag collisions is called anti-collision algorithm [4]. Previous classification of algorithms based on multiple access techniques [7]. They are: SDMA (Space Division Multiple Access), FDMA (Frequency Division Multiple Access), CDMA (Code Division Multiple Access) and TDMA (Time Division Multiple Access). TDMA includes the largest group of anti-collision algorithms, these are divided into reader-driven (reader speaks first) and tag-driven (tag speaks first) algorithms. In Reader Talk First (RTF), tags remain silent until addressed by the reader. In Tag Talk First (TTF), a tag presents itself to the reader by transmitting its identifiers. Of the two, TTF is very slow. RTF has two types such as ALOHA and Tree.ALOHA has three types: Pure Aloha (PA), Slotted Aloha (SA) and Framed SA (FSA). FSA is divided into three types. These are: Basic FSA (BFSA), Dynamic FSA (DFSA) and Enhanced DFSA (EDFSA). The next major type is tree-based protocols. The types are Tree Splitting (TS), Query Tree (QT), Binary Search (BS) and Bitwise Arbitration (BTA) [7].TS sp...... middle of paper ...... read 20 tags ).As shown in fig. 4, the collision cycles are reduced but it cannot identify all the tags in case of re-entry, due to a tag shortage problem (PRB requires 14 cycles to read 19 tags out of 20).VI. CONCLUSIONThe re-entry issues involved in RFID tags are explained. The comprehensive investigation and classification of anti-collision algorithms of RFID tags have been described. This paper also analyzes the effect of the tag re-entry problem in variants of binary tree algorithms. Due to the occurrence of collisions, the ABS and SRB algorithms are ineffective in dealing with the tag re-entry problem and it is also difficult for the PRB algorithm due to the lack of tags. As a result, ABS, SRB and PRB algorithms are essential to handle the label re-entry problem. These issues can be resolved by including frame information in the label after the identification process.