page_title: Singulation
section_title: Tree walking
hierarchical_section_title: Singulation / Tree walking
caption_attribution_description: Schematic representation of typical RFID search environments, where large numbers of tags can be probed and located by means of simple primitive commands such as "respond if bit i of your tag is 0/1". The diagram shows a possible search path to find a certain device or group of devices.
context_page_description: Singulation is a method by which an RFID reader identifies a tag with a specific serial number from a number of tags in its field. This is necessary because if multiple tags respond simultaneously to a query, they will jam each other. In a typical commercial application, such as scanning a bag of groceries, potentially hundreds of tags might be within range of the reader.
  When all the tags cooperate with the tag reader and follow the same anti-collision protocol, also called singulation protocol,
  then the tag reader can read data from each and every tag without interference from the other tags.
context_section_description: There are different methods of singulation, but the most common is "tree walking", which involves asking all tags with a serial number that starts with either a 1 or 0 to respond. If more than one responds, the reader might ask for all tags with a serial number that starts with 01 to respond, and then 010. It keeps doing this until it finds the tag it is looking for. Note that if the reader has some idea of what tags it wishes to interrogate, it can considerably optimise the search order. For example with some designs of tags, if a reader already suspects certain tags to be present then those tags can be instructed to remain silent, then tree walked without interference from them, and then finally they can be queried individually.
  This simple protocol leaks considerable information, because anyone able to eavesdrop on the tag reader alone can determine all but the last bit of a tag's serial number. Thus a tag can be (largely) identified so long as the reader's signal is receivable, which is usually possible at much greater distance than simply reading a tag directly. Because of privacy and security concerns related to this, the Auto-ID Labs have developed two more advanced singulation protocols, called Class 0 UHF and Class 1 UHF, which are intended to be resistant to these sorts of attacks. These protocols, which are based on tree-walking but include other elements, have a performance of up to 1000 tags per second.
  The tree walking protocol may be blocked or partially blocked by RSA Security's blocker tags.
caption: Tree walking singulation
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