Communication

Anti-Collision

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A protocol mechanism that allows an NFC reader to individually select and communicate with one tag when multiple tags are present in the RF field simultaneously. Uses a binary tree-based selection algorithm.

    


    
    
    

      Also known as:
      
      anti-collision
      
      singulation
      
    

    
  


  




  
  

    
What Is Anti-Collision?


Anti-collision is a protocol mechanism enabling an NFC reader to identify and communicate with individual tags when multiple tags are present in the RF field simultaneously. Without it, overlapping tag responses would produce garbled data.


Why It Is Needed


Multiple tags commonly share a field: a wallet with multiple contactless cards, stacked NFC-labeled products, overlapping event wristbands, or products with multiple tags. All would respond simultaneously without anti-collision, causing signal collisions.


Anti-Collision Algorithms


ISO 14443 Type A (NFC-A) — Binary Tree


    

  1. REQA: Reader broadcasts Request. All Type A tags respond with ATQA.
    2. 

  2. Anti-collision loop: Reader sends SELECT with partial UID. Matching tags respond. Collisions detected at bit level trigger branching.
    3. 

  3. SELECT: Full UID identified, reader selects that specific tag.
    4. 

  4. HALT: Selected tag is halted after communication, allowing the next tag to be selected.
    5. 



ISO 14443 Type B (NFC-B) — Slotted ALOHA


Tags randomly select from N time slots (1-16) and respond only in their slot. Collisions in the same slot trigger re-queries with more slots.


ISO 15693 (NFC-V) — Mask-Based


Reader sends an Inventory command with a UID mask. Only matching tags respond. The mask is refined on collision.


Performance






Protocol
Algorithm
Tags/Second






ISO 14443A
Binary tree
10-20




ISO 14443B
Slotted ALOHA
15-30




ISO 15693
Mask-based
30-50






Smartphone Behavior


Most smartphones handle anti-collision automatically but present only one tag at a time, typically resolving to the strongest signal. Dedicated industrial readers can enumerate multiple tags sequentially in a single field activation.


Security Note


The UID transmitted during anti-collision is sent in plaintext. It should never be used as a sole authentication factor. Always combine UID identification with cryptographic authentication using AES after tag selection.

  

  

  
  
  




  
  
Related Terms

  

  

    
    UID (Unique Identifier)
    
    Reader/Writer Mode
    
  





  

  
  

  
  

  
  
  

    
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Frequently Asked Questions


  

    
    

      
      

        
The NFC glossary is a comprehensive reference of technical terms, acronyms, and concepts used in Near Field Communication technology. It is designed for developers, product managers, and engineers who work with NFC and need clear definitions of terms like NDEF, APDU, anti-collision, and ISO 14443.

      

    

    
    

      
      

        
Each glossary term is cross-referenced with related NFC chips, standards, and other terms. For example, the term 'AES-128' links to chips that support AES encryption (NTAG 424 DNA, DESFire EV2/EV3), and the term 'ISO 14443' links to all chips compliant with that standard.

      

    

    
    

      
      

        
Yes. NFCFYI provides glossary definitions in 15 languages including English, Korean, Japanese, Chinese, Spanish, Portuguese, Hindi, Arabic, French, Russian, German, Turkish, Vietnamese, Indonesian, and Thai. Use the language selector in the header to switch languages.