Anti-Collision
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.
What Is Anti-Collision?
Anti-collisionAnti-collisionProtocol for selecting individual tags from multiple in RF fieldView full → 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
- REQA: Reader broadcasts Request. All Type A tags respond with ATQA.
- Anti-collision loop: Reader sends SELECT with partial UID. Matching tags respond. Collisions detected at bit level trigger branching.
- SELECT: Full UID identified, reader selects that specific tag.
- HALT: Selected tag is halted after communication, allowing the next tag to be selected.
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 15693ISO 15693Standard for vicinity-range smart cards, 1+ meter read rangeView full → | 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 authenticationauthenticationIdentity verification of NFC tags/readers via passwords or cryptographyView full → factor. Always combine UID identification with cryptographic authentication using AES after tag selection.
Related Terms
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Questions fréquemment posées
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.
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