Smart Card Data Encoding Formats: TLV, APDU and NDEF on Contactless Cards
Smart Card Data Encoding Formats: TLV, APDU and NDEF on Contactless Cards
Publish Date: 2026-09-27
A smart card is only as useful as the data you put on it and the rules for reading it back. Under the plastic sit three ideas that decide how an application talks to the chip: TLV (the tagged data layout), APDU (the command envelope sent over the air) and NDEF (the friendly record format phones understand). Confusing them leads to cards a terminal cannot parse or a phone cannot open. This article explains TLV, APDU and NDEF so you can choose the right encoding format for your contactless card.
Layout, Command and Message
TLV (Tag-Length-Value) is the low-level data structure inside the chip: each piece of information is labeled with a tag, given a length, then carries its value, so parsers can walk a file without fixed offsets. APDU (Application Protocol Data Unit) is the command wrapper — the ISO 7816 envelope of CLA, INS, P1, P2 and data that a reader sends to invoke an instruction such as read, write or authenticate. NDEF (NFC Data Exchange Format) sits on top for NFC: a standardized message of records (text, URI, MIME) that an Android or iOS phone reads automatically and turns into a web link or action. In practice a card stores TLV files, is driven by APDU commands, and exposes an NDEF message through the NFC Type 4 tag abstraction so both terminals and phones work.
Contactless card data layers, from TLV files and APDU commands inside the chip to the NDEF message a phone reads on tap.
What Each Layer Does
TLV and APDU
TLV organizes stored data into self-describing fields; APDU is the instruction set that reads, writes and secures them. Together they power payment, ID and access applets where the terminal controls the dialog and cryptography.
NDEF
NDEF is the phone-friendly layer that maps a tap to a URL, contact or action with no custom app. Ideal for marketing, pairing and lightweight interaction; relies on TLV/APDU underneath on a Type 4 tag.
Comparison Parameters
Designing the right encoding format for your card?
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