Beginning NFC

Chapter 4. Introducing NDEF

In order to understand NFC, you need to know about the NFC Data Exchange Format (NDEF), which is the lingua franca for NFC devices and tags. In this chapter, you’ll learn about the structure of NDEF and the records it carries. You’ll also write a couple of apps that read and write NDEF-formatted messages.

NDEF Structure

NDEF is a binary format structured in messages, each of which can contain several records, as shown in Figure 4-1. Each record is made up of a header, which contains metadata about the record, such as the record type, length, and so forth, and the payload, which contains the content of the message. Think of an NDEF message like a paragraph, and records like the sentences within it. A well-formed paragraph is made up of sentences pertaining to one topic. Similarly, it’s good practice to use one NDEF message made up of several records to describe one subject, say, an address book entry.

NFC transactions are generally short. Each exchange generally consists of only one message, and each tag carries just one message. Keep in mind the physical circumstances of an NFC exchange: you tap your device to another device or tag, and the whole exchange happens while you’re in contact with the other device or tag. You don’t want to send a whole novel in a single exchange, so think of your NDEF messages as paragraph-length, not book-length. You’ll see a workaround to this for sending large files in one of the final chapters of this book, but for now, consider ...