2019-04-23 NFC概览

Near Field Communication (NFC) is a set of short-range wireless technologies, typically requiring a distance of 4cm or less to initiate a connection. NFC allows you to share small payloads of data between an NFC tag and an Android-powered device, or between two Android-powered devices.
近场通信是一组短程无线技术，通常需要4cm或者更小的距离去发起连接。NFC允许您在NFC标签和Android驱动设备或者两个Android驱动设备之间之间共享小的有效数据负载。
Tags can range in complexity. Simple tags offer just read and write semantics, sometimes with one-time-programmable areas to make the card read-only. More complex tags offer math operations, and have cryptographic hardware to authenticate access to a sector. The most sophisticated tags contain operating environments, allowing complex interactions with code executing on the tag. The data stored in the tag can also be written in a variety of formats, but many of the Android framework APIs are based around a NFC Forum standard called NDEF (NFC Data Exchange Format).
标签的复杂程度很高。简单的标签只提供读和写语义，有时用一次性可编程区域使卡只读。更复杂的标记提供数学操作，并具有加密硬件来验证对扇区的访问。最复杂的标记包含操作环境，允许与在标记上执行代码进行复杂的交互。存储在标签中的数据也可以以多种格式写入，但许多Android框架API都基于围绕[NFC论坛]（http://www.nfc-forum.org/）名为ndef（NFC数据交换格式）的标准写入。
Android-powered devices with NFC simultaneously support three main modes of operation:
集成NFC的安卓设备同时支持三种主要操作模式：
1.Reader/writer mode, allowing the NFC device to read and/or write passive NFC tags and stickers.
1.读/写模式，允许NFC设备去读和/或写被动的NFC标签。
2.P2P mode, allowing the NFC device to exchange data with other NFC peers; this operation mode is used by Android Beam.
2.点对点模式，允许NFC设备去和其他的NFC端交换数据；该操作模式由Android Beam使用。
3.Card emulation mode, allowing the NFC device itself to act as an NFC card. The emulated NFC card can then be accessed by an external NFC reader, such as an NFC point-of-sale terminal.
3.仿真卡模式，允许将NFC设备自身作为一个NFC卡。它可以被外部的NFC读卡器访问，例如NFC销售点终端。

NFC Basics：NFC基础
This document describes how Android handles discovered NFC tags and how it notifies applications of data that is relevant to the application. It also goes over how to work with the NDEF data in your applications and gives an overview of the framework APIs that support the basic NFC feature set of Android.
本文档描述了Android如何处理发现的NFC标签，以及它如何通知应用程序与应用程序相关的数据。它还介绍了如何在应用程序中使用ndef数据，并概述了支持Android基本NFC功能集的框架API。

Advanced NFC：高级NFC
This document goes over the APIs that enable use of the various tag technologies that Android supports. When you are not working with NDEF data, or when you are working with NDEF data that Android cannot fully understand, you have to manually read or write to the tag in raw bytes using your own protocol stack. In these cases, Android provides support to detect certain tag technologies and to open communication with the tag using your own protocol stack.
本文介绍了允许使用Android支持的各种标签技术的API。当您不使用ndef数据时，或者当您使用android无法完全理解的ndef数据时，您必须使用自己的协议栈以原始字节手动读取或写入标记。在这些情况下，Android提供了检测某些标签技术和使用您自己的协议栈打开与标签的通信的支持。

Host-based Card Emulation：基于主机的仿真卡
This document describes how Android devices can perform as NFC cards without using a secure element, allowing any Android application to emulate a card and talk directly to the NFC reader.
本文档描述了Android设备如何在不使用安全元素的情况下作为NFC卡执行，允许任何Android应用程序模拟卡并直接与NFC读卡器对话。