Introduction of IoT(3): RFID

Radio Frequency Identification 射频识别技术 ，利用射频信号通过空间耦合（交变磁场或电磁场）实现无接触信息传递并通过所传递的信息达到识别目的。

不用接触，且不用像条码一样依靠“有形”得几何图案进行“无形”的识别，是RFID的优势。现如今RFID已广泛应用于身份识别、物品管理与跟踪等方面。

RFID

Is a wireless non-contact system that uses radio-frequency electromagnetic fields to transfer data from a tag attached to an object for the purpose of automatic identification and tracking.

• Uniquely identify an individual item beyond just its product type.
• Identify items without direct line-of-sight.
• Identify many items simultaneously.
• Identify items within a vicinity of between a few centimeters to several meters.

consists of :

• RFID reader
• RFID antenna
• RFID tags
• network
• workstation

阅读器实际上包括传送器、接收器、微处理器

Communication process of RFID

• Host manages Reader and issues Commands
• Readers and tags communicate via RF signal
• Carrier signal generated by the reader
• Carrier signal sent out through the antennas
• Carrier signal hits the tag
• Tag receive the modulated carrier signal–“send back” modulated signal
• Antennas receive the modulated signal and send them to the Reader
• Reader decodes the data
• Results returned to the host application

Different types of RFID systems

1. Frequency band:
   • Low Frequency (LF) 30~300kHz
   • High Frequency (HF) 3~30 Mhz
   • Ultra-high Frequency (UHF) 300 MHz~1Ghz
   • Super-high Frequency (SHF)( or Microwave?) 2~30Ghz

LF

small amounts of data at slow speed and minimal distances

HF

small amounts of data and minimal distance BUT higher data rate

BUT government regulated frequency

前⾯两个都可以穿过⽔、纸，UHF、MW不能

UHF

distance > 1m

high data rate, large amounts of data

典型应用：ETC

Microwave

high data rate, large amounts of data

BUT share spectrum with other technologies such as microwave ovens, WLAN, TV

2. Power supply: Active, Passive, Semi-active/passive

Different types of RFID tags

1. Active, Passive, Semi-Active/Passive

   被动标签：由读写器天线辐射形成的电磁场传递能量，线圈发生电磁感应后发送出存储在芯片中的电子编码给读写器天线。

   主动标签：读写不需要读写器提供能量，自身携带的电池就可以提供能量。通信距离比被动标签更远，但体积大、成本高、寿命短。

   半主动标签：内部携带电池，可以进行计算或携带传感器，检测环境参数。但通信并不依靠自身电池，而是像被动式标签一样通过阅读器发射的电磁波获取通信能量。

2. Read-only, Write once Read many (WORM), Read/Write

EPC code ——RFID tags里唯一的标识

The electronic product code is designed as a universal identifier that provides a unique identity for every physical object anywhere in the world, for all time.
不是实际存在的，只是一串存在RFID中的数字

• Header 标头（8）

  代表EPC编码的版本，从中可以知道编码的长度与结构

• EPC Manager 厂商识别代码（28）

  识别生产商

• Object class 对象分类代码（24）

  标识产品类型

• Serial Number 序列号（36）

  标识每一件产品

98 bit code.

EPC码最后还会上传到ONS服务器。类似DNS服务器，要形成根服务器、顶级服务器到本地服务器的多层结构；与DNS服务器相互依存。

Readers functions

前三个是通信的过程

1. Remotely power tags
2. Establish a bidirectional data link
3. can read 100-300 tags per second
4. Inventory tags, filter results
5. communicate with networked server

The advantages of RFID over barcodes

• No line of sight required for reading
• Multiple items can be read with a single scan
• Each tag can carry a lot of data( read/write)
• Passive tags have a virtually unlimited lifetime
• Active tags can be read from great distances
• 体积小且形状多样，不用配合纸张尺寸、印刷品质等因素；
• 抗水油污性能好；
• 可被回收而重复使用；
• 穿透力强；
• 会通过循环冗余校验的方法保证标签发送的数据准确性。

The relationship between RFID and IoT

In the IoT, RFID will embed intelligence in the physical object, so that simple physical objects can also “say”.
In the IoT, RFID allows a physical object to be uniquely identified in a way similar to the “IP Address” of a computing node in the Internet.
In the IoT, RFID provides a low-cost communication way to achieve effective communication between nodes.
In the IoT, RFID makes the physical objects in a passive environment achieve "passive intelligence", providing fundamental guarantee for the “thing-thing connection”.