射频识别即RFID（Radio Frequency IDentification）技术，又称电子标签、无线射频识别，是一种通信技术，可通过无线电讯号识别特定目标并读写相关数据，而无需识别系统与特定目标之间建立机械或光学接触。
Radio Frequency Identification (RFID) is an automatic identification method, relying on storing and remotely retrieving data using devices called RFID tags or transponders. An RFID tag is an object that can be attached to or incorporated into a product, animal, or person for the purpose of identification using radio waves. Chip-based RFID tags contain silicon chips and antennae. Passive tags require no internal power source, whereas active tags require a power source.
RFID标签的类别
Types of RFID tags
RFID标签分为被动，半主动（也称作半被动），主动三类。
RFID cards are also known as "proximity", "proxy" or "contactless cards" and come in three general varieties: passive, semi-passive (also known as semi-active), or active.
被动式(Passive)
被动式标签没有内部供电电源。其内部集成电路通过接收到的电磁波进行驱动，这些电磁波是由RFID阅读器发出的。当标签接收到足够的信号时，可以向阅读器发出数据。这些数据不仅包括ID号(全球唯一标示ID），还可以包括预先存在于标签内EEPROM中的数据。
由于被动式标签具有价格低廉，无需电源的优点。目前市场的RFID标签主要是被动式的.
半主动式(Semi-Active)
一般而言，被动式标签的天线有两个任务，第一：接收阅读器所发出的电磁波，藉以驱动标签IC；第二：标签回传信号时，需要靠天线的阻抗作切换，才能产生0与1的变化。问题是，想要有最好的回传效率的话，天线阻抗必须设计在“开路与短路”，这样又会使信号完全反射，无法被标签IC接收，半主动式标签就是为了解决这样的问题。半主动式类似于被动式，不过它多了一个小型电池，电力恰好可以驱动标签IC，使得IC处于工作的状态。这样的好处在于，天线可以不用管接收电磁波的任务，充分作为回传信号之用。比起被动式，半主动式有更快的反应速度，更好的效率。
主动式(Active)
射频识别技术包括了一整套信息技术基础设施，包括：
* 射频识别标签，又称射频标签、电子标签，主要由存有识别代码的大规模集成线路芯片和收发天线构成，目前主要为无源式，使用时的电能取自天线接收到的无线电波能量；
* 射频识别读写设备以及
* 与相应的信息服务系统，如进存销系统的联网等。
将射频类别技术与条码（Barcode）技术相互比较，射频类别拥有许多优点，如：
* 可容纳较多容量。
* 通讯距离长。
* 难以复制。
* 对环境变化有较高的忍受能力。
* 可同时读取多个标签。
相对地有缺点，就是建置成本较高。不过目前透过该技术的大量使用，生产成本就可大幅降低。
技术及性能参数
射频识别标签是目前射频识别技术的关键。射频识别标签可存储一定容量的信息并具一定的信息处理功能，读写设备可通过无线电讯号以一定的数据传输率与标签交换信息，作用距离可根据采用的技术从若干厘米到1千米不等。
识别标签的外形尺寸主要由天线决定，而天线又取决于工作频率和对作用距离的要求。目前有四种频率的标签在使用中比较常见。他们是按照他们的无线电频率划分：低频标签（125或134.2千赫），高频标签（13.56兆赫），超高频标签（868到956兆赫）以及微波标签（2.45GHz）。由于目前尚未制定出针对超高频标签使用的全球规范，所以此类标签还不能够在全球统一使用。而超高频标签的应用目前也最受人们的最受注意，此类标签主要应用在物流领域。频率越高，作用距离就越大，数据传输率也就越高，识别标签的外形尺寸就可以做得更小，但成本也就越高。目前面向消费者的识别标签外形尺寸需求，一般以信用卡或商品条形码为准。
2005年初每标签的价格仍在30欧分左右，大批量（十亿个以上）生产的射频识别标签的价格可望在2008年降至10欧分以下。
鉴于标签和读写设备之间无需建立机械或光学接触，密码技术在整个射频识别技术领域中的地位必将日益提高。随着射频识别的普及，不同厂家的标签和读写设备之间的兼容性也将成为值得关注的问题。
此外，使用寿命、使用环境和可靠性也是重要参数。
Current uses:
Passports,
Transport payments,
Product Tracking,
Automotive,
Animal identification,
RFID in inventory systems,
RFID mandates,
Human implants.
Security concerns
A primary security concern surrounding RFID technology is the illicit tracking of RFID tags. Tags which are world-readable pose a risk to both personal location privacy and corporate/military security. Such concerns have been raised with respect to the United States Department of Defense's recent adoption of RFID tags for supply chain management. More generally, privacy organizations have expressed concerns in the context of ongoing efforts to embed electronic product code (EPC) RFID tags in consumer products.
A second class of defense uses cryptography to prevent tag cloning. Some tags use a form of "rolling code" scheme, wherein the tag identifier information changes after each scan, thus reducing the usefulness of observed responses. More sophisticated devices engage in challenge-response protocols where the tag interacts with the reader. In these protocols, secret tag information is never sent over the insecure communication channel between tag and reader. Rather, the reader issues a challenge to the tag, which responds with a result computed using a cryptographic circuit keyed with some secret value. Such protocols may be based on symmetric or public key cryptography. Cryptographically-enabled tags typically have dramatically higher cost and power requirements than simpler equivalents, and as a result, deployment of these tags is much more limited. This cost/power limitation has led some manufacturers to implement cryptographic tags using substantially weakened, or proprietary encryption schemes, which do not necessarily resist sophisticated attack. For example, the Exxon-Mobil Speedpass uses a cryptographically-enabled tag manufactured by Texas Instruments, called the Digital Signature Transponder (DST), which incorporates a weak, proprietary encryption scheme to perform a challenge-response protocol.
Still other cryptographic protocols attempt to achieve privacy against unauthorized readers, though these protocols are largely in the research stage. One major challenge in securing RFID tags is a shortage of computational resources within the tag. Standard cryptographic techniques require more resources than are available in most low cost RFID devices. RSA Security has patented a prototype device that locally jams RFID signals by interrupting a standard collision avoidance protocol, allowing the user to prevent identification if desired. Various policy measures have also been proposed, such as marking RFID tagged objects with an industry standard label.
Controversy.
How would you like it if, for instance, one day you realized your underwear was reporting on your whereabouts?
— California State Senator Debra Bowen, at a 2003 hearing
The use of RFID technology has engendered considerable controversy and even product boycotts by consumer privacy advocates such as Katherine Albrecht and Liz McIntyre of CASPIAN who refer to RFID tags as "spychips". The four main privacy concerns regarding RFID are:
* The purchaser of an item will not necessarily be aware of the presence of the tag or be able to remove it
* The tag can be read at a distance without the knowledge of the individual
* If a tagged item is paid for by credit card or in conjunction with use of a loyalty card, then it would be possible to tie the unique ID of that item to the identity of the purchaser
* The EPCglobal system of tags create globally unique serial numbers for all products.
Most concerns revolve around the fact that RFID tags affixed to products remain functional even after the products have been purchased and taken home and thus can be used for surveillance and other purposes unrelated to their supply chain inventory functions. [citation needed]
Another privacy issue is due to RFID's support for a singulation (anti-collision) protocol. This is the means by which a reader enumerates all the tags responding to it without them mutually interfering. The structure of some collision-resolution (Medium Access Control) protocols is such that all but the last bit of each tag's serial number can be deduced by passively eavesdropping on just the reader's part of the protocol. Because of this, whenever the relevant types of RFID tags are near to readers, the distance at which a tag's signal can be eavesdropped is irrelevant; what counts is the distance at which the much more powerful reader can be received. Just how far this can be depends on the type of the reader, but in the extreme case some readers have a maximum power output of 4 W, enabling signals to be received from tens of kilometres away.[citation needed] However, more recent UHF tags employing the EPCglobal Gen 2 (ISO 18000-6C) protocol, which is a slotted-Aloha scheme in which the reader never transmits the tag identifying information, are not subject to this particular attack.
RFID Shielding(射频识别屏蔽）
A number of products are available on the market in the US that will allow a concerned carrier of RFID-enabled cards or passports to shield their data. Simply wrapping an RFID card in aluminum foil, essentially creating a Faraday cage, is claimed to make transmission more difficult, yet not be completely effective at preventing it.
Shielding is again a function of the frequency being used. Low-frequency tags, like those used in implantable devices for humans and pets, are relatively resistant to shielding, though thick metal foil will prevent most reads. High frequency tags (13.56 MHz -- smart cards and access badges) are more sensitive to shielding and are difficult to read when within a few centimetres of a metal surface. UHF tags (pallets and cartons) are very difficult to read when placed within a few millimetres of a metal surface, although their read range is actually increased when they are spaced 2-4 cm from a metal due to positive reinforcement of the reflected wave and the incident wave at the tag. UHF tags can be successfully shielded from most reads by being placed within an anti-static plastic bag.
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