The term RFID denotes a range of wireless identification technologies, not a single device type. One way to classify tags is according to their source of power. The most inexpensive and compact tags are passive, meaning that they derive all of their transmission power from the reading device. Passive tags are also the most physically robust RFID tags. Active tags contain batteries, and are capable of broadcasting at much longer distances than passive ones. (Loosely speaking, your mobile phone is a sophisticated active RFID tag.) Semi-active tags make use of battery power to run local circuitry, but use reader power for communication.
Another important axis of classification is the frequency at which an RFID tag operates. In general, lower frequencies have shorter associated ranges, but offer better penetration of materials; higher frequencies offer greater range, but are subject to greater physical interference. The two most important RFID-frequency categories are as follows:
Ultra-High Frequency (UHF): UHF tags operate in the 868-956 Mhz frequency band. This is the same part of the radio spectrum in which cordless phones and some mobile phones operate. UHF RFID tags will see the widest use in supply-chain and retail applications. One of the big benefits of passive UHF tags is that they have a range, in many environments, of over ten feet (and sometimes as much as tens of feet). Additionally, RFID readers can scan hundreds of UHF tags simultaneously.
A major drawback of UHF tags is that they cannot be easily read in the presence of high concentrations of liquids, as found such things as beverage containers and human beings!
High-Frequency (HF): By comparison with UHF tags, passive HF tags have the drawback of low transmission range -- generally on the order of just over a foot. In general, they are also larger than UHF tags; flat HF tags are typically about 50mm by 100mm in size. HF tags, however, have the advantage of being readable in the presence of water.
HF tags operate at 13.56 Mhz, a frequency known as the industrial-scientific-medical (ISM) band. HF tags are popular in some smartcard applications and also for various industrial uses.
Other frequencies: RFID tags also come in a low-frequency (LF) variety operating at 120-140 Khz. These tags tend to be popular for use in building-access badges and animal tagging. RFID tags can also operate at higher UHF frequencies, most notably at 2.45 GHz.
In order for an RFID reader to identify many tags in its read range, it must engage with the tags in what is known as an anti-collision or singulation protocol. If all tags were to transmit to the reader simultaneously, then their signals would interfere with one another, rendering reading ineffective. A singulation protocol addresses this problem by enabling tags to take turns in transmitting to a reader.
For UHF tags, singulation is generally a variant of a protocol known as tree-walking. Briefly stated, in tree-walking, the space of k-bit identifiers is viewed as the leaves in a tree of depth k. A reader traverses the tree, asking subsets of tags to broadcast a single bit at a time. A feature of the basic tree-walking protocol is that the RFID reader broadcasts tag serial numbers over very large distances, which can introduce vulnerability to eavesdropping.
The anti-collision protocol used in HF tags is generally a variant of the classic ALOHA protocol. Briefly stated, tags in the ALOHA protocol transmit their identifiers to the reader at a variety of randomly determined times so as to avoid transmission collisions. ALOHA-based RFID reading leaks less information than most UHF tree-walking protocols. On the other hand, most HF readers are capable of scanning only several dozen tags simultaneously.
The least expensive RFID tags, such as basic EPC tags, are read-only. Writeable tags are more expensive, while rewritable tags (containing EEPROM) are still more expensive. In a highly networked environment, however, large amounts of information can easily be associated with read-only tags in a database; in this case the tag simply serves as a pointer to an associated database entry.
Cryptography and Security
The tags that will be most inexpensive and most prevalent, such as basic EPC tags, lack the computing power to perform even basic cryptographic operations. (They will have about 500-5000 gates, many devoted to the basic tag functions. By contrast, the Advanced Encryption Standard (AES) requires some 20,000-30,000 gates.) Such tags are at best capable of employing static keys, i.e., PINs and passwords as security mechanisms. For example, the "kill codes" used to disable EPC tags for purposes of privacy, are secured by PINs. The limited capabilities of such RFID tags make privacy and security enforcement a special challenge.
More expensive RFID tags are capable of advanced functionality, and often include the ability to perform basic cryptographic algorithms, such as symmetric-key encryption and challenge-response identification protocols. (Public-key cryptographic is expensive, and used on few RFID tags.)