The majority of RFID tags produced today are passive RFID tags, comprised basically of a micro-circuit and an antenna. They are referred to as passive tags because the only time at which they are actively communicating is when they are within relatively close proximity of a passive RFID tag reader or interrogator.
Another type of common RFID tag in the marketplace today is known as the active RFID tag, which usually contains a battery that directly powers RF communication. This onboard power source allows an active RFID tag to transmit information about itself at great range, either by constantly beaconing this information to a RFID tag reader or by transmitting only when it is prompted to do so. Active tags are usually larger in size and can contain substantially more information (because of higher amounts of memory) than do pure passive tag designs. The tables shown in Figure 6-1 provide a quick reference of common comparisons between active and passive RFID tags. Within these basic categories of RFID tags can be found subcategories such as semi-passive RFID tags.
The terms beacon and beaconing have been used in the RFID industry for some time, predating the establishment of the formal 802.11 standards. When an active RFID tag periodically beacons, it is simply transmitting a tag message (much like any other messages the tag might send) at a set interval. Despite the use of similar terminology, this should not be confused with an 802.11 Beacon. An 802.11 Beacon is a management frame that the 802.11 access point (or the beacon sender in an IBSS) transmits to provide time synchronization and PHY-specific parameters in order to facilitate mobile stations locating and identifying a BSS or IBSS.
Recent market developments have brought yet another category of RFID tag into the spotlight. Known as hybrid or multimode tags, these combine several different tag technologies into a versatile package that can be tracked by one or more location technologies. Multimode RFID tags are typically low power, small form factor devices that allow a single physical tag to assume multiple personalities and perform tasks that previously would have required several individual physical tags to be attached to the asset. A multimode tag, for example, may combine multiple active tag subcategories along with a passive tag into a single homogenous product.
Passive RFID Tags
Passive RFID tags typically do not possess an onboard source of power. Instead, the passive RFID tag receives its power from the energizing electromagnetic field of an RFID reader (or interrogator). The energy coupled from the electromagnetic field undergoes rectification and voltage multiplication in order to allow it to be used to power the passive tag’s microelectronics. In the typical passive RFID tag design, the tag cannot communicate with host applications unless it is within the range of an RFID reader.
Interrogators come in many forms, with two common examples being handheld reader-interrogators (shown on the left in Figure 6-2) and large stationary models capable of reading many tags simultaneously as they pass (shown in the center of Figure 6-2). Embedded sub-miniature passive RFID readers and tags (shown on the right in Figure 6-2) can be used in applications requiring immediate action verification. Examples of this might include immediate verification of proper supply-line hose connections. In these types of applications, passive RFID tags and microreaders embedded into hose plugs and receptacles ensure that the proper supply hoses are connected to the proper material sources at all times. Should an incorrect connection be made, the mismatch is detected and the system refuses to open an electromagnetic flow control.