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A brief description of the different technology issues.
Single bit transponders are used generally in Electronic Article Surveillance (EAS) systems. The single bit status conveys an ON or an OFF status to a reader, being suitable for use in anti-shoplifting systems. Often this tag comprises a strip of magnetised material. or alternatively a tuned circuit which is destroyed to switch to the off state. The magnetic versions state is changed using magnetic fields, and can be switched either on or off according to the magnetic orientation of the particles.
This is the most basic form of transponder. In its most simple form,
it could comprise of some read only memory, a power rectifying circuit,
an onboard oscillator,a variable antenna loading circuit (for backscatter
modulation) and driving logic. As the memory is often permanent, the tags
do not need power to retain their identity and often would be passive in
nature (that is no battery). Transponders in this form can be made very
cheaply, often comprising only a single integrated circuit which is attached
to an antenna. EAS features can be added to the circuitry. The read-only
transponders are effective where the identity of an object is required,
or can be used with a computer network for situations where the transponder
provides a reference to a database and the database contains the variable
The permanent memory of read only tags can be programmed by the factory and in some cases by the users.
Read-write requirements introduce many new levels of complexity over
the read only transponders. Read-write transponders find application in
situations where the information to be carried by the transponder is variable,
and might be altered along the route.
For a tag to be suitable for use in read-write situations, it must have some form of static memory, and a method of retaining that state when not in the interrogation field. Such memory might be battery backed up memory, or alternatively magnetic based memory that will retain its magnetic status when not powered.
A read-write tag needs some form of receiver on board to receive communications from the reader that contains the data to be stored. This introduces new complexities and might mean that the tag loses its frequency agility. The tag will also need some means of unique identity to ensure that the correct data is written to the correct tag in situations where many tags are present in the reading field.
The next major complication is catering for variations between the clock rate of the transponder and the reader. As the data is likely to be a serial data stream, there being only one data communication channel often between the reader and the transponder, it is necessary to synchronise the data rate so that the correct data is stored and interpretted by the transponder. This might be achieved by using a manchester encoded type format, crystal controlled clocks in both the transponder and the reader, or some method of calibrating the transponder relative to the reader which often involves additional tuned circuits, logic and even microprocessors on the tags.
Read-wrtite transponders find application particularly in the more expensive transponder market, such as with toll roads where developments are leading to their integration with smart cards for the financial transactions.
The more expensive read-write transponders are also to be found using
spread spectrum communication for the datalinks rather than using tuned
Read-write transponders due to their complexity can cost many tens of dollars each versus the sub- dollar read only possibilities.
Smartcards are in reality a merging of microprocessor technology, memory
technology and packaging. Often the connections to the smartcard are via
direct contacts or lately via short range magnetic coupling. The later
being called contactless smartcards.
The benefits of the intelligence in the smartcard is that it can execute specific programs allowing high levels of security where the answer supplied by the application matches some algorithm computed in the card. More recently banks have started to use smartcards as cashcards, where electronic money is in effect carried in the card securely as the contents are encrypted. The cards are recharged by means of automatic teller machines.
Smartcards are a distant technology from transponders, and are mentioned here merely to place their technology in perspective.
This technology is often associated with identification systems such
as barcoding but it has little to do with the identification technology.
RFDC is a communication system utilising radio techniques that allows a terminal/scanner/display or any computer device, to be linked to a computer sysetem in situations where a direct wire between the device and the computer network is not viable.
The RFDC protocol allows many devices to share the radio communication system at the same time. In an identification scenario, a communications hub might be attached to the roof of the warehouse or store, and operators with hand scanners fitted with RFDC could roam the floor passing data via the RFDC link to the computer system.
RFDC systems typically operate at about 2.5GHz and by using spread spectrum or frequency hopping techniques, allow many devices to operate at the same time without interferring with each other and without requiring specialist operators to manage instalation or the provision of complex frequency management systems.
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