AIM WP-98/002R
Radio Frequency Identification - RFID A basic
primer
Document Type: AIM, Inc. White Paper Document
Version: 1.11, 1999-09-28 AUTOMATIC IDENTIFICATION
MANUFACTURERS - AIM
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V1.11
Part I : Basics
Introduction
RFID, its application, standardisation, and innovation are
constantly changing. Its adoption is still relatively new and
hence there are many features of the technology that are not
well understood by the general populace. Developments in RFID
technology continue to yield larger memory capacities, wider
reading ranges, and faster processing. It's highly unlikely
that the technology will ultimately replace bar code - even
with the inevitable reduction in raw materials coupled with
economies of scale, the integrated circuit in an RF tag will
never be as cost-effective as a bar code label. However, RFID
will continue to grow in its established niches where bar code
or other optical technologies aren't effective. If some
standards commonality is achieved, whereby RFID equipment from
different manufacturers can be used interchangeably, the
market will very likely grow exponentially.
This document tries to set out the basic information about
RFID in a simple format that can be understood by everyone.
AIM's purpose is to provide education on RFID and hence
increase the use of the technology. This is part one of a
three part series that will also include:
Part II : Application Case Studies
Part III : Getting Started in RFID - A Step approach
A moment's thought about radio broadcasts or mobile
telephones and one can readily appreciate the benefits of
wireless communication. Extend those benefits to communication
of data, to and from portable low cost data carriers, and one
is close to appreciating the nature and potential of radio
frequency identification (RFID). RFID is an area of automatic
identification that has quietly been gaining momentum in
recent years and is now being seen as a radical means of
enhancing data handling processes, complimentary in many ways
to other data capture technologies such bar coding. A range of
devices and associated systems are available to satisfy an
even broader range of applications. Despite this diversity,
the principles upon which they are based are quite straight
forward, even though the technology and technicalities
concerning the way in which they operate can be quite
sophisticated. Just as one need not know the technicalities of
a mobile phone or personal computer to use it, it is not
necessary to know the technicalities to understand the
principles, considerations and potential for using RFID.
However, a little technical appreciation can provide advantage
in determining system requirements and in talking to
consultants and suppliers.
What is RFID?
The object of any RFID system is to carry data in suitable
transponders, generally known as tags, and to retrieve data,
by machine-readable means, at a suitable time and place to
satisfy particular application needs. Data within a tag may
provide identification for an item in manufacture, goods in
transit, a location, the identity of a vehicle, an animal or
individual. By including additional data the prospect is
provided for supporting applications through item specific
information or instructions immediately available on reading
the tag. For example, the colour of paint for a car body
entering a paint spray area on the production line, the set-up
instructions for a flexible manufacturing cell or the manifest
to accompany a shipment of goods.
A system requires, in addition to tags, a means of reading
or interrogating the tags and some means of communicating the
data to a host computer or information management system. A
system will also include a facility for entering or
programming data into the tags, if this is not undertaken at
source by the manufacturer. Quite often an antenna is
distinguished as if it were a separate part of an RFID system.
While its importance justifies the attention it must be seen
as a feature that is present in both readers and tags,
essential for the communication between the two.
To understand and appreciate the capabilities of RFID
systems it is necessary to consider their constituent parts.
It is also necessary to consider the data flow requirements
that influence the choice of systems and the practicalities of
communicating across the air interface. By considering the
system components and their function within the data flow
chain it is possible to grasp most of the important issues
that influence the effective application of RFID. However, it
is useful to begin by briefly considering the manner in which
wireless communication is achieved, as the techniques involved
have an important bearing upon the design of the system
components.
Wireless communication and the air interface
Communication of data between tags and a reader is by
wireless communication. Two methods distinguish and categorise
RFID systems, one based upon close proximity electromagnetic
or inductive coupling and one based upon propagating
electromagnetic waves. Coupling is via 'antenna' structures
forming an integral feature in both tags and readers. While
the term antenna is generally considered more appropriate for
propagating systems it is also loosely applied to inductive
systems.
Inductive Coupling |
Propagation Coupling |
Transmitting data is subject to the vagaries and influences
of the media or channels through which the data has to pass,
including the air interface. Noise, interference and
distortion are the sources of data corruption that arise in
practical communication channels that must be guarded against
in seeking to achieve error free data recovery. Moreover, the
nature of the data communication processes, being asynchronous
or unsynchronised in nature, requires attention to the form in
which the data is communicated. Structuring the bit stream to
accommodate these needs is often referred to as channel
encoding and although transparent to the user of an RFID
system the coding scheme applied appears in system
specifications. Various encoding schemes can be distinguished,
each exhibiting different performance features.
To transfer data efficiently via the air interface or space
that separates the two communicating components requires the
data to be superimposed upon a rhythmically varying
(sinusoidal) field or carrier wave. This process of
superimposition is referred to as modulation, and various
schemes are available for this purposes, each having
particular attributes that favour their use. They are
essentially based upon changing the value of one of the
primary features of an alternating sinusoidal source, its
amplitude, frequency or phase in accordance with the data
carrying bit stream. On this basis one can distinguish
amplitude shift keying (ASK), frequency shift keying (FSK) and
phase shift keying (PSK).
In addition to non-contact data transfer, wireless
communication can also allow non-line-of-sight communication.
However, with very high frequency systems more directionality
is evident and can be tailored to needs through appropriate
antenna design.
Carrier frequencies
In wired communication systems the physical wiring
constraints allow communication links and networks to be
effectively isolated from each other. The approach that is
generally adopted for radio frequency communication channels
is to separate on the basis of frequency allocation. This
requires, and is generally covered by government legislation,
with different parts of the electromagnetic spectrum being
assigned to different purposes. Allocations may differ
depending on the governments concerned, requiring care in
considering RFID applications in different countries.
Standardisation efforts are seeking to obviate problems in
this respect.
Three frequency ranges are generally distinguished for RFID
systems, low, intermediate (medium) and high. The following
table summarises these three frequency ranges, along with the
typical system characteristics and examples of major areas of
application.
Table 1. Frequency Bands and Applications
Frequency Band |
Characteristics |
Typical Applications |
Low 100-500 kHz |
Short to medium read
range Inexpensive low reading speed |
Access control Animal
identification Inventory control Car
immobiliser |
Intermediate 10-15 MHz |
Short to medium read
range potentially inexpensive medium reading
speed |
Access control Smart
cards |
High 850-950
MHz 2.4-5.8 GHz |
Long read range High
reading speed Line of sight required Expensive |
Railroad car
monitoring Toll collection
systems |
A degree of uniformity is being sought for carrier
frequency usage, through three regulatory areas, Europe and
Africa (Region 1), North and South America (Region 2) and Far
East and Australasia (Region 3). Each country manages their
frequency allocations within the guidelines set out by the
three regions. Unfortunately, there has been little or no
consistency over time with the allocation of frequency, and so
there are very few frequencies that are available on a global
basis for the technology. This will change with time, as
countries are required to try to achieve some uniformity by
the year 2010.
Three carrier frequencies receiving early attention as
representative of the low, intermediate and high ranges are
125kHz, 13.56 MHz and 2.45 GHz. However, there are eight
frequency bands in use around the world, for RFID
applications. The applications using these frequency bands are
listed in Table 2.
Not all of the countries in the world have access to all of
the frequency bands listed above, as some countries have
assigned these bands to other users. Within each country and
within each frequency range there are specific regulations
that govern the use of the frequency. These regulations may
apply to power levels and interference as well as frequency
tolerances.
Data transfer rate and bandwidth
Choice of field or carrier wave frequency is of primary
importance in determining data transfer rates. In practical
terms the rate of data transfer is influenced primarily by the
frequency of the carrier wave or varying field used to carry
the data between the tag and its reader. Generally speaking
the higher the frequency the higher the data transfer or
throughput rates that can be achieved. This is intimately
linked to bandwidth or range available within the frequency
spectrum for the communication process. The channel bandwidth
needs to be at least twice the bit rate required for the
application in mind. Where narrow band allocations are
involved the limitation on data rate can be an important
consideration. It is clearly less of an issue where wide
bandwidths are involved. Using the 2.4 - 2.5 GHz spread
spectrum band, for example, 2 megabits per second data rates
may be achieved, with added noise immunity provided by the
spread spectrum modulation approach. Spread spectrum apart,
increasing the bandwidth allows an increase noise level and a
reduction in signal-to-noise ratio. Since it is generally
necessary to ensure a signal is above the noise floor for a
given application, bandwidth is an important consideration in
this respect.
Range and Power Levels
The range that can be achieved in an RFID system is
essentially determined by:
- The power available at the reader/interrogator to
communicate with the tag(s)
- The power available within the tag to respond
- The environmental conditions and structures, the former
being more significant at higher frequencies including
signal to noise ratio
Although the level of available power is the primary
determinant of range the manner and efficiency in which that
power is deployed also influences the range. The field or wave
delivered from an antenna extends into the space surrounding
it and its strength diminishes with respect to distance. The
antenna design will determine the shape of the field or
propagation wave delivered, so that range will also be
influenced by the angle subtended between the tag and
antenna.
In space free of any obstructions or absorption mechanisms
the strength of the field reduces in inverse proportion to the
square of the distance. For a wave propagating through a
region in which reflections can arise from the ground and from
obstacles, the reduction in strength can vary quite
considerable, in some cases as an inverse fourth power of the
distance. Where different paths arise in this way the
phenomenon is known as "multi-path attenuation". At higher
frequencies absorption due to the presence of moisture can
further influence range. It is therefore important in many
applications to determine how the environment, internal or
external, can influence the range of communication. Where a
number of reflective metal 'obstacles' are to encountered
within the application to be considered, and can vary in
number from time to time, it may also be necessary to
establish the implications of such changes through an
appropriate environmental evaluation.
The power within the tag is generally speaking a lot less
than from the reader, requiring sensitive detection capability
within the reader to handle the return signals. In some
systems the reader constitutes a receiver and is separate from
the interrogation source or transmitter, particularly if the
'up-link' (from transmitter-to-tag) carrier is different from
the 'down-link' (from tag-to-reader).
Although it is possible to choose power levels to suit
different application needs is not possible to exercise
complete freedom of choice. Like the restrictions on carrier
frequencies there are also legislative constraints on power
levels. While 100 - 500mW are values often quoted for RFID
systems actual values should be confirmed with the appropriate
regulatory authorities, in the countries where the technology
is to be applied. The authorities will also be able to
indicate the form in which the power is delivered, pulsed or
continuous, and the associated allowed values.
Having gained some grasp of the data communication
parameters and their associated values it is appropriate to
consider, in a little more detail, the components of an RFID
system.
RFID System Components
Transponders/Tags
The word transponder, derived from TRANSmitter/resPONDER,
reveals the function of the device. The tag responds to a
transmitted or communicated request for the data it carries,
the mode of communication between the reader and the tag being
by wireless means across the space or air interface between
the two. The term also suggests the essential components that
form an RFID system - tags and a reader or interrogator. Where
interrogator is often used as an alternative to that of
reader, a difference is sometime drawn on the basis of a
reader together with a decoder and interface forming the
interrogator.
The basic components of a transponder may be represented as
shown below. Generally speaking they are fabricated as low
power integrated circuits suitable for interfacing to external
coils, or utilising "coil-on-chip" technology, for data
transfer and power generation (passive mode).
Basic features of an RFID transponder:
The transponder memory may comprise read-only (ROM), random
access (RAM) and non-volatile programmable memory for data
storage depending upon the type and sophistication of the
device. The ROM-based memory is used to accommodate security
data and the transponder operating system instructions which,
in conjunction with the processor or processing logic deals
with the internal "house-keeping" functions such as response
delay timing, data flow control and power supply switching.
The RAM-based memory is used to facilitate temporary data
storage during transponder interrogation and response.
The non-volatile programmable memory may take various
forms, electrically erasable programmable read only memory
(EEPROM) being typical. It is used to store the transponder
data and needs to be non-volatile to ensure that the data is
retained when the device is in its quiescent or power-saving
"sleep" state.
Data buffers are further components of memory, used to
temporarily hold incoming data following demodulation and
outgoing data for modulation and interface with the
transponder antenna. The interface circuitry provides the
facility to direct and accommodate the interrogation field
energy for powering purposes in passive transponders and
triggering of the transponder response. Where programming is
accommodated facilities must be provided to accept the data
modulated signal and perform the necessary demodulation and
data transfer processes.
The transponder antenna is the means by which the device
senses the interrogating field and, where appropriate, the
programming field and also serves as the means of transmitting
the transponder response to interrogation.
A number of features, in addition to carrier frequency,
characterise RFID transponders and form the basis of device
specifications, including:
- Means by which a transponder is powered
- Data carrying options
- Data read rates
- Programming options
- Physical form
- Costs
Powering tags - For tags to work they require power, even
though the levels are invariably very small (micro to
milliwatts). Tags are either passive or active, the
designation being determined entirely by the manner in which
the device derives its power.
Active tags are powered by an internal battery and are
typically read/write devices. They usually contain a cell that
exhibits a high power-to-weight ratio and are usually capable
of operating over a temperature range of -50° C to +70° C. The
use of a battery means that a sealed active transponder has a
finite lifetime. However, a suitable cell coupled to suitable
low power circuitry can ensure functionality for as long as
ten or more years, depending upon the operating temperatures,
read/write cycles and usage. The trade-off is greater size and
greater cost compared with passive tags.
In general terms, active transponders allow greater
communication range than can be expected for passive devices,
better noise immunity and higher data transmissions rates when
used to power a higher frequency response mode.
Passive tags operate without an internal battery source,
deriving the power to operate from the field generated by the
reader. Passive tags are consequently much lighter than active
tags, less expensive, and offer a virtually unlimited
operational lifetime. The trade-off is that they have shorter
read ranges than active tags and require a higher-powered
reader. Passive tags are also constrained in their capacity to
store data and the ability to perform well in
electromagnetically noisy environments. Sensitivity and
orientation performance may also be constrained by the
limitation on available power. Despite these limitations
passive transponders offer advantages in terms of cost and
longevity. They have an almost indefinite lifetime and are
generally lower on price than active transponders.
Data carrying options - Data stored in data carriers
invariable require some organisation and additions, such as
data identifiers and error detection bits, to satisfy recovery
needs. This process is often referred to as source encoding.
Standard numbering systems, such as UCC/EAN and associated
data defining elements may also be applied to data stored in
tags. The amount of data will of course depend on application
and require an appropriate tag to meet the need. Basically,
tags may be used to carry:
- Identifiers, in which a numeric or alphanumeric string
is stored for identification purposes or as an access key to
data stored elsewhere in a computer or information
management system, or
- Portable data files, in which information can be
organised, for communication or as a means of initiating
actions without recourse to, or in combination with, data
stored elsewhere.
In terms of data capacity tags can be obtained that satisfy
needs from single bit to kilobits. The single bit devices are
essentially for surveillance purposes. Retail electronic
article surveillance (EAS) is the typical application for such
devices, being used to activate an alarm when detected in the
interrogating field. They may also be used in counting
applications.
Devices characterised by data storage capacities up to 128
bits are sufficient to hold a serial or identification number
together, possibly, with parity check bits. Such devices may
be manufacturer or user programmable. Tags with data storage
capacities up to 512 bits, are invariably user programmable,
and suitable for accommodating identification and other
specific data such as serial numbers, package content, key
process instructions or possibly results of earlier
interrogation/response transactions.
Tags characterised by data storage capacities of around 64
kilobits may be regarded as carriers for portable data files.
With increased capacity the facility can also be provided for
organising data into fields or pages that may be selectively
interrogated during the reading process.
Data read rate - It has been mentioned already that data
transfer rate is essentially linked to carrier frequency. The
higher the frequency, generally speaking the higher the
transfer rates. It should also be appreciated that reading or
transferring the data requires a finite period of time, even
if rated in milliseconds, and can be an important
consideration in applications where a tag is passing swiftly
through an interrogation or read zone.
Data programming options - Depending upon the type of
memory a tag contains the data carried may be read-only, write
once read many (WORM) or read/write. Read-only tags are
invariably low capacity devices programmed at source, usually
with an identification number. WORM devices are user
programmable devices. Read/write devices are also
user-programmable but allowing the user to change data stored
in a tag. Portable programmers may be recognised that also
allow in-field programming of the tag while attached to the
item being identified or accompanied.
Physical Form - RFID tags come in a wide variety of
physical forms, shapes sizes and protective housings. Animal
tracking tags, inserted beneath the skin, can be as small as a
pencil lead in diameter and ten millimetres in length. Tags
can be screw-shaped to identify trees or wooden items, or
credit-card shaped for use in access applications. The
anti-theft hard plastic tags attached to merchandise in stores
are also RFID tags, as are heavy-duty 120 by 100 by 50
millimetre rectangular transponders used to track inter-modal
containers, or heavy machinery, trucks, and railroad cars for
maintenance and tracking applications.
Costs - The cost of tags obviously depends upon the type
and quantities that are purchased. For large quantities (tens
of thousands) the price can range from less than a few tens of
pence for extremely simple tags to tens of pounds for the
larger and more sophisticated devices.
Increasing complexity of circuit function, construction and
memory capacity will influence cost of both transponders and
reader/programmers.
The manner in which the transponder is packaged to form a
unit will also have a bearing on cost. Some applications where
harsh environments may be expected, such as steel mills,
mines, and car body paint shops, will require mechanically
robust, chemical and temperature tolerant packaging. Such
packaging will undoubtedly represent a significant proportion
of the total transponder cost.
Generally, low frequency transponders are cheaper than high
frequency devices, passive transponders are usually cheaper
than active transponders.
The Reader/Interrogator
The reader/interrogators can differ quite considerably in
complexity, depending upon the type of tags being supported
and the functions to be fulfilled. However, the overall
function is to provide the means of communicating with the
tags and facilitating data transfer. Functions performed by
the reader may include quite sophisticated signal
conditioning, parity error checking and correction. Once the
signal from a transponder has been correctly received and
decoded, algorithms may be applied to decide whether the
signal is a repeat transmission, and may then instruct the
transponder to cease transmitting. This is known as the
"Command Response Protocol" and is used to circumvent the
problem of reading multiple tags in a short space of time.
Using interrogators in this way is sometimes referred to as
"Hands Down Polling". An alternative, more secure, but slower
tag polling technique is called "Hands Up Polling" which
involves the interrogator looking for tags with specific
identities, and interrogating them in turn. This is contention
management, and a variety of techniques have been developed to
improve the process of batch reading. A further approach may
use multiple readers, multiplexed into one interrogator, but
with attendant increases in costs.
RF Transponder Programmers
Transponder programmers are the means by which data is
delivered to write once, read many (WORM) and read/write tags.
Programming is generally carried out off-line, at the
beginning of a batch production run, for example.
For some systems re-programming may be carried out on-line,
particularly if it is being used as an interactive portable
data file within a production environment, for example. Data
may need to be recorded during each process. Removing the
transponder at the end of each process to read the previous
process data, and to programme the new data, would naturally
increase process time and would detract substantially from the
intended flexibility of the application. By combining the
functions of a reader/interrogator and a programmer, data may
be appended or altered in the transponder as required, without
compromising the production line.
The range over which the programming can be achieved is
generally less than the read range and in some systems near
contact positioning is required. Programmers are also
generally designed to handle a single tag at a time. However,
developments are now satisfying the need for selective
programming of a number of tags present within the range of
the programmer.
RFID System Categories
RFID systems may be roughly grouped into four categories:
- EAS (Electronic Article Surveillance) systems
- Portable Data Capture systems
- Networked systems
- Positioning systems
Electronic Article Surveillance systems are typically a one
bit system used to sense the presence/absence of an item. The
large use for this technology is in retail stores where each
item is tagged and a large antenna readers are placed at each
exit of the store to detect unauthorised removal of the item
(theft).
Portable data capture systems are characterised by the use
of portable data terminals with integral RFID readers and are
used in applications where a high degree of variability in
sourcing required data from tagged items may be exhibited. The
hand-held readers/portable data terminals capture data which
is then either transmitted directly to a host information
management system via a radio frequency data communication
(RFDC) link or held for delivery by line-linkage to the host
on a batch processing basis.
Networked systems applications can generally be
characterised by fixed position readers deployed within a
given site and connected directly to a networked information
management system. The transponders are positioned on moving
or moveable items, or people, depending upon application.
Positioning systems use transponders to facilitate
automated location and navigation support for guided vehicles.
Readers are positioned on the vehicles and linked to an
on-board computer and RFDC link to the host information
management system. The transponders are embedded in the floor
of the operating environment and programmed with appropriate
identification and location data. The reader antenna is
usually located beneath the vehicle to allow closer proximity
to the embedded transponders.
Areas of Application for RFID
Potential applications for RFID may be identified in
virtually every sector of industry, commerce and services
where data is to be collected. The attributes of RFID are
complimentary to other data capture technologies and thus able
to satisfy particular application requirements that cannot be
adequately accommodate by alternative technologies. Principal
areas of application for RFID that can be currently identified
include:
- Transportation and logistics
- Manufacturing and Processing
- Security
A range of miscellaneous applications may also be
distinguished, some of which are steadily growing in terms of
application numbers. They include:
- Animal tagging
- Waste management
- Time and attendance
- Postal tracking
- Airline baggage reconciliation
- Road toll management
As standards emerge, technology develops still further, and
costs reduce considerable growth in terms of application
numbers and new areas of application may be expected.
Some of the more prominent specific applications include:
- Electronic article surveillance - clothing retail
outlets being typical.
- Protection of valuable equipment against theft,
unauthorised removal or asset management.
- Controlled access to vehicles, parking areas and fuel
facilities - depot facilities being typical.
- Automated toll collection for roads and bridges - since
the 1980s, electronic Road-Pricing (ERP) systems have been
used in Hong Kong.
- Controlled access of personnel to secure or hazardous
locations.
- Time and attendance - to replace conventional "slot
card" time keeping systems.
- Animal husbandry - for identification in support of
individualised feeding programmes.
- Automatic identification of tools in numerically
controlled machines - to facilitate condition monitoring of
tools, for use in managing tool usage and minimising waste
due to excessive machine tool wear.
- Identification of product variants and process control
in flexible manufacture systems.
- Sport time recording
- Electronic monitoring of offenders at home
- Vehicle anti-theft systems and car immobiliser
A number of factors influence the suitability of RFID for
given applications. The application needs must be carefully
determined and examined with respect to the attributes that
RFID and other data collection technologies can offer. Where
RFID is identified as a contender further considerations have
to be made in respect of application environment, from an
electromagnetic standpoint, standards, and legislation
concerning use of frequencies and power levels.
Standardisation
If the unique advantages and flexibility of RFID is the
good news, then the proliferation of incompatible RFID
standards is the corresponding bad news. All major RFID
vendors offer proprietary systems, with the result that
various applications and industries have standardized on
different vendors' competing frequencies and protocols. The
current state of RFID standards is severe disarray - standards
based on incompatible RFID systems exist for rail, truck, air
traffic control, and tolling authority usage. The US
Intelligent Transportation System and the US Department of
Defense (DOD) Total Asset Visibility system are among other
special-interest applications.
The lack of open systems interchangeability has severely
crippled RFID industry growth as a whole, and the resultant
technology price reductions that come with broad-based
inter-industry use. However, a number of organizations have
been working to address and hopefully bring about some
commonality among competing RFID systems, both in the U.S. and
in Europe where RFID has made greater market inroads.
Meanwhile in the U.S.A., ANSI's X3T6 group, comprising major
RFID manufacturers and users, is currently developing a draft
document based systems' operation at a carrier frequency of
2.45 GHz, which it is seeking to have adopted by ISO. ISO has
already adopted international RFID standards for animal
tracking, ISO 11784 and 11785.
Just as standardisation enabled the tremendous growth and
widespread use of bar code, cooperation among RFID
manufacturers will be necessary for promoting the technology
developments and refinements that will enable broad-based
application growth.
Further Information This document has been published by
AIM International with input and co-operation from all AIM
Affiliates. On the next page, you will find the contact
information for all of the AIM Affiliates world-wide. You are
invited to contact your local affiliate to determine the
latest work that is in progress in the RFID arena or any other
Automatic Identification and Data Capture Technology.
A list of all AIM publications can be obtained by visiting
the AIM Global web site at http://www.aimglobal.org/
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