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Cascading RFID Tags

by Jeffrey D. Lindsay and Walter Reade

The following article on nested or cascading RFID tags was published Dec. 23, 2003 on IP.com as Article 21112D, available at https://priorart.ip.com/viewPub.jsp?pubID=IPCOM000021112D.

Related articles at JeffLindsay.com on radiofrequency identification (RFID) technology:


Table of Contents


Cascading RFID Tags

Jeffrey D. Lindsay and Walter Reade
Nov. 7, 2003

Abstract

RFID chips can be employed at multiple hierarchical levels in an integrated supply chain model to convey redundant information for error detection and security purposes. The use of "cascading" or nested sets of RFID tags can provide a number of practical advantages to business integrating RFID technology in the supply chain.

Introduction

A revolution in supply chain management may be achieved through the use of Radio Frequency Identification (RFID) technology or smart tags. RFID technology refers to passive or active smart tags (miniature antennae-containing tags requiring no internal power supply) that can be embedded in or attached to a product or material to convey information that can be read by a scanner. In general, RFID systems consist of readers and tags in which the tags generate an electromagnetic response to an electronic signal from a reader. The response signal can be read by the reader, typically with a readable range on the order of a few feet, though broader or narrower ranges are possible. The signal generated by the smart tag can contain information (an electronic product code) that identifies the tag or the article including the tag. Using protocols, standards, and systems being implemented at MIT's Auto-ID Center and elsewhere, many corporations are working to implement RFID to improve the tracking of goods in the supply chain and to improve inventory handling, reduce out-of-stock events, and reduce the time required to provide goods to the customer.

In pilot trials done by Gillette and Wal-Mart, several problems in implementing RFID technology have been encountered. In a warehouse or distribution center, some tags fail to be read by scanners due to shielding of radio signals resulting from radio frequency (RF) blocking materials (e.g., metal and liquids), interference between multiple tags, nodes in the distribution of emitted radio signals, distance between tags and scanners, and other factors. For example, in a pallet with multiple stacked cases of product, products in the center may not be read easily, while those on the outer portions of the pallet may be readily detected by scanners. Failure to scan and read all products in a case, on a pallet, in a truck, or in a warehouse can result in inefficiencies and loss. An improved system is needed to ensure that products are detected even when there is RF shielding or other problems that cause some tags in a group of products not to be read.

Cascading RFID Tags

We propose that improved applications of RFID technology in the supply chain can be achieved by incorporating cascading smart tags, wherein groups of products such as cases, pallets, or truckloads are associated with a "macro tag" that provides information about smaller groupings of products or individual products and their associated tags. For example, a case of 24 tagged products can have a macro tag on the case that can provide information about the 24 individual units in the case. The tag can contain a code that can point to a database with previously scanned information for each of the 24 units (e.g., scanned when the units were assembled into the case, or before collection). A pallet of such cases (say, 12 cases) can be provided with a higher-level macro tag that contains or points to information for each of the macro tags for the cases. Thus, information for each case can be retrieved by scanning the single macro tag for the pallet, and information for each of the units in any of the cases can be obtained once the code for the case is scanned or known from reading the pallet macro tag. The hierarchical structure of the tag-related information is shown in Figure 1. Depicted is a pallet containing multiple cartons, each of which contains multiple product packages. Each product package has an RFID label, as does each carton and the pallet itself. The case RFID tags provide information about the enclosed packages, and the pallet tag provides information about each of the case tags.

Pallet of goods showing nested RFID tags.

Figure 1. Hierarchy of RFID tags on a pallet showing three levels of tags on products, cases, and the pallet.

In general, RFID chips can be used to track products grouped in various hierarchies: (1) individual items or single packages containing multiple items for consumer purchase; (2) cartons or cases of multiple items; (3) pallets of multiple cartons or cases; and (4) loads (e.g., truckloads, shiploads, or railcar loads) of multiple pallets. The products at each of these levels may be assigned an RFID label that is associated with information pertaining to at least one adjacent hierarchical level. For example, an RFID label on a pallet may be associated in a database with the RFID labels for each carton on the pallet, or may be associated with data pertaining to the RFID label from the truckload.

A related concept to the cascading RFID system has been proposed by Savi Technology (Sunnyvale, California), which announced a collaborative effort with Matrics Technology to combine short- and long-range RFID tags for "nested visibility" in the supply chain, as reported in the news item, "Passive, Active RFID Tags Linked," RFID Journal, July 24, 2003, available at http://www.rfidjournal.com/article/articleprint/512/-1/1/, as viewed July 25, 2003. In the past, the article notes, Savi Technology's customer would write information scanned from bar codes on individual boxes and write the summary information to a read-write active tag (Savi's EchoPoint tag). By replacing the barcodes on boxes with passive RFID chips, the contents of a container or trailer having multiple boxes will be more visible, allowing the RFID tags to be scanned within the container or trailer for comparison to the written information on the active tag. This comparison can enable detection of missing items or point out possible tampering. Savi proposes replacing barcodes at both the carton and pallet level with passive tags.

The concept of nested or "cascading" RFID systems can be extended even further, or offered in other combinations. Here we propose a number of additional variations that might be of benefit to the supply chain and other applications.

Multiple Tag Technologies

Among expected advances in nested or cascaded tag systems, we propose that multiple types of tags can be used in addition to homogenous systems using one tag type. For example, macro tags at any level may operate in substantially the same way as the tags at other levels or may differ from the product tags or tags at any other level in any of several ways. For example, a macro tag may operate at a different frequency than a product tag (e.g., 13.2 GHz vs. 2 GHz, or UHF versus 100 kHz), or operate using ultra-wide band wireless technology while product tags use conventional RFID (single frequency) technology. A macro tag may be active while product tags are passive, or may have a substantially longer range than product tags (e.g., 100 yards versus 10 feet), which can be achieved, for example, with differences in antenna size (larger antennae generally have larger ranges). In another embodiment, macro tags at one level may be programmable while product tags (or tags at other levels) may have fixed codes (e.g., standard RFID technology).

By using tags with two or more technologies, it may be possible to read macro tags only--without interference from product tags--or to read tags at only one level of the hierarchy. Reading tags at a high level can be used to reliably extract information at lower levels, or to verify the accuracy of the information obtained from lower level scans. Again, the scans at different levels of the hierarchy may employ differing frequencies or signal strengths to simplify the scanning process and keep it targeted at the level of interest.

Further, the various levels of the system need not all employ RFID tags or rely exclusively on RFID tags for identification. Any one or more of the labels may, for example, use bar codes or other vision-based identification systems instead of or in addition to RFID technology.

Writing Information to Databases

Writing the summary information about the contents of a container or trailer to an active RFID tag on a container is a known method, but the information can also or alternatively be stored in a database where it can be associated with an identifier code (e.g., an electronic product code) from the container's tag. In this case, the container's tag need not be a writeable tag, but only need contain a unique identifier that can served as a pointer to the associated information in a database. The information associated with the container's tag may be an abbreviated summary of the contents, a complex listing of the encoded information on the RFID codes of the contents, or other levels of information about the contents (e.g., links to MSDS data, manufacturing data, etc.) and other information such as the manufacturing history and shipping history, or previously obtained GPS data for shipments. In one embodiment, the stored data can include factors such as the measured or expected weight of the cases, cartons, or pallets for subsequent comparison to scale-measured weight (e.g., upon receipt) as another means to automatically detect shrinkage or other errors.

Multiple Levels of Nesting

Nesting of cartons, pallets, and containers or trailers has been proposed, but nesting can be extended to further levels. For example, individual products within the carton may be tagged, and the RFID tag on the carton may then either contain a list of the electronic product codes of the items within it or have an electronic product code that is linked in a database to the read contents of the items within the carton. Further, an individual product may include multiple components that have been assembled, and one or more of the components may also have an RFID tag, whose code or other information is contained within the information on the product's RFID tag or can be accessed using the electronic product code of the product as a pointer. In addition, multiple containers with their RFID tags may be stored within a larger container, which again may have an RFID tag that contains or can be used to access information about the container nested therein. Thus, there may be nesting of RFID tags for the following levels: components, products, cartons of products, pallets of cartons, containers of pallets, and super-containers. If desired, the RFID tag at any level could contain or provide access to information about all lower nested layer, or, if desired, only about one or more of lower levels. This concept can be extended to containers within a container zone such as a section of a ship, or for one or more ships within a certain region such as a harbor, or for other such variations. Nesting can also be done at levels beyond supercontainers such as ships, trucks, or fleets of trucks.

In some cases, the cascading structure may not be truly nested. For example, one macro tag may describe the contents of two or more related shipments on two or more pallets that are not physically joined, while each pallet may further have macro tags describing the contents of the pallets, in a nested relationship with the cartons and products of the pallet.

Forms of Nesting

Nesting of active and passive RFID tags has been proposed, but nesting can be done with any combination of RFID tag types and frequencies. Examples include:

The active/passive characteristic is not all that can be varied. Whether they are active or passive, the tags in the nested system can also vary in terms of other features, such as frequency, writeability, and range (e.g., antenna size). Thus, one or more levels of a nested system may operate at UHF frequencies (e.g., 920 MHz, 2.45 GHz, or 5.48 GHz), while other levels operate at, for example, 13.56 MHz, which would allow differentiation of levels by using different readers or coordinated systems within the same reader. Writeable tags could be used for one or more levels, if desired, or the entire system could be based on passive RFID tags.

In addition to considering a combination of active and passive tags, semi-active tags could be used at any level as well. For example, PowerPaper's Power ID semi-active tag system could be used for improved sensitivity to RFID reading.

Redundancy for Improved Accuracy

The use of cascading levels of smart tag-linked information to track products can be analogous to "checksum" methods used in data transmission technology to validate the accuracy of data that has been sent or received. Macro tags can be encoded with checksum information or with detailed listings of what should have been scanned at the lower level, and this can be used to validate the scan. Inconsistencies may be due to detection difficulty (e.g., tags shielded by metal or water), or may indicate tampering or theft has occurred. Further actions may be taken to determine the cause of the inconsistency, such as measuring the weight of a pallet, calling for manual inspection of the shipment, or activating a machine vision system to verify the presence of items and to resolve the inconsistency.

For example, if an RFID tag on one level contains or is associated with all the information from the RFID tags at the next lower level, a scan of the higher-level tag can provide the same information as a scan of the previous level tags, and conducting both scans can be used to verify the accuracy of the scan, or possibly to identify missing or damaged items. Scanning of items at multiple levels, such as three or more, for comparison of redundant information can also be done as a check on the accuracy of scanning or as a means of identifying other problems.

Summary

Multiple hierarchical levels of RFID tags can be applied in a wide variety of forms to enhance logistics and information flow in the supply chain. Benefits of improved product tracking, ease of scanning (e.g., by receiving information from easily-read active macro tags), and error checking can be expected as various cascading RFID systems are employed.

Appendix: RFID Basics

Basic principles of RFID technology are given by Raghu Das, "RFID Explained: An Introduction to RFID and Tagging Technologies," white paper from IDTechEx Ltd. available at www.idtechex.com, dated 2003, as viewed Aug. 19, 2003. The author defines RFID as follows:

Radio Frequency Identification (RFID) is the use of radio frequencies to read information on a small device known as a tag.... Radio frequency Identification (RFID) is a term used for any device that can be sensed at a distance by radio frequencies with few problems of obstruction or misorientation. The origins of the term lie in the invention of tags that reflect or retransmit a radio-frequency signal. In its current usage, those working below 300Hz and those working above 300MHz, such as microwave (GHz) tags, are included. For example, one type of chipless tag works at only a few hertz and Inkode chipless taggants operate at around 20-25 GHz. Higher frequencies such as visible and infrared devices are excluded as these systems have very different properties and are frequently sensitive to obscuration, heat, light and orientation.

The term "tag" is used to describe any small device -- shapes vary from pendants to beads, nails, labels or microwires and fibres that can be incorporated into paper and even special printed inks on, for example, paper.

Examples of potential applications for smart chips attached to products and wireless communication systems are described by C.R. Schoenberger in "The Internet of Things," Forbes, March 18, 2002, pp. 155-160. Further information on these technologies is provided at http://www.mccombs.utexas.edu/news/magazine/01f/alien.asp and http://www.mindfully.org/Food/The-Code.htm, and more technical information has been provided by MIT's Auto-ID Labs (formerly the Auto-ID Center), Cambridge, Mass. Useful information from the Auto-ID Labs can be accessed from www.autoidlabs.org/ and other information can be obtained at http://archive.epcglobalinc.org/aboutthetech.asp. See also "Toward the 5 Tag" by Sanjay Sarma, Auto-ID Center, Nov. 2001, available at http://www.autoidlabs.org/whitepapers/mit-autoid-wh-006.pdf, and "Integrating the Electronic Product Code (EPC) and the Global Trade Number (GTIN)," by David L. Brock, Auto-ID Center , Nov. 2001, available at http://www.autoidlabs.org/whitepapers/mit-autoid-wh-004.pdf.

An overview of the history of RFID is given by Jeremy Landt in "Shrouds of Time: The History of RFID," AIM (Association for Automatic Identification and Data Capture Technologies), Pittsburgh, Pennsylvania, Oct. 1, 2001, available at http://www.aimglobal.org/technologies/rfid/resources/shrouds_of_time.pdf.

RFID chips can be used to track products grouped in various hierarchies: (1) individual items or single packages containing multiple items for consumer purchase; (2) cartons or cases of multiple items; (3) pallets of multiple cartons or cases; and (4) loads (e.g., truckloads, shiploads, or railcar loads) of multiple pallets. The products at each of these levels may be assigned an RFID label that is associated with information pertaining to at least one adjacent hierarchical level. For example, an RFID label on a pallet may be associated in a database with the RFID labels for each carton on the pallet, or may be associated with data pertaining to the RFID label from the truckload.

RFID tags of any known type may be used, including:

While RFID tags typically include a semiconductor chip associated with an electronic code, chipless RFID technologies are also known.

Examples of low-cost technologies for producing chip-based RFID systems listed by Raghu Das (2003) include:

For chipless RFID, Das lists the following as available low-cost RFID technologies:

Transistor-based chipless circuits can include laminar transistor circuits, including polymer film circuits such as those of Plastic Logic, Inc. or Philips Research Laboratories, thin film silicon, bioelectronics, etc. Magnetic wire and fiber systems for chipless RFID can include the products of MXT (Canada), HID (US), British Technology Group (UK), Fuji Electric (Japan), REMOSO Development (Netherlands), and Advanced Coding Systems (Israel). Laminar transistorless circuits can include diode circuits, printed electronics, EM tags of ACS, surface acoustic wave technology such as that of RF SAW, Inc., and LC arrays (swept RF) such as the devices of CWOSRFID (US) and Lintec (Japan), Miyake (Japan), Checkpoint (US), MIT Medialab (US), the rewritable chipless tag of Navitas (Japan). Thin magnetic films for chipless RFID can include conventional magnetics for non-contact reading, or the films of Flying Null (UK), 3M Intelligent Transportation Systems (US), Scipher TSSI (UK), etc. Magnetic assembly can also be used, such as the technologies of Scientific Generics (UK). The self-generation RFID technology of Siemens Roke Manor Research can also be employed.

In general, RFID chips may be read-only chips, which include a fixed electronic code, or they may be read-write chips, which allow new information to be added. The chips may also be associated with sensors to read sensor information and transmit a signal responsive to the information, such as a value from a biosensor. Exemplary smart labels including RFID technology associated with a sensor are the active labels of KSW Microtec (Dresden, Germany), including TempSens® active smart labels for measuring and recording temperature. KSW Microtec also offers smart labels produced by flip chip assembly methods.

RFID tags can take many physical formats, such as a microchip from 30 to 100 microns thick and from 0.1 to 1 mm across, joined to a minute metal antenna such as the Hitachi 2.45 GHz Mew chip, or they can be in the form of deposited alloys 0.5 to 5 microns thick on a 20 micron polyester ribbon 1 mm across as used in some banknote security ribbons. Another form is the "Coil-on-Chip" system from Maxell (Tokyo, Japan), which is a 2.5 mm square IC with a coil mounted directly on the chip. The chip is a read-write chip with 108 bytes of storage.

In addition, related devices such as the PENI tag of the University of Pittsburgh can also be considered for identifying objects wirelessly.

Exemplary RFID vendors include Matrics, Intermec, Alien Technology, Philips Semiconductor, and Texas Instruments. Manufacturing can be done by robotic techniques (e.g., "flip-chip"/"pick and place" techniques), fluidic self-assembly (FSA), the Philips "I-connect" method or the Philips "vibratory assembly" method, the Matrics PICA system (Parallel Integrated Chip Assembly, as described in the news item "New High-Speed RFID Tag Machine," RFID Journal, Sept. 19, 2003, available online for subscribers at http://www.rfidjournal.com/article/articleview/586/1/1/) or other known processes. (See also L. Frisk, J. Jarvinen, and R. Ristolainen, "Chip on Flex Attachment with Thermoplastic ACF for RFID Applications," Microelectronics Reliability, 42(9-11): 1559-1562 (Sept.-Nov. 2002)). Also of potential use for tracking and finding objects is the "mu-chip" of Hitachi with a built-in antenna on a sub-millimeter chip having a 128-bit serial number, as described by Jonathan Collins, "Hitachi Unveils Integrated RFID Tag," RFID Journal, Sept. 4, 2003, available at http://www.rfidjournal.com/article/articleview/556/1/1/. Exemplary RFID reader manufacturers include Intermec Technologies, Symbol Technologies, Matrics, AWID (e.g., their multi-protocol reader that can operate at various frequencies), and others. Software systems to support RFID systems are provided by IBM Global Services (which has acquired PriceWaterhouseCoopers), Texas Instruments, Manhattan Associates (particularly for integrated supply chain executions), SAP, and others. Printed RFID labels can be made using equipment from Zebra Technologies and other vendors.

Readers may also be integrated into or added onto a laptop, a PDA device, a cell phone, or other electronic device. Suitable readers may include the readers of AWID (see Jeffrey Jacobsen, op. cit.) or the RFID reader on a compact flash card marketed by Syscan International for reading 13.56 MHz ISO-compliant tags or for other frequencies, as described in the news item, "Get RFID Readers in a Flash (Card)," RFID Journal, April 22, 2003, available online for subscribers at http://www.rfidjournal.com/article/articleview/393/1/1/.

Chip-based RFID systems need not be limited to silicon chips, but can also include printed electronics, particularly polymer electronics (organic electronics) such as organic field effect transistors (OFETs), and other technologies. Principles of polymer electronics are given by J. M. Shaw and P. F. Seidler, "Organic Electronics: Introduction," IBM Journal of Research and Development, Vol. 45, No. 1, 2001 (http://www.research.ibm.com/journal/rd/451/shaw.html). See also PCT publication WO 99/19883, published April 22, 1999 by S. Babinec et al. of Dow Chemical. A representative manufacturer of printed electronics technology is Precisia, LLC (Ann Arbor, Michigan), a business unit launched by Flint Ink (Ann Arbor, Michigan). Precisia, LLC produces printed electronics for RFID systems, including smart packaging, lighting, and displays. Conductive inks manufactured by Precisia, LLC including conductive particles of silver or carbon have been proposed for use in printed RFID antennas. Such inks can be applied by screen printing, flexographic printing, lithographic printing, gravure printing, ink-jet printing, and the like. Plastic Logic (Cambridge, England) is another firm producing printable electronics suitable for RFID applications.

Other components associated with RFID systems can also include polymer electronics or printed electronics. For example, display graphics can include organics LEDs (OLEDs), printed electroluminescent displays, printed organics application specific integrated circuits (organic ASICs), polymer thin film transistors (pTFTs), the light-emitting polymers (LEPs) of Dow Corporation (see www.lumation.com and Appl. Phys. Letters, Vol. 77, 2000, p. 406), and the like.

Power sources may include printed batteries, such as those produced by PowerPaper (Einat Israel) or Cymbet Corp. (Elk River, Minnesota--see http://www.rfidjournal.com/article/view/94) or may rely on energy harvesting techniques that convert RF energy into useful electrical energy.

RFID tags can be assembled using flip chip technology, in which chips from an RFID wafer are inverted and placed in contact with an antenna. Exemplary processes include the Matrics PICA process for chip attachment to the antenna.

The RFID system may follow the systems proposed by the MIT Auto-ID Center, including the use of an electronic product code (EPC); a Savant system to manage the codes being read with a distributed architecture and processes such as data smoothing, reader coordination, data forwarding, data storage, and task management; and Object Name Service (ONS) for matching EPC information to item information, typically using a Domain Name Service (DNS) to route computers to Internet sites; and Physical Markup Language (PML) to describe information about a product.

Other vendors of integrated RFID systems or other tools for RFID include CheckPoint Systems, Tyco Sensormatic, Escort Memory Systems, Psion Teklogix (particularly for software systems to assist in logistics), SAMSys Technologies, Savi Technology, SCS Corporation, TAGSYS, ThingMagic LLC, and others. Supply-chain software can be provided by Crimson Software, Descartes Systems, EXE Technologies, Globe Ranger, Manhattan Associates, IBM Global Services, SAP, and others. RFID readers include those of Alien Technology, Matrics, Intermec, iPico, and AWID (Applied Wireless Identification Group, Hollister, California).

For a given retail supply chain, the RFID system can operate for all products that have an RFID tag. The tag may be provided by the manufacturer, the retailer, or others, and may be embedded in the product, attached to the surface of the product by a label or adhesive means, or be otherwise physically associated with the product. The RFID tag may have a unique electronic product code or other code, or optionally may include more extensive information. As the product is received by a retailer, the product may be scanned and its code or other information from the chip can be obtained. Logged information about manufacturing, shipments, etc., may also be accessed from a database using the electronic product code as a pointer, or may be otherwise associated with the RFID tag, and this information may be screened or downloaded from a database by the retailer. As the retail product is received and stocked or shelved by the retailer, its location may also be recorded in a database, where such information can be associated with the electronic product code or with an entry for the product.

RFID-enabled products can be integrated into supply chain systems using any known tools. One exemplary software tool is Auto-ID Infrastructure (AII) marketed by SAP AG (Walldorf, Germany) for integration with SAP and non-SAP software systems, as described in the news article, "SAP Takes RFID into the Enterprise" by Bob Violino, RFID Journal, Oct. 13, 2003 (available online at http://www.rfidjournal.com/article/articleprint/613/-1/6/, as viewed Oct. 31, 2003). According to Violino:

A key function of the infrastructure will be managing the massive amounts of data that will be generated by item-level tagging. But SAP says its Auto-ID Infrastructure will have other important functions. It will capture, filter and publish data--including product location, shelf life, price and inventory level--from many readers. It will aggregate and store information about shipping containers, pallets, cases and items. And it will receive and maintain data specifying the location and physical relationships between items, such as a particular item's location within a particular container.

Finally, an alternative to typical low-range readers is to use long-range readers with directional antennas that sweep selected areas to identify objects based on their RFID code. For such systems, highly sensitive electronics can be used to resolve faint signals at larger distances. By using advanced antennas in particular, RFID tags may, in some cases, be read from larger distances than previously recognized. The US military, for example, has developed sensitive electronics to extend the read range of passive RFID tags to large distances, as described in the article, "RFID Sensors: From Battlefield Intelligence To Consumer Protection," RFID Journal, Aug. 12, 2002, available online at http://www.rfidjournal.com/article/view/182. Such ultra-sensitive radio frequency receivers rely on filtering systems to separate the RFID signal from background noise.

One antenna technology that may be particularly useful in amplifying weak signals and determining the direction of a radio signal source is smart antenna technology, such as adaptive antenna arrays including a plurality of antennas cooperatively associated with processors that continually readjust the radio signals from each of the antennas to create radio systems with extremely precise directionality and the ability to greatly amplify detected signals. Basic information about such smart antenna systems is provided by Martin Cooper in "Antennas Get Smart," Scientific American, Vol. 289, No. 1, July 2003, pp. 49-55. Further information is given by Martin Cooper and Marc Goulburg, "Intelligent Antennas: Spatial Division Multiple Access," 1996 Annual Review of Communications, pp. 999-1002 (see www.arraycomm.com/Company/spatial_division.pdf); in Joseph C. Liberti and T.S. Rappapport, Smart Antennas for Wireless Communications: IS-95 and Third Generation CDMA Applications, New York: Prentice Hall, 1999; and in various papers available at www.arraycomm.com/Company/white_papers.html. By way of example, manufacturers of adaptive antenna arrays include ArrayComm (San Jose, California) and Lucent Technologies (Murray Hill, New Jersey). Other antenna arrays can also be used, such as switched-beam arrays. Other beam-forming and beam steering technologies can be applied as well, such as the directional steerable antenna systems of Antenova Ltd. (Cambridge, England, see www.antenova.com/).


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