GB2387744A

GB2387744A - Transponder alarm system

Info

Publication number: GB2387744A
Application number: GB0204999A
Authority: GB
Inventors: William Swire
Original assignee: SNITCH Ltd
Current assignee: SNITCH Ltd
Priority date: 2002-03-04
Filing date: 2002-03-04
Publication date: 2003-10-22
Also published as: GB0204999D0

Abstract

A method of monitoring a zone comprises interrogating the zone to detect tags (transponders) present in said zone, providing an alarm upon detection of a first tag (PT1) and no second tag (CT1) in said zone, and providing no alarm upon detecting said second tag (CT1) in said zone. A timing period is started when the first tag (PT1) is detected in said zone. The alarm is inhibited until the timing period elapses whereby no alarm is provided if the second tag (CT1) is detected in said zone before said timing period elapses. The tags may also transmit identification data. The tags may further transmit data relating to the identity of tags which are authorized to partner the tag. For example, a Parent tag (PT1) may transmit the identies of the Child Tags (CT1-4) that it can accompany.

Description

\ -1 ALARM SYSTEM

The present invention relates to a method of monitoring a zone, to a method of indicating unauthorized 5 removal of an asset through a zone, to a system for monitoring a zone, a system for indicating unauthorized removal of an asset from a zone, to a set of transponders and to a method of identifying a target transponder.

10 Alarm systems using transponders are well known. Such transponders are also sometimes known as labels and tags and are referred to hereinafter as tags.

Tags are of two types, passive tags and active tags.

15 Passive tags are devices that have no power source of their own and include tags that couple an excitation field from a

transmitting antenna to a reception antenna, and tags that respond to an excitation field of an excitation antenna so

as to output harmonics for pick up by a reception antenna.

20 Active tags have their own power supply, for example in the form of a battery, or may alternatively derive power from an interrogating field for temporary storage in for example

a capacitor, and use that power to supply a responding circuit. Tags may respond to radio frequencies (RF tags) or 25 low frequencies (inductive tags); some tags respond to one type of frequency (e.g. low frequency) and provide an output in a different frequency range. Tags are also known which are responsive to sound or light.

30 The present invention is not restricted to any particular type of tag, but is instead more concerned with the way tags may be used together.

-2- Embodiments of the invention relate to systems and methods in which tags are used in relation to one another to enhance security of tagged items, and at the same time 5 provide improved convenience in use.

Conventional tag/reader applications use the information on a single tag to inform security or stocks control systems about what assets are passing through the 10 field of the reader. This is typically used in

warehousing/logistics and retail environments. The amount of data stored within each tag differs per application and manufacturer; tags typically store between 1 bit and 128 Kbits of information.

Tags with smaller storage are typically used in a retail environment where an alarm system is triggered when an article for sale is taken out of a store without having been paid for - for example, the single bit, set to '1'/ 20 'ON' for articles on shelves is set of '0' / 'OFF' when the article is paid for, and therefore if a tag passes through the RF portal (typically the exit of a store) with the bit set '1' / 'ON' then an alarm is triggered alerting store staff to the passage on an unpaid object. Tags with larger 25 storage can be used in more advanced 'supply chain' solutions where information about the route/flow of a managed (tagged) object is stored on that object's tag.

Another use of tags, this time in a public 30 environment, is in library systems, where books are labelled/tagged with numbers identifying an individual book (numbering system dependent upon implementation). In

this case, an automated library system can track assets in and out of the library.

Embodiments of the invention described here may be 5 used to attract the attention of owners of possessions, of managers of premises, or security staff to the movement of a stolen item (or any other item without its owner) through the entrance, exit or another zone. A number of applications exist additional to the core system when a 10 stolen or like article is discovered passing through the entrance or exit, including but not limited to alarm sounds, lights, video camera, snapshots, asset logging, etc. The system may be used to reduce theft through a reliable network of units which can identify unauthorized 15 passage of assets.

The described embodiments relate to an application of Electronic Article Surveillance ('EAS'), based upon the relatively mature Radio Frequency Identification (RFID) 20 technology, as manufactured by such companies as Texas Instruments, Motorola, Philips, and others. Whilst many companies manufacture both RFID tags (also known as transponders, cards, labels) and readers (antennae, reader units, base stations and related hardware) the embodiment 25 relates to how a processing unit uses relationships between tags passing through the RF field (also known as RF portal,

reader field) to infer the relationship between an Owner

(parent tag) and its associated assets (child tags). A mismatch between these tags or a missing parent/owner tag 30 within a short time period of its child/asset tags passing through a reader field may trigger an alarm.

-4- It will of course be clear that other identification technologies could be substituted for RFID.

According to a first aspect of the present invention 5 there is provided a method of monitoring a zone, the method comprising interrogating the zone to detect tags present in said zone, providing an alarm upon detection of a first tag and no second tag in said zone, and providing no alarm upon detecting said second tag in said zone.

In this way, a tagged asset in the zone without its tag-carrying owner will trigger an alarm.

Preferably, the method comprises starting a timing 15 period when the first tag is detected in said zone; and inhibiting the step of providing an alarm until said timing period elapses whereby no alarm is provided if the second tag is detected in said zone before said timing period elapses. If the alarm were triggered immediately a first tag were detected in the absence of a second tag, the system would constantly be switching to the alarm state, since statistically even where both first and second tags were 25 present, the first tag would be likely to be noted before the second around 50 per cent of the time. By providing a timing window each time a first tag is detected, incipient false alarms are prevented.

30 Advantageously the method comprises extending the timing period when a further first tag is detected in said zone.

- - This allows for two or more assets to be detected in the presence of their owner and before detection of the owner. According to a second aspect of the invention there is provided a method of indicating unauthorized removal of an asset through a zone, comprising providing the asset with a first tag; providing a person authorised to remove the 10 asset with a second tag; interrogating the zone to detect tags present in said zone; and based upon the interrogation results, outputting an alarm upon detection of the first tag and no second tag in said zone; and not outputting the alarm upon detection of the second tag in said zone.

Preferably the system comprises an interrogation device for interrogating the zone to detect tags present in said zone, and first and second tags, the device in use providing a first response if the first tag is detected in 20 said zone but the second tag is not detected in said zone, and a second response if the second tag is detected in said zone. Advantageously the interrogation device comprises 25 delay means for inhibiting the first response for a set time period such that if the second tag is detected during said set time period, the interrogation device provides only said second response.

30 According to a third aspect of the invention there is provided a system for indicating unauthorized passage of an asset through a zone, comprising a first tag secured to the asset; a second tag carried by a person authorised to

-6- remove the asset; and an interrogation device disposed to detect tags present in said zone, the device in use providing a first response if the first tag is detected in said zone but the second tag is not detected in said zone, 5 and a second response if the second tag is detected in said zone. According to a fourth aspect of the invention there is provided a method of indicating unauthorized removal of an 10 asset through a zone by the use of plural tags, comprising a tagging step, the tagging step comprising providing the asset with a first tag and providing a person authorised to remove the asset with a second tag; interrogating the zone to detect tags present in said zone; identifying any 15 detected tags and based upon the identification results; outputting an alarm if the first tag is detected in said zone but a second tag is not detected in said zone; and not outputting the alarm if the second tag is detected in said zone. Preferably each of said plural tags has a respective identity and each of said plural tags stores data representative of its identity; the tagging step comprises storing on the first tag additional data representative of 25 the identity of the second tag; and the step of identifying comprises reading said data and said additional data.

In another embodiment each of said plural tags has a respective identity and each of said plural tags stores 30 data representative of its identity; the tagging step comprises storing in a database a relationship between the identity of the first tag and the identity of the second

-7- tag; and the step of identifying comprises accessing said database. According to a further aspect of the invention there 5 is provided a set of transponders comprising a first transponder and a second transponder, each transponder storing data indicative of the identity of itself, wherein data stored on the first transponder is indicative of the identity of the second transponder.

The transponders are thus related together, and access to the data on the first allows the second to be identified, and discriminated from other transponders that are not so related.

According to yet a further aspect of the invention there is provided a set of transponders comprising a first plurality of transponders, each of said first plurality of transponders having an identity, each transponder storing 20 data indicative of its identity, and a second plurality of transponders, each of said second plurality of transponders having an identity and each storing data indicative of its identity and further storing data indicative of the identity of a respective one of the transponders of said 25 first plurality.

According to a still further aspect of the invention there is provided a method of identifying a target transponder from a first plurality, the method comprising 30 providing a first plurality of transponders wherein each of said first plurality of transponders has an identity and each transponder stores data indicative of its identity; providing a second plurality of transponders wherein each

-8- of said second plurality of transponders has an identity and each of said second plurality of transponders stores data indicative of its identity and further stores data indicative of the identity of a respective one of the 5 transponders of said first plurality; interrogating a transponder of said second plurality whereby said transponder responds with said data indicative of the target transponder.

10 Thus it will be seen that a feature of embodiments of the present invention is the use of a parent-child relationship between plural identified tags passing through a reader's field (within a short period of time) to alert

the RAID reader to the movement of a possession or asset 15 without its owner.

Exemplary embodiments of the invention will now be described with reference to the accompanying drawings, in which: 20 Figure 1 shows a block diagram illustrating of a hierarchical parent-child relationship between tags; Figure 2 shows a block diagram of an alarm device embodying the invention; Figure 3 shows an example of a Tag Processing pipeline 25 useable in the invention; and Figure 4 shows timing diagrams illustrating scenarios and their outcomes (including detail of session activity).

Reading and Writing of tag data may be achieved using 30 existing tag and reader equipment (from the aforementioned manufacturers, as an example). The parent-child relationship between tags may be encoded within either a programmable reader or an attached processing device.

9 - Modern RFID readers can discriminate between many tags passing through the RF portal in a short time - the speed of tag reads has increased steadily in the lifetime of this technology: this allows a device in accordance with the 5 invention to match parent-child and child- parent relationships for tags moving through the RF portal field

within the available timeframe.

To illustrate, consider the example of Figure 1.

10 Figure 1 shows a parent tag (PT1) and four child tags (CT1, CT2, CT3, CT4) related to parent tag PT1. If PT1 moves through a field within a fixed time period of all of CT1,

CT2, CT3 or CT4 (in any sequence of CT1..4 and PT1) then an owner (parent tag) is accompanying its assets (child tags).

15 In this safe situation any security alarm warning of assets without owners remains inactive.

If, however, any instance of CT1..4 should pass through the RP Portal without the identification of their 20 parent tag PT1 then the security/alarm system is triggered, as an asset is moving through the portal without its owner present. An alternative and more exact time proximity mechanism 25 is described later herein with respect to Figure 4. Thus in accordance with this embodiment of the invention, individuals may be provided with unique, permanent 'parent' tags (i.e. PT1 above) and such individuals may tag assets which are correctly moved with them but whose movement 30 without them would be unwanted. By use of the method and system of the invention, a warning may be given of the presence/movement of an asset without its owner.

-10 This brings about a fundamental change in the way that such devices may be deployed: whereas current RFID security systems are installed to protect assets in fixed locations, 5 the invention makes it possible to protect movement of owners' assets at any/many locations. The role of reader/base unit changes from guarding the assets on a premises to guarding the assets of a (mobile) owner. The present embodiment is based upon RFID technology. The 10 frequency of the tags used, and whether tags are active or passive is immaterial to operation of the embodiment, although these parameters may affect the usability of the solution. 15 A preferred embodiment of the system uses 13.56MHz passive tags which offer a cost-effective and standardized solution with good scan range and penetration, low susceptibility to noise and tag orientation. Using passive tags decreases tag maintenance and increases usability.

A logical schematic of this system is shown in Figure 2. Referring to Figure 2, a tag 1 is capable of interrogation via antennae 2,3,4. Each antenna 2,3,4 is connected to a respective reader 5,6,7. The readers are 25 connected to a common processing unit 10. The common processing unit 10 is coupled to a data storage device 11, a data input device 12 and to an alarm device 13.

In the embodiment, and as previously discussed, the 30 tag 1 receives power from an RF field created by one or

more of the antennae 2,3,4. After receiving power, the antenna emits an interrogation signal, and in reply to this

the tag 1 emits a signal including the identity of the tag on a frequency of 13.56MHz.

The antennae 2,3,4 provide the RF field for powering

5 the tags, transmit interrogation signals to tags passing within the transmission field, and receive identification

responses from tags. In the embodiment, each antenna connects to a reader through an impedance-matched coaxial cable 8.

The readers 5,6,7 power the respective antenna to which it is connected. Each reader includes logic and processing circuitry for powering tags, interrogating tags, and receiving and interpreting responses from tags passing 15 through the RF field of the antenna. The reader selected

for this embodiment is a Texas Instruments S6550A long range HE reader, which can scan passive tags up to 1.5m from the antenna. The internal logic of the reader is capable of managing tag transmission contention and 20 transmits tag information (Identities/data from tags read) back to the processing unit 10. As known to those skilled in the art, data other than the pure identity data may be transferred between the tag and reader - for example parity and checksum information, but the significant information 25 content to the present invention is the passage of identity data. In the present embodiment, the readers provide outputs to the processing unit 10 by an asynchronous serial 30 transmission protocol.

Transmission protocols between processing unit and reader differ for each reader manufacturer, but are well

-12 defined by each manufacturer in reference guides and generally rely on an asynchronous serial line for communication. It may also be noted that a small number of manufacturers use a standard Ethernet connection, and use a 5 well-defined TCP/IP-based protocol for this communication (very similar in structure to the asynchronous serial protocols). These are interchangeable and do not affect the technical effect of the device.

10 Thus in other embodiments, the reader connects to one or more antennae through an impedance-matched Coaxial cable (RF signals) and to a processing unit via an Ethernet cable. 15 The processing unit 10 runs the software application required to support the embodiment, and consists of a central processing unit (CPU) which performs the low-level logical operations required by the application, random access memory ("Memory") for storing the application code 20 and for storing variables and constants used by the application program, and a firmware component responsible for starting the unit and providing low-level functions for connection to other devices (communication with storage, reader, data input device and alarm device for example).

25 The processing unit 10 also includes a timer device used by the central processing unit for timing events.

Typically, and in order to lower costs, the processing unit 10 is a programmed general-purpose computer, i.e. low 30 end personal computer or equivalent. A PC becomes a good option for this when structured data requirements are considered as it provides strong processing capability with the ability to manage multiple simultaneous devices and to

-13 multi-process, at a low initial cost and maintenance cost for the above functions.

The data storage device ll is a hard disk which stores 5 the application program required by the processing unit, the processing unit's operating system, and data required by the application.

The data input device 12 is a PC/laptop or equivalent, 10 which connects temporarily to interrogate or update the unit through a selected communication channel - likely choices are Ethernet/network (TCP/IP), serial (wire connection or wireless connection, either a bespoke protocol or TCP/IP over PPP). A simple option is a standard 15 5-wire serial connection.

The data input device 12 is used to both interrogate and update the processing unit. Updates include data updates (updates to the parent/child data sets, updates to 20 the key required to decode tag Identities, update location/owner information), and application updates (altering the application which runs on the processing unit itself). Interrogation of the processing unit, through the data input device, includes reading out alarm violations 25 which have been monitored by the unit, with any associated data (for example, digital images of the theft/event taking place). The alarm device 13 of the embodiment is a 30 piezoelectric sounder of conventional type. Other devices are possible, including sirens, bells and the like, and visible warnings may be afforded by flashing lights and the like. The alarm device may also include a digital camera

-14 for recording the scene at the RF field when an alarm is

taking place (Such images can be recorded within the storage device by the processing unit).

Session Management 5 The processing unit maintains an in-memory cache of tags which have recently been identified. The period during which the unit 'remembers' tags identified will herein be called a tag's "Session".

10 The processing unit is capable of maintaining a very large number of simultaneous sessions (limited by memory size of the processing unit and performance constraints dictated by the processing unit's CPU).

15 Sessions may be of two types - Parent or Child sessions. Sessions also have a status - authorized (allowed to pass through the field) and unauthorized.

A parent session is created when a new parent tag is 20 identified in the zone. Parent sessions always have status authorized' (parent tags are allowed to pass through the zone). A child session is created when a new child tag is 25 identified in the zone. Child sessions, when created, may have an 'unauthorized' status if there are no active parent sessions within the cache which can authorize that child session. A child session will become authorized if a parent session (triggered by a parent tag related to the 30 child session's child tag) is created during its lifetime.

-15 The rules governing session behaviour in the system are as follows: A session is started when a new tag is identified by 5 the reader. When a session is started, it is also named (its name is set to the identified tag which started the session), its status is set (to 'authorized' if the tag is a parent tag or if there is an active session for the identified tag's parent).

A session is authorized: - when it is started, if the session is a parent session; - if the session is a child session, the session is 15 authorized when a parent session (triggered by a parent tag related to the child session's tag) is started.

A Parent Session is extended when a child tag (related to a parent session's tag) is identified in the zone.

A session ends through expiry, a constant time period after the last activity against that session. "Child sessions" time out a constant period after creation/start; "parent sessions" time out a constant period after start or 25 last extension.

In the security application, an alarm is triggered when a session ends with an 'unauthorized' status.

30 The two principal processes occurring in the application software are: 1. The tag identification process.

2. The session expiry process.

-16 Tag Identification Process The process triggered by identification of a tag is 5 shown in Figure 3. Each step in this process is explained below. Step 1: Identify tag 10 This step is performed by the reader, and is governed by international standards which describe processes by which tag identification (including contention) should be handled (ISO 15693). The reader powers the tag via the antenna and derives from the tag and again via the antenna, 15 identity data stored on the tag. It forwards the identity data to the processing unit.

Step 2: Decode tag 20 In the present embodiment, this step is performed by the processing unit. Whilst identifying a tag is within the functional realm of the reader unit, it is likely that - if a reader is placed in a public area - tags from other systems will be identified (given standardized tag 25 frequencies and identification mechanisms), and therefore the application must be able to distinguish between tags from other systems and tags intended to be interpreted by the present system. In the present embodiment tag identities are thus encoded.

In the described embodiments the information stored on each tag is protected by use of a public key cryptosystem

(asymmetric encryption). A brief description of how this

technology works is as follows: There exist two keys - conventionally known as public 5 and private keys. A key is a number of arbitrary length typically between 64 and 4096 bits. The relation between the numbers forming the private and public keys is such that when a piece of data (in this case, a tag's ID) is encrypted by use of the public key, it can only be 10 decrypted using the public key's corresponding private key.

Thus only the holder of the private key can retrieve the encrypted data. In normal data transmission applications, a public key is handed out to all who wish to send data to a party by the party wanting to receive transmissions, 15 ensuring that anyone can send data but only the party himself can interpret it.

In the present embodiment, an ID is encrypted onto a card using the public key - the encrypted number becomes 20 the major component of the card's own ID as identified by the reader. When the card has been identified, and the application needs to decode the card ID into its corresponding ID, then the private key is used by the device to decrypt the card ID (corresponding to the public 25 key used to encrypt the card ID originally). Contrary to the naming convention, in the embodiment both public and private keys are kept secret. The RSA public-key crypto system is used for encryption and decryption purposes: the keys remain secret to the device producer.

-18 Step 3: Identify Parent(s) In the present embodiment, this step is performed by the processing unit. For each child tag there exists one 5 (or more) parent tags. In order for the system to understand whether to trigger an alarm or not, it calculates or otherwise establishes the parent identity of a parent tag when a child tag is read, such that it can look for the presence of that parent tag.

In one embodiment the parent-child relationship is maintained "on-tag", by inferring the parent identity from a property (or sub-part) of the child identity stored on the child tag. In another embodiment, the parent- child 15 relationship is stored "off-tag" and is established by looking up the parent identity (or identities) in a local database on the processing unit.

In embodiments that maintain parent-child details "ON 20 TAG", the identification of a parent tag (PT) and child tag (CTl..n) ensures that the reader can infer a relationship by examining the two identifications through rule(s) rather than referring to a maintained database of parentchild relationships, with local representation on the processing 25 unit/storage device 11. This may be preferable for consumer applications, as it lowers reader/processor complexity and therefore cost.

When storing relationships between tags on the tags 30 themselves ("ontag"), it is not required that updates are made to data on each reader required to prove the relationships. Instead, tag reprogramming is required for each change of relationship. This option does limit the

-19 number of 'parent' tags which can 'own' a child tag due to space limitations within tag storage.

Maintaining parent-child relationships "off-tag" 5 requires updates to every database in the reader network when new tags or new relationships are issued, although it increases relationship flexibility. Such relationships can be maintained in a database accessible by the processing unit 10 for example in the storage device 11.

In one embodiment, a structure is provided on the tag.

Assuming that each tag is capable of storing 128 bits, the following bit map is selected: 15 Bits Usage 0-63 Parent Tag ID 64-111 Child Tag ID 112127 Issuer ID 20 The Issuer ID is a unique number to identify issuer as being authentic and admissible by device. In this embodiment, the mode of representation of each number (signed/unsigned, etc.) is not of concern.

25 Using the rule "if Child Tag ID = false then tag is a parent" (2exp64)1 parents and (2exp48)-2 children of each parent may be represented. No lockups are required, and the tags storage is maximised by not using a bit as a parent flag. Note that in this case, the system is limited to one 30 parent tag only for each child (Parent-child relationship is a simple hierarchy).

-20 In an embodiment that maintains parent-child details "OFF-TAG", identities of parent tag and child tags of a parent are maintained not within the tags themselves but within a database accessed to prove relationships of tags 5 passing through the RF portal. This solution is preferable where relationships between person and asset (parent/child) aredynamic parent-child relationships or are more complex than one-to- many, i.e. many-to-many (example: corporate managers, all of whom are able to move corporate assets 10 through specified portals). The solution is slightly more secure, since when relationships between tags are held in a private database, it is not possible to falsely manufacture tags [hence relationships between tags] when access to the database is not also granted. By contrast, in the on-tag 15 embodiment, given a child tag and an available mechanism to read and write tags (i.e. an RFID programming unit), it is possible to create a parent tag for that child tag thus allowing the free passage of the child tag through an alarm gateway. Placing the data for inter-tag relationships off-tag is likely to be more expensive to maintain but less tamper-

prone and more flexible.

25 In an embodiment of the OFF-TAG situation, a single tag ID is stored, with an issuer ID, e.g. a bitmap is: Bits Usage 0-ill Tag ID 30 112-127 Issuer ID In the off-tag embodiment, the tag's information (it's own ID, its parent tag(s), etc.) is stored off-tag, in a

-21 database, typically of hierarchical type for speed of lockups and record set flexibility.

An exemplary record in such a database is: Record name: ''Tag" Unique Tag ID (128-bit unique number) Owner or asset tag (flag) Name (text/string, finite length) 10 Parent tag set - any number of parent UIDs stored against tag [reference to tag record].

In this embodiment, there is a single record per tag, which can be looked up quickly by the processing unit 15 within the database storage as it is indexed by tag ID. The record for each tag tells the processing unit a) whether the tag is a parent tag or not, and b) the Tag IDs of any parent tags associated with the tag.

20 Embodiments of the invention are also envisaged that store the relationships partly on-tag and partly off-tag.

Step 4: Extend Session 25 Referring now to Figure 4, this Figure shows a number of situations showing how a session is extended depending on the tags identified by the reader, and when. Each diagram explains a different scenario of tag identification over time, with the result of the session shown for each 30 scenario. In this embodiment, the parent- child relationships are stored on-tag; however it will be clear to those skilled in the art that database storage could be substituted.

-22 Diagram (1): Person (parent tag) passes through field

with no tagged possessions (child tags).

A single parent tag PT1 is taken through the reader 5 field at time t1, starting, naming and authorising a

session. No child tags are passed through the field

(either within the session time or outside the session time). The authorised session ends at time t2, a fixed default time period after session start. Thus the alarm 10 system is not triggered. The system reports or logs that a parent tag passed through the field with no associated

child tags.

Diagram (2): Tagged Possession passes through reader 15 field without a person (owner) tag.

A single child tag CTlpasses through reader field at

time t1, thus starting a session of default length, having a name CT1. As no parent tag capable of authorising CT1 passes through the field within this time period, the session

20 remains unauthorized. The unauthorized session times out at t2, triggering an alarm. The system logs the identity of the tagged asset that has been taken through the reader.

Diagram (3): Person followed by an owned possession 25 passes through the reader.

The identification of parent tag PT1 starts, names and authorises a session at t1. When child tag CTl passes through the reader at t2 (which is before the session PT1 times out) session CTlis started and authorized (session PT1 is active 30 and capable of authorizing the child session) and session PT1is extended in length to t3. Both sessions expire at t3 authorized', so no alarm is triggered. The system logs that PTl has moved CT1 through the field.

-23 Dia ram (4). Owned ossession followed b its owner g P Y passes through the reader.

By passing through the field, child tag CT1 starts a

5 session at time tl, and as the tag is a child tag, this is unauthorized. The parent tag PT1 is identified in the reader field at time t2 which authorizes the child session CT1.

This child session expires at t3 as an authorised session so no alarm occurs. The system logs that PT has moved CT1 10 through the field.

Dia ram (5) Co lex scenario with no alarm: Owner g. mp with four tagged possessions moves through the reader.

At tl: Session ({CT1}, status unauthorized) is created 15 by child tag CT1 moving through the field.

At t2: Session (|CT2}, status unauthorized) is created by child tag CT2 moving through the field.

At t3: Session ({PT1}, status authorised) is created by parent (owner) tag PT moving through the field. Creation

20 of this parent session authorizes related tags' sessions CT1 and CT2 (which are both authorised at t3).

At t4: Session ({PT1), authorised) extended by child tag CT3 moving through field. Session ({CT3}, status

authorised) is created by child tag CT3 moving through the 25 field. The child session is authorised because a relevant

parent session is active, i.e. PT1.

At Us: Session ({PT1}, authorised) extended by child tag CT4 moving through field. Session ({CT4}, status

authorised) is created by child tag CT3 moving through the 30 field. The child session is authorised because a relevant

parent session is active, i.e. PT1.

At t6: Session ({PT1}, authorised) expires. Also, session ({CT4}, authorised) expires. As both sessions were

-24 authorised at expiry, no alarm is triggered.

The system logs that PT has moved through the field

with possessions CT1, CT2, CT3, CT4.

5 Diagram (6): Complex scenario with alarm: A (tagged) owner moves through field with child tags which do not

belong to him/her.

In this example, unlike previous diagrams, two parent tags are shown: PT1 and PT2. Assume that child tags CT1 10 and CT2 have parent tag PT1. In this diagram, PT2 is identified in the field with CT1 and CT2 (i.e. the owner

PT1 is not present).

At t1: Session ({CT1}, unauthorized) is created by child tag CT1 moving through the field.

15 At t2: Session ({CT2}, unauthorized) prolonged by child tag CT2 moving through the field.

At t3: Session ({PT2}, authorized) is created by parent tag PT2 moving through the field.

At t4: Session ({CT1}, unauthorized) expires, 20 unauthorized, triggering an alarm.

At t5: Session ({CT2}, unauthorized) expires, unauthorized, triggering an alarm.

Session ({PT2}, authorized) times out, authorized (no alarm). In this scenario, the system logs that CT1 and CT2 were moved through the field without the associated parent

(owner) tag, and further logs that PT2 has moved through the field (without child tags in this instance).

-25 Session Expiry Process When a session ends, the system invokes a process to examine whether that session had been authorised (see step 5 4 "augment session" and, if not, then the system triggers the alarm and records the event to a log. A typical log entry may include: Loca ti on ( reader/antenna, premises) 10 Time and date Parent ID required but not present Child IDs triggering the session (i.e. child tag Ids which passed through a field without a parent key).

Embodiments of the invention have now been described to help in understanding the invention. It will be understood that the invention is not limited to features described above, but extends to the full scope of the appended claims.

Claims

-26 Claims

1. A method of monitoring a zone, the method comprising 5 interrogating the zone to detect tags present in said zone, providing an alarm upon detection of a first tag and no second tag in said zone, and providing no alarm upon detecting said second tag in said zone.

10

2. A method according to Claim 1, comprising starting a timing period when the first tag is detected in said zone; and inhibiting the step of providing an alarm until said timing period elapses whereby no alarm is provided if the second tag is detected in said zone before said timing 15 period elapses.

3. A method according to Claim 2, comprising extending the timing period when a further first tag is detected in said zone.

4. A method of indicating unauthorized removal of an asset through a zone, comprising providing the asset with a first tag; providing a person authorised to remove the asset with a second tag; interrogating the zone to detect 25 tags present in said zone; and based upon the interrogation results, outputting an alarm upon detection of the first tag and no second tag in said zone; and not outputting the alarm upon detection of the second tag in said zone.

30 5. A system for monitoring a zone, the system comprising an interrogation device for interrogating the zone to detect tags present in said zone, and first and second tags, the device in use providing a first response if the

-27 first tag is detected in said zone but thesecond tag is not detected in said zone, and a second response if the second tag is detected in said zone.

5

6. A system according to Claim 5, wherein the interrogation device comprises delay means for inhibiting the first response for a set time period such that if the second tag is detected during said set time period, the interrogation device provides only said second response.

7. A system for indicating unauthorized passage of an asset through a zone, comprising a first tag secured to the asset; a second tag carried by a person authorized to remove the asset; and an interrogation device disposed to 15 detect tags present in said zone, the device in use providing a first response if the first tag is detected in said zone but the second tag is not detected in said zone, and a second response if the second tag is detected in said zone.

8. A method of indicating unauthorized removal of an asset through a zone by the use of plural tags, comprising a tagging step, the tagging step comprising providing the asset with a first tag and providing a person authorized to 25 remove the asset with a second tag; interrogating the zone to detect tags present in said zone; identifying any detected tags and based upon the identification results; outputting an alarm if the first tag is detected in said zone but a second tag is not detected in said zone; and not 30 outputting the alarm if the second tag is detected in said zone.

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9. A method according to Claim 8, wherein each of said plural tags has a respective identity and each of said plural tags stores data representative of its identity; wherein the tagging step comprises storing on the first tag 5 additional data representative of the identity of the second tag; and wherein the step of identifying comprises reading said data and said additional data.

10. A method according to Claim 8, wherein each of said 10 plural tags has a respective identity and each of said plural tags stores data representative of its identity; wherein the tagging step comprises storing in a database a relationship between the identity of the first tag and the identity of the second tag; and wherein the step of 15 identifying comprises accessing said database.

11. A set of transponders comprising a first transponder and a second transponder, each transponder storing data indicative of the identity of itself, wherein data stored 20 on the first transponder is indicative of the identity of the second transponder.

12. A set of transponders comprising a first plurality of transponders, each of said first plurality of transponders 25 having an identity, each transponder storing data indicative of its identity, and a second plurality of transponders, each of said second plurality of transponders having an identity and each storing data indicative of its identity and further storing data indicative of the 30 identity of a respective one of the transponders of said first plurality.

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13. A method of identifying a target transponder from a first plurality, the method comprising providing said first plurality of transponders, wherein each of said first plurality of transponders has an identity and each of said 5 plurality of transponders stores data indicative of its respective identity; providing a second plurality of transponders, wherein each of said second plurality of transponders has an identity, each of said second plurality of transponders stores data indicative of its identity and 10 further stores data indicative of the identity of a respective one of the transponders of said first plurality; interrogating a transponder of said second plurality whereby said transponder responds with said data indicative of the target transponder.

14. A system arranged substantially as herein described with reference to the accompanying drawings.