United States Patent [191
`Cummings et al.
`
`US005406260A
`[11] Patent Number:
`[45] Date of Patent:
`
`5,406,260
`Apr. 11, 1995
`
`[54]
`
`[75]
`
`[73]
`
`[21]
`[22]
`[51]
`[52]
`[53]
`
`NETWORK SECURITY SYSTEM FOR
`DETECTING REMOVAL OF ELECTRONIC
`EQUIPMENT
`Inventors: Marshall B. Cummings, Ann Arbor,
`Mich.; Christopher R. Young, Austin,
`Tex.
`Assignee: ChriMar Systems, Inc., Ann Arbor,
`Mich.
`992,
`924
`Dec. 18, 1992
`
`Appl. No.:
`Filed:
`
`............................... .. G08B 21/00
`Int. Cl.6 ........... ..
`340/568; 340/687
`US Cl. ........... ..
`Field of Search ........ ..'.'.'... 340/568, 571, 572, 652,
`340/664, 687
`
`[56]
`
`References Cited
`U_.S. PATENT DOCUMENTS
`11/1971
`3,618,065
`1/1976
`3,932,857
`4,654,640
`3/1987
`8/1987
`4,686,514
`4,736,195
`4/1988
`7/1988
`4,760,382
`5,034,723
`7/1991
`1/1991
`5,059,948
`11/1991
`5,066,942
`8/1992
`5,136,580
`7/1993
`5,231,375
`
`Matsuo ..... ..
`
`5,243,328 9/1993 Lee et a1. .......................... .. 340/568
`
`FOREIGN PATENT DOCUMENTS
`
`357482 3/1990 European Pat. on". .......... .. 340/568
`4203304 8/1992 Germany .......................... .. 340/568
`Primary Examiner-John K. Peng
`Assistant Examiner—Thomas J. Mullen, Jr.
`Attorney, Agent, or Firm—Harness, Dickey & Pierce
`[57]
`ABSTRACT
`A system and method are provided for monitoring the
`connection of electronic equipment, such as remote
`computer workstations, to a network via a communica
`tion link and detecting the disconnection of such equip
`ment from the network. The system includes current
`loops internally coupled to protected pieces of equip
`ment so that each piece of associated equipment has an
`associated current loop. A low current power signal is
`provided to each of the current loops. A sensor moni
`tors the current ?ow through each current loop to de
`tect removal of the equipment from the network. Re
`moval of a piece of hardware breaks the current ?ow
`through the associated current loop which in turn may
`activate an alarm. This invention is particularly adapted
`to be used with an existing 10BaseT communication link
`or equivalent thereof, employing existing wiring to
`form the current loops.
`
`19 Claims, 3 Drawing Sheets
`
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`U.S. Patent
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`Apr. 11, 1995
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`US. Patent
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`Apr. 11, 1995
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`[III/III’,
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`NETWORK SECURITY SYSTEM FOR DETECTING
`REMOVAL OF ELECTRONIC EQUIPMENT
`
`30
`
`35
`
`BACKGROUND OF THE INVENTION
`1. Technical Field
`This invention relates generally to theft protection
`security systems and, more particularly, to a network
`security system for detecting the unauthorized removal
`of remotely located electronic equipment from a net
`work.
`2. Discussion
`There has been an ever increasing need to provide
`security for electronic equipment against the unautho
`rized removal or theft thereof. Computer systems have
`become a major capital expenditure for users which
`commonly include businesses, educational institutions
`and governmental entities, among other users. Ad
`vancements in technology have signi?cantly reduced
`the size and weight of complex computer equipment,
`thus making expensive computer equipment more easily
`portable. As a consequence, modern computer equip
`ment is generally more compact and more easily trans
`portable, which further makes it more difficult to secure
`against the unauthorized removal or theft thereof.
`Today, computer network systems are frequently
`employed to provide efficient computing capabilities
`throughout a large work area. Existing computer net
`work systems generally include a number of remotely
`located work stations coupled via a data communica
`tion link to a central processing center. For instance,
`many educational institutions such as universities com~
`monly provide a large number of individual work sta
`tions at different locations throughout the university
`campus so as to allow easy computing access to the
`computer network system. However, the wide dissemi
`nation of such equipment at remote locations has made
`the equipment an accessible target for computer thieves.
`Accordingly, a number of methods have been devel
`oped for guarding against the unauthorized removal of
`.40
`electronic equipment. Early methods of protection have
`included the physical attachment of a security cord to
`each piece of protected equipment. However, the secu
`rity cord generally may be cut or physically detached
`from its secured position and is usually considered to be
`45
`a non-appealing aesthetic addition to the equipment.
`Another method of protection includes the attachment
`of a non-removal tag to the equipment which also re
`quires cooperating sensing devices responsive to the tag
`which are properly located at exit locations from the
`premises. However, this approach requires rather ex
`pensive sensing devices and is generally not very feasi
`ble especially when multiple exit points exist.
`Other methods of theft protection have included
`installing a special electronic card inside each computer
`machine which responds to polls from an external moni
`toring station. Upon removal of the machine, the card
`stops responding to the polling of the central station and ,
`an alarm is initiated. Another approach involves mount
`ing a sensing device on or into the machine to detect
`movement of the machines. These approaches, how
`ever, are generally undesirable since they require the
`incorporation of additional components into each ma
`chine.
`More recent methods of theft protection have in
`cluded the sensing of a current loop coupled to the
`protected equipment. One such method is discussed in
`U.S. Pat. No. 4,654,640 issued to Carll et al which dis
`
`60
`
`65
`
`2
`closes a theft alarm system for use with a digital signal
`PBX telephone system. This method includes a plural
`ity of electronic tethers which are connected to individ
`ual pieces of protected equipment by way of connectors
`which in turn are bonded to the surface of the protected
`equipment. Each tether includes a pair of conductors
`which are connected together to form a closed current
`loop via a series resistor and conductive foil which is
`adhesively bonded to the outside of the equipment.
`However, this method requires the addition of an exter
`nally mounted current loop, and it is conceivable that
`the current loop may be carefully removed without any
`detection.
`It is therefore desirable to provide for an enhanced
`network security system which detects unauthorized
`removal of remotely located pieces of hardware from a
`network. More particularly, it is desirable to provide for
`such a security system which feasibly employs separate
`current loops provided through an existing data com~
`munication link to monitor the presence of remotely
`located computer equipment. In addition, it is desirable
`to provide for a security network system which may be
`easily and inexpensively implemented in an existing
`network system and may not be easily physically re
`moved or detached from tile system without detection.
`
`SUMMARY OF THE INVENTION
`In accordance with the teachings of the present in
`vention, a security system is provided for detecting
`unauthorized removal of electronic equipment from a
`network. The system includes current loops internally
`coupled to protected pieces of equipment so that each
`piece of associated equipment has an associated current
`loop. A low current power signal is applied to each of
`the current loops. A detector monitors current flow
`through each of the current loops so as to detect a drop
`in current ?ow which represents removal of equipment
`from the network. Detection of removal of a piece of
`equipment may in turn activate an alarm. This invention
`is particularly adapted to be used in conjunction with a
`computer network having an existing communication
`wiring scheme coupling each piece of equipment to the
`network, and which may be used to form the current
`loops.
`BRIEF DESCRIPTION OF THE DRAWINGS
`Other objects and advantages of the present invention
`will become apparent to those skilled in the art upon
`reading the following detailed description and upon
`reference to the drawings in which:
`FIG. 1 is a block diagram which illustrates a network
`security system coupled in to a computer network in
`accordance with the present invention;
`FIG. 2 is a circuit diagram which illustrates the net
`work security system coupled to the computer network
`in accordance with the present invention; and
`FIG. 3 is a schematic diagram which illustrates instal
`lation of the network security system into an existing
`computer network in accordance with the present in
`vention.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`Turning now to FIGS. 1 and 2 a network security
`system 24 is provided therein for achieving theft protec
`tion of electronic computer equipment associated with a
`computer network 10. In general, the network security
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`system 24 monitors remotely located electronic work
`stations such as personal computers 12a through 12d via
`current loop continuity so as to detect the removal of
`any of the personal computers 12a through 12d from the
`computer network 10. The network security system 24
`described herein is particularly adapted to be easily
`implemented in conjunction with an existing computer
`network 10 without the need for substantial modi?ca
`tions and while realizing minimal interference to the
`computer network 10.
`Remotely located personal computers 12a through
`12d are each connected to the computer network 10 so
`as to provide widespread remote user access to the
`computer network 10. The computer network 10 shown
`herein is of the conventional type which includes a
`network ?le server 18 connected to a network back
`bone 16. The computer network 10 may include most
`any type of backbone such as, for instance, an Ether
`net ® backbone manufactured by Xerox Corporation.
`A plurality of hubs such as hubs 20, 21, 22 and 23 are
`generally coupled to the network ?le server 18 or back
`bone 16 to provide communication links therewith. The
`remotely located personal computers 120 through 12d
`are shown connected to hub 20 via a data communica
`tion link 14. Data communication link 14 includes a
`plurality of transmit and receive data communication
`lines for communicating information between each of
`remotely located personal computers 12a through 12d
`and the network ?le server 18 via network backbone 16
`and hub 20.
`The invention described herein is particularly suited
`to be implemented in conjunction with a computer
`network 10 which preferably employs a conventional
`wiring approach of the type which may include 10Ba
`seT wiring. Wiring schemes of the 10BaseT type are
`commonly employed to provide data communication
`lines for electronic computer equipment. In accordance
`with conventional wiring approaches, data communica
`tion link 14 generally includes a plurality of pairs of
`transmit wires 44 and 46 as well as a plurality of pairs of
`40
`receive wires (not shown) connected to each of per
`sonal computers 12a through 12d. Each pair of transmit
`wires 44 and 46 are internally coupled to an associated
`personal computer 12 via one winding 53 of an inter
`nally located isolation transformer 52. Each pair of
`45
`transmit wires 44 and 46 along with isolation trans
`former 52 thereby form a current loop 50 through the
`personal computer 12 which is advantageously em
`ployed in accordance with the approach described
`herein. However, the same approach could be imple
`mented with the pairs of receive wires without depart
`ing from the scope of this invention.
`The network security system 24 includes an isolation
`power supply 26 which supplies a continuous direct
`current (DC) power signal to each of current loops 500
`through 50d. The DC power signal has a low current
`preferably on the order of magnitude of less than one
`milliamp (1 mA) and, more speci?cally includes a pre
`ferred current of approximately ?fty microamps (50
`[1A). The isolation power supply 26 includes an input
`terminal 25 for receiving a low voltage signal VIN.
`which has a magnitude of approximately ?ve (5) volts.
`A plurality of parallel connected capacitors C1, C2, and
`C3 are connected to input terminal 25. In addition, a
`plurality of power supply lines 280 through 28d are
`provided, each of which has one of capacitors C4(a—d)
`coupled thereto, and all of which are coupled to parallel
`connected capacitors C1 through C3. Capacitors C1
`
`4
`through C4 operate as a power supply ?lter to ?lter out
`any undesirable AC signals such as network operating
`signals. Each of power supply lines 28a through 28d is
`further coupled in series to one of resistors R1(a—d) and
`one of inductors L1(a—d), respectively. Each of resistors
`R1(a—d) has a preferred resistance of about one kiloohm
`(1 k0) which ensures a low current ?ow thereacross.
`Accordingly, inductors L1(a—d) provide isolation to
`power supply 26 by blocking unwanted AC signals
`from transmitting through lines 280 through 28d. Ac
`cording to one embodiment, capacitors C1 through C3
`have respective values of 100 pF, 0.1 uF and 1.0 uF,
`while capacitors C4(a-d) each have values of 0.1 ptF and
`inductors L1(a—d) each have values of 120 mH.
`The power supply lines 280 through 28d each are
`electrically coupled to respective transmit wires 440
`through 44d found within data communication link 14.
`Receive power lines 30a through 30d are likewise elec
`trically coupled to transmit wires 46a through 46d also
`found within the data communication link 14. Transmit
`wires 440 through 44d and 460 through 46d are existing
`wires found within data communication link 14 that are
`selectively tapped as pairs in accordance with the pres
`ent invention to provide current loops 50a through 50d.
`As a consequence, power supply line 28a continu
`ously supplies a low current DC power signal to remote
`personal computer 12a via transmit wire 44a. The low
`current power signal ?ows through an internal path
`provided by existing circuitry in personal computer
`12a. The low current power signal then exits the remote
`personal computer 12a via transmit wire 46a and in turn
`is picked up by receive power line 30a. The low current
`power signal is continuously supplied to current loops
`50a through 500' at all times regardless of whether the
`computer network 10 or any personal computers 12a
`through 12d are operating or not. In addition, the very
`low current DC power signal is so small that it does not
`interfere with or adversely effect the operation of the
`associated computers 12a through 12d or computer
`network 10. To prevent the ?ow of DC current to or
`from hub 20, each of transmit wires 44a through 44d
`and 46a through 46d are further coupled to DC block
`ing capacitors C5 between each of current loops 50a
`through 50d and hub 20. DC blocking capacitors C5
`thereby prevent unwanted DC current paths through
`hub 20.
`-
`The return power signals tapped from transmit wires
`46a through 46d via receive power lines 300 through
`30d are then applied to a signal isolation device 32. The
`signal isolation device 32 includes an RLC circuit made
`up of inductors L2(a—d) coupled in parallel to grounded
`pairs of parallel connected resistors R2 and capacitors
`C5 which are coupled to each of receive power lines 30a
`through 30d. Accordingly, the signal isolation device 32
`helps to prevent network operating signals from inter
`fering with one another. According to one preferred
`embodiment, resistors R2 and capacitors C6 each have
`preferred values of 100 k0 and 0.33 uF, respectively,
`while inductors L2(a—d) each have preferred values of
`120 mH each.
`'
`Op-amp voltage to current converters 34a through
`34d are further connected to receive power lines 30a
`through 30d, respectively. The voltage to current con
`verters 34A through 34D each convert the return
`power signal to a desired magnitude current signal via
`an operational transconductance ampli?er. A signal
`conditioning unit 36 in turn is connected to the output
`of the voltage to current converter 34. The signal condi
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`tioning unit 36 includes Schmidt trigger buffers 36a
`through 36d which further ensure a smooth DC signal
`response.
`The signal conditioning unit 36 has an output con
`nected to digital alarm logic 38 which essentially in
`cludes a NAND gate 38. The NAND gate 38 has four
`inputs for receiving a signal from each of receive power
`lines 30(a-d) and generates a NAND logic operation in
`response thereto. The output of the NAND gate 38 in
`turn provides an alarm output signal to an alarm 40.
`Accordingly, a “high” signal on each NAND gate input
`which is indicative of unbroken current loop continuity
`will result in a “low” alarm output signal. Whereas, a
`“low” signal on any input which is indicative of a cur
`rent loop discontinuity will result in a “high” alarm
`output signal. The alarm 40 includes a reset 42 for dis
`abling the alarm 40 when so desired. In addition, the
`alarm output signal may be further used to activate the
`operation of additional security related functions which
`may include alarm status noti?cation to designated au
`thorities via a telephone link amongst other possible
`functions known throughout the ?eld.
`In addition, each of receive power lines 30a through
`30d is further coupled to one end of light emitting di
`odes 480 through 48d. The other end of light emitting
`diodes 480 through 48d are coupled to a voltage'power
`supply V+. As a consequence, each of light emitting
`diodes 4811 through 48d provides an energized light
`indication whenever the associated current loop 50 is
`broken so as to indicate which. of the personal comput
`ers 12a through 12d are disconnected from the com
`puter network 10.
`FIG. 3 illustrates the connection of the network secu
`rity system 24 to an existing computer network 10. The
`network security system 24 is substantially enclosed
`within a housing 60 which is connected between data
`communication link 14 and hub 20. The housing 60 has
`one or more female receptacles 62 each for receiving a
`male plug 66 that is connected to one end of the data
`communication link 14. The housing 60 further includes
`one or more additional female receptacles 64 for receiv
`ing a male plug 68 from an additional data communica
`tion extension line 70 which in turn connects to female
`receptacle 74 in hub 20 via male plug 72. For purposes
`of maintaining a secure system, the network security
`system 24 is preferably located in a secure area separate
`from personal computers 12a through 12d. This further
`’ ensures against unwanted tampering with the network
`security system 24.
`To implement the present invention, the network
`security system 24 is easily installed into an existing
`computer network 10 such as that employing a l0BaseT
`hub to workstation communication link 14. In doing so,
`the housing 60 enclosing the network security system 24
`is connected between data communication link 14 and
`hub 20 so that male plug 66 is removed from female
`receptacle 74 in hub 20 and inserted into female recepta
`cle 62 in housing 60. The additional data communica
`tion extension link 70 is in turn connected between
`housing 60 and hub 20. As a consequence, power supply
`lines 280 through 28d and receive power lines 30a
`through 30d are easily tapped into selected pairs of
`existing transmit wires 44(a-d) and 46(a-d) found in
`data communication link 14. The selected pairs of trans
`mit wires 44 and 46 enable current to flow through
`current loops 50a through 50d internally coupled to
`personal computers 12a through 12d, respectively.
`
`6
`In operation, the isolation power supply 26 supplies a
`continuous low current DC power signal to each of
`power supply lines 28a through 28d. The low current
`power signal ?ows through current loops 50a through
`50d via pairs of transmit wires 44 and 46 and existing
`circuitry such as isolation transformers 52 within each
`of the remote personal computers 120 through 12d
`being monitored. The return signal in each of current
`loops 50a through 50d is applied to a signal isolation
`device 32 for preventing signal interference among the
`separate communication channels and then is further
`coupled to an op-amp voltage to current converter 34.
`Voltage to current converter 34 converts the voltage to
`a desired current level which in turn is applied to a logic
`NAND gate 38. The logic NAND gate 38 detects dis
`continuities in the current loops 500 through 50d being
`monitored and provides an output indication to an
`alarm 40 which indicates removal of one or more of
`remote personal computers 12a through 12d from the
`computer network 10. In addition, detection of a cur
`rent ?ow discontinuity further energizes the appropri
`ate light emitting diodes‘44a through 44d associated
`with the disconnected personal computer 12.
`While this invention has been described herein in
`connection with a network security system 24 for de
`tecting continued connection of remotely located per
`sonal computers 12a through 12d to a computer net
`work 10, it is conceivable that other electronic equip
`ment may likewise be detected without departing from
`the spirit of this invention. In addition, any number of
`pieces of equipment may be monitored with the net
`work security system 24 and any number of network
`security systems may be coupled to a given network or
`a plurality of networks to handle large numbers of re
`motely located pieces of equipment.
`In view of the foregoing, it can be appreciated that
`the present invention enables the user to achieve anti
`theft protection for remotely located electronic equip- I
`ment connected to an existing network system. Thus,
`while this invention has been disclosed herein in combi
`nation with a particular example thereof, no limitation is
`intended thereby except as de?ned in the following
`claims. This is because a skilled practitioner recognizes
`that other modi?cations can be made without departing
`from the spirit of this invention after studying the speci
`?cation and drawings.
`What is claimed is:
`1. A security system for detecting disconnection of
`electronic equipment from a network, said security
`system comprising:
`current loop means including separate current loops '
`associated with different pieces of monitored
`equipment, each of said current loops employing a
`pair of data communication lines which connect
`one of the associated pieces of equipment to the
`network and which are coupled to existing internal
`circuitry within the associated piece of monitored
`equipment, and wherein respective pairs of data
`communication lines are associated with different
`ones of the associated pieces of equipment;
`source means for supplying a low DC current signal
`to each of said current loops; and
`detector means for monitoring the current signal
`through each of said current loops and detecting a
`change in said current signal through one of said
`current loops which represents disconnection of
`said associated piece of equipment from the net
`work.
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`2. The security system as de?ned in claim 1 wherein
`said electronic equipment comprises computer worksta
`tions each connected to a network ?le server and lo
`cated remote from the network ?le server.
`3. The security system as de?ned in claim 1 wherein
`each of said current loops includes existing pairs of data
`communication lines used ‘by said network for commu
`nicating data between the associated pieces of equip
`ment and a network ?le server.
`4. The security system as de?ned in claim 1 wherein
`said network includes an Ethernet ® network and said
`respective pairs of data communication lines include
`I existing IOBaseT wiring connecting the different ones
`of the associated pieces of equipment to said network.
`5. The security system as de?ned in claim 1 wherein
`said existing internal circuitry includes an isolation
`transformer having a ?rst winding coupled between
`said pair of data communication lines so as to allow said
`current signal to ?ow therethrough when the associated
`piece of equipment is connected to the network.
`6. The security system as de?ned in claim 1 wherein
`said system further comprises high frequency ?lter
`means coupled to each of said current loops for provid
`ing isolation to each of said current loops.
`7. The security system as de?ned in claim 1 further
`25
`comprising DC blocking capacitors coupled to each of
`said current loops for preventing said current signal
`through one of said current loops from interfering with
`other of said current loops.
`8. The security system as de?ned in claim 1 further
`comprising alarm indicator means responsive to said
`current detection for providing an alarm signal indica
`tive of a disconnected piece of said equipment when
`said change in the current signal through one of said
`current loops is detected.
`9. A security system for detecting unauthorized dis
`connection of electronic equipment that is connected to
`a network communication link having existing pairs of
`data communication lines interconnecting said elec
`tronic equipment to a network, said system comprising:
`current loop means including separate current loops
`associated with different pieces of protected equip
`ment and internally coupled to the associated
`pieces of protected equipment, each of said current
`loops using said existing pair of data communica
`tion lines which are coupled together via existing
`internal circuitry within said associated equipment
`to form a complete circuit path therethrough, and
`wherein respective pairs of data communication
`lines are associated with different ones of the asso
`ciated pieces of equipment;
`source means for supplying a low DC current signal
`to each of said current loops;
`sensing means for sensing current ?ow through each
`of said current loops and detecting a change in said
`current flow through one of said current loops
`which is indicative of disconnection of one of the
`associated pieces of equipment; and
`output means for providing an alarm output signal so
`as to indicate detection of a disconnected one of
`said pieces of equipment.
`
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`8
`10. The security system as de?ned in claim 9 wherein
`.each of said pairs of data communication lines are cou
`pled between one of said associated pieces of equipment
`and a network ?le server.
`11. The security system as de?ned in claim 9 wherein
`said data communication lines are provided via l0Ba
`seT wiring.
`12. The security system as de?ned in claim 9 wherein
`said internal circuitry within said associated equipment
`comprises a ?rst winding of a transformer which is
`coupled between each of said respective pairs of data
`communication lines to form a circuit path through
`each of said pairs of data communication lines.
`13. The security system as de?ned in claim 9 wherein
`said DC current signal has a current of less than 1 milli
`amp.
`14. A method for detecting unauthorized disconnec
`tion of remotely located electronic equipment which
`has existing data communication lines connecting the
`equipment to a network, said method comprising:
`selecting respective pairs of the existing data commu
`nication lines for associated pieces of monitored
`equipment so that each of said selected pairs of data
`communication lines forms a current loop through
`the associated pieces of monitored equipment,
`wherein said respective pairs of data communica
`tion lines are associated with different ones of the
`associated pieces of equipment;
`supplying a low DC current signal to each current
`loop so as to achieve continuous current flow
`through each current loop while each of said asso
`ciated pieces of equipment is physically connected
`to said network via the data communication lines;
`and
`sensing said DC current signal in each of said current
`loops so as to detect a change in current ?ow indic
`ative of disconnection of one of said pieces of asso
`ciated equipment.
`15. The method as de?ned in claim 14 further com
`prising the step of providing an alarm signal when said
`disconnection of one of said pieces of equipment is
`detected.
`16. The method as de?ned in claim 14 further com
`prising the step of:
`selectively tapping into each of said selected pairs of
`existing data communication lines at a location
`which is remote from said associated pieces of
`equipment.
`17. The method as de?ned in claim 14 wherein said
`existing data communication lines comprise IOBaseT
`wiring.
`18. The method as de?ned in claim 14 wherein each
`of said pieces of electronic equipment comprises a com
`puter workstation connected to an Ethernet ® net
`work.
`19. The method as de?ned in claim 14 wherein each
`of said current loops includes existing circuitry within
`the associated piece of equipment and coupled between
`the one of said associated pairs of data communication
`lines to provide a circuit path therebetween.
`*
`* * *
`*
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`55
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`D-Link-1010
`Page 8 (of 8)