Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L63/00—Network architectures or network communication protocols for network security
  • H04L63/10—Network architectures or network communication protocols for network security for controlling access to devices or network resources
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W12/00—Security arrangements; Authentication; Protecting privacy or anonymity
  • H04W12/08—Access security
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W12/00—Security arrangements; Authentication; Protecting privacy or anonymity
  • H04W12/12—Detection or prevention of fraud
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W12/00—Security arrangements; Authentication; Protecting privacy or anonymity
  • H04W12/12—Detection or prevention of fraud
  • H04W12/121—Wireless intrusion detection systems [WIDS]; Wireless intrusion prevention systems [WIPS]
  • H04W12/122—Counter-measures against attacks; Protection against rogue devices
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W4/00—Services specially adapted for wireless communication networks; Facilities therefor
  • H04W4/80—Services using short range communication, e.g. near-field communication [NFC], radio-frequency identification [RFID] or low energy communication
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W74/00—Wireless channel access
  • H04W74/08—Non-scheduled access, e.g. ALOHA
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W84/00—Network topologies
  • H04W84/02—Hierarchically pre-organised networks, e.g. paging networks, cellular networks, WLAN [Wireless Local Area Network] or WLL [Wireless Local Loop]
  • H04W84/10—Small scale networks; Flat hierarchical networks
  • H04W84/12—WLAN [Wireless Local Area Networks]
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W88/00—Devices specially adapted for wireless communication networks, e.g. terminals, base stations or access point devices
  • H04W88/08—Access point devices
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L63/00—Network architectures or network communication protocols for network security
  • H04L63/14—Network architectures or network communication protocols for network security for detecting or protecting against malicious traffic
  • H04L63/1408—Network architectures or network communication protocols for network security for detecting or protecting against malicious traffic by monitoring network traffic
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W48/00—Access restriction; Network selection; Access point selection
  • H04W48/20—Selecting an access point

Definitions

• the present invention generally relates to wireless local area networks. More particularly, the present invention relates to detecting a counterfeit access point in a wireless local area network.
• WLANs wireless local area networks
• the IEEE 802.11 standard was developed as an international standard for WLANs. Generally, the IEEE 802.11 standard was designed to present users with the same interface as an IEEE 802 wired LAN, while allowing data to be transported over a wireless medium.
• WLANs provide users with increased mobility over wired LANs
• the security of communications over a WLAN can vary for reasons that are not present in wired LANs.
• a counterfeit access can pose as an authorized access point in the WLAN.
• Stations in the WLAN can mistakenly associate with the counterfeit access point and can send confidential information to the counterfeit access point, without knowing that the counterfeit access point is unsecure. Consequently, the counterfeit access point can obtain confidential information from stations in the WLAN. Accordingly, the presence of a counterfeit access point can present security problems in a WLAN.
• a counterfeit access point in a wireless local area network is detected by receiving beacon frames at a detector in the wireless local area network, where the beacon frames are transmitted over the wireless local area network by one or more access points.
• the received beacon frames are analyzed at the detector to detect the counterfeit access point.
• Fig. 1 shows an exemplary Open Systems Interconnection (OSI) seven layer model
• FIG. 2 shows an exemplary extended service set in a wireless local area network ("WLAN");
• WLAN wireless local area network
• FIG. 3 is an exemplary flow diagram illustrating various states of stations in a WLAN
• FIG. 4 shows an exemplary embodiment of an access point sending a beacon frame
• Fig. 5 shows an exemplary embodiment of an access point and a counterfeit access point sending beacon frames
• FIG. 6 shows an exemplary flow diagram of a process for detecting a counterfeit access point in a WLAN
• FIG. 7 shows another exemplary flow diagram of a process for detecting a counterfeit access point in a WLAN.
• FIG. 8 shows another exemplary flow diagram of a process for detecting a counterfeit access point in a WLAN.
• an exemplary Open Systems Interconnection (OSI) seven layer model is shown, which represents an abstract model of a networking system divided into layers according to their respective functionalities.
• the seven layers include physical layer 102 corresponding to layer 1, data link layer 104 corresponding to layer 2, network layer 106 corresponding to layer 3, transport layer 108 corresponding to layer 4, session layer 110 corresponding to layer 5, presentation layer 112 corresponding to layer 6, and application layer 114 corresponding to layer 7.
• Each layer in the OSI model only interacts directly with the layer immediately above or below it, and different computers 100 and 116 can communicate directly with each other only at the physical layer 102. However, different computers 100 and 116 can effectively communicate at the same layer using common protocols.
• computer 100 can communicate with computer 116 at application layer 114 by propagating a frame from application layer 114 of computer 100 through each layer below it until the frame reaches physical layer 102. The frame can then be transmitted to physical layer 102 of computer 116 and propagated through each layer above physical layer 102 until the frame reaches application layer 114 of computer 116.
• the IEEE 802.11 standard for wireless local area networks operates at the data link layer 104, which corresponds to layer 2 of the OSI seven layer model, as described above. Because IEEE 802.11 operates at layer 2 of the OSI seven layer model, layers 3 and above can operate according to the same protocols used with IEEE 802 wired LANs.
• layers 3 and above can be unaware of the network actually transporting data at layers 2 and below. Accordingly, layers 3 and above can operate identically in the IEEE 802 wired LAN and the IEEE 802.11 WLAN. Furthermore, users can be presented with the same interface, regardless of whether a wired LAN or WLAN is used.
• an exemplary extended service set 200 which forms a WLAN according to the IEEE 802.11 standard, is depicted having basic service sets ("BSS") 206, 208, and 210.
• BSS basic service sets
• Each BSS can include an access point (“AP”) 202 and stations 204.
• a station 204 is a component that can be used to connect to the WLAN, which can be mobile, portable, stationary, and the like, and can be referred to as the network adapter or network interface card.
• a station 204 can be a laptop computer, a personal digital assistant, and the like.
• a station 204 can support station services such as authentication, deauthentication, privacy, delivery of data, and the like.
• Each station 204 can communicate directly with an AP 202 through an air link, such as by sending a radio or infrared signal between WLAN transmitters and receivers.
• Each AP 202 can support station services, as described above, and can additionally support distribution services, such as association, disassociation, distribution, integration, and the like. Accordingly, an AP 202 can communicate with stations 204 within its BSS 206, 208, and 210, and with other APs 202 through medium 212, called a distribution system, which forms the backbone of the WLAN.
• This distribution system 212 can include both wireless and wired connections.
• each station 204 must be authenticated to and associated with an AP 202 in order to become a part of a BSS 206, 208, or 210. Accordingly, with reference to Fig. 3, a station 204 begins in State 1 (300), where station 204 is unauthenticated to and unassociated with an AP 202. In State 1 (300), station 204 can only use a limited number of frame types, such as frame types that can allow station 204 to locate and authenticate to an AP 202, and the like.
• station 204 can be elevated to State 2 (302), where station 204 is authenticated to and unassociated with the AP 202. In State 2 (302), station 204 can use a limited number of frame types, such as frame types that can allow station 204 to associate with an AP 202, and the like. [0022] If station 204 then successfully associates or reassociates 308 with AP 202, then station 204 can be elevated to State 3 (304), where station 204 is authenticated to and associated with AP 202. In State 3 (304), station 204 can use any frame types to communicate with AP 202 and other stations 204 in the WLAN.
• station 204 can be transitioned to State 2. Furthermore, if station 204 then receives deauthentication notification 312, then station 204 can be transitioned to State 1. Under the IEEE 802.11 standard, a station 204 can be authenticated to different APs 202 simultaneously, but can only be associated with one AP 202 at any time. [0023] With reference again to Fig. 2, once a station 204 is authenticated to and associated with an AP 202, the station 204 can communicate with another station 204 in the WLAN. In particular, a station 204 can send a message having a source address, a basic service set identification address ("BSSED"), and a destination address, to its associated AP 202.
• BSSED basic service set identification address
• the AP 202 can then distribute the message to the station 204 specified as the destination address in the message.
• This destination address can specify a station 204 in the same BSS 206, 208, or 210, or in another BSS 206, 208, or 210 that is linked to the AP 202 through distribution system 212.
• Fig. 2 depicts an extended service set 200 having three BSSs 206, 208, and 210, each of which include three stations 204, it should be recognized that an extended service set 200 can include any number of BSSs 206, 208, and 210, which can include any number of stations 204.
• station 204 first locates the AP 202.
• an exemplary system that can be used to locate an AP 202 using beacon frames in a WLAN is shown. More particularly, according to the current IEEE 802.11 standard, AP 202 can transmit beacon frames 400 across transmission range 402.
• beacon frames 400 can include information such as frame type, beacon frame interval/rate, sequence number, timestamp, capability information, SSID, supported rates, one or more PHY parameter sets, direct sequence (DS) parameter set, frequency hopping (FH) parameter set, and the like.
• sending beacon frames 400 from AP 202 can be optional. However, some functionality in the WLAN can be lost if AP 202 does not send beacon frames 400.
• station 204 may not be able to locate AP 202 by passively listening for signals from AP 202. Instead, station 204 can send a probe request to locate AP 202. However, more bandwidth and time can be required if each station 204 in the WLAN individually sends a probe request to locate AP 202. Furthermore, for roaming stations 204, if AP 202 does not send beacon frames 400 periodically, the roaming stations 204 can send probe requests periodically in order to locate the AP. However, periodically sending probe requests from these roaming stations 204 can consume even more bandwidth and time.
• sending beacon frames 400 from AP 202 can be optional, sending beacon frames 400 from AP 202 can improve the functionality of the WLAN.
• sending beacon frames from APs in a WLAN can also compromise the security of communications over the WLAN.
• WLANs can provide users with increased mobility, in comparison to wired LANs, but the security of communications over a WLAN can vary for reasons that are not present in wired LANs.
• a counterfeit AP 500 can obtain confidential information from a station 204 by posing as an authorized AP 202. More particularly, counterfeit AP 500 can transmit beacon frame 504 across a transmission range 502. Beacon frame 504 can include information such as frame type, beacon frame interval/rate, sequence number, timestamp, and the like. Stations 204 located within this transmission range 502 can detect beacon frame 504. After detecting beacon frame 504, station 204 can associate with counterfeit AP 500, without realizing that counterfeit AP 500 is not an authorized AP 202. Once associated with counterfeit AP 500, station 204 can transmit confidential information to counterfeit AP 500.
• a counterfeit AP 500 can pose as an authorized AP 202.
• counterfeit AP 500 can determine information about authorized AP 202, such as the SSID for authorized AP 202, the MAC address for authorized AP 202, and the like.
• Counterfeit AP 500 can then be configured with the same SSID as authorized AP 202.
• counterfeit AP 500 can obtain and use the MAC address of authorized AP 202.
• counterfeit AP 500 can locate itself near authorized AP 202 to avoid detection in the WLAN.
• counterfeit AP 500 can transmit a stronger signal across the WLAN in order to entice stations 204 to associate with it instead of authorized AP 202.
• counterfeit APs 500 can obtain confidential information from stations 204 by posing as authorized APs 202, counterfeit APs 500 can create unacceptable security problems in a WLAN. Accordingly, detecting counterfeit APs 500 in a WLAN can be used to improve security in the WLAN.
• detecting counterfeit APs 500 in a WLAN can be used to improve security in the WLAN.
• FIG. 6 an exemplary process for detecting a counterfeit AP is depicted. With reference to Fig. 5, assume for the sake of example that AP 202 is an authorized AP and that counterfeit AP 500 is an unauthorized AP attempting to pose as authorized AP 202. As described above, AP 202 sends beacon frames 400 and counterfeit AP 500 sends beacon frames 504 in an effort to associate with stations that would associate with authorized AP 202.
• beacon frames 504 can include similar information as beacon frames 400 in an effort to pose as beacon frames 400.
• beacon frames 504 can have the same sender MAC address (i.e., the MAC address of authentic AP 202) and the same beacon frame rate.
• detector 506 receives frames from APs having transmission ranges that include detector 506. As such, in the exemplary scenario depicted in Fig. 5, detector 506 receives beacon frames 400 and 504 from authorized AP 202 and unauthorized counterfeit AP 500, respectively.
• detector 506 measures the rate at which frames are received to determine a measured frame rate. For example, in one configuration, detector 506 can count the number of beacon frames received during a period of time. For the sake of example, assume that detector 506 counts a total of 100 beacon frames, which in the exemplary scenario depicted in Fig. 5 would include beacon frames 400 and 504, during a 5 second interval. As such, in this example, the measured beacon frame rate is 20 frames per second.
• step 604 detector 506 compares the measured frame rate to the stated frame rate.
• the stated frame rate can be obtained from the information provided in the frame itself.
• the stated beacon frame rate in beacon frame 400 is 10 frames per second.
• the measured frame rate is 20 frames per second.
• step 606 detector 506 determines if a counterfeit AP is detected based on the comparison of the measured frame rate to the stated frame rate. Again, in the present example, the measured frame rate is 20 frames per second and the stated frame rate is 10 frames per second. As such, in the present example, detector 506 determines that a counterfeit AP has been detected based on the difference in the measured frame rate and the stated frame rate.
• Fig. 7 another exemplary process for detecting a counterfeit AP is depicted.
• AP 202 is an authorized AP and that counterfeit AP 500 is an unauthorized AP attempting to pose as authorized AP 202.
• unauthorized counterfeit AP 500 can obtain the MAC address of authorized AP 202.
• Counterfeit AP 500 can then use the MAC address of authorized AP 202 as the sender MAC address in beacon frames 504 in an effort to associate with stations that would associate with authorized AP 202.
• detector 506 receives frames from APs having transmission ranges that include detector 506. As such, in the exemplary scenario depicted in Fig. 5, detector 506 receives beacon frames 400 and 504 from authorized AP 202 and unauthorized counterfeit AP 500, respectively.
• detector 506 compares the sequence number of a received frame to the sequence number of a previously received frame with the same sender MAC address. More specifically in the present example, when detector 506 receives a beacon frame, it determines the sender MAC address of the beacon frame. If the sender MAC address of the received beacon frame matches the sender MAC address of an authorized AP, detector 506 compares the sequence number of the received beacon frame to the sequence number of a previously received beacon frame from the same authorized AP, which was stored earlier.
• step 704 detector 506 determines if a counterfeit AP is detected based on the comparison of the sequence number of the received frame to the sequence number of a previously received frame. If the sequence number of the received frame is consistent with that of the previously received frame, then detector 506 saves the sequence number of the received frame as the sequence number of a previously received frame. However, if the sequence number of the received frame is not consistent with that of the previously received frame, then detector 506 determines that a counterfeit AP has been detected. [0041] More particularly, in accordance with current IEEE 802.11 standard, APs send frames with sequence numbers that follow an incremental pattern.
• beacon frames 400 having sequence numbers in ascending order such as 100, 101, 102, and the like.
• detector 506 first receives beacon frame 400 having sequence number 100.
• detector 506 examines the sender MAC address of beacon frame 400 to confirm that the sender MAC address matches that of an authorized AP, which in this example is that of authorized AP 202.
• beacon frame 400 having sequence number 100 is the first beacon frame received from AP 202.
• the sequence number of the received beacon frame 400 is stored as the new sequence number of a previously received beacon frame 400.
• detector 506 receives a beacon frame 504 from counterfeit AP 500, which is unauthorized and attempting to pose as authorized AP 202. Also assume that counterfeit AP 500 has sent beacon frame 504 using the sender MAC address of authorized AP 202. However, assume that the sequence number for beacon frame 504 sent by counterfeit AP 500 is 50. Accordingly, when detector 506 compares the sequence number of the received beacon frame, which in this example is 50, to the sequence number of the previously received beacon frame, which in this example is 100, they are not consistent. As such, detector 506 determines that a counterfeit AP 500 has been detected.
• detector 506 determines that the sequence number of the received frame and the sequence number of the previously received frame are consistent, then the sequence number of the received frame replaces the sequence number of the previously received frame, and the new sequence number is stored. For example, if the sequence number of the received frame is 101, then 506 stores 101 as the new sequence number of a previously received frame.
• the IEEE 802.11 standard is a family of specifications, which includes the 802.11, 802.11a, 802.11b, and 802.1 lg specifications.
• the 802.11 specification provides 1 or 2 Mbps transmission in the 2.4 GHz band using either frequency hopping spread spectrum (FHSS) or direct sequence spread spectrum (DSSS).
• FHSS frequency hopping spread spectrum
• DSSS direct sequence spread spectrum
• the 802.1 la specification which is an extension to the 802.11 specification, provides up to 54 Mbps in the 5GHz band using an orthogonal frequency division multiplexing encoding scheme rather than FHSS or DSSS.
• the 802.1 lb specification which is also an extension of the 802.11 specification and commonly referred to as 802.11 High Rate or Wi-Fi, provides 11 Mbps transmission (with a fallback to 5.5, 2 and 1 Mpbs) in the 2.4 GHz band using DSSS.
• the 802.1 lg specification which is the most recent extension of the 802.11 specification, provides 20+ Mbsp in the 2.4 GHz band. Undoubtedly, further extensions and therefore additional 802.11 specifications are likely to be established and available in the future.
• channels 36, 40, 44, 48, 52, 56, 60, 64 are used.
• channels 34, 38, 42, and 46 are used.
• channels 1-14 a total of 14 channels are defined (i.e., channels 1-14).
• channels 1-11 are used.
• channels 1-13 are used.
• channels 1- 14 are used.
• an authorized AP 202 operates in a single channel in a given 802.11 specification. For example, if authorized AP 202 operates using the 802.1 la specification, authorized AP 202 uses one of the defined channels in the 802.11a specification (i.e., channels 34, 36, 38, 40, 42, 44, 46, 48, 52, 56, 60, 64, 159, 153, 157, and 161). Similarly, if authorized AP 202 operates using the 802.1 lb specification, authorized AP 202 uses one of the defined channels in the 802.1 lb specification (i.e., channels 1-14). Authorized AP 202 can also operate in dual-mode in which case it uses both the 802.1 la and 802.1 lb specification.
• authorized AP 202 uses one channel in the 802.1 la specification and one channel in the 802.1 lb specification.
• FIG. 8 another exemplary process for detecting a counterfeit AP is depicted.
• AP 202 is an authorized AP and that counterfeit AP 500 is an unauthorized AP attempting to pose as authorized AP 202.
• unauthorized counterfeit AP 500 can obtain the MAC address of authorized AP 202.
• Counterfeit AP 500 can then use the MAC address of authorized AP 202 as the sender MAC address in beacon frames 504 in an effort to associate with stations that would associate with authorized AP 202.
• step 800 (Fig. 8) of the present exemplary process, detector 506 receives frames from APs having transmission ranges that include detector 506. As such, in the exemplary scenario depicted in Fig. 5, detector 506 receives beacon frames 400 and 504 from authorized AP 202 and unauthorized counterfeit AP 500, respectively.
• detector 506 determines the channels used to send the beacon frames. More specifically, when detector 506 receives beacon frames, it determines the sender MAC addresses of the beacon frames. If the sender MAC addresses of the beacon frames are the same, then detector 506 determines the channels used to send the beacon frames. In accordance with the 802.1 lb specification, the channel used to send the beacon frame is included in the beacon frame. Thus, detector 506 can examine the channel field in the beacon fame to determine the channel used to send the beacon frame. Alternatively, detector 506 can determine the channel on which the beacon frame was received to determine the channel used to send the beacon frame. In accordance with the 802.1 la specification, the channel used to send the beacon frame is not included in the beacon frame.
• detector 506 can determine the channel on which the beacon frame was received to determine the channel used to send the beacon frame. [0053] In step 804 (Fig. 8), detector 506 determines if a counterfeit AP is detected based on the channels used to send the beacon frames. More specifically, as noted above, authorized AP 202 operates using a single channel. Thus, in one embodiment, detector 506 determines that a counterfeit AP has been detected when at least two beacon frames with the same MAC addresses are detected that were sent using two different channels.
• detector 500 is configured to operate in multiple modes. More specifically, in a first mode, detector 506 determines that a counterfeit AP has been detected when at least two beacon frames with the same MAC addresses are detected that were sent using two different channels without regard to whether the two different channels are in different 802.11 specifications. In a second mode, detector 506 determines that a counterfeit AP has been detected when at least two beacon frames with the same MAC addresses are detected that were sent using two different channels and the two different channels are in the same 802.11 specification. [0055] In the present exemplary embodiment, the mode in which detector 500 operates can be selected based on whether authorized AP 202 operates in dual mode.
• detector 500 can be selected to operate in the first mode described above (i.e., determining that a counterfeit AP has been detected when at least two beacon frames with the same MAC addresses are detected that were sent using two different channels without regard to whether the two different channels are in different 802.11 specifications). If authorized AP 202 is known to operate in dual mode, detector 500 can be selected to operate in the second mode described above (i.e., determining that a counterfeit AP has been detected when at least two beacon frames with the same MAC addresses are detected that were sent using two different channels and the two different channels are in the same 802.11 specification).
• the exemplary processes described above for detecting a counterfeit AP in a wireless local area network can be performed using software and/or hardware installed on a detector in the wireless local area network.
• the detector is a station in the wireless local area network.
• the station can be mobile, portable, stationary, and the like.
• the station can be a laptop computer, a personal digital assistant, and the like.
• the station can be used by a user as a diagnostic tool, by an administrator as an administrative tool, and the like, to assess the quality of communications in the WLAN.
• One advantage of the present embodiment includes allowing the station to passively monitor the WLAN to detect a counterfeit AP. By passively monitoring the WLAN in this manner, the station can detect a counterfeit AP in the WLAN without burdening AP 202, consuming bandwidth, or interfering with traffic over the WLAN.

Landscapes

• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Computer Security & Cryptography (AREA)
• Computer Hardware Design (AREA)
• Computing Systems (AREA)
• General Engineering & Computer Science (AREA)
• Mobile Radio Communication Systems (AREA)
• Small-Scale Networks (AREA)

Priority Applications (5)

Applications Claiming Priority (2)

Publications (1)

Family

ID=28453323

Family Applications (1)

Country Status (8)

Cited By (22)

Families Citing this family (64)

Citations (2)

Family Cites Families (4)

• 2002
  • 2002-03-29 US US10/112,402 patent/US7236460B2/en not_active Expired - Fee Related
• 2003
  • 2003-03-28 WO PCT/US2003/009914 patent/WO2003084255A1/en not_active Ceased
  • 2003-03-28 CN CNA038074176A patent/CN1650642A/zh active Pending
  • 2003-03-28 CA CA002478402A patent/CA2478402A1/en not_active Abandoned
  • 2003-03-28 JP JP2003581520A patent/JP4284192B2/ja not_active Expired - Fee Related
  • 2003-03-28 KR KR1020047014527A patent/KR100956192B1/ko not_active Expired - Fee Related
  • 2003-03-28 EP EP03714483A patent/EP1491059A4/en not_active Withdrawn
  • 2003-03-28 AU AU2003218479A patent/AU2003218479A1/en not_active Abandoned
• 2007
  • 2007-06-14 US US11/818,793 patent/US7539146B2/en not_active Expired - Fee Related

Patent Citations (2)

Non-Patent Citations (4)

Also Published As

Similar Documents

Legal Events