System and method for detection of fraud in a wireless telephone system

• US 5,956,634 A
• Filed: 02/28/1997
• Issued: 09/21/1999
• Est. Priority Date: 02/28/1997
• Status: Expired due to Fees

• Current Assignee: Cellular Technical Services Co., Inc. (Asensus Surgical, Inc.)
• Original Assignee: Cellular Technical Services Co., Inc. (Asensus Surgical, Inc.)
• Inventors: Otterson, Scott D., Kaplan, Dmitry
• Primary Examiner(s): Eisenzopf, Reinhard J.
• Assistant Examiner(s): Aoki, Makoto
• Application Number:
  US08/807,794
• Time in Patent Office: 935 Days
• Field of Search: 455/410, 455/411, 455/418, 455/423, 455/424, 455/425, 380/23, 380/25, 379/189, 340/825.34, 706/52, 706/900, 395/726
• US Class Current: 455/410
• CPC Class Codes: H04M 15/47 Fraud detection or prevention means
• CPC Class Codes: H04M 2215/0148 Fraud detection or prevention means
• CPC Class Codes: H04M 2215/32 Involving wireless systems
• CPC Class Codes: H04W 12/126 Anti-theft arrangements, e.g. protection against subscriber identity module [SIM] cloning

Abstract

A system and method for combining the results of different analysis portions to produce a single YES/NO decision regarding the validity of a present cellular telephone call. A soft data analysis portion of the system analyzes soft data, such as the results of a signature analyzer and a customer call profile and determines the likelihood that the particular call is a valid call from an authorized cellular telephone or an invalid call from a fraudulent cellular telephone. In addition, the system performs a deterministic analysis of the particular phone call to independently designate the particular phone call as valid or invalid. The outputs from the deterministic analyzer and the soft data analyzer portion are combined in a combinatorial logic circuit to produce a single binary output having one logic level for valid calls from the authorized cellular telephone and the opposite logic level for invalid calls from the fraudulent cellular telephone. In addition, the deterministic analyzer may include a control line to override the decision of the soft analyzer to permit calls to designated numbers, such as emergency numbers, even if the independent analysis indicates that the particular call is invalid. In one embodiment, the soft data analyzer portion is a fuzzy logic classifier receiving data inputs and confidence values from the various soft data portions of the system. The fuzzy logic classifier combines the various soft data inputs to produce a single binary output. A sensitivity input is used to vary the threshold of the binary output. The binary output of the fuzzy logic analyzer is combined with the outputs of the deterministic analyzer by the combinatorial logic circuit.

Claims

1. A system for the authorization of a present telephone communication with a wireless telephone in a wireless telephone system, the authorization system comprising:

a signature analyzer to classify an unauthenticated wireless telephone as an authorized wireless telephone or a fraudulent wireless telephone based on analysis of a transmission characteristic of the unauthenticated wireless telephone detected during the present communication with the unauthenticated wireless telephone, the signature analyzer calculating a first data value indicative of a similarity with which the transmission characteristic of the unauthenticated wireless telephone matches a stored reference characteristic, and a first confidence value indicative of a measure of reliability of the first calculated data value;

a soft analyzer receiving and analyzing the first calculated data value and first confidence value from the signature analyzer to determine a likelihood that the present communication is a valid communication with the authorized wireless telephone or an invalid communication with the fraudulent wireless telephone, the soft analyzer generating a first authorization signal having a first binary level if the analysis determines that the present communication is the valid communication and a second binary level different from the first binary level if the analysis determines that the present communication is the invalid communication;

a deterministic analyzer analyzing the present communication in a deterministic fashion to determine if the present communication with the unauthenticated wireless telephone is the valid communication or the invalid communication, the deterministic analyzer generating a second authorization signal at a third binary level if the deterministic analyzer determines that the present communication is the valid communication and a fourth binary level different from the third binary level if the deterministic analyzer determines that the present communication is the invalid communication, the soft analyzer analyzing the first calculated data value and the first confidence value independently of the analysis by the deterministic analyzer; and

a combinatorial logic unit to combine the first and second authorization signals and generate an output signal at a fifth binary level to authorize the present communication only if the first and second authorization signals are both at the first and third binary levels, respectively, and generating the output signal at a sixth binary level different from the fifth binary level if either of the first and second authorization signals is at the second and fourth binary levels, respectively, to cause termination of the present communication as the invalid communication, the logic unit generating the output signal at the sixth binary level to cause termination of the present communication as the invalid communication regardless of the analysis by the soft analyzer if the deterministic analyzer determines that the present communication is the invalid communication.

2. The system of claim 1 wherein the first confidence value is used as a weighting factor for the calculated first data value.

3. The system of claim 1 wherein the signature analyzer analyzes a plurality of different transmission characteristics of the unauthenticated wireless telephone detected during the present communication with the unauthenticated wireless telephone and calculates a plurality of respective data values indicative of the similarity with which each of the plurality of transmission characteristics match a plurality of corresponding stored reference characteristics, and a plurality of confidence values indicative of a measure of reliability of each of the plurality of calculated data values, the soft analyzer using the plurality of calculated data values and the plurality of confidence values to generate the first authorization signal.

4. The system of claim 1 wherein the soft analyzer generates the first authorization signal at the first binary level if the analysis determines that the present communication is the valid communication with a probability exceeding a predetermined probability threshold.

5. The system of claim 4 wherein the soft analyzer further includes an adjustable sensitivity control adjustable to selectively alter the predetermined probability threshold.

6. The system of claim 4 wherein said soft analyzer scales said first calculated value to alter the sensitivity of the soft analyzer.

7. The system of claim 1 wherein the deterministic analyzer always generates the second authorization signal at the third binary level unless the deterministic analyzer determines that the present communication is the invalid communication with the fraudulent wireless telephone.

8. The system of claim 7 wherein the present communication with the unauthenticated wireless telephone includes a plurality of dialed digits indicative of an intended destination telephone number, the system further including a data storage area containing a list of known fraudulent destination telephone numbers and wherein the deterministic analyzer generates the second authorization at the signal fourth binary level whenever the deterministic analyzer determines that the dialed digits of the present communication matches one of the known fraudulent destination telephone numbers.

9. The system of claim 1 wherein the deterministic analyzer generates a third authorization signal at a seventh binary level if the deterministic analyzer determines that the present communication should be processed regardless of the first and second authorization signals, the combinatorial logic unit combining the first, second, and third authorization signals and generating the output signal at the fifth binary level regardless of the first and second authorization signals if the third authorization signal is at the seventh binary level and generating the output signal at the sixth binary level if the third authorization signal is at an eighth binary level different from the seventh binary level and one of the first and second output signals are at the second and fourth binary levels, respectively.

10. A system for the authorization of a present telephone communication with an unauthenticated wireless telephone in a wireless telephone system based on soft data generated by analysis of characteristics of the unauthenticated wireless telephone with respect to stored characteristics associated with an authorized wireless telephone and based on deterministic data generated by analysis of characteristics of the present communication, the authorization system comprising:

a soft analyzer to receive and analyze the soft data to determine a likelihood that the present communication is a valid communication with the authorized wireless telephone or an invalid communication with a fraudulent wireless telephone, the soft analyzer generating a first authorization signal at a first binary level if the analysis determines that the present communication is the valid communication and at a second binary level different from the first binary level if the analysis determines that the present communication is the invalid communication;

a deterministic analyzer to receive and analyze the deterministic data to determine if the present communication with the unauthenticated wireless telephone is the valid communication or the invalid communication, the deterministic analyzer generating a second authorization signal at a third binary level if the deterministic analyzer determines that the present communication is the valid communication and a fourth binary level different from the third binary level if the deterministic analyzer determines that the present communication is the invalid communication, the soft analyzer analyzing the soft data independently of the analysis by the deterministic analyzer; and

a combinatorial logic unit to combine the first and second authorization signals and generate an output signal at a fifth binary level to authorize the present communication only if the first and second authorization signals are both at the first and third binary levels, respectively, and generating the output signal at a sixth binary level different from the fifth binary level if either of the first and second authorization signals is at the second and fourth binary levels, respectively, to cause termination of the present communication as the invalid communication, the logic unit generating the output signal at the sixth binary level to cause termination of the present communication as the invalid communication regardless of the analysis by the soft analyzer if the deterministic analyzer determines that the present communication is the invalid communication.

11. The system of claim 10, further including a signature analyzer to generate the soft data by classifying the unauthenticated wireless telephone as the authorized wireless telephone or the fraudulent wireless telephone based on analysis of a transmission characteristic of the unauthenticated wireless telephone detected during the present communication with the unauthenticated wireless telephone, the signature analyzer calculating a first data value indicative of a statistical likelihood with which the transmission characteristic of the unauthenticated wireless telephone matches a stored reference characteristic, the soft analyzer generating the first authorization signal based at least in part on the first data value.

12. The system of claim 11 wherein the signature analyzer analyzes a plurality of different transmission characteristics of the unauthenticated wireless telephone in the present communication with the unauthenticated wireless telephone and calculates a plurality of data values indicative of the degree to which each of the plurality of transmission characteristics match a plurality of corresponding stored reference characteristics, respectively, the soft analyzer using the calculated first data value and the calculated plurality of data values to generate the first authorization signal.

13. The system of claim 12 wherein the signature analyzer also calculates a first confidence value indicative of a statistical measure of reliability of the first data value and a plurality of confidence values indicative of a statistical measure of reliability of each of the plurality of data values, the soft analyzer using the first confidence value and the plurality of confidence values as weighting factors for the calculated first data value, and the calculated plurality of data values, respectively.

14. The system of claim 10 wherein the soft analyzer generates the first authorization signal at the first binary level if the analysis determines that the present communication is the valid communication with a probability exceeding a predetermined probability threshold.

15. The system of claim 14 wherein the soft analyzer further includes an adjustable sensitivity control adjustable to selectively alter the predetermined probability threshold.

16. The system of claim 14 wherein said soft analyzer scales said first calculated value to alter the sensitivity of the soft analyzer.

17. The system of claim 10 wherein the deterministic analyzer always generates the second authorization signal at the third binary level unless the deterministic analyzer determines that the present communication is the invalid communication with the fraudulent wireless telephone.

18. The system of claim 17 wherein the present communication with the unauthenticated wireless telephone includes a plurality of dialed digits indicative of an intended destination telephone number, the system further including a data storage area containing a list of known fraudulent destination telephone numbers as the deterministic data and the deterministic analyzer generates the second authorization signal at the fourth binary level whenever the deterministic analyzer determines that the dialed digits of the present communication matches one of the known fraudulent destination telephone numbers.

19. The system of claim 10 wherein the deterministic analyzer generates a third authorization signal at a seventh binary level if the deterministic analyzer determines that the present communication should be processed regardless of the first and second authorization signals, the combinatorial logic unit combining the first, second, and third authorization signals and generating the output signal at the fifth binary level regardless of the first and second authorization signals if the third authorization signal is at the seventh binary level and generating the output signal at the sixth binary level if the third authorization signal is at an eighth binary level different from the seventh binary level and one of the first and second output signals are at the second and fourth binary levels, respectively.

20. A system for the authorization of a present telephone communication with a wireless telephone in a wireless telephone system, the authorization system comprising:

a signature analyzer to classify an unauthenticated wireless telephone as an authorized wireless telephone or a fraudulent wireless telephone based on analysis of a transmission characteristic of the unauthenticated wireless telephone detected during the present communication with the unauthenticated wireless telephone, the signature analyzer calculating a first data value indicative of a similarity with which the transmission characteristic of the unauthenticated wireless telephone matches a stored reference characteristic, and a first confidence value indicative of a measure of reliability of the first calculated data value;

a fuzzy logic classifier receiving and analyzing the first calculated data value and first confidence value from the signature analyzer to determine a likelihood that the present communication is a valid communication with the authorized wireless telephone or an invalid communication with the fraudulent wireless telephone, the fuzzy logic classifier generating a first authorization signal having a first binary level if the analysis determines that the present communication is the valid communication and a second binary level different from the first binary level if the analysis determines that the present communication is the invalid communication;

a deterministic analyzer analyzing the present communication in a deterministic fashion to determine if the present communication with the unauthenticated wireless telephone is the valid communication or the invalid communication, the deterministic analyzer generating a second authorization signal at a third binary level if the deterministic analyzer determines that the present communication is the valid communication and a fourth binary level different from the third binary level if the deterministic analyzer determines that the present communication is the invalid communication, the fuzzy logic classifier analyzing the first calculated data value and the first confidence value independently of the analysis by the deterministic analyzer; and

a combinatorial logic unit to combine the first and second authorization signals and generate an output signal at a fifth binary level to authorize the present communication only if the first and second authorization signals are both at the first and third binary levels, respectively, and generating the output signal at a sixth binary level different from the fifth binary level if either of the first and second authorization signals is at the second and fourth binary levels, respectively, to cause termination of the present communication as the invalid communication, the logic unit generating the output signal at the sixth binary level to cause termination of the present communication as the invalid communication regardless of the analysis by the fuzzy logic classifier if the deterministic analyzer determines that the present communication is the invalid communication.

21. A system for the authorization of a present telephone communication with an unauthenticated wireless telephone in a wireless telephone system based on soft data generated by analysis of characteristics of the unauthenticated wireless telephone with respect to stored characteristics associated with an authorized wireless telephone and based on deterministic data generated by analysis of characteristics of the present communication, the authorization system comprising:

a fuzzy logic classifier to receive and analyze the soft data to determine a likelihood that the present communication is a valid communication with the authorized wireless telephone or an invalid communication with a fraudulent wireless telephone, the fuzzy logic classifier generating a first authorization signal at a first binary level if the analysis determines that the present communication is the valid communication and at a second binary level different from the first binary level if the analysis determines that the present communication is the invalid communication;

a deterministic analyzer to receive and analyze the deterministic data to determine if the present communication with the unauthenticated wireless telephone is the valid communication or the invalid communication, the deterministic analyzer generating a second authorization signal at a third binary level if the deterministic analyzer determines that the present communication is the valid communication and a fourth binary level different from the third binary level if the deterministic analyzer determines that the present communication is the invalid communication, the fizzy logic classifier analyzing the soft data independently of the analysis by the deterministic analyzer; and

a combinatorial logic unit to combine the first and second authorization signals and generate an output signal at a fifth binary level to authorize the present communication only if the first and second authorization signals are both at the first and third binary levels, respectively, and generating the output signal at a sixth binary level different from the fifth binary level if either of the first and second authorization signals is at the second and fourth binary levels, respectively, to cause termination of the present communication as the invalid communication, the logic unit generating the output signal at the sixth binary level to cause termination of the present communication as the invalid communication regardless of the analysis by the fuzzy logic classifier if the deterministic analyzer determines that the present communication is the invalid communication.

22. The system of claim 21, further including a signature analyzer to generate the soft data by classifying the unauthenticated wireless telephone as the authorized wireless telephone or the fraudulent wireless telephone based on analysis of a transmission characteristic of the unauthenticated wireless telephone detected during the present communication with the unauthenticated wireless telephone, the signature analyzer calculating a first data value indicative of a similarity with which the transmission characteristic of the unauthenticated wireless telephone matches a stored reference characteristic, the fuzzy logic classifier generating the first authorization signal based at least in part on the first data value.

23. The system of claim 22 wherein the signature analyzer analyzes a plurality of different transmission characteristics of the unauthenticated wireless telephone in the present communication with the unauthenticated wireless telephone and calculates a plurality of data values indicative of the degree to which each of the plurality of transmission characteristics match a plurality of corresponding stored reference characteristics, respectively, the fuzzy logic classifier using the calculated first data value and the calculated plurality of data values to generate the first authorization signal.

24. The system of claim 23 wherein the signature analyzer also calculates a first confidence value indicative of a measure of reliability of the first calculated data value and a plurality of confidence values indicative of a measure of reliability of each of the respective calculated plurality of data values, the fuzzy logic classifier using the first confidence value and the plurality of respective confidence values as additional data values and generating the first authorization signal using the first calculated data value, the plurality of calculated data values, the calculated first confidence value and the plurality of respective calculated confidence values.

25. The system of claim 21 wherein the fuzzy logic classifier generates the first authorization signal at the first binary level if the fuzzy logic classifier determines that the present communication is the valid communication with a likelihood exceeding a predetermined threshold.

26. The system of claim 25 wherein the fuzzy logic classifier further includes an adjustable sensitivity control adjustable to selectively alter the predetermined probability threshold.

27. The system of claim 21 wherein the deterministic analyzer always generates the second authorization signal at the third binary level unless the deterministic analyzer determines that the present communication is the invalid communication with the fraudulent wireless telephone.

28. The system of claim 21 wherein the deterministic analyzer generates a third authorization signal at a seventh binary level if the deterministic analyzer determines that the present communication should be processed regardless of the first and second authorization signals, the combinatorial logic unit combining the first, second, and third authorization signals and generating the output signal at the fifth binary level regardless of the first and second authorization signals if the third authorization signal is at the seventh binary level and generating the output signal at the sixth binary level if the third authorization signal is at an eighth binary level different from the seventh binary level and one of the first and second output signals are at the second and fourth binary levels, respectively.

29. A system for the authorization of a present telephone communication with an unauthenticated wireless telephone in a wireless telephone system based on soft data generated by analysis of characteristics of the unauthenticated wireless telephone with respect to stored characteristics associated with an authorized wireless telephone and based on deterministic data generated by analysis of characteristics of the present communication, the authorization system comprising:

a first analyzer to receive and analyze the soft data to determine a likelihood that the present communication is a valid communication with the authorized wireless telephone or an invalid communication with a fraudulent wireless telephone, the first analyzer generating a first authorization signal at a first binary level if the first analyzer determines that the present communication is the valid communication and at a second binary level different from the first binary level if the first analyzer determines that the present communication is the invalid communication;

a second analyzer to receive and analyze the deterministic data to determine if the present communication with the unauthenticated wireless telephone is the valid communication or the invalid communication, the second analyzer generating a second authorization signal at a third binary level if the second analyzer determines that the present communication is the valid communication and a fourth binary level different from the third binary level if the second analyzer determines that the present communication is the invalid communication, the first analyzer analyzing the soft data independently of the analysis by the second analyzer; and

a combinatorial logic unit to combine the first and second authorization signals and generate an output signal at a fifth binary level to authorize the present communication only if the first and second authorization signals are both at the first and third binary levels, respectively, and generating the output signal at a sixth binary level different from the fifth binary level if either of the first and second authorization signals is at the second and fourth binary levels, respectively, to cause termination of the present communication as the invalid communication, the logic unit generating the output signal at the sixth binary level to cause termination of the present communication as the invalid communication regardless of the analysis by the first analyzer if the second analyzer determines that the present communication is the invalid communication.

30. The system of claim 29 wherein the first analyzer is a soft analyzer that combines the data generated by the analysis of characteristics of the telephone communication and the unauthenticated wireless telephone with respect to stored characteristics associated with the authorized wireless telephone, the soft analyzer calculating a likelihood that the present communication is the valid communication and generating the first authorization signal at the first binary level if the analysis determines that the present communication is the valid communication.

31. The system of claim 29 wherein the first analyzer is a neural network analyzer that combines the soft data generated by the analysis of characteristics of the telephone communication and the unauthenticated wireless telephone with respect to stored characteristics associated with the authorized wireless telephone, the neural network analyzer calculating a likelihood that the present communication is the valid communication and generating the first authorization signal at the first binary level if the neural network analyzer determines that the present communication is the valid communication.

32. The system of claim 29 wherein the first analyzer is a fuzzy logic classifier that combines the soft data generated by the analysis of characteristics of the telephone communication and the unauthenticated wireless telephone with respect to stored characteristics associated with the authorized wireless telephone, the fuzzy logic classifier calculating a likelihood that the present communication is the valid communication and generating the first authorization signal at the first binary level if the fuzzy logic classifier determines that the present communication is the valid communication.

33. A method for the authorization of a present telephone communication with an unauthenticated wireless telephone in a wireless telephone system based on soft data generated by analysis of characteristics of the unauthenticated wireless telephone with respect to stored characteristics associated with an authorized wireless telephone and based on deterministic data generated by analysis of characteristics of the present communication, the authorization method comprising the steps of:

analyzing the soft data to determine a likelihood that the present communication is a valid communication with the authorized wireless telephone or an invalid communication with a fraudulent wireless telephone;

generating a first authorization signal at a first binary level if the step of analyzing the soft data determines that the present communication is the valid communication and at a second binary level different from the first binary level if the step of analyzing the soft data determines that the present communication is the invalid communication;

analyzing the deterministic data to determine if the present communication with the unauthenticated wireless telephone is the valid communication or the invalid communication;

generating a second authorization signal at a third binary level if the step of analyzing the deterministic data determines that the present communication is the valid communication and at a fourth binary level different from the third binary level if the step of analyzing the deterministic data determines that the present communication is the invalid communication, wherein the steps of analyzing the soft data and generating the first authorization signal occur independently of the steps of analyzing the deterministic data and generating the second authorization signal; and

combining the first and second authorization signals to generate an output signal at a fifth binary level to authorize the present communication only if the first and second authorization signals are both at the first and third binary levels, respectively, and to generate the output signal at a sixth binary level different from the fifth binary level if either of the first and second authorization signals is at the second and fourth binary levels, respectively, to cause termination of the present communication as the invalid communication, wherein the step of combining to generate the output signal at the sixth binary level occurs regardless of the binary level of the first authorization signal if the step of analyzing the deterministic data determines that the present communication is the invalid communication.

34. The method of claim 33 wherein the step of analyzing the soft data is performed by a soft analyzer that performs the steps of combining the soft data generated by the analysis of characteristics of the telephone communication and the unauthenticated wireless telephone with respect to stored characteristics associated with the authorized wireless telephone, and calculating a likelihood that the present communication is the valid communication and generating the first authorization signal at the first binary level if the analysis determines that the present communication is the valid communication.

35. The method of claim 33 wherein the step of analyzing the soft data is performed by a neural network analyzer that performs the steps of combining the soft data generated by the analysis of characteristics of the telephone communication and the unauthenticated wireless telephone with respect to stored characteristics associated with the authorized wireless telephone, and calculating a likelihood that the present communication is the valid communication and generating the first authorization signal at the first binary level if the neural network analyzer determines that the present communication is the valid communication.

36. The method of claim 33 wherein the step of analyzing the soft data is performed by a fuzzy logic classifier that performs the steps of combining the soft data generated by the analysis of characteristics of the telephone communication and the unauthenticated wireless telephone with respect to stored characteristics associated with the authorized wireless telephone, the fuzzy logic classifier applying a set of relational rules to calculate a likelihood that the present communication is the valid communication and generating the first authorization signal at the first binary level if the fuzzy logic classifier determines that the present communication is the valid communication.

37. The method of claim 33, further including the step of generating the soft data by classifying the unauthenticated wireless telephone as the authorized wireless telephone or the fraudulent wireless telephone based on an analysis of a transmission characteristic of the unauthenticated wireless telephone detected during the present communication with the unauthenticated wireless telephone, the transmission characteristic analysis calculating a first data value indicative of a similarity with which the transmission characteristic of the unauthenticated wireless telephone matches a stored reference characteristic, the step of generating the first authorization signal generating the first authorization signal based at least in part on the first data value.

38. The method of claim 37 wherein the step of generating the soft data analyzes a plurality of different transmission characteristics of the unauthenticated wireless telephone in the present communication with the unauthenticated wireless telephone and calculates a plurality of data values indicative of the similarity with which each of the plurality of transmission characteristics matches a plurality of corresponding stored reference characteristics, respectively, the step of generating the first authorization signal based at least in part on the calculated first data value and the calculated plurality of data values.

39. The method of claim 38 wherein the step of generating the soft data also calculates a first confidence value indicative of a measure of reliability of the first data value and a plurality of confidence values indicative of a measure of reliability of each of the plurality of data values, the step of analyzing the probabilistic data using the first confidence value and the plurality of confidence values as weighting factors for the calculated first data value, and the calculated plurality of data values, respectively.

40. The method of claim 33 wherein the step of generating the first authorization signal generates the first authorization signal at the first binary level if the step of analyzing the soft data determines that the present communication is the valid communication with a probability exceeding a predetermined probability threshold.

41. The method of claim 40, further including the step of altering an adjustable sensitivity control adjustable to selectively alter the predetermined probability threshold.

42. The method of claim 33 wherein the step of generating the second authorization signal always generates the second authorization signal at the third binary level unless the step of analyzing the deterministic data determines that the present communication is the invalid communication with the fraudulent wireless telephone.

43. The method of claim 33, further including the step of generating a third authorization signal at a seventh binary level if the step of analyzing the deterministic data determines that the present communication should be processed regardless of the first and second authorization signals, the step of combining authorization signals combining the first, second, and third authorization signals and generating the output signal at the fifth binary level regardless of the first and second authorization signals if the third authorization signal is at the seventh binary level and generating the output signal at the sixth binary level if the third authorization signal is at an eighth binary level different from the seventh binary level and one of the first and second output signals are at the second and fourth binary levels, respectively.