Telephonic communications – Call or terminal access alarm or control – Fraud or improper use mitigating or indication
Reexamination Certificate
1999-11-05
2004-07-13
Isen, F. W. (Department: 2644)
Telephonic communications
Call or terminal access alarm or control
Fraud or improper use mitigating or indication
C379S406010
Reexamination Certificate
active
06763099
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of telecommunications and telephone systems. More particularly, the present invention relates to a system and method for detecting three way calls using spread spectrum technology.
BACKGROUND OF THE INVENTION
The proliferation of private telephone systems and the increasingly sophisticated services offered by such systems have created a need to monitor various events occurring on the telephone lines of the system. This is particularly true with telephone systems designed to provide users with only limited access to the public switched telephone network (PSTN). For example, prison telephone systems are tightly controlled to prevent inmates from contacting unauthorized parties or using the telephone system for illegal purposes. Prison telephone systems typically include some type of authorization mechanism to prevent inmates from dialing unauthorized numbers directly. However, once connected to an authorized number, the inmate may attempt to be connected to a third party at an unauthorized number using three way calling. These calls can only be prevented if the three way calling event is detected and intercepted.
A three way call is initiated when the non-local or remote party (the party outside the private telephone system) has a telephone equipped with three way calling services. The remote party usually depresses the hook switch on the remote telephone, generating a hook flash signal. This hook flash signal instructs the telephone central office to put the local party (the party within the private telephone system) on hold and provide a dial tone to the non-local party. On receipt of the dial tone, the non-local party dials the number of a third party to be added to the telephone conversation. Once a connection is established between the non-local party and the third party, the non-local party will typically depress the hook switch a second time, generating a second hook flash signal which instructs the telephone central office to complete the connection by bringing the local party back onto the active telephone line. Once the connection is completed, the local party and the third party can communicate through the connection made between the non-local party and the third party.
In order to detect three way calling events, prior art three way call detection systems have typically used one of two different methods. In a first method, some conventional three way call detection systems monitor the local telephone connection for the presence of a hook flash or associated central office signals which typically occur during a three way call. A hook flash signal will normally fall in a frequency band outside the ordinary frequency range of signal frequencies produced by the human voice. Prior art three way call detection systems which use this approach monitor signals on the local telephone line through a frequency filter which is designed to pass audio signals within the range of signal frequencies produced by the human voice. If the system detects signals falling within a frequency band outside the normal range of frequencies produced by the human voice, a three way call event is indicated.
U.S. Pat. No. 5,796,811, entitled “Three Way Call Detection,” discloses a system and method for detecting three way calls by monitoriong audio signals for features that distinguish ordinary human voice signals from audio signals produced by events associated with three way call events, such as hook flash clicks followed by an interval of silence.
However, this type of system which uses a frequency filtering method for detecting three way call events which may not be very accurate for a number of reasons. First, the frequency filtering method is designed to detect known frequencies of three way call events. However, the frequencies of hook flash signals or other signals generated by activating central office switches, are often modified by transmission through switches and along loaded lines such that the known frequency characteristics of a hook flash signal are substantially distorted by the time the hook flash signal reaches the three way call detection system. Second, some carriers condition telephone lines to suppress certain undesired signal frequencies. This conditioning tends to suppress the low frequency signal produced by a hook flash click. Finally, prison inmates have learned to mask hook flash clicks by yelling into the telephone receiver. If the inmate can make a sound that is louder than the hook flash click, the detection scheme can be defeated.
In a second method, a three way call detection system may be configured to measure changes in the parameters used by an echo-canceller in order to detect a three way call event. In an echo canceller, a replica of an echo is synthesized using coefficients which are determined by known echo characteristics. The synthesized echo is then used to cancel out the actual echo. If the synthesized echo is equal to the actual echo then the actual echo is essentially filtered out from the signal. However, when the actual echo changes, the synthesized echo will no longer cancel out the actual echo. Accordingly, the synthesized echo needs to be altered. In order to determine a three way calling event, this three way call detection system monitors the set of coefficients used to generate the synthesized echo over the duration of a telephone call. Significant changes in the set of coefficients, beyond a predetermined threshold, are used to signify a three way calling event.
One difficulty in this three way call detection system is the need for accurate detection of the echo or reflection. This system may not be able to accurately detect small echoes beyond the range of the echo canceller's ability. Accordingly, very faint changes in the echo characteristics which may be caused by a third party calling event may not be detected and will have no effect on the set of coefficients used by the echo-canceller. Therefore, such a system may not be able to accurately detect a three way calling event.
Accordingly, there is a need to provide a reliable system and method for indicating the occurrence of three way call events without relying on a hook flash click detection scheme. Moreover, such a system and method should be able to accurately determine a three way calling event by detecting a change in the echo characteristics without relying on an echo-canceller. Finally, the system should be adapted to detect very faint echoes so that the system can more accurately detect any change in the echo characteristics which may be caused by a three way call event in order to properly indicate the occurrence of the three way calling event.
SUMMARY OF THE INVENTION
The present invention is directed to an advanced three way call detection system which measures delay times associated with multiple echoes of a reference signal transmitted over a two way call. A reference signal is initially transmitted over the two way call when the two way connection is first established and the echo characteristics of the two way connection are measured and recorded in an initial echo profile. The initial echo profile represents the number of and timing of each detected echo of the reference signal after transmission over the two way call.
In a preferred embodiment, the present invention continuously transmits the reference signal at various timing intervals during the telephone conversation and samples the incoming line to detect echoed versions of the reference signal in order to create subsequent echo profiles. Whether a three way calling event has occurred is determined by virtue of changes in the delay times and number of echoes in each subsequent echo profile when compared with the initial echo profile.
Because of delays imposed by the switching electronics used in modern telecommunications systems, far more delay is imposed by routing signals through the switches of the public switched telephone network than from actual propagation characteristics associated with transmission lines. Every time a new connection is establis
Fenwick & West LLP
Harold Jefferey
Isen F. W.
T-Netix, Inc.
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