METHODE POUR EMPECHER LES FRAUDES DANS LES SYSTEMES TELEPHONIQUES A SIGNALISATION INTRABANDE ET MODULE DE LIAISON UTILISANT CETTE METHODE

METHOD FOR PREVENTING FRAUD IN AN INBAND SIGNALLING TELEPHONY SYSTEM AND TRUNK MODULE REALIZING SUCH A METHOD

Abrégé anglais

The disclosed method prevents fraud in an inband
signalling telephony system, such as a No. 5 signalling
system. It includes a first step of detecting a fraudulent
inband signal at a predetermined location of an outgoing
signal path between an input terminal Os and an output
terminal Oss of a trunk module T included in the inband
signalling telephony system. The predetermined location is
situated between the input terminal Os and an injection
terminal I of the outgoing signal path where an inband
signal can be injected. In a second step, the fraudulent
inband signal is removed from the outgoing signal path by
interrupting this path during a time interval at least
equal to the duration of the inband signal.
The trunk module realizing this method includes a
receiver similar to the receivers used in the inband
signalling path of the trunk module to detect incomming
inband signals and to filter these out before transmitting
the signal to a user station connected to the trunk module.
This receiver is coupled betwwen the above mentioned input
terminal Os and the injection terminal I.

Revendications

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CLAIMS
1. Method for preventing fraud in an inband
signalling telephony system, characterized in that said
method includes the steps of :
- detecting a fraudulent inband signal at a
predetermined location of an outgoing signal path between
an input terminal (Os) and an output terminal (Oss) of a
trunk module (T) included in said inband signalling
telephony system, said predetermined location being
situated between said input terminal (Os) and an injection
terminal (I) of said outgoing signal path where an inband
signal can be injected; and
- removing a said fraudulent inband signal from
said outgoing signal path when detected.
2. Method according to claim 1, characterized in
that said step of removing said fraudulent inband signal is
realized by interrupting said outgoing signal path between
said input terminal (Os) and said injection terminal (I),
for a predetermined time interval.
3. Method according to claim 2, characterized in
that said predetermined time interval is at least equal to
the time duration of said fraudulent inband signal.
4. Trunk module (T) included in an inband
signalling telephony system, said trunk module including an
input terminal (Os) and an output terminal (Oss) between
which an outgoing signal path can be established, and an
inband signal generation means (SEN) coupled to an
injection terminal of said outgoing signal path and adapted
to generate inband signals, characterized in that said
trunk module additionally includes an inband signal
detection means (REC2) coupled to said outgoing signal path
between said input terminal (Os) and said injection
terminal (I) and adapted to detect inband signals and a
removal means (REC2) coupled between said input terminal
and said injection terminal (I) and adapted to remove a
said inband signal from said outgoing signal path under

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control of an output signal of said inband detection means,
when detected.
5. Trunk module according to claim 4,
characterized in that said removal means includes
interruption means to interrupt said outgoing signal path
during a predetermined time interval.
6. Trunk module according to claim 4,
characterized in that said trunk module additionally
includes a second inband signal detection means and a
second removal means (REC1) similar to the first mentioned
signal detection means and the first mentioned removal
means to detect and remove inband signals on and from an
incoming signal path of said trunk module respectively.
7. Trunk module according to claim 5,
characterized in that said predetermined time interval is
at least equal to the duration of said detected inband
signal.

Description

2171076
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METHOD FOR PREVENTING FRAUD IN AN INBAND SIGNALLING
TELEPHONY SYSTEM AND TRUNK MODULE REALIZING SUCH A METHOD
The present invention relates to a method for
preventing fraud in an inband signalling telephony system.
The problem of fraud with respect to inband
signalling telephony systems, e.g. No. 5 signalling
systems, is well known and especially applies to
international systems and to "free-phone", i.e.
international toll-free phone. So called "hackers" can via
a normal phone inject inband signals in an outbound
signalling path of a telephony system in such a way that
they get access to this telephony system. A detailed
description of the fraud problem in No. 5 signalling
systems can be found in the paper distributed at the
International Network Abuse Prevention meeting held in
Brussels on 6-7 May 1992 "CCITT No. 5 Bluebox Fraud" by
A.S. Foote of AT&T. The problem is also acknowledged in the
CCITT Temporary Document XI/1-16 of Working Party XI/1,
Geneva 10 - 12 March 1992, Question 7/XI p. 3, item 10.
In the CCITT Recommendations Q.141 Fascicle VI.2,
chapter II, p. 58, item h) measures are proposed to detect
such fraud attempt and to possibly react thereon. However,
these measures rely on actions to be taken by both sides of
a communication. As such a communication can be an
international one this implies an agreement to be followed
by different national operators or administrations which is
often difficult to realize.
Therefore, the object of the present invention is
to provide a method for preventing fraud in the above type
of telephony networks, but which does not rely on the
goodwill of operators or administrations.
This object is realized due to the fact that said
method includes the steps of :
- detecting a fraudulent inband signal at a
predetermined location of an outgoing signal path between
an input terminal and an output terminal of a trunk module

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included in said inband signalling telephony system, said
predetermined location being situated between said input
terminal and an injection terminal of said outgoing signal
path where an inband signal can be injected; and
- removing a said fraudulent inband signal from
said outgoing signal path when detected.
When in the first mentioned step an inband signal
is detected, this means that someone has in a fraudulous
way injected this signal, since normally, this signal
should only appear after the injection terminal. This
signal is then removed from the outgoing signal path.
An additional feature of the present invention is
that said step of removing said fraudulent inband signal is
realized by interrupting said outgoing signal path between
said input terminal and said injection terminal, for a
predetermined time interval. This time interval has to be
such that the fraudulent inband signal is intercepted
without losing speech signals, i.e. it has at least to be
equal to the duration of the fraudulent signal, but it may
not overlap the time interval used for the transmission of
speech signals.
A second object of the invention is to provide a
trunk module realizing the above method.
This is achieved by means of a trunk module
including an input terminal and an output terminal between
which an outgoing signal path can be established, and an
inband signal generation means having an output coupled to
an injectior terminal of said outgoing signal path and
adapted to g~nerate inband signals, said trunk module
additionally lncluding an inband signal detection means
coupled to said outgoing signal path between said input
terminal and said injection terminal and adapted to detect
inband signals and a removal means coupled between said
input terminal and said injection terminal and adapted to
remove a said inband signal from said outgoing signal path

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under control of an output signal of said inband detection
means, when detected.
An additional characteristic feature of this
trunk module is that said removal means includes
interruption means to interrupt said outgoing si?nal path
during a predetermined time interval. As mentioned earlier
this predetermined time interval has to be such that the
inband signal intercepted without losing speech signals.
Yet another characteristic feature of the trunk
module according to the invention is that said trunk module
additionally includes a second inband signal detection
means and a second removal means similar to the first
mentioned signal detection means and the first mentioned
removal means to detect and remove inband signals on and
from an incoming signal path of said trunk module
respectively.
It is well known that trunk modules of inband
signalling telephony systems include inband signal
detection circuitry and removal circuitry to detect and
extract inband signals on and from incomming signalling
paths. This circuitry is included in well known receivers
of the trunk module. Similar circuitry can be used to
detect the fraudulent inband signals in the outgoing signal
path and remove it from that path. As a result, no
additional development effort is required to realize the
first mentioned detection circuitry and removal circuitry
thereby realizing a very efficient design of the subject
trunk module.
The above mentioned and other objects and
features of the invention will become more apparent and the
invention itself will be best understood by referring to
the following description of an embodiment taken in
conjunction with the accompanying drawing which
schematiccally represents a trunk module T according to the
invention.

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The trunk module T is a No. 5 signalling trunk
module included in a No. 5 signalling telephony system (not
shown). It includes a receiver module REC1 coupled between
an input terminal Iss and an output terminal Is. An input
terminal Os is connected to an input of another receiver
module REC2, similar to REC1, and an output of REC2 is
connected to an input of a sender module SEN, an output of
which is coupled to an output terminal Oss of T.
The trunk module T is able to receive at its
input terminal Iss a likenamed input signal consisting of a
speech signal and an inband signal. The inband signal is in
the receiver module REC1 filtered out and only the speech
signal Is is provided at the output terminal IS to be
transmitted to a subscriber station (not shown).
A signal Os applied at the terminal Os by a
subscriber station is applied to REC2 in order to detect
and filter out possible inband signals. A resultant signal
is applied to SEN which adds a No. 5 signal thereby
obtaining the outgoing signal Oss. The location at which
the No. 5 signal is added is in the figure represented by
an injection terminal I located in the circuitry of SEN.
Normally the signal Os should only contain speech
signals since the No. 5 signalling is added to the signal
provided by the subscriber station by SEN. Detection in
REC2 of an inband signal thus means that someone is trying
to get access to the system in a fraudulous way. This
signal is then filtered out by REC2.
SEN and, REC1 and REC2, are a sender ind
receivers realized according to the CCITT Recommendations
Q143 and Q144 respectively. Their realization according to
the latter standard being obvious for a person skilled in
the art, they are not described in further details.
While the principles of the invention have been
described above in connection with specific apparatus, it
is to be clearly understood that this description is made

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only by way of example and not as a limitation on the scope
of the invention.

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