WO1996013913A1

WO1996013913A1 - A method for eliminating the effect of disturbance frames, and a base station

Info

Publication number: WO1996013913A1
Application number: PCT/FI1995/000589
Authority: WO
Inventors: Tuomo VÄÄNÄNEN
Original assignee: Nokia Telecommunications Oy
Priority date: 1994-10-26
Filing date: 1995-10-25
Publication date: 1996-05-09
Also published as: FI945047A; FI97263B; FI97263C; FI945047A0; AU3747795A

Abstract

This invention relates to a method for eliminating the effect of disturbance frames in a radio communication system including a base station (BS) which comprises a radio receiver (RX) and telecommunication means for forwarding signals received from the radio path through a telecommunication connection, and in which system the signalling of the base station (BS) occurs on the speech path. In the method according to the invention, radio frequency signals received by the receiver (RX) of the base station (BS) are supervised in order to detect disturbance frames, and when a disturbance frame is detected, the forwarding of signals received by the receiver (RX) that received the disturbance frame is interrupted. The invention further relates to a base station.

Description

A method for eliminating the effect of disturbance frames, and a base station

This invention relates to a method for eliminating the effect of disturbance frames in a radio communication system including a base station which comprises a radio receiver and telecommunication means for forwarding signals received from the radio path through a telecommunication connection, and in which system the signalling of the base station occurs on the speech path. The invention further relates to a base station of a radio communication system which comprises a receiver for receiving radio frequency signals; telecommunication means for forwarding the received signals to a switching centre; signalling means for transmitting signalling frames to the switching centre on the speech path; and interruption means responsive to the signalling means for breaking a telecommuni¬ cation connection between the receiver and the switching centre for the duration of the transmission of a signalling frame.

In this application disturbance frames refer generally to such frames that are not intended to be received fro*n the radio path, and which significantly disturb or complicate the operation of the system when a base station forwards them to a switching centre.

This invention relates especially to the deliberate disturbance of a cellular radio system, such as the NMT system (Nordisk Mobil Telefon) . A person somewhat familiar with the technique can cause significant damage to the NMT system, for example, by transmitting base station alarm frames with a mobile phone or with a radio transmitter operating on the frequencies in question. When a mobile phone is switched on, it searches for a frequency channel that transmits a calling channel identifier. It is relatively easy to modify a mobile phone so that after it has found a calling channel it will start transmitting base station alarm frames on the transmission frequency that corresponds to the calling channel of the base station. Because in the NMT system, for example, signalling between a base station and a mobile services switching centre occurs on the speech path (that is, there is no separate signalling channel) , the mobile services switching centre cannot distinguish where the signalling transmitted to it originates from. That is, if blocking base station alarm frames are transmitted with a mobile phone on a suitable frequency channel, said frames will travel, forwarded by the base station, to a mobile services switching centre that will assume that they originate from the base station in question. When the mobile services switching centre receives a blocking alarm, it disables the channel in question and enables another channel as a calling channel. When the mobile phone loses a calling channel, it will immediately start searching for a new one, after which it can start again transmitting base station alarm frames. A vandal can thus block all available channels within a very short period of time.

The object of the present invention is to solve the above-mentioned problem and introduce a method for eliminating the effect of disturbance frames. This object is achieved with the method of the invention which is characterized in that radio frequency signals received by the receiver of the base station are supervised in order to detect disturbance frames, and when a disturbance frame is detected, the forwarding of signals received by the receiver that received the disturbance frame is interrupted. The invention further relates to a base station with which the method of the invention can be applied. The base station of the invention is characterized in that the base station comprises a supervisory unit for supervising the signals received by the receiver in order to detect predetermined disturbance frames, whereby the interruption means are arranged to break the telecommunication connection between the receiver that received a disturbance frame and the switching centre when the supervisory unit detects a disturbance frame.

The invention is based on the idea that when means are arranged to the base station for supervising the signals received from the radio path, with which means a disturbance frame can be identified, the proceeding of a disturbance frame from the base station to the switching centre can be prevented by breaking the connection for a moment between the receiver in question and the switching centre. Therefore, the most significant advantage of the method and the base station of the invention is that the deliberate disturbance of a radio system will be significantly more difficult since the base station is able to identify such frames that it should not receive at all from the radio path.

The preferred embodiments of the method and the base station of the invention are disclosed in the appended dependent claims 2 to 3 and 5 to 6. In the following, the invention will be described in more detail by way of example by means of one preferred embodiment of the base station of the invention with reference to the accompanying figures, in which

Figure 1 illustrates a block diagram of the NMT system, and Figure 2 illustrates the structure of the frames of the NMT system.

Figure 1 illustrates a block diagram of a part of the NMT system. A mobile services switching centre MTX shown in Figure 1 can be connected to a base station BS for example by means of a 2 Mbit/s PCM connection (Pulse Code Modulation) . The base station of Figure 1 transmits a calling channel with a transmitter TX and receives callings with a receiver RX. In the case of Figure 1 there are integrated into the receiver RX a demodulator for receiving radio frequency signals and a telecommunication unit for adapting the received signals to such a form that they can be transmitted to the mobile services switching centre MTX through a telecommunication connection. There are also several (not shown in the figures) other transmitter and receiver units in the base station for transmitting calls between mobile phones MS in the area and the mobile services switching centre MTX.

The signalling between the base station BS and the mobile services switching centre MTX is accomplished in the NMT system on the speech path, that is, no special signalling channel is reserved from the telecommunication connection between the base station and the switching centre. Therefore, a switching means SI is arranged to the base station BS by means of which a signalling unit 1 (an FSK modem) of the base station BS breaks the telecommunication connection between the receiver RX and the mobile services switching centre MTX for the period of time required for the transmission of a signalling frame, after which the switching means SI is closed in order to restore the telecommunication connection between the receiver RX and the mobile services switching centre MTX.

In order that the disturbance frames trans- mitted with the mobile phone MS would not be able to proceed from the base station BS to the switching centre, a supervisory unit 2 is arranged to the base station of Figure 1. A similar FSK modem 3 can be utilized as a supervisory unit that is used as a signalling unit in the switching centre MTX for receiving and decoding the signalling transmitted from the base stations of the system.

In the case of Figure 1 the supervisory unit 2 is arranged to supervise the telecommunication signals which are received and supplied further by the receiver RX. In the case of Figure 1 these signals are frames in compliance with the NMT specifications (cf. Figure 2) . In this case the supervisory unit 2 especially supervises the fifth sign Z of the frames. It is stipulated in the NMT specifications that if the value of the fifth sign of the frame to be transmitted to the switching centre MTX is "F" , expressed in hexadecimals, it means that said frame is a signalling frame transmitted by the base station. Therefore, the supervisory unit 2 should never in a normal situation detect a frame in which the value of the fifth sign is F (hex) . However, if the supervisory unit detects a frame such as this, it means that it is a disturbance frame, and that the operation of the base station is being disturbed. Then the supervisory unit 2 immediately supplies a control signal to the switch SI, whereby the switch SI switches to an open state and prevents the forwarding of the frame to the switching centre MTX by breaking the connection. The level indicator of the switching centre MTX detects then that the frame did not arrive, whereby the disturbance frame will not be received. After a certain period of time the supervisory unit 2 supplies a new control signal to the switch SI, whereby it switches to a closed state and the signals received by the receiver RX are again transmitted to the switching centre MTX.

Figure 2 illustrates the structure of the signalling frames of the NMT system. According to the NMT specifications, the NMT system uses a continuous phase FSK datamodulation and signalling frames of 166 bits in which there are 144 bits of information, preceded by a synchronizing part of 26 bits. The duration of a frame is 138 ms.

The upper frame of Figure 2 illustrates the signalling frames which the mobile phone MS transmits to the mobile services switching centre MTX. The upper frame of Figure 2 is a signalling frame 13 = line signal/break-off connection. The lower frame of Figure 2 is a signalling frame 28 = base station alarm, transmitted by the base station BS to the mobile services switching centre. The structure of both of the frames disclosed in Figure 2 is analogous, that is, N1N2N3 = channel number P = frame identifier Z = country code of the subscriber number of the phone. If Z=15 (F hex) , the frame is from a base station. XI-X6 = the subscriber number of the mobile phone. A frame in which Z=15 (F hex) should never originate from a mobile phone. If a frame like this should for some reason appear (for example due to the deliberate disturbance of the system) and its proceeding is not prevented in the base station, the mobile services switching centre MTX will assume that the frame originates from the base station.

It is to be understood that the specification above and the figures related thereto are only intended to illustrate the present invention. The different variations and modifications of the invention will be apparent for those skilled in the art without deviating from the scope and spirit of the invention disclosed in the appended claims.

Claims

1. A method for eliminating the effect of disturbance frames in a radio communication system including a base station (BS) which comprises a radio receiver (RX) and telecommunication means for forwarding signals received from the radio path through a telecommunication connection, and in which system the signalling of the base station (BS) occurs on the speech path, c h a r a c t e r i z e d in that radio frequency signals received by the receiver (RX) of the base station (BS) are supervised in order to detect disturbance frames, and when a disturbance frame is detected, the forwarding of signals received by the receiver (RX) that received the disturbance frame is interrupted.

2. A method according to claim 1, c h a r a c¬ t e r i z e d in that said base station (BS) is connected to a switching centre (MTX) through said telecommunication connection.

3. A method according to claim 1 or 2, c h a r a c t e r i z e d in that said radio communication system is the NMT system, whereby the fifth sign of the frames to be received, which frames are in compliance with the NMT specifications, is supervised, and the receiver (RX) is detected to have received a disturbance frame when the hexadecimal value of the fifth sign of the frame is "F", whereby the operation of the receiver (RX) that received the disturbance frame is interrupted for a predetermined period of time.

4. A base station (BS) of a radio communication system which comprises a receiver (RX) for receiving radio frequency signals; telecommuni- cation means for forwarding the received signals to a switching centre (MTX) ; signalling means (1) for trans¬ mitting signalling frames to the switching centre on the speech path; and interruption means (SI) responsive to the signalling means (1) for breaking a telecommuni- cation connection between the receiver (RX) and the switching centre (MTX) for the duration of the transmission of a signalling frame, c h a r a c t e r¬ i z e d in that the base station comprises a super¬ visory unit (2) for supervising the signals received by the receiver (RX) in order to detect predetermined disturbance frames, whereby the interruption means (SI) are arranged to break the telecommunication connection between the receiver (RX) that received a disturbance frame and the switching centre (MTX) when the supervisory unit (2) detects a disturbance frame.

5. A base station according to claim 4, c h a r a c t e r i z e d in that said base station is a base station (BS) of the NMT system, that the supervisory unit (2) is arranged to supervise the fifth sign of the frames to be received, which frames are in compliance with the NMT specifications, whereby the interruption means (SI) are arranged to interrupt the forwarding of the received signals for a predetermined period of time when the hexadecimal value of the fifth sign (Z) of the frame to be received is "F" .

6. A base station according to claim 5, c h a r a c t e r i z e d in that the interruption means comprise a switching means (SI) which is responsive to the supervisory means (2) and connected to the output of the receiver (RX) in order to break the telecommunication connection between the output of the receiver (RX) and the switching centre (MTX) when the supervisory means (2) detect a disturbance frame.