FR2745144A1 - GSM digital data link establishment method between modems - Google Patents

Abstract

The method involves establishing a connection between the two modems. The emitting modem (22) is configured in a calling mode, and the receiving modem is arranged to be in a called mode. The signal level at the emitting modem is set within 10% of the maximum recommended level for a GSM network, and the receiving modem is connected to the GSM network across a switched telephone network at an emission level fixed in the range 4.5 to 5.5 V, peak to peak. The test sequence includes an activating header, and the chosen sequence is formed by the ASCII reference characters 85 (U ) and 255 (white) to respectively test the detection and separation of octets, as well as the equal distribution of the zeros and units, and to test the presence of a false zero. The test data also includes the topological position of the emitter.

Description

Method for establishing a digital data transmission link through the GSM radiotelephone network and test method
the quality of the link.

The present invention relates to a method for establishing a digital data transmission link through the radiotelephone network.
GSM.

The GSM radiotelephone network of a territory is made up of a plurality of radio stations distributed over the territory and linked together by switching systems and the PSTN switched telephone network. Any subscriber in the radio coverage area of a station can establish communication with the GSM network and move around the territory.

A radio network of this type has a certain number of faults: - possibility of local absence of radio coverage, due to the relief for example, - possibility of failure of an attempt to access the network, due to saturation or failing this, - possibility of impromptu disconnection of communication, - sometimes poor sound quality.

The problem posed by the applicant, which is at the origin of the invention, without however having to limit the scope of the present application, was to test the proper functioning of the GSM network.

To identify the faults mentioned above, it was known to manually launch calls from a mobile to a monitoring center and establish the percentages of the various faults, by reporting them if necessary on a topographic map. Such a process is slow and costly in labor. It was also known to carry out, by means of two automata, one of which on board a car, the tests relating to the establishment and maintenance of communication.

However, the measurement of the sound quality could not be carried out, due to the impossibility of transmission by the GSM network of a pure test frequency between a transmitting modem and a receiving modem. In fact, the GSM network is suitable for transmitting voice signals after processing the signal which eliminates redundancies, which considerably reduces the bandwidth required. Only the frequency variations of the voice signal are transmitted, so that a test tone signal at a fixed frequency is not transmitted. However, the test requires a transmission of digital data between modems, at fixed frequencies, in order to perfectly synchronize the receiving automaton with the transmitter; a recorded analog voice signal does not allow this synchronization.

The requester then thought of modulating the test frequency, in order to create signal variations which are transmitted by the network. However the network
GSM transmits only the frequency variations exceeding a low threshold AFb, below which the signal is considered to be stable, ie free from variations. In addition, the frequency variations exceeding a high threshold AFh are of no interest for reconstituting the phonic signal at the output of the network and are therefore not transmitted.

However, it was found that the "high" threshold for frequency variation was lower than the "low" threshold, so that there was no spectral window in which a carrier frequency could be modulated.

In other words, a voice signal has frequency variations allowing it to pass through the network, but these same variations prevent its precise recognition in order to synchronize the receiving automaton, whereas a test signal, having the desired stability for synchronization is not transmitted.

It was then that the applicant also noted that the GSM network did not exhibit immutable transmission characteristics, the high and low thresholds of frequency variations depending on the signal level. The requester was then able to determine a transmission "tunnel" limited at the bottom and at the top by two respective values of signal level thresholds and limited in frequencies by two thresholds at the bottom and at the top, the "high" threshold being greater than the "low" threshold. ". This tunnel provides the bandwidth necessary for transmitting frequency or phase modulated signals, allowing the test signals to be transmitted.

The invention therefore relates first of all to a method for testing the quality of a digital data transmission link of the GSM network, in which a digital data transmission link is established through the GSM radiotelephone network, between a modem transmitter, adjustable level, of a radio transceiver and a modem of a receiving equipment, process in which: - a radio link is established between the two modems - the transmitting modem is configured in called mode and the modem receiver in calling mode, - the transmission of a predetermined sequence of commissioning data by the transmitting modem is commanded, - the data received by the receiving modem is compared with the data sequence and, - if the data received and the data sequence has a correlation below a determined threshold, the level of the signal transmitted is adjusted to reach the correlation threshold, then - a predetermined sequence is emitted of test data and, at the output of the receiving modem, the data received is compared with the predetermined sequence of test data, available locally, to detect transmission errors and to determine the quality of transmission.

It is thus possible to automatically transmit digital test data between test machines, without human intervention, which reduces the cost of the tests and frees the driver of a vehicle transporting a mobile transmitting machine from all constraints, while allowing a rapid sequence between successive calls.

The concept of the above invention is in fact generally applicable to the establishment of links for the transmission, through the GSM network, of any type of digital data.

The invention therefore also relates, as an intermediate step, to a method for establishing a digital data transmission link through the GSM radiotelephone network, between a transmitting modem, with adjustable level, of a transceiver radio and a modem of a receiving equipment, process in which: - a radio link is established between the two modems, - the transmitting modem is configured in called mode and the receiving modem in calling mode, - the transmission of a predetermined sequence of data for commissioning by the sending modem, - the data received by the receiving modem is compared with the data sequence and, - if the data received and the data sequence have a correlation below a determined threshold, the level of the signal transmitted is adjusted to reach the correlation threshold.

The invention will be better understood with the aid of the following description of a preferred embodiment of the method of the invention, with reference to the appended drawing, in which: - Figure 1 is a schematic representation of the GSM network ; - Figure 2 is a block diagram of a mobile network test machine
GSM; and - Figure 3 illustrates the transmission tunnel mentioned above.

The GSM network, represented inside the dotted frame 1, comprises radio 2 transmission-reception stations distributed over a territory and each ensuring radio coverage of a territory area. The stations 2 are connected by clusters to concentrators 3 which are themselves connected to a central framework 4 of switches.

A test automaton 10 of the GSM network 1, belonging to a management and monitoring center, is connected to the switches 4, through the switched telephone network PSTN 9 in this example.

It will be understood that the links purely internal to the GSM network 1 between the stations 2 and the switches 4, can wholly or partly borrow the PSTN.

A mobile radio test automaton 20, on board a car 19, is intended to transmit test messages to the automaton 10 through the GSM network 1.

As shown in FIG. 2, the automaton 20 includes a computer 25, here a personal computer, connected to a radio transmission-reception interface 21 by a modem 22, here meeting the CCITT V 21 standard (300 bits / s ).

The output level of the radio signal emitted by the interface 21 can be adjusted, here by a command input 23 of the level of the modem 22.

The computer 25 includes a central processing unit 26, a program memory 27, and a data sequence memory 28 to be transmitted. The computer 25 controls, by a link 29, the transmission of the interface 21. The interface 21 ensures in known manner the frequency transposition between the voice, low frequency, and the carrier of the radio signal transmitted or received.

The controller 10 has a structure similar to that of the controller 20.

The transmission quality test of the GSM 1 network is carried out according to the following method.

A radio link is established between the automata 10 and 20, by a call, coming from any one of them, through the GSM network 1.

Next, a logical link for the transmission of digital data between the modems (22) of the respective automata 10 and 20 is established on this "physical" link.

For this, the sending modem 22 is configured in called mode and the receiving modem of the controller 10 in calling mode, for data exchange according to the CCITT V 21 standard.

Then the transmission of a predetermined sequence of data for activating the link is ordered, here that of the memory 28, the data received by the receiving modem 10 is compared with the predetermined sequence, available locally, and, if the received data and the predetermined sequence of data have a correlation below a determined threshold, the level of the transmitted signal is adjusted until said threshold is reached. This latter information is transmitted to the controller 20, for example directly by the modem 10 after it has itself adjusted its level, or by an operator, by voice for example.

In this example, the useful data (28), for testing the GSM 1 network, are also those used for putting the logical link into service, that is to say reading activating or waking up vocoders ensuring compression of the sound signal at the input of the GSM 1 network, so that they detect variations in the signal from the modem 22.

It will be understood that, for standard equipment, having known emission level characteristics, the adjustment of the emission level can be carried out once and for all at the factory or, by the user, when establishing 'a first connection.

FIG. 3 very schematically illustrates the shape of the transmission tunnel and represents two frequency threshold curves 31, 32, respectively low and high, determining the width of the frequency range F transmitted, as a function of the amplitude A of the radio signal transmitted. It can thus be seen that there is a data transmission tunnel between two amplitude values A1 and A2, in which frequency or phase modulation of a carrier can take place.

Tests have also shown that there is a range of emission levels in the range A1-A2 compatible with all types of network equipment (24).
GSM, that is to say that any modem 22 can be preset (23) within this range, without risk of excessive transmission failure. This range corresponds substantially to the maximum level of saturation, which it is recommended not to exceed, + 10%, that is to say that, surprisingly, the GSM network has an ability to transmit data outside. of its specified amplitude range. The corresponding level of the modem 22 depends on the gain of the interface 21, which can vary from one manufacturer to another. The modem 22 and the interface 21 must therefore be considered as a single functional unit with regard to the level adjustment.

Thanks to this method, the modem 22 can thus transmit a carrier with voice frequency and the frequency and phase modulation of the latter corresponding to the succession of the bits to be transmitted.

On the PLC 10 side, it has been found, in a homologous manner, that a transmission level, on the PSTN, close to the nominal saturation level of this one, is compatible with all of the above equipment. The corresponding level range is 5 volts peak-to-peak + 10% on 600 ohms and also extends outside the range specified for PSTN. The modem of PLC 10 only has the transmitting function here for transmitting service signals to modem 22.

In this example, the memory 28 comprises a sequence comprising only the only characters "U" and "blank" corresponding to the ASCII code words of numbers 85 (10100101 in binary) and 255 (1111 1111). The alternation, with symmetry, of 0 and 1 of the character "U" allows easy detection and separation between bytes, as well as a test of the equal distribution of 0 and 1. The code word 1111 1111 allows to very simply detect any defect by the presence of a 0.

To determine the transmission quality of the GSM network 1, the receiving automaton 10 compares, by means of its central unit, the data received with the sequence contained in its own memory, identical to that of memory 28, to detect errors of transmission and determine the quality of transmission. The latter can be expressed by the rate of erroneous bits.

In this example, it is associated with the test data of the topographic position data provided by a GPS receiver on board the car.

Claims (7)

1. Method for establishing a digital data transmission link through the GSM radiotelephone network, between a transmitting modem (22), with adjustable level, of a radio transceiver (21) and a modem of a receiving equipment (10), method in which: - a radio link is established between the two modems, - the transmitting modem (22) is configured in called mode and the receiving modem in calling mode, - the transmission of a predetermined sequence of data for commissioning by the sending modem (22), - the data received by the receiving modem is compared with the data sequence and, - if the data received and the data sequence have a correlation below a threshold determined, the level of the signal transmitted is adjusted to reach the correlation threshold. 2. Method according to claim 1, in which the signal level of the transmitting modem (22) is fixed in a range ranging substantially from - 10% to + 10% of the maximum level recommended for the GSM network. 3. Method according to one of claims 1 and 2, wherein, the receiver modem being connected to the GSM network through the switched telephone network (9), its emission level is fixed in a range of 4.5 to 5 , 5 volts peak to peak. 4. Method for testing the quality of a digital data transmission link of the GSM network established according to the method of claim 1, in which - a predetermined sequence of test data is sent and, - at the output of the receiving modem ( 10), the data received is compared with the predetermined sequence of test data, available locally, to detect transmission errors and to determine the transmission quality. 5. Method according to claim 4, in which the sequence comprises a binding activation header. 6. Method according to one of claims 4 and 5, in which the data of the sequence are chosen from the group consisting of the ASCII characters of reference 85 (U) and 255 (white). 7. Method according to one of claims 4 to 6, in which is associated with the test data topographic position data of the transmitting modem (22).