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Description  |
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FIELD OF THE
INVENTION
This invention concerns testing of a subscriber connection in a system implementing a wireless subscriber connection and comprising base stations and wireless terminals to which a subscriber terminal equipment is connected and wherein a part of
the subscriber connection is formed by radio link between the terminal equipment and the base station.
BACKGROUND OF THE INVENTION
In fixed telephone networks the subscriber line network is formed so that twin cables from several subscriber terminal equipments are taken into a crossbar switch, cables from several crossbar switches are combined in another crossbar switch and
a cable from this is taken to an exchange. The signalling interface facing the subscriber lines of the exchange is standardized and is either a connection of 2-wire analog subscriber lines, a multiplexer interface in accordance with the CCITT V2
recommendation or a message-based multiplexer interface in accordance with the ETSI V5.1 recommendation. Subscriber lines are a big cost item both in the building of the network and in its maintenance. The condition of subscriber lines must be
monitored constantly by performing automatic loop measurements. In all exchanges, whether the exchange is a local one or a PBX exchange, each subscriber line is connected to the exchange by way of the SLIC (Subscriber Line Interface Circuit). The SLIC
circuit performs the so-called BORSHT functions, which are Battery Feed, Overvoltage Protection, Ringing, Supervision, Hybrid and Test. There is a good commercial supply of integrated SLIC circuits.
FIG. 1 illustrates a known arrangement for testing the condition of a subsriber line. The typical telephone in a fixed network can be represented by terminating resistor R1 for the purpose of loop measurements and by a ringing circuit formed by
capacitance C and resistor R.
In impedance measurement performed with direct voltage the exchange sees the telephone as a very high impedance in an on-hook state. This is due to the ringing circuit capacitance. When the hook is off, the ringing circuit does not affect,
whereby the telephone is seen as a low resistance. In the SLIC circuit there is a loop identification based on a comparison of the subscriber line's current consumption with the limit value. This function finds out the on-hook or off-hook state.
For measurements at audio frequency the impedance of the line telephone and the SLIC impedance are clearly defined in standards. In most countries 600 W is defined as the terminal impedance for the telephone's audio range in the off-hook state,
even if differences exist between countries. Depending on the frequency, the audio range impedance in the on-hook state is in the magnitude of 1.5 kOhm. Thus, in FIG. 1 terminal resistance R1, depending on the situation, is a direct voltage impedance
or an alternating voltage impedance with the hook off.
The telephone is connected to the exchange with a subscriber line, which is usually a twin cable AB. Directed by a special relay 10, subscriber line AB is branched before the SLIC circuit in the exchange to measuring device 12, which measures
the subscriber loop resistance and capacitance. Of the SLIC circuit only the receiver circuit is shown. By disconnecting the subscriber line from the SLIC input with a relay, the measuring device is able to measure the subscriber loop resistance using
direct voltage, whereby a possible short-circuit is detected, and to measure the capacitance using alternating voltage, thus finding out the condition of the telephone's ringing circuit. Often the loop's resistance to earth is also measured to find out
possible earth faults. It should be noted that by using just one measuring device it is possible in the exchange to perform loop measurements of a great number of subscriber connections.
In building a telephone network the installation of subscriber lines between the exchange and the subscriber terminal equipments is both a considerable cost item and a work demanding much time. Especially in a region with scarce population the
costs for the individual connection may become big. One solution for bringing down costs is by replacing the fixed subscriber loop between exchange and subscriber terminal equipment for radio link. Hereby the subscriber terminal equipment in the fixed
network is connected to a terminal unit containing a radio transmitter/receiver and converting the audio-frequency signal of the subscriber terminal equipment into a radio-frequency signal and transmitting it by radio to the base station and,
correspondingly, converting the received radio-frequency signal into an audio-frequency signal which the subscriber terminal equipment will understand.
In a solution of this kind the radio system may be a pure analog or digital cellular system, wherein the terminal units are communicating with the base station, or it may be a modified cellular system from which some network elements have been
removed and some functions have been simplified. In the present application this solution is called WLL or Wireless Local Loop System.
The WLL system may be constructed in such a way that structural parts of some existing cellular system are applied. A mobile telephone system may be, for example, an analog NMT or TACS system or a digital GSM system. Hereby, the signalling of
the WLL system is in accordance with the system in question, base stations are standard base stations of the system in question and the radio unit of the terminal is similar to the radio unit of a mobile station or the terminal may be the mobile station
in a mobile telephone system. The important component of the WLL system is the subscriber network element which connects subscribers to a local exchange meeting the standards. In a pure cellular system it is a mobile switching center and in an applied
system it is a network element to be described later. The subscriber network element changes the signalling of the WLL network, for example, NMT or GSM signalling, into signalling suitable for the fixed network (for example, PSTN) and, correspondingly,
it adapts the signalling of the fixed network to the WLL network interface.
FIG. 2 shows a principle of the WLL system. A wireless fixed terminal T comprises a radio unit with an aerial and a telephone adaptor 4 connecting a standard subscriber terminal equipment 5 to the terminal. The subscriber terminal equipment may
be an ordinary telephone set, a telefax or a modem. It is connected to the terminal by pushing a standard connecting plug into the terminal's adaptor connection. The user uses subscriber terminal equipment 5 as in an ordinary fixed network, although a
main part of the subscriber line connection is formed by the radio link between terminal T and base station 2 or 3. The subscriber terminal equipment and the terminal may be combined to form a single physical device 6, for example, as a mobile station
used in that system on which the WLL system is based. Each base station may serve numerous subscriber terminal equipments. The base stations are connected to a special subscriber network element 1 which for its part is connected to a standard telephone
exchange.
The terminal receives its power supply either directly from the power supply mains, if one is available at the terminal's place of location, or from a battery. In the former case a standby battery is also used and a charger then keeps the
standby battery charged. In case of power failure the terminal will have its power supply from the battery and will thus be able to continue its operation.
Network element 1 is connected to a local exchange with a multiplexer connection using an open 2 Mbit/s PCM system of the V2 or V5.1 type. If the local exchange supports a 2-wire connection only, the network element is connected to the exchange
by converting the V2 signalling into an analog 2-wire subscriber connection interface. Signalling between the network element and the base stations connected to it is the signalling of the applied mobile telephone network, however, modified in such a
way that functions typical of the cellular network, such as cell handover and roaming, are left out. Thus, the subscriber must stay in the coverage area of his named base station. Routing of incoming and outgoing calls is based on the subscriber file
of the network element. The operation of the network element is similar to the operation of a concentrator: a call is routed from the subscriber connection to the exchange while analysis of numbers, calculation and other functions are performed in the
exchange.
In accordance with the above, a WLL network may be based on the known GSM system. GSM is a digital cellular system based on time division multiple access wherein a traffic channel TCH is always allocated together with a SACCH (Slow Associated
Control Channel) connected to it. The mobile station must measure the power of the received signal, signal strength of BCCH carriers of neighbour base stations, and it must calculate the bit error ratio describing the quality of the received signal. It
must report measurement results in accordance with the GSM recommendation no less than once a second on the above-mentioned SACCH (Slow Associated Control Channel). If the SACCH channel is used only for the purpose of reporting, the mobile station may
report measurement results twice a second. In a WLL application the terminal may perform the same measurements as the mobile station, although measurements relating to handover are unnecessary.
It is true that the WLL system is able from the measurement results transmitted by the terminal in some degree to conclude whether there are errors in the terminal's operation. If no results are received, then it is of course obvious that the
terminal is not operating. But the system will not know whether there are disturbances in ringing, in the passing on of DTMF signals, in responses to on-hook and off-hook signals of the subscriber terminal equipment etc. In addition, the system is not
able to find out if there are faults in the operation of the subscriber terminal equipment. The subscriber terminal equipment, which may be a conventional fixed network telephone, may be short-circuited, its hook is left off, the plug is not connected
to the terminal etc. Nor has the system any means for finding out whether the terminal's standby battery is okay and whether the battery charger is operating as it should. In some cases the subscriber, without the operator knowing, may use batteries of
a wrong type which may cause disturbances in the terminal's operation. The subscriber makes an error report to the operator who must then find out the cause of the trouble, which is simple as such.
The international patent application WO 94/22255, applicant Telular International, Inc., describes a testing device suitable for a system of the WLL type which automatically performs programmed tests on the terminal. Physically connected to the
terminal, the testing device monitors the operation of the terminal and the radio unit, adaptor and power supply and checks whether their operation is correct. When connected to the adaptor it simulates functions of the subscriber terminal equipment
(telephone, modem, fax) concluding from responses of the adaptor whether the adaptor is working as it should. It should be mentioned as regards simulation that the testing device generates an off-hook signal for the adaptor and checks whether the
adaptor responds with a dialling tone. It directs the ringing tone generated by the adaptor to the terminal, then generates an off-hook signal and checks whether the adaptor turns off the ringing tone in response to the off-hook signal. The testing
device may connect to a transmitter/receiver and command it to generate an outgoing call to its own number, in other words, to call through the network to itself, whereby the device checks whether the transmitter/receiver generates a busy tone in
response to the call.
The prior art testing device has some drawbacks. Firstly, the network operator can not start a test sequence by remote control nor receive test results concerning the terminal's operation, but the test takes place locally either automatically or
started by the user. It notifies the party ordering the connection of the test results, for example, by indicator lights. Secondly, the testing device does no test at all on the operation of the terminal and of the subscriber line connecting this to
the subscriber terminal equipment, nor this way the operation of the subscriber terminal equipment, so when a fault occurs in the subscriber line or in the subscriber terminal equipment, the user must call for a service man. This is unnecessary, for
example, if the telephone plug is not pushed well into the adaptor. Thirdly, the operator may need to perform loop measurements on the subscriber line, as is done also on the fixed network, but this is not possible in the WLL system, because a part of
the subscriber line is radio link. Should the known method shown in FIG. 1 be applied, this would mean that every WLL terminal ought to have its own resistance/capacitance meter. This would be economically expensive and yet there are no means of
passing on measurement data to the operator. Fourthly, it is not possible with the testing device to test the condition of the terminal's standby battery.
Thus, it is an objective of the present invention to bring about a method of testing the operation of both the terminal and its standby battery and the condition of the subscriber line and subscriber terminal equipment and where the test may be
started by radio from the network and the test results passed on by radio to the network. A connection implementing the method is another objective.
The established objectives are achieved through solutions defined in the independent claims.
BRIEF SUMMARY OF THE INVENTION
According to the invention, a connection arrangement is contained in a WLL terminal to receive from the network a test message transmitted to it by using a suitable protocol, to perform diagnosing functions stated in the message and to transmit
the results to the network using a suitable protocol.
Diagnosing functions include a terminal test which may contain tests of the same type as those performed in a prior art arrangement, such as on-hook/off-hook measurements and signal level measurements. In particular, diagnosing functions include
loop measurements of the subscriber loop and a function checking the condition of the terminal's standby battery.
To carry out loop measurement the connection arrangement contains a SLIC circuit, which is used to perform the subscriber connection, that is, the twin cable from the subscriber terminal equipment is brought to this circuit. In parallel with the
2-wire input of the SLIC circuit and with the subscriber line a series connection is located which comprises a signal generator and a resistance. The resistance value is approximately the same as the input impedance of the SLIC circuit. The signal
generator supplies an alternating voltage to the subscriber line, which depending on the purpose of measurement is either of a low frequency or of a high frequency. The state of the subscriber loop determines the effect of the supplied signal in the
SLIC input and, correspondingly, in its output. Results of low and high frequency measurements are compared with each other and it is easy to find out from these results if there is a short-circuit in the subscriber line, if the line has broken, if the
telephone is off-hook and if the telephone's ringing circuit is faulty.
The SLIC circuit itself contains a loop identification based on a comparison of the subscriber line's current consumption with a limit value. This function finds out whether the hook is off or on. Digital terminals already use a signal
processor to which an A/D converter, usually a PCM codec, is connected which for its part is connected to the SLIC output. The signal processor may be given the task to perform necessary calculations. A separate calculation circuit may be used in
analog terminals.
For finding out the condition of the standby battery, such a logic unit is added to the terminal or to its power supply unit, which performs a battery condition test in response to a message from the network which starts the test. The test
includes at least a battery load test. The terminal then forms a message wherein information on the battery condition is transmitted to the network.
For sending messages starting the test from the network to the terminal and, correspondingly, for sending reports from the terminal to the network, the cellular network's existing protocol is used, if at all possible. An analog NMT network uses
certain frames defined in the specifications. A digital network uses a SMS protocol (Short Message Service) or any transmission protocol developed on a SMS basis. The test may be started by some network element, such as, for example, a remote operation
center or a subscriber network element, but a base station may also start the test and analyze the results. Another applicable protocol used in a digital cellular network is called Unstructured Supplementary Service Data (USSD). This protocol has been
proposed by European Telecommunication Standards Institute (ETSI) in the specification ETS 300 625, January 1995. Both a mobile station and the network are able to form, send and receive unstructured strings. After the mobile station has received the
string, it realises operations instructed by the network, forms a new string including results and send the string to the network.
After the terminal has received the test message it searches that function specification from a stored table to which there is a reference in the message and performs the tasks defined in the specification. Upon completion of the task and
calculation of the results the terminal forms a message according to the protocol and transmits it to the network.
Test functions of the terminal may be started as required either with regular repetition, at a certain time only, such as in the night, and concerning some terminals only, or concerning all terminals. A test instruction may concern an individual
measurement or a whole series of measurements.
LIST OF FIGURES
The invention is described more closely in the following with references to the enclosed schematic Figures, of which
FIG. 1 shows the known principle of loop measurement,
FIG. 2 shows a WLL system,
FIG. 3 is a simplified view of a connection according to the invention,
FIG. 4 shows a practical embodiment of the connection,
FIG. 5 shows a connection according to the invention in a digital terminal,
FIG. 6 depicts a first arrangement for testing a terminal's standby battery, and
FIG. 7 depicts another arrangement for testing a terminal's standby battery,
FIG. 8 is a flow diagram of functions performed by a terminal,
FIGS. 9A and 9B show frames suitable for message transmission in an analog cellular network, and
FIG. 10 shows a digital cellular network supplemented with elements according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 3 shows a connection arrangement in a WLL terminal which allows loop measurements. The markings in the Figure correspond with the markings in FIG. 1 in applicable parts. An essential difference as regards the subscriber line is that the
subscriber line in FIG. 1 running from the telephone to the exchange is long, even tens of kilometers, whereas in the WLL case in the figure the length of the subscriber line, that is, the line running from the subscriber terminal equipment to the
terminal, is short, typically only a few meters, and that the subscriber connection thereafter is a radio link.
Only circuits essential to the invention are shown in the WLL terminal demarked by a broken line. They comprise a commercial SLIC circuit intended for telephone exchanges and available from several suppliers, and alternating voltage generator
31, switch 32 and resistance 33. The SLIC input is formed by wires A and B of the twin cable which is the subscriber line. A series connection of alternating voltage generator 31, controlled switch 32 and resistance 33 is connected between the input
poles of the circuit. The resistance value of resistance 33 is equal to that of the SLIC input impedance R1, which for its part is approximately equal to the terminal impedance of the subscriber terminal equipment's audio range, that is, the impedance
measured at low frequencies (also reference number R1) in an off-hook state. When the subscriber line's plug is pushed into the adaptor socket of the terminal, both the subscriber loop, the alternating voltage generator and the SLIC circuit are
connected and fitted in parallel as seen from the connecting point.
It was noted in connection with the prior art description that commercial SLIC circuits as such contain loop identification based on a comparison of the subscriber line's current consumption with a limit value which can be set. This function
allows identification of the telephone's hook state, that is, whether it is on-hook or off-hook. The method in accordance with the invention also allows separation from one another of
the states a) off-hook telephone (that is, there is a call in progress, the hook has been forgotten on the table or is badly replaced) and b) a short-circuit in the subscriber line
and the states c) subscriber line off (that is, the line is really off or broken or the telephone's ringing circuit is faulty or broken) and d) the hook is on and the ringing circuit can be seen (that is, all is okay).
These states can be identified just by feeding alternating voltages of different frequencies to the subscriber line from a separate signal generator. The states can be concluded by comparing with each other at different frequencies signal levels
obtained from the output of the SLIC amplifier. Concluding may be done in several ways of which an advantageous one will be presented hereinafter.
When no loop measurements are done, switch 32 is open and the SLIC circuit is in a passive state. When the terminal receives from the network a message prompting loop measurement, switch 32 is closed and alternating voltage generator 32 is
activated.
Several different frequencies may be formed with the generator and their effect is illustrated below with the aid of Table 1.
TABLE 1 Generator SLIC output frequency level Hook Conclusion A(f) 0 off/on short-circuit A(f) 1/2 A off/on subscriber line off or loose A(f) > ringing reducing on ringing circuit OK circuit cutoff frequency A(f) > ringing does not
on fault in ringing circuit circuit cutoff change frequency A(f) < ringing minimum value off/on DC impedances are circuit limiting received frequency A(f) > ringing default values off/on audio range impedances circuit cutoff are received
frequency
When the generator supplies any frequency A(f), wherein A is the amplitude and f the frequency, and the SLIC output level is 0, it is known that the subscriber line is in short-circuit. This is so because as seen from the terminal the resistance
between wires A and B is zero or a few ohms, so this short-circuits the generator branch, and for this reason no signal is obtained from the SLIC receiver output. This is the case whether the hook is off or on.
When the generator supplies any frequency A(f) and the SLIC output level is 1/2A, it is known that the subscriber line is off or loose. This is so because since the impedance towards the subscriber line is infinite, the voltage given by the
generator is divided equally between generator resistance 33 and the SLIC input resistance R1 (the resistances are equal). Such a voltage is thus received from the SLIC output whose amplitude is exactly one-half of amplitude A of the voltage of
generator 31. This is the case whether the hook is off or on. The frequency given by the generator has no significance in this case either.
The frequency is of significance when the subscriber line is okay and when the operation of the telephone's ringing circuit, the telephone's direct voltage resistance and audio range impedances are examined. The cutoff frequency of the
telephone's ringing circuit, which is shown as a RC series connection in FIG. 3, is in the range 0.5 . . . 1 kHz. When the frequency of the alternating voltage supplied by generator 31 is increased above this, that is, A(f)>cutoff frequency of the
ringing circuit, and A(f) is, for example, an alternating voltage of 4 kHz, capacitor C of the ringing circuit represents only a low resistance, so the subscriber line impedance is reduced, leading to a reduction of the SLIC input voltage and so also of
the output voltage. It can be concluded from this that the ringing circuit is in order. The on-hook state is known.
If in the previous measurement, wherein A(f)>cutoff frequency of the ringing circuit, the SLIC output level is not changed, the conclusion will be that the ringing circuit is faulty.
When the generator frequency is reduced below the cutoff frequency of the ringing circuit, that is, A(f)<cutoff frequency of the ringing circuit, and A(f) is, for example, an alternating voltage of 50 Hz, it is possible to conclude direct
current impedances corresponding to the hook state. If the hook is on, the SLIC input sees the subscriber line DC impedance as high and if the hook is off, whereby the ringing circuit has no effect, the SLIC circuit sees the value of the telephone
terminal impedance R1.
By having the generator supply the frequency A(f)>cutoff frequency of the ringing circuit one finds out whether the audio range impedances are such as they should be.
When the SLIC circuit is connected to a PCM codec, it could be thought that the codec's signal generator would be used whose signal is available out from the circuit through the SLIC's internal supply amplifier. However, it can not be used as a
signal generator 31 in accordance with the invention, because the hybrid function of the SLIC circuit prevents the codec's signal from being heard in the receiver branch, if no phase difference is brought about between the sent signal and the received
signal. A phase difference may result naturally when using the SLIC circuit in the traditional manner in the telephone exchange of a fixed network, whereby the subscriber line is often long enough and the sent signal is reflected back from the
subscriber terminal equipment end. In the WLL case, the subscriber line is far too short for any phase difference to be | | |
