Remote test of a subscriber connection in a system...

Details Remote test of a subscriber connection in a system... Remote test of a subscriber connection in a system...

1997-03-06

2001-01-02

Maung, Nay (Department: 2744)

Telecommunications

Transmitter and receiver at separate stations

Having measuring, testing, or monitoring of system or part

C455S423000, C455S425000, C379S027010

Utility Patent

active

06169883

ABSTRACT:

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 R
1
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 R
1
, 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, modifie

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