Maintenance termination unit for ISDN

Abstract: Disclosed is a Maintenance Termination Unit (MTU) for an Integrated Services Digital Network (ISDN). A tone ringer (13) is provided at the end of the customer loop to generate an AC signal. The signal is applied through a transformer (T₁) and rectifier (D₁) to charge a capacitor (C₁). The resulting voltage across the capacitor causes a normally closed MOS switch (Q₁, Q₂) to open and disconnect the customer until the capacitor discharges.

Description: This invention relates to Maintenance Termination Units (MTUs). Maintenance termination units are utilized to disconnect the customer from the telecommunications network in order to determine if any problems which exist are in the customer equipment or in the network equipment. Present Plain Old Telephone Service (POTS) MTUs generally rely on the presence of a DC current on the line to close a sold state switch when the customer is off-hook. Integrated Services Digital Network (ISDN) is a digital transmission system which will provide voice, data and video information over customer lines. One of the problems with such a network is how to provide an MTU in the customer loop. Since there is sometimes no current to the customer, use of standard MTUs would result in a disconnection while the customer is off-hook. According to this invention there is provided a maintenance termination unit as claimed in claim 1. The invention will now be described with reference to the accompanying drawing in which: FIG. 1 is a schematic circuit diagram of an MTU embodying the invention; FIG. 2 is a schematic circuit diagram of another MTU embodying the invention; and FIG. 3 is a schematic circuit diagram of a portion of an MTU embodying the invention. A maintenance termination unit embodying the invention is illustrated in FIG. 1. Data, voice, or video signals are supplied from the central office to input tip and ring conductors 11 and 12. The incoming signals are supplied to a tone ringer circuit 13. The tone ringer circuit is a commercially available circuit designed to produce an AC signal (tone) in response to a ringing signal on the lines 11 and 12. One such circuit, which is generally used to create AC tones on speakerphones, is made by Motorola under the designation MC 34017-2P. Typically, the AC signal will have a frequency in the range 100-1000 Hz and a peak amplitude in the range 1-5 volts. The output of the tone ringer is coupled to a transformer, T 1 , which is used to isolate the remainder of the unit from voltages on the tip and ring conductors. Typically, the transformer, T 1 , has a turn ratio of 1:1. The output of the transformer is coupled to a rectifying means which, in this example, is a standard silicon rectifying diode D 1 . A capacitor, C 1 , has one plate coupled to conductor 14 extending from the cathode of the diode D 1 and its other plate coupled to conductor 15 extending from the secondary winding of the transformer T 1 . Generally, the capacitance of C 1 should be within the range 2-10 microfarads. Coupled in parallel with the capacitor C 1 is a resistor R 1 which, for reasons to be discussed, has a resistance in the range 10-30 megohms. A zener diode D 2 is also coupled in parallel with capacitor C 1 and resistor R 1 . A pair of MOS Field Effect Transistors (MOSFETs), Q 1 and Q 2 , have a common gate electrode G coupled to conductor 15 and a common source region coupled to conductor 14. The drain region of transistor Q 1 is coupled to the tip conductor 11, and the drain of transistor Q 2 is coupled to the output tip conductor 16 which is connected to the customer equipment. The transistors Q 1 and Q 2 are normally conducting, depletion mode MOSFETs. A capacitor C 2 is coupled across the two transistors Q 1 and Q 2 to minimize losses of high frequency signals through the circuit during normal transmission by providing a path which bypasses Q 1 and Q 2 for such signals. C 2 generally has a value in the range 0.1 to 1 microfarads. During the normal operation of the customer equipment, transistors Q 1 and Q 2 will be conducting so that the circuit of FIG. 1 is transparent to the central office and customer. If, however, a service problem should develop, or if the central office merely wants to conduct a routine test, a ringing signal is transmitted from the central office and appears on conductors 11 and 12. This signal, which normally has a frequency of approximately 20 Hz and an RMS amplitude of approximately 84 RMS, is not used in the normal operation of ISDN systems. The tone ringer, 13, detects the presence of the ringing signal and, in response, produces an AC signal at its output. This signal is applied to transformer T 1 and to diode D 1 for rectification. The rectified signal is used to quickly charge up capacitor C 1 . When the voltage across the capacitor reaches a certain level, in this example approximately 2 volts, the depletion mode MOSFETs Q 1 and Q 2 will be rendered non-conductive as a result of the corresponding voltage established between the gate G and source regions of the MOSFETs. The customer is thereby cut off from the network while standard tests can be performed to determine if any problem exists in the network. The MOSFETs will remain non-conducting for a period of time depending on the value of R 1 since the capacitor C 1 will be discharging through R 1 during this disconnect interval. It is generally desirable to keep the customer disconnected for a period of 30-180 seconds. Consequently, a resistance of R 1 in the range 6-30 megohms is appropriate. Once the capacitor C 1 has discharged below a certain level, MOSFETs Q 1 and Q 2 will resume their normal conducting state, and further tests can then be performed on the customer equipment. The diode D 2 serves as a protector element. In the event that too much voltage is generated across conductors 14 and 15, D 2 will break over and conduct current away from the capacitor. FIG. 2 illustrates an alternative embodiment where elements corresponding to those of FIG. 1 are similarly numbered. In this embodiment, a second pair of MOSFETs, Q 3 and Q 4 , are coupled to the output ring conductor 17 to provide an additional switch. The additional switch is operated by a similar combination of transformer T 2 coupled to the tone ringer, 13, a rectifying diode D 3 , capacitors C 3 and C 4 , a resistor R 2 and a zener diode D 4 . This circuit permits both central office tip and ring conductors 11 and 12 to be disconnected from the customer. The circuit will also add additional impedance to the lines, which can be minimized by the presence of capacitors C 2 and C 4 to bypass transistors Q 1 - Q 4 for high frequency signals. Various additional modifications will become apparent to those skilled in the art. In the FIG. 2 embodiment, a single 3-winding transformer can replace the two transformers T 1 and T 2 . Further, a MOSFET with one input and two outputs, each coupled to one of the output tip and ring conductors, could simplify the circuitry of FIG.2 by eliminating T 2, D 3, C 2 , R 2 and D 4 . Also, as shown in FIG. 3 a bridge circuit including tip, ring, and ground conductors can be used to provide the incoming signal to the tone ringer 13. This modification would ensure that the MTU would operate even if there is a short between tip and ring by applying the signal between tip or ring and ground. All such variations which basically rely on the teachings through which the invention has advanced the art are properly considered within the scope of the invention.

Claims: A maintenance termination unit CHARACTERIZED BY means (13) responsive to a ringing signal for generating an AC signal, means (D 1 ) for rectifying the AC signal, a capacitor (C 1 ) coupled to the rectifying means, and a normally closed semiconductor switch (Q 1 , Q 2 ) coupled to the capacitor such that when the capacitor is charged the switch will open for a period of time. A unit according to claim 1 wherein the switch comprises a pair of normally conducting MOS field effect transistors (Q 1 , Q 2 ). A unit according to claim 2 wherein the MOSFETs have a common gate electrode (G 1 ) coupled to one plate of the capacitor (C 1 ), and a common source region coupled to the other plate of the capacitor. A unit according to claim 3 wherein the drain of one MOSFET (Q 1 ) is coupled to a ring conductor (11) adapted for coupling to a central office, and the other MOSFET (Q 2 ) has a drain coupled to a ring conductor (16) adapted for coupling to a customer's equipment. A unit according to claim 1,2,3 or 4 wherein the means for generating an AC signal comprises a tone ringer circuit (13). A unit according to any preceding claim wherein the rectifying means comprises a semiconductor diode (D 1 ). A unit according to any preceding claim comprising a transformer (T 1 ) coupled between the means for generating the AC signal and the means for rectifying the AC signal. A unit according to any preceding claim comprising a resistor (R 1 ) coupled in parallel with the capacitor. A unit according to claim 8 wherein the resistor has a resistance in the range 10-30 megohms. A unit according to any preceding claim wherein the capacitor has a capacitance in the range 2-10 microfarads. A unit according to any preceding claim comprising a tip conductor (11) and ring conductor (12), the switch being coupled to disconnect a customer from the tip conductor, and an additional switch (Q 3 ,Q 4 ) coupled to disconnect the customer from the ring conductor.

Inventors: DEBALKO GEORGE ANDREW, DEBALKO, GEORGE ANDREW
Applicants: AT & T CORP, AT&T CORP.
Application year: 1992
Main classification: H04M3
IPC codes: H04M3, H04Q11