LINE ACCESS APPARATUS FOR IN-BAND COMMUNICATION

Abstract: Line access apparatus (10) for in-band communication includes a line coupler that functions as a high voltage isolation device and has a resonant frequency of the signal transmission band. A carrier signal of about the resonant frequency is modulated at an in-band frequency to convey information across the line coupler. The apparatus comprises a plug (18) adapted to be inserted between the main distribution frame (42) and a plurality of high voltage protector blocks. The plug includes a plurality of pins (20) of which each pair is intended to bridge a respective line pair. With the pins (20) are associated piezoelectric devices (24) each serving as both a voltage-isolating device and a signal coupler.

Description: The present invention relates to communications apparatus comprising line access apparatus for in-band communication and comprising a voice communication system having a predetermined operating bandwidth. The introduction of and continued demand for non-voice communication services is well recognised. As a result, entire non-voice communication networks have been developed to serve both the individual consumer and large users. In general, the provision of such services has entailed one of two approaches. One is the provision of a separate and distinct communication system; the other is the expanded use of existing forms of communication. The provision of separate and distinct communication systems is, of course, expensive as it requires, inter alia, complete new wiring, whereas the use of existing forms of communication, most frequently telephone lines, requires techniques that do not interfere with existing voice services. The line access apparatus for in-band communication described and discussed hereinafter follows the second approach. Currently, the essential problems encountered when attempting to use existing telephony systems are two-fold.The first problem is access and the second problem is the cost and reliability of equipment capable of functioning at the relatively high voltages required of telephone line circuits. Existing telephony systems are composed of a number of switches, each switch serving a large number of telephone subscribers. These switches are geographically distributed and are generally referred to as "central office switches". Each central office switch has a main distribution frame (MDF) whereat each subscriber line pair served by that switch enters one side thereof. The subscriber line pairs are then routed through the MDF to the switch mechanism. This configuration is also found in smaller, usually privately owned, switches serving a rather geographically concentrated group of subscribers, such as, for example, a university campus. These smaller switches are generally referred to as private branch exchanges (PBXs). Regardless of the size, most telephone company switches are presently operating at near peak capacity. Because of the near peak capacity it would be impossible to provide each subscriber with a second pair of wires for non-voice communication via the MDF. Even if the capacity were available, the cost of the existing wiring militates against such an approach and massive disruption of existing voice communication would most likely occur. However, to make efficient use of existing communication lines, the MDF is clearly a most advantageous location to access a large number of subscriber lines. It is primarily for this reason that alternatives to rewiring an MDF have been developed for accessing existing subscriber lines. One particular development is shown and described in U.S. Patent Applications Serial Numbers 595,108 and 595,120. In these applications a main distribution frame access device is discussed and described. The MDF access device typically includes a plug-in unit that is inserted at the main distribution frame between the subscriber line termination points and the high voltage protection points. The use of such a device can provide immediate access to the subscriber pairs of wires within the main distribution frame without disturbing or disrupting the internal wiring thereof. Such an access device is particularly useful in systems wherein additional services are to be provided via the subscriber lines, for example, for providing an auxiliary data switch or providing a remote meter reading system or other telemetry applications. Telemetry systems have been described and discussed in U.S. Patent Applications Serial Numbers 648,542; 667,527 and 667,789. In such systems the main distribution frame access device can be used to provide access to the subscriber lines and connected, via a multiplexing means, to the data processing equipment of a utility company. The provision of data services by the use of a similar main distribution frame access device is described in U.S. Patent Application Serial Number 705,462. In this particular application a system is provided wherein a full range of data services is supplied to subscribers of, for example, an analog switching system. The second problem in implementing systems that make use of existing communication means is that telephone subscriber line circuits are designed to sustain comparatively high voltages, such as, for example, -48 volts D.C. commonly used in the talking circuits thereof, as well as a superimposed 75 RMS A.C. ringing voltage. This voltage is conventionally provided by the central office battery and ringing generator, and exists intermittently on the subscriber lines when rung in response to a telephone call made thereto. In addition, for obvious reasons, telephone subscriber line circuits are also designed to survive lightning strikes and other high voltage transients. The fact that such line circuits are subjected to and must sustain relatively high voltages is a detriment to adding auxiliary services at the MDF via a main distribution frame access plug. For example, when multiplexing/demultiplexing is performed at the MDF, high voltage large scale integrated circuits are presently required. Currently, high voltage large scale integrated circuits, in addition to being expensive compared with low voltage large scale integrated circuits, are somewhat limited in both manufacture and reliability. Furthermore, currently available high voltage large scale integrated circuits are, themselves, extremely susceptible to high voltage transient signals and could exhibit catastrophic failure due to transients occurring at, or near, the main distribution frame. Further, if an existing communication media has loaded or long subscriber lines the operating bandwidth thereof is often quite restricted. In the instance of telephone communication systems, the bandwidth is limited to the voice band. This places a further restriction on any auxiliary communication system in tandem therewith. Hence, to implement a system using such existing communication media the communication must be made within the operating bandwidth thereof, i.e., in band. Consequently, in order to take full advantage of the ability to access subscriber line pairs at a main distribution frame, it is highly desirable to isolate high voltage signals while, nevertheless, exchanging information with the subscriber lines. In British Patent Specification No. 21370564 there is disclosed a communications apparatus which provides coupling between acoustic signal modulated carrier generating/receiving circuits of remotely located terminal stations by way of mains wiring carrying mains voltage and extending between terminal stations. For the purpose of isolating the acoustic signal modulated carrier generating/receiivng circuits from the mains voltage at the mains wiring but at the same time provide for the transmission/reception of the acoustic signal modulated carrier to and from remote terinal stations a piezoelectric coupling device is used. The coupling device is required to allow the passage of both transmitted and received acoustic signal modulated carrier and therefore has a resonant frequency tuned to the carrier modulated by the acoustic signals which are outside the operating bandwidth of the mains wiring extending between the remote terminal stations. However, it is an object of the present invention to provide a line access apparatus for in-band communication for use at a main distribution frame of a communication network. According to the invention, therefore, there is provided communications apparatus comprising a transmission medium, means for modulating a carrier signal with voice frequency signals and piezoelectric coupling means for coupling the modulating means to the transmission medium and providing high voltage isolation therebetween, characterised in that the apparatus comprises line access apparatus for providing in-band communication comprising a voice communication system in which the transmission medium includes a communication medium having a predetermined operating bandwidth, in that the piezo-electric coupling means comprises at least one coupling device having a resonant frequency outside of the predetermined operating bandwidth, in that modulating means is provided for modulating the carrier signal with an information signal having a frequency within the operating bandwidth and is connected to one side of the piezoelectric coupling device, the carrier signal having a frequency about equal to the resonant frequency of the or each piezoelectric coupling device whereby the information signal can be conveyed across the or each piezoelectric coupling device with the carrier signal and in that means are arranged between the or each piezoelectric coupling device and the communication medium for connecting the or each piezoelectric coupling device to the communication medium and for removing the carrier signal whereby only the information signal is injected onto the communication medium. Embodiments of the invention will now be described by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a perspective view, not drawn to scale, of line access apparatus for in-band communication according to an embodiment of the present invention, Figure 2 is a block diagram of the line access apparatus for in-band communication shown in Figure 1, and Figure 3 is a circuit diagram of a portion of the apparatus shown in Figure 2. The line access apparatus for in-band communication, generally indicated at 10 in Figure 1 and embodying the principles of the present invention, includes means 12 for accessing a plurality of subscriber lines, means 14 (Figure 2) for intercepting a plurality of subscriber lines via the line access means 12 and means 16 for multiplexing/demultiplexing signals from/to the subscriber lines. In this embodiment, the subscriber line access means 12 includes a plug 18 adapted to be inserted between the main distribution frame, not shown in Figure 1, and a plurality of high voltage protector blocks not shown in the drawings. The plug 18 includes a plurality of pins 20 extending through openings 22 in the plug 18. Each subscriber line pair is bridged by a pair of the pins 20. The pins 20, in addition to being continuous through the plug 18 are also connected to the subscriber line bridging means 14. The bridging connection can be made using known techniques in the connector industry and essentially includes a means 15 (Figure 2) for connecting to a transmit side of the means 16 and a means 17 for connecting to a receive side of the means 16. Preferably, the subscriber line access means 12 is provided in accordance with the main distribution frame access device shown and described in the above referenced U.S. Patent Applications Serial Numbers 595,108 and 595,120. Essentially, the use of the access means 12 allows the bridging of all of the subscribers line pairs that terminate at, for example, a selected telephone central office switch. Each subscriber line pair can then be selectively accessed via the bridging means 14 at the main distribution frame as desired. In a preferred embodiment, the means 14 for bridging to the subscriber lines includes, on the transmit side thereof, a plurality of piezoelectric coupling devices 24. preferably, the devices 24 are stacked and retained by a containment means 26. The means 26 can be formed using known fabrication techniques. Further, the means 26 can be mounted on a base member 28 that can be integrally attached to the plug 18. Preferably, the means 26 is disposed near to the plug 18 and includes a plurality of pairs of wires 30 extending therefrom. The pairs of wires 30 are, as more fully discussed below and as shown in Figures 2 and 3, connected to the means 16 for multiplexing/demultiplexing signals from/to subscriber lines. In the preferred embodiment, the number of devices 24 in each containment means 26 is at least equal to the number of sets of pins 20 in the plug 18 associated therewith. One device 24 particularly adaptable for use in the apparatus 10 is shown and described in U.S. Patent Number 4,529,904. In general, the device described therein for use in the device 10, provides the means 24 for coupling a low voltage electronic circuit to a subscriber line adapted to sustain a relatively high voltage, such as -48 volts DC. Such a piezoelectric device 24 allows signals to be transferred thereacross only in a relatively narrow bandwidth about the resonant frequency of the piezoelectric disk. Further, such a piezoelectric disk is capable of withstanding high voltages, i.e., 2000 to 4000 volts. Hence, the piezoelectric device 24 is both a high voltage isolating device and a signal coupler. Such a coupling device 24 is inexpensive and physically small. Consequently, a plurality of lines can be coupled in an extremely small space such that, for example, the multiplexing/demultiplexing means 16 can be located at the main distribution frame of a central office switch as well as be fabricated using low voltage circuitry elements. A signal demodulator 25 shown in Figures 2 and 3, is electrically connected between the coupling device 25 and the means 15. As more fully discussed below, the signal demodulator 25 removes the high frequency carrier signal from the output of the coupling device 24 thus leaving an in-band signal. The means 16 for multiplexing/demultiplexing signals from/to a subscriber line preferably includes a low voltage large scale integrated circuit having the plurality of pairs of wires 30 from the coupling device 24 at one terminus thereof, the means 17 at a second terminus thereof, and a multiplexed bus 32 at another terminus thereof. In the preferred embodiment, the multiplexed bus 32 is time-division multiplexed (TDM) and includes a transmit bus 34, a receive bus 36 and a control bus 38. As is known, the transmit bus 34 and the receive bus 36 could be combined into a single bus although a more rapid exchange of information is achievable with the separated buses, 34 and 36, if, as more fully discussed below, separate transmit and receive circuitry is provided. The provision of such separate circuitry is within the state of the art of low voltage integrated circuits. Naturally, if physical space becomes a very limiting consideration then a single TDM bus could be used. However, this usually entails a rather more complicated signal control scheme. In fact , the specific implementation of the multiplexed bus 32 is a matter of design choice and other multiplexing/demultiplexing techniques can be used. In the embodiment shown in Figure 2, the plug 18, although depicted as a plurality of separate elements, has a plurality of subscriber pairs 40, commonly referred to as tip (T) and ring (R) in telephony, connecting into the single plug 18. As shown, the plug 18 is, in this embodiment, on the subscriber side of the main distribution frame 42. Each piezoelectric coupling device 24 is connected, via the signal demodulator 25 and the means 15, to a subscriber pair 40 on one side thereof and is connected, on the other side, to the means 16 having, for each piezoelectric device 24, a means 44 for transmitting a signal to the subscriber pair 40. A means 46 for receiving a signal from the subscriber pair 40 is also connected. As more fully discussed below, the transmit and receive means, 44 and 46, respectively, communicate with a communication network 48 and via the transmit and receive busses, 34 and 36, respectively. Preferably, all needed control signals are provided to the means 16 from the network 48 via the control bus 38. In this embodiment, the piezoelectric coupling device 24 is designed to have a resonant frequency of about 144 kilohertz. The resonant frequency of such a piezoelectric disk is determined not only by the material of the disk itself but also by the diameter, thickness and position of the electrodes connected thereto. These particular parameters can be determined and selected by techniques well known in the art. The coupling device 24 thus functions as a band pass filter passing only signals of 144 kilocycles and a surrounding narrow bandwidth. Hence, voice and other signals including D.C. carried on the subscriber pairs 40 do not pass through the device 24 to the means 16. Thus, the device 24 serves not only as a coupling device but also as a high voltage isolating device.Apparatus, also useful for digital communication on existing communication media that utilises a similar coupling device can be designed to inject digital signals at the resonant frequency of the coupling device 24 onto the pairs of wires 40 of the existing communication system. One example of such apparatus is discussed and described in U.S. Patent Application Serial Number 799,519. In the embodiments shown in Figure 3, each of the means 44 for transmitting to the subscriber pairs 40 from, for example, the communications network 48, includes the signal demodulator 25, the coupling means 24 and means 50, for enabling the transmission to the subscriber pairs 40. Preferably, the enablement means 50 includes a pair of switches, 52 and 54, having inputs from a control means 56 that provides, in effect, clock counter switch control functions. The receive means 46 includes a high voltage isolator 58, a differential amplifier 60 for amplifying signals from the isolator 58, and a comparator amplifier 62 that receives the output signals from the differential amplifier 60 and converts them to a digital output against a standardising reference signal. The output from the comparator 62 is directed to the receive bus 36 via a switch 64 enabled by the control means 56. In addition, a modulator 66 is included between the network 48 and the transmit bus 34 via an isolator transformer 68 and a subscriber loop status detector 70 is included between the receive bus 36 and the network 48. The receive bus 36 provides signals to the network 48 via an isolator transformer 72. Operationally, information is directed to the modulator 66 from the network 48 where in the carrier signal, at the resonant frequency of the coupling means 24 is modulated with the signal information. The modulated carrier is impressed, via the isolating transformer 68 onto the transmit bus 34. The switch controller 56 clocks the TDM signal from the bus 34 by enabling the switches, 52 and 54, at the appropriate time slot. The modulated signal is then passed across the coupling device 24 and the carrier signal is removed therefrom by the signal demodulator 25. The in-band signal is then propagated along the subscriber pair of wires 40. When no active transmission is occurring the receive means 46 is enabled and the status of activity on the subscriber pair 40 is monitored by the loop status detector 70. In order to avoid collisions, the network 48 via the detector 70 determines if a signal is present on the subscriber pair 40. A signal detected by the differential amplifier 60 has the output thereof measured against a convenient reference of the comparator amplifier 62. The reference level is set to allow the passage of all signals on the pair 40 thereacross. The switch 64, as stated, is preferably enabled except when digitised information is being transmitted. The digitised information from the comparator 62 is injected into the appropriate time slot on the TDM receive bus 36 via the isolating transformer 72. In one specific discrete element implementation the low voltage switches, 52, 54 and 64, can be CD4097B CMOS devices manufactured and marketed by RCA, Corp. of Somerville, New Jersey. The switch means 56 can be CD4067B or CD4015B manufactured and marketed by RCA, Corp. of Somerville, New Jersey, the modulator 66 can be a CA3054 marketed and manufactured by RCA, Corp. of Somerville, New Jersey. The remainder of the elements not specified immediately above are available as off-the-shelf items. Although the above described discrete device implementation is readily available and functional, it is preferred that the means 16 be implemented via a low voltage CMOS large scale integrated circuit. In one particular arrangement, apparatus 10, in addition to being associated with an MDF access plug 18, is located at each subscriber subset 74 for the extraction of the digitised signals that are then routed to a data device either directly or via a modem. Advantageously, the particular coupling device 24 and the resultant ability to use low voltage large scale integrated circuitry allows the data extraction at the subscriber premises to be located either within the telephone subset 74 or at any other convenient place. Another advantage of the apparatus 10 described herein is that it can be made physically small and thus readily used as a multiplexer/demultiplexer at the main distribution frame of a telephone central office, or PBX, without damage to or extended disruption of existing voice services. In addition, it is inexpensive and readily implemented by, for example, the use of currently available low voltage CMOS large scale integrated circuit technology.

Claims: Communications apparatus comprising a transmission medium, means for modulating a carrier signal with voice frequency signals and piezo-electric coupling means for coupling the modulating means to the transmission medium and providing high voltage isolation therebetween, characterised in that the apparatus comprises line access apparatus (10) for providing in-band communication comprising a voice communication system in which the transmission medium includes a communication medium (14,15,17,40) having a predetermined operating bandwidth, in that the piezo-electric coupling means comprises at least one coupling device (24) having a resonant frequency outside of the predetermined operating bandwidth, in that modulating means (66) is provided for modulating the carrier signal with an information signal having a frequency within the operating bandwidth and is connected to one side of the piezoelectric coupling device (24), the carrier signal having a frequency about equal to the resonant frequency of the or each piezoelectric coupling device (24) whereby the information signal can be conveyed across the or each piezoelectric coupling device with the carrier signal and in that means (25) are arranged between the or each piezoelectric coupling device (24) and the communication medium (14,15,17,40) for connecting the or each piezoelectric coupling device to the communication medium and for removing the carrier signal whereby only the information signal is injected onto the communication medium. Apparatus as claimed in claim 1, characterised in that means (46) are provided for monitoring the activity on the communication medium. Apparatus as claimed in claim 1, or claim 2, characterised in that means (50,56) are provided for enabling the information to be injected onto the communication medium (14,15,17,40) only if the communication medium is inactive. Apparatus as claimed in claim 3, characterised in that means (46) are provided for receiving information from the communication medium and means (56,64) are provided for disabling the receiving means when the information injection means is enabled. Apparatus as claimed in any preceding claim, characterised in that the communication medium includes a plurality of subscribers lines (40) each comprising a pair of wires and in that a plurality of high voltage isolating piezoelectric coupling devices (24) are provided, means (50,56) also being provided for selectively connecting the carrier signal modulating means (66) to one side of the piezoelectric coupling devices (24) which allow the information signal with the carrier signal to be conveyed thereacross and further means (25) being provided for connecting each piezoelectric coupling device to a subscriber line (40) and for removing the carrier signal whereby only the information signal is injected onto the subscriber lines of the communication medium. Apparatus as claimed in claim 5, characterised in that the means for connecting comprises an access plug (18) having a plurality of sets of pins (20) associated therewith connected to the subscriber lines (40). Apparatus as claimed in claim 5, characterised in that the means for connecting and removing comprises a plurality of demodulators (25) each of which is arranged between one of the piezoelectric devices (24) and the subscriber line (40) associated therewith. Apparatus as claimed in claim 7, characterised in that a transmit TDM bus (32,34,36,38) is connected to the carrier modulating means (44), the selectively connecting means (50,56) having an input thereof connected to the transmitted TDM bus (38) and means (46), associated with each subscriber line (40) for receiving information therefrom. Apparatus as claimed in claim 8, characterised in that means (58) is provided for affording high voltage isolation between each receiving means (46) and the subscriber line (40) associated therewith.

Inventors: HARGRAVE, FRANKLIN, MIDDLETON, FRANCISCO ARTURO, ZELLER, DAVID ANDREW, JR.
Applicants: ALCATEL N.V.
Application year: 1986
Main classification: H04M3
IPC codes: H04M3, H04M11, H04Q1, H04Q3