CA2042675C - Enhanced subscriber line interface circuit - Google Patents

Abstract

An enhanced subscriber line interface circuit for a digital switching system over which a connection is established between a digital switching system and a subscriber instrument via a subscriber line. The enhanced subscriber line interface circuit includes a line configuration and protection circuit connected to the subscriber line and to a thick-film hybrid module. The thick-film hybrid module comprises in combination a high voltage interface circuit and a PCM conversion circuit.
The high voltage interface circuit provides power to the subscriber line, converts received analog voice signals transmitted from the subscriber instrument into differential voltage voice signals and detects the subscriber lines status. The PCM conversion circuit is connected to the high voltage interface circuit and is disposed to convert received differential voice signals into PCM
digital signals. A line control interface circuit is also provided located remote from the thick-film module.
The line control interface circuit receives control signals from a peripheral processor of the digital switching system and controls the operation of the circuits of the thick-film hybrid module. Additionally, the line control interface circuit receives subscriber line information from the high voltage interface circuit which it passes on to the peripheral processor. A time slot assignment circuit connected to the peripheral processor is arranged to develop and transmit to the PCM conversion circuit time slot assignment information and to connect the PCM
digital signals from the enhanced subscriber line interface circuit to the digital switching system.

Description

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AN ENHANCED SUBSCRIBER LINE INTERFACE CIRCUIT
CROSS-REFERENCE TO RELATED APPLICATIONS
Cross reference is made to the following related Application entitled: "An Enhanced High Voltage Line Interface Circuit", Canadian Ser. No. 2,042,683-7; "A
Hybrid Balance And Combination Codec Filter Circuit", Canadian Ser. No. 2,042,680-2; "An Over-Current Detector Circuit For An Enhanced Subscriber Line Interface", Canadian Ser. No. 2,042,682-0; and "A Ringing Signal Control Circuit For An Enhanced Subscriber Line Inter-face", Canadian Ser. No. 2,042,676-4, filed on the same date (May 15, 1991) as the instant Application, and having a common assignee.

BACKGROUND OF THE lN V~N~llloN
1. Field of the Invention The present invention relates in general to the field of telecommunications, and more particularly, to an enhanced Subscriber Line Interface Circuit (SLIC) architecture.

2. Description of the Prior Art.
Subscriber line interface circuits are customarily found in the central office exchange of a telecommunica-tions network. The SLIC weds the digital switching net-work of the central office exchange to a plurality of analog subscriber lines. The analog subscriber lines connect to subscriber stations or telephone instruments found at subscriber locations remote from the central office exchange.
The SLIC functions to supply power to a subscriber station and to transmit and receive voice signals between the digital switching network and the subscriber station.
Modern solid state SLICs are constructed using spe-cialized integrated circuits. This construction dis-penses with the need for inductive components in the analog end of the interface. The operating environment of the SLIC includes a necessity to provide high voltages _ and currents, used for analog voice transmission and - . ? ~ -1-,. ~

2~ 42675 ~ substation signalling, as well as, low voltage digital logic signals used for the transmission of digital data between the SLIC and the digital switching network. The high voltage requirements of -48V battery feed is accom-plished using a specialized High Voltage Subscriber Line Interface Circuit (HVSLIC). Such as the HVSLIC circuit described in Canadian patent application Serial No.
2,029,139-7, filed November 1, 1990, entitled "High Voltage Subscriber Line Interface Circuit", having a common assignee with the present invention.
The translation of the analog voice signals to PCM
encoded digital signals and the interface of the PCM
digital signals between the line circuit and the digital switching system is accomplished using a specialized integrated circuit known as a CODEC/FILTER. The CODEC/
FILTER converts analog voice signals received from a subscriber line and the HVSLIC to PCM encoded digital signals. Similarly, PCM encoded digital signals from the digital switching system are converted into analog voice signals for transmission on the subscriber line. One such device is the commercially available as the CODEC/
FILTER COMBO~, TP30XX family of COMBO~ devices manufac-tured by the National Semiconductor Company.
The SLIC must also provide certain signalling and detection functions in order to allow the digital switch-ing system to communicate with a subscriber station.
These signaling and detection functions include ringing signal control, ring-trip and loop sense detection as well as detection of abnormal loop conditions.
These functions are normally provided by a low volt-age companion circuit of the HVSLIC usually referred to as a Low Voltage Subscriber Line Interface Circuit (LVSLIC). One such LVSLIC circuit is described in Cana-dian Patent Application Ser. No. 2,029,140-1, filed No-35 vember 1, 1991, entitled "Control Circuit For A Solid State Telephone Line Circuit", having a common assignee with the present invention. The LVSLIC communicates via a data and address bus with a central controller of the digital switching system. Information pertaining to the ~IA
, status of the SLIC and the subscriber line are transmit-ted from the LVSLIC to the central controller. Operating commands from the central controller are received by the LVSLIC for execution by the SLIC.
Presently available SLICs conform to a circuit architecture that connects in combination an integrated circuit HVSLIC, an LVSLIC, and CODEC/FILTER with discrete components that provide subscriber line configuration and protection. This SLIC combination connects a single sub-scriber line to the central office exchange.
Such a SLIC is described in Canadian Patent Applica-tion Serial No. 2,031,180-1, filed November 29, 1990, en-titled "A Solid State Telephone Line Circuit", having a common assignee with the present invention.
The SLIC just described is normally one circuit of a plurality of SLICs that are assembled on a line card.
The line card connects a plurality of subscriber lines to the digital switching network. Typically, eight or more SLICs can be found on a single line card. However, each SLIC is susceptible to catastrophic damage due to the en-vironment of the associated subscriber line, such as lightning strikes, power surges, etc. A failure of one SLIC circuit necessitates the replacement of the line card. It is advantageous therefore to be able to replace only those circuits of the line card that are damaged by the aforementioned environmental factors and not the entire line card.
Further, since the LVSLIC acts primarily as a local controller and signal detector between the central con-troller of the digital switching system and the HVSLICand COMBO, a certain amount of economy in circuit compo-nents can be realized by removing the LVSLIC from the above mentioned combination. By placing the LVSLIC in a more central location the LVSLIC can provide control and detection functions to two or more HVSLIC and COMBO
circuits.
Accordingly, it is an object of the present inven-tion to provide a new and more effective SLIC architec-ture that can effectively and efficiently interface the '-''''A

~ digital switching network of a central office ~Yc-h~nge to an analog subscriber line.

SUNMARY OF THE lN Vh~. LlON
The above and other objects, advantages, and capa-bilities are realized in an enhanced subscriber line in-terface circuit for a digital switching system over which a ~o~ll.oction is established between the digital switching system and a c~lh~c-riber instrument, via a subscriber line. The enhanced subscriber line interface circuit of the present invention includes a line configuration and protection circuit, connected to the subscriber line, to a ring bus and to a test bus. The line configuration and protection circuit includes a destructive voltage protec-tion circuit, which protects the subscriber line inter-face circuit from destructive voltages that may be in-duced onto the subscriber line. The line configuration and protection circuit further includes a ring relay, which connects ringing current transmitted over the ring bus, from a ringing generator to the subscriber line.
The ringing current is used to signal the telephone in-strument. A test relay applies test signals transmitted over the test bus from a test controller to the sub-scriber line interface circuit, or alternatively, to the subscriber line.
A thick-film hybrid module is connected to the line configuration and protection circuit and comprises in combination of at least a first high voltage interface circuit and a first PCM conversion circuit. The first high voltage interface circuit is further connected to a source of high voltage potential and includes a sub-scriber line power circuit that provides power from the source of high voltage potential to the subscriber line.
The first high voltage interface circuit further includes a differential voltage circuit, which is disposed to con-vert received analog voice signals transmitted from the subscriber instrument to differential voltage voice sig-nals, a detector circuit for monitoring the status of the ~ sllh~criber line and circuits for activating the ring and test relays.
The first PCM conversion circuit is connected to the first high voltage interface circuit and to a source of voltage potential which is substantially less than the source of high voltage potential. The first PCM conver-sion circuit is arranged to receive the differential voice signals from the high voltage interface circuit and to convert the differential voice signals to PCM digital signals.
The thick-film hybrid module is further connected to a line control interface circuit that is located off the thick-film hybrid module. The line control interface circuit is also connected to the digital switching system via a control bus and is arranged to receive control sig-nals from a peripheral processor of the digital switching system. The received control signals are used to operate the first high voltage interface circuit including the operation of the circuits for activating the ring and test relays. The control signals from the peripheral proceCcor also operate the first PCM conversion circuit.
Additionally, the PCM digital signals from the first PCM
conversion circuit are p~c~e~ to the line control inter-face. Finally, the line control interface circuit is ar-ranged to receive status information from the detectorcircuit and to convey the status information to the peripheral processor over the control bus.
The subscriber interface circuit of the present in-vention further includes a time slot assignment circuit connected to the line control interface circuit, the con-trol bus, and to a PCM bus of the digital switching system. The time slot assignment circuit connects the PCM digital signals from the line control interface cir-cuit to the PCM bus for transmission to the digital switching system. Additionally, the time slot assignment circuit receives control information from the peripheral processor that is used by the time slot assignment cir-cuit to generate PCM time slot synchronization signals for operating the first PCM conversion means.

~ In the present arrangement, the thick-film hybrid module contains two high voltage interface circuits and two PCM conversion circuits con~ected together as indi-vidual stand alone pairs. Each high voltage interface and PCM conversion circuit pair connects to a respective line configuration and protection circuit, thereby, per-mitting the connection of two subscriber lines to the thick-film hybrid module. Each module connects to a single line control interface circuit since the control interface circuit is capable of controlling two high voltage interface circuits and two PCM conversion circuits .

BRIEF DESCRIPTION OF THE DRAWINGS
A better understAn~ng of the invention may be had from the consideration of the following detailed descrip-tion taken in conjunction with the accompanying drawings in which:
FIG. 1 is a simplified block diagram of a T-S-T
central office switching system where the ~nhAnce~ SLIC
in accordance with the present invention is used to advantage.
FIG 2. is a block diagram of a line card including the ~nhAnce~ SLIC in accordance with the present invention.

DESCRIPTION OF A PRr;rr;~K~v EMBODINENT
Turning now to FIG. 1 of the included drawings a central office switching system or central office ex-change of the type to which the invention is used to ad-vantage is illustrated. The central office switching system includes a time division multiplexed Time-Space-Time (T-S-T) digital switching network 10 consisting minimally of an originating time switch and control unit, a space switch unit and a terminating time and control unit (not shown). The T-S-T network 10, connects to Analog Facility Interface Units (AFIU) 30 and 40 via a (Pulse Code Modulation) PCM BUS. Each AFIU 30 and 40 allows, in this embodiment, the connection of subscriber ~ lines 3 and 4 respectively to the network 10. Subscriber lines 3 and 4 further connect to subscriber telephone in-struments A and B, respectively. The network 10 and each AFIU 30 and 40 is further connected to a Peripheral Pro-cessor (PP) 20, via a CONTROL BUS. The PP 20 maintains control of its environment by CcAnning the AFIUs 30 and 40 and controlling the associated network time and con-trol units. The total control of the network connections within the network 10 is the responsibility of the Cen-tral Controller (CC) 50. That is, the central controller 50, via the PP 20, informs the time and control units of the time switch connections and also informs the space switch unit of the space switch connections. In addition the CC 50 maintains a data base of the call processing and administrative software for the central office switching system.
Analog voice signals from the transmitter of sub-scriber instrument A are transmitted to AFIU 30 where they are converted to PCM encoded digital signals. The PCM encoded digital signals are then inserted into avail-able çh~nnels on the PCN BUS and transmitted to the digi-tal switching network 10. Under control of the PP 20 the PCM encoded digital signals from subscriber A are switched through the digital switching network 10 and transmitted to AFIU 40 on the PCM BUS. The received PCM
enco~e~ digital signals are converted back to analog voice signals and transmitted via subscriber line 4 to the receiver of subscriber instrument B. The SLIC of the present invention is located within the AFIU.
Turning now to FIG. 2 of the included drawings a block diagram of a line card including the enhanced SLIC
in accordance with the present invention is shown. The line card shown in FIG. 2 includes eight SLICs. Each SLIC is arranged to interface a single subscriber line comprised of a Tip (T) lead and Ring (R) lead to the T-S-T digital switching network 10. The line card in-cludes four thick-film transmission hybrid (XBRID) modules 100. All critical analog components of the SLIC
are contained on the XBRID module 100. Each XBRID module ~042675 ~ 100 further includes two High Voltage Line Interface (HVLI) circuits such as HVLI 101 and two Hybrid Combo (HCOMBO) circuits such as HCOMBO 102 along with matched resistor networks and battery feed transistors (not shown). An HVLI 101 and a HCOMBO 102 form one functional SLIC that in combination provide most of the so-called BORS~nT-functions. In particular, the function of the subscriber power supply (Battery), line status monitoring (Signals, Supervision), analog-digital conversion (Coding) and filter functions and two wire-four wire transition (Hybrid) are provided by the HVLI 101 and HCOMBO 102 of each XBRID module 100.
In addition to powering the subscriber line, the HVLI 101 performs the essential two wire-four wire hybrid function of splitting the balanced signal on the T and R
leads to separate transmit/receive paths. The differen-tial audio signals are transmitted and received to/from the HCOMBO 102. The HVLI 101 further performs loop su-pervision, ring trip detection, ground sensing and over-current detection. A more detailed explanation of theelectrical structure and operation of HVLI 101 may be had by reference to co-pending Canadian Patent Application Serial No. 2,042,683-7.
The HCOMBO 102 synthesizes the input impedance of the SLIC and performs hybrid balance echo cancellation.
Additionally, the HCOMBO 102 converts the differential audio signals from the HVLI 101 to PCM encoded digital signals and from PCM encoded digital signals to differen-tial audio signals. A more detailed explanation of the electrical structure and operation of HCOMBO 102 may be had by reference to co-pending Canadian Patent Applica-tion Serial No. 2,042,680-2.
The remainder of the BORSCHT functions are provided by the Line Configuration and Protection (LCP) circuit 103, located off the XBRID module 100. The LCP 103 ap-pears between the T and R leads of a subscriber line and the HVLI 101. The LCP 103 functions to provide overvolt-age protection (Overvoltage), the application of ringing current to the subscriber line (Ringing) and testing of a -8-A

2 ~ ~ 2 6 7 5 '- the subscriber line as well as the SLIC (Test). The LCP
103 contains ring relays which under software control inject ringing current from a ringing generator (not shown) to the subscriber line. Similarly the subscriber line as well as the SLIC can be isolated and tested via a test relay (not shown) that provides facility test (Out Test) and circuit test (In Test). The test relay allows the application of test signals from a test program which resides in the CC 50.
10In a digital common logic area of the line card re-sides a plurality of Line Control Interface (LCI) cir-cuits 104. Each LCI 104 performs all of the control and sense logic functions for the two SLICs of an associated XBRID module 100. In particular, each LCI 104 functions to administer and control the ringing and test relays, hook status reporting, dial pulse detection, ring trip, ground sensing, and over-current detection. Each LCI 104 connects its associated XBRID module 100 to PP 20 via a control and sense bus. A more detailed explanation of the electrical structure and operation of LCI 104 may be had by reference to co-pending Canadian Patent Applica-tions Serial No. 2,042,682-0 and 2,042,676-4.
Digital logic buffers 105 transmit data from the HCOMBO 101 of XBRID module 100, to the PCM bus of the network, and conversely, receive PCM data from the net-work PCM bus to a respective HCOMBO 102 via CODEC Control circuit 106. CODEC Control circuit 106 is an octal Time Slot Assigner Circuit (TSAC) device that functions to generate transmit and receive frame sync pulses to its connected HCOMBO devices 102. One TSAC 106 connects to all eight HCOMBOs of the line card. The network provides a required 1.544 MHz clock with a nominal 50% duty cycle for the purpose of shifting PCM data in and out of the HCOMBO 102 data registers. A frame (125~sec) provides for 24 eight bit PCM channels with one framing bit for synchronization. One such TSAC device is the TP3155 Time Slot Assigner Circuit manufactured by the National Semiconductor Corporation.

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As can be seen in FIG. 2, the four XBRID modules 100 on the line card, include eight SLIC circuits that via each SLICs associated LCP 103 connect to eight subscriber lines. Further, each of the four LCI 104 circuits is connected via a control and sense bus to an associated XBRID module 100 and the HVLI 101 and HCOMBO 102 pair.
Still further, a single CODEC controller 106 connects to all eight HCOMBO devices 102 via a PCM and CODEC data bus, allowing the CODEC controller 106 to control the PCM
time slot allocation for each of the eight SLICs.
Finally, the PCM data and control and sense data is transmitted to the digital switching network 10 via the TTL buffers 105 and the network bus.
As can be appreciated from the above organizational description of the line card architecture, any destruc-tive voltages or currents induced into a subscribers line and which are not blocked by the protection circuitry of the LCP 103 would damage only the effected XBRID 100.
RepA i r of the damaged SLIC would entail the replacement of the damaged XBRID module 100 only. Further, it can be appreciated by those skilled in the art that the XBRID
modules 100 can be manufactured as plug-in devices easily allowing the removal and replacement of the XBRID modules 100 on the line card.
Furthermore, it will be obvious to those skilled in the art that numerous modifications to the present inven-tion can be made without departing from the scope of the invention as defined by the appen~e~ claims. In this context, it should be recognized that the essence of the invention resides in a new and more effective SLIC archi-tecture that can effectively and efficiently interface the digital switching network of a central office ex-change to analog subscribers lines.

Claims (9)

1. A subscriber line interface circuit for a digital switching system over which a connection is established between said digital switching system and a subscriber instrument via a subscriber line, comprising:
a line configuration and protection circuit connected to said subscriber line, to a ring bus and to a test bus, said line configuration and protection circuit including destructive voltage protection means for protecting said subscriber line interface circuit from destructive voltages induced onto said subscriber line, ring mean for connecting ringing current transmitted over said ring bus from a ringing generator to said subscriber line, and test means for applying test signals transmitted over said test bus from a test controller to said subscriber line interface circuit, or alternatively to said subscriber line:
a thick-film hybrid module comprising in combination:
at least a first high voltage interface circuit connected to said line configuration and protection circuit and to a source of high voltage potential, said first high voltage interface circuit including means for providing power from said source of high voltage potential to said subscriber line, differential voltage means arranged to convert received analog voice signals transmitted from said subscriber instrument to differential voltage voice signals, detector means for monitoring the status of said subscriber line, and means for activating said ring means and said test means; and at least a first PCM conversion circuit connected to said first high voltage interface circuit and to a source of voltage potential substantially less than said source of high voltage potential, said first PCM
conversion circuit arranged to receive said differential voice signals and to convert said differential voice signals to PCM digital signals;

a line control interface circuit connected to said thick-film hybrid module and to said digital switching system via a control bus, said line control interface circuit arranged to receive control signals from a peripheral processor of said digital switching system and to control the operation of said first high voltage interface circuit including operation of said means for activating said ring means and said test means, and said first PCM conversion means, and alternatively said line control interface circuit arranged to receive status information from said detector means and to convey said status information to said peripheral processor over said control bus, said line control interface circuit further receiving said PCM digital signals from said first PCM
conversion circuit; and a time slot assignment circuit connected to said line control interface circuit, said control bus, and to a PCM bus of said digital switching system, said time slot assignment circuit connecting said PCM digital signals from said line control interface circuit to said PCM
bus for transmission to said digital switching system, and said time slot assignment circuit further arranged to receive control information from said peripheral processor and to generate PCM time slot synchronization signals for operating said first PCM conversion circuit.

2. A subscriber line interface circuit as claimed in claim 1, wherein said time slot assignment circuit connects PCM digital signals from said digital switching system PCM bus to said line control interface circuit, and said PCM digital signals are transferred to said first PCM conversion circuit, whereby, said first PCM
conversion circuit converts said PCM digital signals into differential voice signals which are input to said differential voltage means and converted to analog voice signals for transmission along said subscriber line to said subscriber instrument.

3. A subscriber line interface circuit as claimed in claim 2, said thick-film hybrid module further includes in combination:
at least a second high voltage interface circuit connected to a respective additional line configuration and protection circuit and to a source of high voltage potential, said second high voltage interface circuit including means for providing power from said source of high voltage potential to an additional subscriber line, differential voltage means arranged to convert received analog voice signals transmitted from a subscriber instrument connected to said additional subscriber line to differential voltage voice signals, detector means for monitoring the status of said additional subscriber line, and means for activating said ring means and said test means of said additional line configuration and protection circuit; and at least a second PCM conversion circuit connected to said second high voltage interface circuit and to a source of voltage potential substantially less than said source of high voltage potential, said second PCM conversion circuit arranged to receive said differential voice signals from said second high voltage interface circuit, and to convert said differential voice signals to PCM
digital signals.

4. A subscriber line interface circuit as claimed in claim 3, wherein said line control interface circuit controls the operation of said first and said second high voltage interface circuits and said first and said second PCM conversion means, and alternatively, said line control interface circuit is arranged to receive status information from said detector means of said first and said second high voltage interface circuits and to convey said status information to said peripheral processor over said control bus, said line control interface circuit further receiving said PCM digital signals from said first and said second PCM conversion means.

5. A subscriber line interface circuit as claimed in claim 4, wherein said time slot assignment circuit connects PCM digital signals from said digital switching system PCM bus to said line control interface circuit, and said PCM digital signals are transferred to said second PCM conversion circuit whereby said second PCM
conversion circuit converts said PCM digital signals into differential voice signals which are input to said second high voltage interface circuit differential voltage means and converted to analog voice signals for transmission along said subscriber line to said subscriber instrument.

6. A subscriber line interface circuit for a digital switching system over which a connection is established between said digital switching system and a subscriber instrument via a subscriber line, comprising:
line means connected to said subscriber line, said line means including destructive voltage protection means for protecting said subscriber line interface circuit from destructive voltages induced onto said subscriber line;
a thick-film hybrid module comprising in combination:
interface circuit means connected to said line means, said interface circuit means arranged to power said subscriber line, and to convert received analog voice signals transmitted from said subscriber instrument into differential voltage voice signals, said interface circuit means further including detector means for monitoring the status of said subscriber line; and PCM conversion means connected to said interface circuit means, said PCM conversion means arranged to receive said differential voice signals and to convert said differential voice signals to PCM digital signals;

controller means connected to said thick-film hybrid module and to said digital switching system via a control bus, said controller means arranged to receive control signals from a peripheral processor of said digital switching system and to control the operation of said interface circuit means and said PCM conversion means, and alternatively said controller means arranged to receive status information from said detector means and to convey said status information to said peripheral processor over said control bus, said controller means further receiving said PCM digital signals from said PCM conversion means; and time slot assignment means connected to said peripheral processor and a PCM bus of said digital switching system, said time slot assignment means arranged to develop and transmit to said PCM conversion means time slot assignment information, transmitting said PCM digital signals from said controller means to said digital switching system.

7. A subscriber line interface circuit for a digital switching system over which a connection is established between said digital switching system and a subscriber instrument via a subscriber line, comprising:
a thick-film hybrid module connected to said subscriber line, said thick-film hybrid module comprising in combination:
means for powering said subscriber line; and means for converting received analog voice signals transmitted from said subscriber instrument into differential voltage voice signals; and means for detecting the status of said subscriber line; and means for converting said differential voice signals to PCM digital signals;

controller means connected to said thick-film hybrid module and to said digital switching system via a control bus, said controller means arranged to receive control signals from a peripheral processor for controlling the operation of said thick-film hybrid module, and alternatively said controller means arranged to receive status information from said means for detecting and to convey said status information to said peripheral processor over said control bus, said controller means further arranged to receive said PCM digital signals from said means for converting; and time slot assignment means connected to said peripheral processor and a PCM bus of said digital switching system, said time slot assignment means arranged to develop and transmit to said means for converting time slot assignment information, and to connect said PCM digital signals from said controller means to said digital switching system.

8. A subscriber line interface circuit as claimed in claim 7, wherein said subscriber interface circuit further includes line means connected between said subscriber line and said thick-film hybrid module arranged to connect said subscriber line to said thick-film hybrid module, said line means including destructive voltage protection means for protecting said subscriber line interface circuit from destructive voltages induced into said subscriber line from external sources.

9. A subscriber line interface circuit as claimed in claim 8, wherein said line means is further connected to a ring bus and a test bus and said line means further includes ring means for connecting ringing current transmitted over said ring bus from a ringing generator to said subscriber line, and test means for applying test signals transmitted over said test bus from a test controller to said subscriber line interface circuit, or alternatively, to said subscriber line.