WITH SMART TELEPHONES

FIELD OF THE INVENTION

The present invention relates generally to the field of telecommunications and relates more particularly to apparatus and methods for interfacing digital telephones (e.g. , paystations) to a central office (CO) . Still more particularly, the present invention relates to a front end processor (FEP) typically collocated with the central office and employing data over voice (DOV) communications with the telephones and analog or digital communications with the central office.

BACKGROUND OF THE INVENTION

Deregulation of the telecommunications industry has led to private ownership of telephone paystations and switching systems. This has resulted in competition for revenue among providers of both local and long distance call services. Both coin collection and credit billing from users of paystations are important sources of such revenue. The ability of a paystation owner to capture this revenue has been enhanced by the development of "smart" telephones capable of operating on a conventional two-wire telephone line. Such telephones typically include a microprocessor and memory for performing functions which otherwise would be performed in a central office. The smart paystations are able to provide enhanced services (for example, automated operator assistance and message delivery) to both the caller and call service provider.

A telephone paystation typically is connected via class-marked trunks, leased lines, or standard business lines to a central office of a local telephone operating company. Local calls are directed by the central office to a destination within a local access and transport area. Long distance calls are carried between distant local telephone operating companies through the AT&T long distance network or through one or more independent interexchange carriers. Local telephone operating companies and other service providers participate in revenue generated from private paystations for their services and equipment utilized to complete a call. This participation is significant for those services the paystation does not offer. For example, calls paid for by coin in advance of transmission from a paystation, referred to as "coin calls" or "sent paid calls," require intelligent coin signaling and accounting which can be handled either at an intelligent paystation or at a central office for a fee.

U.S. Patent No. 5,153,907, October 6, 1992, titled "Telephone System Gateway Interface," discloses an interface between a paystation and an originating central office. The disclosed interface includes a monitoring circuit for monitoring call signals from the paystation, circuits for obtaining coin or card billing information from the paystation, and a control circuit for verifying the payment information and directing the call to a service provider so as to bypass the originating central office. The disclosed interface purportedly enables enhanced call services and call routing functions to be performed from a centralized location on a telephone network line through gateway connections to service providers. Call payment information is validated and the call is delivered to either the originating central office or alternate service providers.

As exemplified by the above-cited U.S. Patent No. 5,153,907, prior art telecommunications systems employing smart telephones require a high degree of sophistication in the individual telephones and thus relatively expensive smart

telephones. In addition, such prior art systems are inefficient in terms of accessing and managing the data stored in the telephones.

SUMMARY OF THE INVENTION Accordingly, a primary goal of the present invention is to reduce cost. This is accomplished by removing processors and database memories from the individual paystations and concentrating these features in a front end processor accessible to multiple paystations. Another goal of the present invention is to improve the functionality of the telecommunications system by providing the FEP direct access to more databases, thereby allowing more sophisticated programming. This also allows programs and other features of the telephones to be changed without requiring technical personnel to visit the individual paystations.

A front end processor (FEP) in accordance with the present invention interfaces a plurality of paystations via corresponding two-wire tip/ring lines to a central office in a telecommunications system. The inventive FEP comprises: (a) a digital line interface module effecting data over voice (DOV) communication with the paystations, the DOV communication comprising the transmission and reception of data and voice signals to and from the paystations and the reception of tone signals from the paystations, the DOV communication employing the tip/ring lines; (b) a database, and (c) a control processor accessing the database to obtain call rating data and collecting per line and per call data for storage in the database.

One presently preferred embodiment of the FEP further comprises an announcement subsystem storing voice messages for transmission to the paystations under the control of the control processor. In addition, this embodiment comprises means for accessing the database from a remote location. The digital line interface module, in the preferred embodiment disclosed herein, comprises a line interface

comprising: a central office interface coupled to a two-wire tip/ring line of the central office; a line interface coupled to the central office interface and to a two-wire tip/ring line of a particular one of the paystations; and a digital signal processor operatively coupled to the central office interface and the line interface. The digital signal processor comprises means for effecting tone decoding and tone generation and analog central office signalling via the interfaces. The digital line interface module of the preferred embodiment also comprises a module control/Tl interface coupled to the line interface and to the control processor. The module control/Tl interface effects TI digital communications with the central office.

The central office interface in the preferred embodiment comprises a codec, a hybrid circuit, a DC signal detection circuit, a surge and power cross protect circuit, and a hook switch relay circuit coupled to the surge and power cross protect circuit and to a two-wire tip/ring line from the central office. The line interface in the preferred embodiment comprises a data/voice interface circuit, a hybrid circuit, a battery feed circuit, a surge and power cross protect circuit, and a reversal/test relay coupled to the surge and power cross protect circuit and to a two-wire tip/ring line of a paystation.

The module control/Tl interface in the preferred embodiment comprises a microprocessor, a digital switching matrix, a set of data channel controllers controlling data flow to the line interface, a TI span interface, tone decoders, and a conference bridge.

The control processor in the preferred embodiment comprises a central processing unit; a clock; program, data, and database memory; and serial input/output circuits providing access ports to the database memory. The FEP may also advantageously comprise means for providing power to the paystations while a call is in progress and while no call is in progress. In addition, the

FEP may comprise means for loading software in the paystations over the two-wire tip/ring lines. This allows features and parameters of the paystations to be modified from the FEP. The FEP may also advantageously include means for storing a call restriction table and means for blocking a call in accordance with the call restriction table. The call restriction table may contain data specifying certain dialed numbers, wherein calls to these numbers are to be blocked by the FEP. Further, the call restriction table may contain data specifying certain debit or cash card numbers, wherein calls associated with (initiated with) these numbers are to be blocked.

The FEP may also comprise means for storing a prescribed card number and means for automatically dialing a prescribed telephone number in response to receiving data from a paystation indicating that a charge or debit or cash card with the prescribed card number has been inserted to initiate the call. This feature allows the FEP to dial a number without the caller actually dialing the number. The number may be stored in the form, NPA-NXX-XXXX and may have a preferred interexchange carrier listed as well. When the card is inserted into the telephone, the call is placed to the indicated number without further action by the caller. The telephone sends the number information to the FEP and the FEP takes appropriate action to complete the call. This may involve deleting the NPA, which would be required for a local call, or it may involve prepending a 1+ or 10XXX+1+ as required. The FEP would perform the proper rating functions and require the appropriate funds on the card to complete the call. In addition, the FEP may comprise means for automatically dialing a prescribed telephone number in response to receiving data from a paystation indicating that a predetermined button on the paystation has been pushed or actuated by a caller. By employing an FEP in accordance with the present invention, many of the elements of a smart telephone are removed from the telephone and concentrated in the FEP. For

example, the announcement subsystem, database memory, and processing circuits are located in the FEP and may be shared by many (e.g., several hundred) telephones, instead of one of each being required for each telephone. In addition, since the database circuits are located in the FEP and are directly accessible to the maintenance system, the database may be updated efficiently in a timely fashion. Access to a remote database during the progress of a call is also possible because of separate data links between the telephone and the FEP and the FEP's data port. Other features and advantages of the present invention are described below.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a block diagram of one embodiment of a telecommunications system comprising a front-end processor 12 in accordance with the present invention.

Figure 2 is a block diagram of one embodiment of a shelf subsystem 12-1 of the front end processor.

Figure 3 is a block diagram of one embodiment of a digital line interface module 20, 22, 24 of the shelf subsystem.

Figure 4 is a block diagram of one embodiment of a line interface card 30, 32, 34, 36 employed by the digital line interface module.

Figure 5 is a block diagram depicting one embodiment of a central office interface 42 and a line interface 44 employed in the line interface card.

Figure 6 is a block diagram of one embodiment of the module control/Tl interface 38 employed in the digital line interface module. Figure 7 is a block diagram of one embodiment of a control processor 26 employed in the shelf subsystem.

Figures 8A and 8B, collectively, are a flowchart of a process for conducting a sent-paid call in accordance with the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A front end processor (FEP) in accordance with the present invention is an in-line call processing device that provides enhanced paystation services on ordinary paystation lines. As shown in Figure 1, the FEP is typically part of an overall system composed of paystations 10, an FEP (or multiple FEPs) 12, a central office (CO) 14, and a maintenance system 16. One presently preferred embodiment of the FEP can interface a number of different types of paystations, including coin, coinless, collect call only, cash card, and debit card. Depending on the particular application, the FEP may screen and restrict calls, offer enhanced services, or perform call rating functions in cash/debit card systems. Referring to Figure 1, one presently preferred embodiment of an FEP 12 in accordance with the present invention comprises a number of shelf subsystems 12-1. Multiple shelf subsystems may be linked or networked (daisy- chained) to present a single port to external local or remote users. The multiple shelf subsystems 12-1 have access to a common database/mux 12-2. Each shelf subsystem operates independently in respect to all other functions. Each set of paystations 10 is coupled to the FEP by a bundle of two-wire tip/ring lines 11. In presently preferred embodiments, each shelf subsystem 12-1 is capable of receiving up to seventy- two two-wire lines. The shelf subsystems 12-1 are coupled to the central office by two-wire lines 13, which may carry digital or analog signals. The shelf subsystems are coupled to the database/mux 12-2 by a 19.2 kilobit per second (kbps) serial link 12-3. The FEP is coupled to the maintenance system 16 by one or more serial links 15, which may comprise ordinary telephone company lines or fixed (leased) lines. The FEP 12 has a flexible, modular architecture that accommodates a variable number of line interface modules 20, 22, 24 (Fig. 2) to match the requirements of a given system. Users may communicate with the FEP through a serial port (e.g., an RS/232 port) coupled to the serial link(s) 15

to the maintenance system 16, the latter typically comprising a terminal or workstation (not shown) for system administration and data collection.

Typical applications of the front end processor 12 include: voice message delivery of non-completed calls;

- inmate collect call restriction in association with a collect call timing device (CCTD) installed in a prison; - cash/debit card phone operation;

- coin telephone management. For example, the FEP could track the coinage in a coin box so that timely scheduling of collection could be arranged. In addition, the FEP could track telephone usage by time of day, providing an indication of a non-working telephone. The FEP could also be employed to provide call rating for local calls, which is not currently available in some central offices, and charging for time for local calls. revenue allocation with the use of alternate carrier funds available to each of the carriers that are used. This may be employed in connection with credit card, debit card, and sent paid (coin) calls. monitoring of telephone usage and calling patterns. For example, the following is a simplified description of the procedures for carrying out a charge card call with a system employing a front end processor in accordance with the present invention. A more detailed description is provided below with reference to Figures 8A and 8B.

1) The telephone goes off-hook. This is signaled ' to the FEP through a data over voice channel.

The FEP connects a dial tone generator to the line and transmits a dial tone to the caller.

2) The caller dials the telephone number of a called party. The telephone sends the dialed number and a credit or debit card number to the FEP. 3) The FEP looks for the dialed number and card number in its database. If the card is valid, the FEP determines the charges for the call. The charges for the call are returned to the telephone through the data over voice channel. 4) A card reader in the telephone examines the card to determine whether sufficient funds remain for the call. a) If the funds are sufficient, the telephone signals the FEP over the data over voice channel. The FEP then outdials the original number and starts monitoring the channel for answer supervision. When answer supervision is detected, the FEP signals the telephone to start charging for the call. b) If the funds are insufficient, the telephone so informs the FEP and the FEP sends a voice announcement to the user indicating that the funds on the card are not sufficient for the call and that the user should either use another card or hang up.

5) The telephone and the FEP then continue to monitor the call. Based on the charging information it originally received, the telephone reduces the funds remaining on the card as the call continues. When disconnect is detected from either end, the call is terminated and statistics are collected and stored by the FEP for later transmission to the maintenance system. The FEP 12 communicates with smart telephones by employing a data over voice (DOV) technique. The voice channel in preferred embodiments is PCM encoded. The FEP

communicates with the central office by either an analog or digital technique, e.g., by employing PCM transmission lines (TI spans). Each digital line interface module 20, 22, 24 (Fig. 2) supplies one or more two-wire lines that carry full duplex voice and data as well as power to charge the battery and operate the circuits within the phones. Preferred embodiments of the invention employ the DOV standard specified in ANSI: T-l-601-1992.

When a call is originated at a paystation phone, a line interface inside the FEP detects an off-hook signal and collects dialing and supervisory information (e.g., dialed digits, coin tones, charge or debit card parameters) . The FEP then processes the call in accordance with a pre- specified procedure. For example, the call may be permitted to pass through a central office interface inside the FEP to the central office and monitored for further information; the call may be blocked and an announcement may be sent back to the caller; the call may be outdialed to a new terminating number; or, as mentioned, call rating information may be calculated and returned to the calling paystation. The FEP will continue to monitor the call and may periodically intervene (e.g., to request coin deposits) or return updated information (e.g., for smart paystation operation). At the conclusion of a call, the FEP will perform any terminating operations (e.g., coin collection, CO disconnect) and generate a call summary report.

A control processor 26 (Fig. 2) in each shelf subsystem 12-1 provides database look-ups for large restriction translation tables or for call rating. The control processor also collects per line and per call data and forwards it to the system database/mux 12-2.

The system database/mux 12-2 provides a common system interface to and from external devices, such as may be associated with the maintenance system 16, by multiplexing and demultiplexing the data streams of the shelf subsystems 12-1. The system database/mux also contains a large storage

capacity for system databases and backup copies of system software.

Figure 2 is a block diagram of one presently preferred embodiment of a shelf subsystem 12-1 of the FEP 12. The shelf subsystem comprises three 24-line digital line interface modules 20, 22, 24. The digital line interface modules are supported by a common control processor 26; an optional database processor 27; an announcement subsystem comprising two quad (four message) announcers 28; and a power converter subsystem 29 for supplying loop power to the paystation telephones. Backplane buses distribute the power, data, and voice signals among the interface modules 20, 22, 24 and supporting subsystems 26-29. In the embodiment depicted in Figure 2, each digital line interface module 20, 22, 24 interfaces up to twenty-four two-wire tip/ring lines from paystations 10 (see lines 11-1, 11-2, 11-3) and up to twenty-four two-wire tip/ring lines to the CO (see lines 13- 1, 13-2, 13-3). The backplane buses include a 2 megabit per second (Mbps) synchronous multi-drop bus and a 19.2 kbps asynchronous multi-drop bus. The 2 Mbps and 19.2 kbps buses provide basically the same capabilities. The 19.2 kbps bus provides backward compatibility with other hardware. It can also function as a backup to the 2 Mbps bus. Both buses are full-duplex asynchronous buses providing a communications medium between the control elements of the system.

Figure 3 is a block diagram of one preferred embodiment of a digital line interface module 20, 22, 24. Each digital line interface module comprises up to four six- line interface cards 30, 32, 34, 36 and one module control card 38. The line interface cards are coupled to a paystation 10 (Fig. 1) by sets of two-wire lines 11-la, 11- lb, 11-lc, 11-ld and communicate with the paystation by employing data over voice. In presently preferred embodiments, each card is capable of receiving six two-wire lines and communicating with six paystation telephones. Each card is coupled to the central office by a set (i.e., one to six) of two-wire lines 13-la. Each digital line interface

module employs a standard ST-BUS 39 for internal digital voice and data information transfer. The ST-BUS is a 32- channel time division multiplexed bus operating at 2.048 Mhz. Clock and framing signals provide synchronization. Two such buses are employed for full duplex operation. The modules 20, 22, 24 are coupled to each other and to the control processor 26 and announcement subsystem 28 (Fig. 2) by the multidrop serial links discussed above.

In one presently preferred embodiment, the digital line interface modules 20, 22, 24 comprise circuitry to interface digital phone lines and analog or digital central office lines. This embodiment of the digital line interface module is characterized by the following features:

1. The line side to phone component of the interface provides a two-wire T/R loop; full duplex digitized voice (64 kbps) ; full duplex 16/64 kbaud data using poled HDLC; surge and power cross protection; battery reversal or tone signalling for phone "wake up"; battery feed with 40 milliamps current limit; dial tone generation; DTMF detection; and announcements.

2. The central office side component of the interface module provides a two-wire T/R loop; analog voice; DC supervision; hookswitch simulation; surge and power cross protection; ringing detection; answer supervision reversal detection; call progress tone decoding; derived answer supervision detection; DTMF generation; OSI detection; and announcements.

3. An ST-BUS to backplane interface is provided for voice and data transfer.

4. Clocks and signals received from the backplane are synchronized. Figure 4 is a block diagram of one presently preferred embodiment of a line interface card 30, 32, 34, 36. As shown, each line interface card contains a digital signal

processor (DSP) module 40 for analog central office signalling detection and tone and voice processing; six analog CO interface circuits 42 (comprising individual interface circuits 42-1, 42-2, etc.); six digital line interface circuits 44 (comprising individual interface circuits 44-1, 44-2, etc.); a power converter 46 for supplying power to the interface circuits; and a timeslot assignment module 48. The analog central office component may be omitted when the card is used with a TI central office interface. In preferred embodiments, the PCM highway includes thirty-two consecutive samples. Each sample is composed of eight bits and is referred to as a "time slot." The time slot assignment module is connected to all elements on the PCM highway and assigns the time slot to be used by each element.

Figure 5 is a block diagram depicting one embodiment of a central office interface circuit 42 and a line interface circuit 44. The central office interface circuit 42 comprises a codec 55, a 2:4 wire hybrid circuit 56, a DC signal detection circuit 57 (having an output to the DSP 40), a surge and power cross protect circuit 58, and a hook switch relay circuit 59. The output of the hook switch relay circuit 59 is adapted to be coupled to a two-wire tip/ring line from the central office. The line interface 44 is coupled as shown to the central office interface 42 via a bus referred to as a "PCM highway." This circuit comprises a data/voice interface circuit 50, a 2:4 wire hybrid circuit 51, a battery feed circuit 52, a surge and power cross protect circuit 53, and a reversal/test relay 54. The reversal/test relay 54 is adapted to be coupled to the two-wire tip/ring line of a smart telephone and is employed to reverse the polarity of the tip/ring line. The line interface 44 provides voice, data, and power to the telephone to which it is connected. The data/voice interface circuit 50 in preferred embodiments is a DOV chip. It receives voice and data from the control processor and converts the voice and data to the

signal sent over the tip/ring line to the telephone. It also receives the signal from the telephone and converts it to a PCM signal and data to be sent to the other elements of the FEP. The 2:4 wire hybrid circuits 51, 56 are hybrid circuits that convert full duplex bidirectional signals to unidirectional signals. Such circuits are well known in the art.

The battery feed circuit 52 connects the battery and ground to the tip/ring pair. This circuit limits the current in the loop and protects the loop from short circuits.

The surge and power cross protect circuits 53, 58 protect the tip/ring line from the effects of lightning strikes and shorts to power lines. Such circuits are well known in the art.

During periods of inactivity, a CCT telephone operates in a low power mode. One function of the reversal relay/test 54 is to "wake up" the telephone and cause it to go into an active mode such that the telephone and FEP can exchange data. This function may also be accomplished by tones that are a provided by an ISDN chip set now available. The reversal/test relay also functions to connect the tip/ring line to the telephone to a tip/ring test line, providing a means for testing the tip/ring line for failures (such as shorts and opens) or noise. Accordingly, in presently preferred embodiments of the invention, the function of the reversal/test relay 54 is to connect the tip/ring pair to test equipment. The codec 55 is a known circuit that converts the analog signal on the CO loop to a PCM signal.

The DC signal detection circuit 57 monitors the DC conditions of the CO line after the hook switch relay goes off-hook to detect such conditions as reversal (answer) by the CO.

The hook switch relay 59 provides a means to take the line "off-hook" toward the CO. This function is equivalent to picking up a telephone handset.

Figure 6 is a block diagram of one preferred embodiment of the module control/Tl interface 38 (Fig. 3) . The module control/Tl interface contains a microprocessor 66 that controls the entire operation of the twenty-four lines associated with that interface module. The module control/Tl interface 38 monitors and controls the line status in both directions, allocates resources, collects dialing information, and accesses the system database 12-2 (Fig. 1) when necessary. The module control/Tl interface also includes a digital switching matrix 60 for making voice and data channel assignments and announcer connections. The switching matrix 60 provides access to the microprocessor 66 for controlling the line interface cards 30, 32, 34, 36 (Fig. 3) . A set of data channel controllers 61 control the data flow to the line interface cards 30, 32, 34, 36. A TI span interface 62, announcer access circuitry comprising codecs, DTMF receiver pool (i.e., tone decoders 63 (note that DTMF tones may also be decoded by the DSP on each line card) ) , conference bridge (DSP) 67, on-board power supply 69, and a timing and sync generator 69a are also included on the module control/Tl interface 38. The data channel controllers 61 in preferred embodiments are special purpose chips that receive data from the DSP and place the data in the PCM data stream. The module control/Tl interface module 38 performs timing and control functions and serves as a digital TI central office interface. One presently preferred embodiment of this interface is characterized as follows:

1. The module control/Tl interface includes a microprocessor for controlling an associated digital line interface module 20, 22, 24 and handling all functions for twenty-four lines. Operating software may be downloaded from the database and stored in memory. A dedicated

serial port to the database may be provided as needed.

2. The module control/Tl interface includes a TI interface to the central office having a standard T-l capability, including SF (super frame) and ESF (extended super frame) . It provides software controlled pulse shaping for line length selection; comprehensive span alarm and status reporting; clock extraction from incoming data streams; and incoming signal jitter attenuation.

3. The module control/Tl interface provides clock and timing signal generation and distribution to the backplane, and includes oscillators and PLLs providing clocks (2, 4, and 15 MHz), frame sync pulses, and time slot sync pulses.

4. The module control/Tl interface includes an announcer interface that interfaces the ST-BUS to twenty-four digitized announcer channels and provides access and PCM conversion onto the ST-BUS for eight analog announcer channels from the backplane.

5. The module control/Tl interface includes a digital switching module that provides an ST- BUS time slot interchange for voice path set¬ up and announcer connection. It also provides access to time slots for data read and write operations and includes the capability of serving as a direct processor interface. 6. The module control/Tl interface includes a pool of digital communication controllers (e.g., three or four HDLC protocol controllers) . 7. The module control/Tl interface provides bi- directional conference bridging (e.g., the playing of announcements over voice) .

Figure 7 is a block diagram of the shelf control processor 26, which is substantially similar to the database/MUX circuit 12-2. This processor comprises a 16 MHz 16-bit central processing unit (CPU) 70; a clock 71; an interface for memory expansion 72; program memory 73; data memory 74; database memory 75; serial input/output circuits 76, 77, 78 providing twelve access ports to the database; and a power supply 79. As shown, the CPU 70 is coupled to both the 19.2 kbps asynchronous multi-drop data bus and to the 2 Mbps synchronous multi-drop data bus.

Voice signals are handled in both analog and digital fashion in the FEP. A smart paystation telephone translates the analog handset signals to/from standard μ-law 64 kbps digital PCM. (μ-law is a North American standard for converting voice to PCM; α-law is used in most other places.) The encoded voice is combined with a 16 kbps data and control channel (8 kbps each) to provide full duplex data over voice transmission between the phone and the FEP. The FEP multiplexes the recovered voice and data into a selected time slot of the internal voice and data buses. In embodiments providing an analog central office interface, a codec (coder/decoder) 55 (Fig. 5) in the central office interface circuits 42 converts the ST-BUS time slot digital signals to/from analog signals. In embodiments providing a digital central office interface, the ST-BUS voice buses connect to a TI interface 62 (Fig. 6) that encodes and decodes the standard TI format. In both embodiments (i.e., those providing analog and digital central office interfaces) , the digital voice buses are routed through the digital switching matrix 60 for any required time slot interchange or assignment. This switching matrix also provides the voice buses access to a conference call module (e.g., a DSP 67 (Fig. 6) ) that places announcements over voice in one or both directions of a call. Voice announcements are generated by the quad announcer cards 28 (Fig. 2) and distributed on the backplane via eight analog buses. Each digital line interface module

20, 22, 24 converts the analog voice buses to PCM channels of the ST-BUS 39 (Fig. 3) . Each of these channels is then available to all line interface cards 30, 32, 34, 36 within the module. A line interface circuit requiring an announcement connects to the assigned ST-BUS slot corresponding to the analog bus on which the proper announcement is being played. In presently preferred embodiments, the analog announcement configuration provides eight announcer channels to service up to seventy-two lines. However, this ratio may cause delays or prove inadequate for announcement-intensive applications and may be modified as needed. Digital announcers may also be employed.

Timing synchronization must be maintained between digital circuits for proper, slip free operation. The phone; digital line interface circuits 30, 32, 34, 36; switching matrix 60; and TI interface 62 compose the primary signal path that must be phase and/or frequency locked. The three digital line interface modules 20, 22, 24 operate independently and need not be synchronized to each other. They are connected only by the synchronous and asynchronous serial communications channels.

In embodiments providing a digital central office interface, a 1.544 MHz clock extracted from the incoming T- span signal provides the primary timing reference for the respective line interface modules 30, 32, 34, 36. A 2.048 MHz clock and frame sync signals required for the ST-BUS operation and 4.096 MHz and 15.36 MHz clocks required for the digital link to a particular phone are all phase locked to the primary 1.544 Mhz signal. The phones operate in slave mode, transmitting data at rates locked to the received rate. In embodiments providing an analog central office interface, the primary timing reference is derived from an on-board free running crystal oscillator (inside block 69a (Fig. 6) ) and all other timing signals are locked to the oscillator.

Each circuit card in the system, except the alarm card 29 (Fig. 2) , contains an on-board DC-to-DC power

converter which transforms a DC battery's voltage into the voltage required to operate that card's analog and digital circuits. A boost converter that supplies the power to the line interface modules 20, 22, 24 to operate the phones is located on the alarm card. In presently preferred embodiments, this converter develops a regulated and protected output with current adequate to power seventy-two phones. The output is distributed down the backplane to all line cards. Figures 8A and 8B are a flow chart of the operation of a system employing the inventive FEP in conducting a sent- paid call. The flowchart indicates the steps performed by the caller, the paystation (which in this example is a charge card terminal (CCT) ) , the FEP, and the central office. A legend corresponding to each of these components is provided at the top of the flowchart. The operating sequence proceeds in the direction of the arrows and from top to bottom. Thus, the first step is for the caller to lift up the handset on the CCT. The CCT recognizes that the handset has been lifted up and makes a loop with the FEP. The FEP recognizes this loop as the beginning of a call. Meanwhile, the CCT receiver is open and the transmitter is mute. The CCT then sends off- hook start text (i.e., a digital message from the telephone to the FEP indicating that the telephone has gone off-hook and is about to initiate a call) employing data over voice to the FEP. The FEP receives this data and transmits a dial tone back to the CCT. The dial tone confirms to the caller that a connection has been made with the central office. The CCT then displays the message "INSERT CARD" on an associated display (not shown) . The caller inserts his card as instructed and the CCT reads the data on the card. The CCT then transmits card-in text (a digital message containing information about the card, including card serial number and monetary value remaining on the card) to the FEP. This transmission is effected by employing data over voice. The FEP receives the data over voice communication, verifies the serial number, and sends card validation text (a digital

message indicating the card has been validated) back to the CCT by data over voice. The CCT receives this information and then displays the message "PLEASE DIAL" along with a message indicating the monetary value remaining on the card. This is confirmed by the caller by dialing the first through last digits of the number to be called. The CCT stores these numbers and transfers them to the FEP by DTMF signals. The FEP receives the dialed digits and retrieves call rating information from memory. The call rating text (a digital message indicating the amount to be charged for the call) is transmitted to the CCT by data over voice.

Referring now to Figure 8B, the FEP then transfers all digits to the central office. The central office then send a ring back tone (RBT) to the call-originating telephone. (The RBT indicates that the connection to the telephone that was called is available and not busy. If the connection is unavailable, other tones, such as busy or reorder, will be sent.) In the meantime, the CCT awaits the battery reversal. (Battery reversal is the standard method used on analog lines and trunks to indicate that the call has been answered at the distant end. Ground is normally connected to the tip wire and battery (-48V) is normally connected to the ring wire, through appropriate impedances. When the call is answered, the tip wire is connected to battery and the ring wire is connected to ground.) The voice data is then sent from the central office to the FEP, which performs derived answer supervision/voice detection. The called party's voice is sent to the CCT and the caller. Meanwhile, the CCT recognizes the battery reversal performed by the FEP and opens its transmitter to allow the caller and the called party to talk to each other. Meanwhile, the CCT reduces the monetary value remaining on the caller's charge or debit card (i.e., depending on the call rating information) . The remaining value on the card is periodically displayed by the CCT. Eventually, the caller hangs up, the CCT recognizes the hang-up and sends hang-up text (a digital message indicating that the phone is on-hook)

by data over voice to the FEP. The CCT also sends card-out text (a digital message indicating that the card has been removed and the monetary amount deducted from the card) to the FEP. The FEP receives these data over voice signals and releases the loop with the central office. The CCT returns the card to the caller and releases the loop with the FEP. Meanwhile, the FEP records the DCR/CDR (card detail record/detail call record, which is later sent to the management system) and then recognizes the release of the loop with the CCT. At this point, the operating sequences of the CCT, FEP, and central office in connection with the sent paid call come to an end.

The foregoing description of the operation of the system in conducting a sent-paid call is an example of how data over voice communications is employed to communicate information between the CCT, FEP, and central office. The particularities of the other steps (e.g., making a loop, digit transfer via DTMF, card reading) are known to those skilled in the art and thus are not explained in detail herein.

Although one presently preferred embodiment of the invention is described in detail herein, the invention is by no means limited to the specific embodiment disclosed. For example, the answer supervision and call charge functions could be off-loaded from a conventional smart paystation and performed by the FEP. Therefore, the paystations do not necessarily have to have a card reader. Further, the paystations may be powered from a local source instead of from the FEP (although the ability to provide power to the paystations even while no call is in progress is an important feature of preferred embodiments) . The FEP's call restriction table and processing circuitry for blocking a call in accordance with the call restriction table are also important but not absolutely essential features of the preferred embodiments. As mentioned above, the call restriction table may contain data specifying dialed numbers to be block and/or debit or cash card numbers to be blocked.

The FEP's ability to automatically dial a predefined number in response to a pre-specified card number being received is also highly useful. For example, this feature could be used to allow a service technician to call his home office, but not other numbers, with the card. Other variations and modifications will be apparent to those skilled in the art after reading this specification. Accordingly, except as they may be expressly so limited, the scope of protection of the following claims is not limited by the particularities specified above.