Patent Publication Number: US-6343126-B1

Title: Method and apparatus for interfacing analog telephone apparatus to a digital, analog or hybrid telephone switching system

Description:
This application claims the benefit of U.S. Provisional Application No. 60/064,382, filed Nov. 6, 1997. This application is a continuation-in-part of U.S. application Ser. No. 08/625,398, filed Mar. 27, 1996, the contents of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to the field of telephony. More particularly, the invention relates to an adaptive interface for interfacing a two-wire or a four-wire analog telephone instrument to a digital, analog or hybrid telephone switching system. 
     BACKGROUND OF THE INVENTION 
     A two-wire analog telephone set of the type commonly found in the homes of telephone service subscribers typically includes a base unit connected to a central office of a telephone service provider via a bi-directional, two-wire, telephone line and also includes a handset connected to the telephone base unit via a four-wire handset cable. The handset cable has four wires because, for two-way voice communication, the handset includes both a microphone and a speaker, each of which requires a pair of wires. Typically, the telephone base supplies audio signals to the speaker and a DC biasing voltage to the microphone, while the telephone base receives audio signals from the microphone. A two-wire to four-wire converter included in the telephone base unit converts the two central office signals into the four handset signals. In addition, the telephone set includes a ring detector for detecting an AC ring signal provided by the central office and a hook-switch for signalling the central office for answering or placing calls. When the handset is removed from its cradle, the hook switch controls draw of DC current from the central office by the telephone set which is detected by the central office. 
     A conventional modem transmits digital data over a two-wire telephone line by modulating an analog carrier signal according to the digital data. Typically, the digital data is generated by a computer or facsimile machine connected to the modem. The carrier signal is a tone within the frequency range of telephone transmission line. Upon reception by a second modem at the other end of the transmission line, the digital data is reconstructed by demodulating the received signal. 
     Business organizations often utilize a telephone switching system for providing telephone service to telephone users within the organization. The telephone switching system can have an all-digital interface with its corresponding compatible telephone sets, such as in a digital private branch exchange (PBX). Alternately, the telephone switching system can have an all-analog interface, such as is provided by an analog line card in a PBX or by a central office. In addition, the telephone switching system can provide a combined digital and analog interface with its corresponding compatible telephone sets, such as in a hybrid PBX or a key telephone system (KTS). For the purposes of this document, the term “PBX” is utilized to encompass equipment similar to those the above-listed types of telephone switching equipment. 
     Telephone sets that are compatible with a particular PBX utilized by a business organization are located on the desks of the users. Each PBX-compatible telephone set is connected to the PBX via a corresponding extension line, while the PBX is connected to a telephone service provider via one or more outside lines. The PBX typically includes capability for appropriately connecting incoming calls to the user telephone sets and for connecting outgoing calls from the user telephone sets to an outside line. In this way, fewer than one outside line per telephone set is needed, thus, reducing the cost of the telephone service. In addition, the PBX typically provides a variety of features to the users of the PBX, such as connecting calls among the users and providing voicemail services. 
     To implement all of the functions of the PBX, certain control and overhead communications must take place between each user&#39;s telephone set and the PBX. These communications typically include digital status, initialization and command signals in addition to the two-way voice signals necessary to carry on a telephone conversation. For example, the PBX must know whether a telephone set is connected to a particular extension line in order to know whether or not to route calls to that extension. As another example, the PBX must interact with the user telephone sets in order for the users to receive incoming calls, initiate outgoing calls, terminate telephone calls and to access voicemail and other features of the PBX. 
     In general, communication protocols utilized for control and overhead communications differ among the various manufacturers of PBX&#39;s. In addition, in an all-digital PBX, the voice signals are communicated between the telephone sets and the PBX as digital samples. Thus, analog voice signals are digitally sampled and encoded according to various different schemes (e.g. μ-law or A-law) before they are communicated. Upon reception, the digital samples are decoded and converted back into analog voice signals. In a hybrid system, voice signals are communicated as analog signals, while control and overhead communications are digital signals. Therefore, a two-wire analog telephone instrument, such as a modem, fax modem, facsimile machine or teleconferencing device, cannot generally interface directly with a PBX. Nor can a four-wire analog telephone instrument, such as a headset, handset or modem, generally interface directly with a PBX. 
     This creates a problem for users of a PBX who wish to use universally available analog telephone instruments, such as modems, fax modems, facsimile machines, teleconferencing devices, headsets or handsets, in addition to their PBX-compatible telephone sets. This problem has intensified by the recent increase in demand for access to the world wide web, which is typically accessed through use of a modem connected to a personal computer. A proposal has been to provide a dedicated outside line for each such analog telephone instrument. This solution is not entirely satisfactory, however, because it negates the savings which result from the PBX limiting the number of required outside lines. Another solution has been to provide an analog line card in the PBX and a separate line connecting the two-wire analog telephone instrument to the PBX. This solution can be costly due to the need to install separate extension lines to connect each of the PBX-compatible telephone set and the analog telephone instrument to the PBX. 
     Another solution has been to provide a device which interfaces a modem with a telephone set through the handset port of the telephone set. For example, U.S. Pat. No. 4,907,267 discloses a modem interface device for use with a telephone set having a base unit and a handset. The telephone set can be a two-wire telephone set or a telephone set designed for use with a PBX. To use the modem interface device, the handset is unplugged from the handset jack of the base and plugged into a handset jack in one end of the device. Extending from the device is a four-wire cable which is connected to the handset jack of the base. The device also includes a modular jack for accepting a two-wire cable which connects the device to a two-wire telephone instrument, such as a modem. A series of switches are manually positioned to select between voice and data communications and to configure the interface device to match the signalling characteristics of the particular telephone set being used. 
     The manually operable switch arrangement described in U.S. Pat. No. 4,907,267 is improved upon in two products manufactured by Unlimited Systems Corp. of San Diego, Calif. A first of these products, the “KONEXX Office Konnector,” connects to the base of a telephone set and to the handset to provide an interface for a two-wire telephone, facsimile machine or modem. The device detects when the two-wire telephone, facsimile machine or modem is placed off-hook for switching between voice and data communications. A second of these products, the “KONEXX Konference,” is similarly connected between the base and handset, but provides an interface for a teleconferencing device. For each of these devices, a manually operable switch is positioned in one of four positions for adjusting the device to the signalling characteristics of the particular telephone set being used. 
     The aforementioned interface devices, however, can be inconvenient for interfacing an analog telephone instrument to a PBX. This is because to install such an interface device, the handset cord of a PBX-compatible telephone set must first be disconnected from its base. Then, the interface device must be connected to both the handset and to the base. Next, the analog telephone instrument must be connected to the interface device. Finally, the switch positions for the interface device must be correctly set. 
     Perhaps a more significant drawback, however, is that each time the analog telephone instrument is used to answer or place a call, the user must manually place the PBX-compatible telephone set off-hook. This is generally accomplished by removing the handset of the PBX-compatible telephone from its cradle. Similarly, when finished using the analog telephone instrument, the user must return the PBX-compatible telephone to its on-hook condition. Otherwise, if the user forgets to return the PBX-compatible telephone to its on-hook condition, incoming calls cannot be connected and will receive a busy indication. In addition, the handset port of the PBX-compatible telephone generally does not provide a ring signal which may be required for automatic answering functions. Another drawback is that some PBX-compatible telephones do not accept DTMF signals through the handset port though DTMF signals may be required by the PBX system for dialing telephone numbers. Thus, for example, auto-dialing features of an analog device will fail to operate. Therefore, the actual telephone keypad must be used to dial for the analog device. Furthermore, the cables required for connecting such an interface device can become tangled and tend to provide a cluttered appearance on the user&#39;s desk. 
     Therefore, what is needed is a technique for interfacing an analog telephone instrument to a PBX that does not require access to the handset port of a PBX-compatible telephone. What is further needed is such a technique that has sufficient flexibility to adapt to the signalling characteristics of a wide variety of commercially available PBX&#39;s. What is still further needed is such a technique that requires a minimum of additional cables to accomplish its functions and that minimizes technical ability required from a user. 
     SUMMARY OF THE INVENTION 
     The invention is an adaptive interface method and apparatus for interfacing a two-wire analog telephone instrument, such as a modem, fax modem, facsimile machine or teleconferencing device, or a four-wire analog telephone instrument, such as a headset, a handset or a modem, to a private branch exchange (PBX). For purposes of this document, the term “analog telephone instrument” will be used to refer to both two-wire and four wire telephone instruments. The interface device according to the present invention is suitable for use with a variety of PBX&#39;s produced by different manufacturers, despite differences in signalling characteristics between the PBX and an associated PBX-compatible telephone. In a preferred embodiment, the invention does not require access to a handset port of the PBX-compatible telephone. 
     A PBX is generally connected to an associated PBX-compatible telephone via a two-wire telephone extension line. An extension line for a hybrid telephone switching system, however, can include up to eight wires. In a first embodiment of the present invention, both the interface device and the PBX-compatible telephone set are connected to the extension line. An analog telephone instrument is then connected to the interface device. The PBX-compatible telephone communicates with the PBX so as to notify the PBX that the extension line is capable of receiving incoming calls. In addition, the PBX-compatible telephone can initiate and receive telephone calls without interference by the interface device. 
     The analog telephone instrument can also initiate and receive telephone calls. To initiate an outgoing telephone call originated by the analog telephone instrument, the interface device detects a current draw (a dial tone request) by the analog telephone instrument, as occurs when the analog telephone instrument goes off-hook. Accordingly, the interface device emulates a central office from the perspective of the analog telephone instrument. In response to detecting the analog telephone instrument going off-hook, the interface device communicates an appropriate instruction to the PBX so as to emulate the PBX-compatible telephone going off-hook. This is accomplished without having to manually take the PBX-compatible telephone off-hook. According to the first embodiment, a telephone number to be called is dialed by using a keypad located on the interface device. 
     To receive an incoming call using the analog telephone instrument, the interface device receives a notification of the incoming call which is sent by the PBX and intended for the PBX-compatible telephone connected to the corresponding extension line. If the analog telephone instrument then goes off-hook, the interface device responds by communicating an appropriate instruction to the PBX so as to emulate the PBX-compatible telephone going off-hook. This is also accomplished without having to manually take the PBX-compatible telephone off-hook. 
     Once a telephone call is connected to the analog telephone instrument via the interface device, the interface device provides a two-way communication path between the analog telephone instrument and the PBX for voice or modem signals. Thus, the interface device receives voice or modem signals from the analog telephone instrument and converts them into a form suitable for reception by the PBX and receives voice or modem signals from the PBX and converts them into a form suitable for reception by the analog telephone instrument. For example, if the PBX is an all-digital PBX, the interface device performs appropriate analog-to-digital and digital-to-analog conversions. 
     When an incoming or outgoing telephone call is complete, the interface device detects that current is no longer drawn by the analog telephone instrument, as occurs when the analog telephone instrument is returned to its on-hook condition. In response, the interface device communicates an appropriate instruction to the PBX so as to emulate the PBX-compatible telephone returning to an on-hook condition. 
     A second embodiment differs from the first embodiment in that the keypad located on the PBX-compatible telephone is utilized to dial a telephone number to be called. According to the second embodiment, a keypad need not be provided on the interface device. 
     A third embodiment differs from the first and second embodiments in that a keypad located on the analog telephone instrument can be utilized to dial a telephone number to be called. The interface device receives dual-tone, multi-frequency (DTMF) signals which are generated by the analog telephone device as the telephone number is dialed. The interface device then converts these signals into a format appropriate for the PBX. 
     A fourth embodiment differs from the other embodiments in that the interface device communicates with the PBX so as to notify the PBX that the telephone connected to the extension line is capable of receiving incoming calls. Similar to the third embodiment, the keypad located on the analog telephone instrument can be utilized to dial a telephone number to be called. Thus, in the fourth embodiment, a PBX-compatible telephone is not required to be connected to the extension line along with the interface device. 
     In order to communicate voice and overhead signals with the PBX using a communication protocol appropriate to the PBX, the interface device must “learn” the characteristics of the PBX. Therefore, when the interface device is coupled to the PBX, a learning technique is performed. 
     A first step of the learning technique requires that the interface device determine whether or not the telephone system to which it is connected communicates voice signals as digital samples, such as an all-digital PBX, or whether the telephone system communicates voice signals in analog form, such as a hybrid PBX, a KTS, or a central office of a telephone service provider. The primary functions of the telephone sets compatible with each of these types of telephone switching systems are powered directly by the associated telephone switching system. The inventor has observed that the power supply characteristics differ for each type of telephone switching system relative to the modular interface terminal locations and the effective DC source resistances. Accordingly, a determination is made by the interface device polling up to eight terminals coupled to the extension line. By discovering which of the polled terminals are active, the interface device distinguishes between hybrid telephone switching systems and other types of telephone switching systems. Assuming the telephone switching system is a hybrid system, the particular model or manufacturer can generally be identified by discovering which polled terminals are active. 
     Assuming the telephone switching system is not a hybrid system, up to three DC source resistance measurements are taken for the extension line via the active terminals. A first measurement is an unloaded DC measurement. For the second two measurements, the extension line is loaded by alternate fixed resistive loads. The interface compares the results of these measurements to pre-stored values to determine whether the telephone switching system is an all-digital system or an analog system. 
     If the system communicates voice signals in a multi-wire hybrid-type format, the interface device configures itself accordingly. Thus, a next step in the learning technique is to emulate an off-hook condition. In response to the emulated off-hook condition, the hybrid PBX provides a dial tone signal to the receive lines of the extension. The interface device detects the dial tone signal and performs level adjustments for both the receive and transmit signal paths. The receive signal path is configured using the dial tone signal and the transmit path is configured using a set of prestored parameters appropriate to the hybrid PBX. This is accomplished by the interface device selecting a stored set of operational parameters from a plurality of such sets. 
     If the system communicates voice signals in an analog format, the interface device also configures itself accordingly. Thus, a next step in the learning technique is to emulate an off-hook condition. In response to the emulated off-hook condition, the PBX analog line card or central office provides a dial tone signal to the interface device. The interface device detects the dial tone signal and performs level adjustments for both the receive and transmit signal paths. The receive path is configured using the dial tone signal and transmit path is configured by implementing Transmit Objective Loudness Rating (TOLR) sensitivity levels. 
     Otherwise, if the system communicates voice signals as digital samples, a next step in the learning technique is to determine the signalling protocol which is utilized for communicating between the PBX and the associated PBX-compatible telephones. This is accomplished by the interface device momentarily open-circuiting the extension line. Then, the interface device monitors signals communicated between the PBX and the PBX-compatible compatible telephone which initialize the PBX-compatible telephone and which notify the PBX that the PBX-compatible telephone connected to the extension line is capable of receiving incoming calls. 
     Then, based upon this determination, the interface device configures itself according to the appropriate signalling protocol. This is accomplished by the interface device selecting a stored set of operational parameters from a plurality of such sets. The sets of operational parameters are pre-stored in a memory device within the interface device. A selected set of operational parameters configures the interface device to communicate with the PBX using a protocol appropriate to the particular PBX being utilized. 
     Thus, the learning technique allows the interface device to automatically adapt itself to variations in signalling characteristics between the PBX and PBX-compatible telephone sets among the different PBX manufacturers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a block schematic diagram of an interface device according to the present invention coupled to a PBX, to a PBX-compatible telephone set and to one or more analog telephone instruments. 
     FIG. 2 illustrates a block schematic diagram of the interface control portion of the interface device according to the present invention. 
     FIG. 3 illustrates a flow diagram of a learning algorithm according to the present invention. 
     FIG. 4 illustrates a schematic diagram of a circuit for measuring a source resistance of the extension lines according to the present invention. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     FIG. 1 illustrates a block schematic diagram of an interface device  100  according to the present invention coupled to a telephone switching system (PBX)  102 , to a PBX-compatible telephone set  104 , to a two-wire analog telephone instrument  106  and to a four-wire analog telephone instrument  108 . The telephone switching system  102  can be an all-digital private branch exchange (PBX), a hybrid PBX, a key telephone system (KTS) or a direct line from a central office  110 . For the purposes of this document, the term “PBX” encompasses all of the above-listed types of telephone switching equipment. And, for the purposes of this document, the term “PBX-compatible telephone set” refers to a telephone set  104  specifically designed to interface directly with a particular PBX  102 . Typically, the PBX  102  and PBX-compatible telephone set  104  are provided by the same manufacturer. Though several manufacturers produce PBX&#39;s and corresponding PBX-compatible telephone sets, a PBX-compatible telephone set provided by a manufacturer is generally not capable of interfacing with a PBX provided by a different manufacturer. 
     The PBX  102  is coupled to a central office  110  of a telephone service provider via one or more outside lines  112  and is also coupled to a PBX port  114  of the interface device  100  via a extension line  116  and a wall jack  118 . The extension line  116  is a two-wire line for most types of telephone switching systems, however, the extension line  116  for a hybrid switching system can include up to eight wires. 
     As an example, the PBX  102  can be centrally located at a business site, such as in a service room or basement. Several extension lines (only one is shown—extension line  116 ) extend to corresponding wall jacks (only one is shown—wall jack  118 ). The wall jacks are typically distributed throughout the business site. The wall jacks can be located in users&#39; offices, conference rooms and reception areas. Conventionally, the PBX-compatible telephone set  104  would be plugged into the wall jack  118 . According to the present invention, however, the interface device  100  is plugged into the wall jack  118 , while the PBX compatible telephone set  104  is plugged into the interface device  100 . 
     The interface device  100  includes an interface control portion  120  which is coupled to the PBX  102  via the PBX port  114 . Internal to the interface device  100 , the PBX port  114  is coupled to the interface control portion  120  and to a first terminal of a switch SW 1 . A second terminal of the switch SW 1  is coupled to a PBX phone port  122 . The switch SW 1  is coupled to be controlled by the interface control portion  120 . Also internally to the interface device  100 , the interface control portion  120  is coupled to a two-wire analog phone port  124  and to a four-wire analog phone port  126 . 
     Externally to the interface device  100 , the PBX-compatible telephone set  104  is plugged into the PBX phone port  122 , the two-wire analog telephone instrument  106  is plugged into the two-wire port  124  and the four-wire analog telephone instrument  108  is plugged into the four-wire port  126 . In certain embodiments of the present invention, it is not necessary for the PBX-compatible telephone set  104  to always be present to obtain the advantages of the present invention. In addition, it is not necessary that both telephone instruments  106 ,  108  be present to obtain the advantages of the present invention. 
     The telephone instruments  106 ,  108  can each be a modem, fax modem, facsimile machine, teleconferencing device, headset, handset or other type of conventional analog telephone instrument. The four-wire telephone instrument  108  differs from the two-wire telephone instrument  106  primarily in that the four-wire telephone instrument  108  transmits analog signals via a first pair of wires and receives analog signals via a second pair of wires (uni-directional signaling), whereas, the two-wire telephone instrument  106  communicates analog signals in both directions (transmit and receive) via a single pair of wires (bi-directional signaling). 
     FIG. 2 illustrates a block schematic diagram of the interface control portion  120  of the interface device  100  illustrated in FIG. 1. A central office emulator  200  is coupled to the two-wire port  124  (FIG.  1 ). The central office emulator  200  provides DC power to the port  124  and detects the on-hook/off-hook condition of the two-wire analog telephone instrument  106  (FIG. 1) depending upon whether it draws current from the central office emulator  200 . The central office emulator  200  provides an indication of the on-hook/off-hook condition of the two-wire analog telephone instrument  106  to a hook switch block  202 . 
     The central office emulator  200  is also coupled to a two-to-four wire converter  204 . Internally to the central office emulator  200 , signals from the two-wire port  124  are routed to the two-to-four wire converter  204 . The two-to-four wire converter  204  can be a conventional circuit, commonly known as a hybrid circuit, which converts the bi-directional signals from the two-wire port  124  into separate transmit and receive signals. These separate transmit and receive signals from the two-to-four wire converter  204  are coupled to a TX/RX audio block  206 . 
     The signals from the four-wire port  126  (FIG. 1) are also routed to the TX/RX audio block  206 . Two-to-four wire conversion is not required for these signals because they are already separated into transmit and receive channels. An indication of the on-hook/off-hook status for the four-wire telephone instrument can be provided by a user interface (not shown), such as a on/off switch, coupled to the hook switch block  202 . 
     The TX/RX audio block  206  performs appropriate level adjustments for both the receive and transmit signal paths. Thus, the TX/RX audio block  206  includes analog signal processing circuits, such as gain-controllable amplifiers. The RX/TX audio block  206  ensures that the levels of the voice or modem signals received from the PBX  102  (FIG. 1) are adjusted for compatibility with the analog telephone instrument  106  or  108  (FIG. 1) and ensures that the levels of the signals received from the analog telephone instrument  106  or  108  are adjusted for compatibility with the PBX  102 . 
     Via the TX/RX audio block  206 , the separate transmit and receive signals from the two-to-four wire converter  204  and from the four-wire port  126  are coupled to a pulse code modulation (PCM) encoder/decoder (CODEC) block  208  and to an analog line interface block  210 . Preferably, the PCM CODEC block  208  is selectively active or inactive depending upon whether the PBX  102  (FIG. 1) communicates voice or modem signals over the extension line  116  (FIG. 1) as digital samples or whether the PBX  102  communicates these signals in analog format. If the PBX  102  communicates these signals as digital samples, then the PCM CODEC block  208  is active. Conversely, if the PBX  102  communicates these signals in analog format, then the CODEC block  208  is inactive. 
     Assuming the PCM CODEC block  208  is active, a digital line transceiver  212  and digital line interface  214  are also active. The PCM CODEC block  208  converts analog voice or modem signals received from the TX/RX audio block  206  into a serial digital data stream. Preferably, this conversion is preformed according to A-LAW or μ-LAW companding techniques. The serial data stream formed by the PCM CODEC block  208  is representative of the voice or modem signal received from the analog telephone instrument  106  or  108  and is provided to the digital line transceiver  212 . 
     The digital line transceiver  212  then combines the digitally sampled voice or modem signals with any necessary overhead or command signals, thereby forming a combined serial data stream. For example, the hook switch block  202  notifies the digital line transceiver  212  of the on-hook/off-hook status of the telephone instrument  106  or  108  (FIG.  1 ). The digital line transceiver  212  responds by including an appropriate command to the PBX  102  in the combined serial data stream. 
     The combined serial data stream formed by the digital line transceiver  212  is then provided to the digital line interface block  214 . The digital line interface block  214  communicates the combined serial data stream to the PBX via a learning block  216 . The digital line interface  214  is preferably controlled by the learning block  216 . 
     Because the combined serial data stream is received by the PBX  102  (FIG.  1 ), it must be in a format that is compatible with, and understandable by, the particular PBX  102  coupled to the interface device  100 . For example, the data must be appropriately synchronized with the PBX  102  and must be appropriately compressed and encoded according to the requirements of the PBX  102 . In addition, the command and overhead information included in the combined serial data stream must be recognizable to the PBX  102 . 
     The specific parameters required for appropriately forming the combined serial data stream, however, generally vary among the various manufacturers of PBX&#39;s. Therefore, the PCM CODEC block  208  and digital line transceiver  212  are preferably pre-configured to perform analog-to-digital conversion appropriately for the particular PBX  102  coupled to the interface device  100 . In addition, the digital line interface  214  is also pre-configured to form the combined serial data stream appropriately for the particular PBX  102  coupled to the interface device  100 . This pre-configuration of the PCM CODEC  208 , digital line transceiver  212  and digital line interface  214  is performed under control of the learning block  216  and according to data stored in manufacturer specific protocol sets  218 . 
     The digital line interface  214  receives a serial stream of digital data generated by the PBX  102  and provides this serial data stream to the digital line transceiver  212 . The digital line transceiver  212  then appropriately separates overhead and commands from voice or modem signals and passes the voice or modem signals to the PCM CODEC  208  for decoding. To perform this function appropriately, the digital line transceiver  212  is pre-configured, under control of the learning block  214  according to data stored in the manufacturer specific protocols block  218 . 
     As an example of operation of the digital line transceiver  212 , if the PBX  102  indicates that an incoming telephone call is to be connected to the extension line  116 , the digital line transceiver  212  recognizes this condition and, in response, communicates this condition to an incoming call detect block  220 . The incoming call detect block  220  then notifies the PCM CODEC block  208  to prepare to receive digital samples from the digital line transceiver  212 . The incoming call detect block  220  can also notify the central office emulator  200  to send a ring signal to the two-wire analog telephone instrument  106  (FIG.  1 ). 
     Then, when the two-wire analog telephone instrument  106  goes off-hook, the central office emulator  200  (FIG. 2) recognizes this condition and, in response, notifies the hook switch block  202 . Alternately, a manual switch notifies the hook switch block when the four-wire analog telephone instrument  108  (FIG. 1) goes off-hook. The hook switch block  202  then appropriately notifies the digital line transceiver  212  which then communicates with the PBX  102  so as to emulate the PBX-compatible telephone set  104  going off-hook. 
     The PCM CODEC block  208  converts the digital samples received from the digital line transceiver  212  into an analog signal. The digital samples are received as a one-bit-wide stream of digital values. Accordingly, the conversion is performed by appropriately parsing the received stream of digital values into a series of digital values, each digital value having an appropriate width. Then, any compression and/or encoding performed by the PBX  102  (FIG. 1) is reversed. Finally, the analog signal is reconstructed from the series of digital values. To perform this conversion appropriately, the PCM CODEC block  208  is pre-configured, under control of the learning block  216  according to a manufacturer specific format and synchronization of the digital samples stored in the manufacturer specific protocols block  218 . 
     The manufacturer specific protocol sets  218  includes a plurality of sets of conversion parameters appropriate for PBX&#39;s produced by various different manufacturers. Each set of parameters includes information relating to an appropriate format and synchronization of the digital samples, decompression and decoding of the digital samples, appropriate compression and encoding of the analog signals into digital samples, generation of commands to the PBX  102  and recognition of commands from the PBX  102 . In general, these parameters are specific to each PBX manufacturer. 
     The analog signal generated by the PCM CODEC block  208  is provided to the TX/RX audio block  206  for routing to the two-wire port  124  via the central office emulator  200  and to the four-wire port  126 . 
     A line filter  222  is coupled to the digital line interface  214  and to the analog line interface  210  for obtaining supply power for the interface device  100  (FIG. 1) from the PBX  102  (FIG. 1) via the extension line  116  (FIG.  1 ). The interface device  100  can also be externally powered. The line filter  222  filters frequency components above a predetermined threshold from the extension line  116  thereby forming an unregulated DC voltage. Alternately, an unregulated DC voltage can be obtained from a battery supply or from a rectified AC line voltage. The unregulated DC voltage is provided to an isolated switching power supply  224 . The isolated switching power supply  224  provides power to the circuits of the interface device  100 , but is electrically isolated from the source of power. When the digital line transceiver  212  is active, the digital line transceiver  212  preferably provides a synchronizing signal to the switching power supply  224 . This synchronizing signal controls switching of the power supply  224  to occur out of phase with digital-to-analog sampling performed by the PCM CODEC block  208  for minimizing sampling errors caused by switching noise. 
     Assuming that the PBX  102  communicates voice or modem signals in analog format, such as when an analog line card is used in the PBX  102  or when the PBX  102  is a hybrid switching system, the PCM CODEC block  208  is preferably inactive. The analog line interface  210  receives analog signals from the PBX  102  via a bi-directional communication path through the learning block  216 . The analog line interface block  210  converts the bi-directional signals into separate uni-directional transmit and receive signal paths. Accordingly, the analog signals are communicated between the analog line interface block  210  and the TX/RX block  206  via separate uni-directional signal paths. 
     Separate uni-directional transmit and receive signal paths connect the TX/RX audio block  206  to the four-wire telephone instrument  108  (FIG.  1 ). For the two-wire telephone instrument  106  (FIG.  1 ), the two-to-four wire converter  204  converts the separate uni-directional signal paths coupled to the TX/RX audio block  206  into a bi-directional signal path through the central office emulator  200 . 
     The analog line interface block  210  monitors the signals originated by the PBX (FIG. 1) for detecting commands from the PBX. For example, the analog line interface block  210  detects whether an incoming call to is to be connected to the extension line  116 . Assuming that the analog line interface block  210  detects an incoming call, the analog line interface block  210  notifies the incoming call detect block  220  of this condition. The incoming call detect block  220  then notifies the TX/RX audio block  206  to prepare to receive incoming voice signals from the PBX  102  (FIG.  1 ). In response, the incoming call detect block  220  can also notify the central office emulator  200  to send a ring signal to the two-wire analog telephone instrument  106 . 
     The analog line interface block  210  also combines the analog voice or modem signals received from the TX/RX audio block  206  with any necessary overhead or command signals. For example, the hook switch block  202  notifies the analog line interface  210  of the on-hook/off-hook status of the telephone instrument  106  or  108  (FIG.  1 ). The analog line interface  210  responds by sending an appropriate command to the PBX  102 , for example, by drawing a DC current from the PBX  102 . 
     Note that for a hybrid PBX, the overhead and command signals sent to the PBX  102  may be in the form of serialized or parallel digital data, though the voice or modem signals are communicated in analog format. Generally the overhead and command signals for a hybrid system are communicated via separate lines within the extension line  116  (FIG. 1) from the lines utilized for communicating voice signals. As mentioned, when the PBX  102  is hybrid switching system, the voice signals are communicated between the PBX  102  and the analog telephone instrument via the analog line interface  210  and TX/RX audio block  206 . For a hybrid switching system, however, a hybrid interface block  226  is provided for communicating overhead and command signals with the PBX  102 . The hybrid interface block  226  is preferably pre-configured under control of the learning block  216  and according to data stored in manufacturer specific protocol sets  218 . 
     As an example of operation of the hybrid interface block  226 , when the PBX  102  sends a command that an incoming call is to be directed to the extension line  116 , the hybrid interface block  226  notifies the incoming call detect block  220 . Also, when the hook switch block  202  indicates to the hybrid interface block  226  that the analog telephone instrument  106  or  108  (FIG. 1) is off-hook, the hybrid interface block  226  requests a dial tone from the PBX  102 . 
     An FSK modem  232  is also coupled to the manufacturer specific protocol sets  218 . The FSK modem  232  allows updates, additions, or modifications to be made to the manufacturer specific protocol sets  218  from a remote location over a telephone line connection. 
     According to a first embodiment of the present invention, a keypad  228  and a dual-tone, multi-frequency (DTMF) generator  230  are provided for initiating telephone calls from the analog telephone instrument  106  or  108  (FIG.  1 ). The keypad  228  is coupled to the DTMF generator  230 . The DTMF generator  230  is coupled to the TX/RX audio block  206  and to the PCM CODEC block  208 . For example, to initiate a telephone call, the two-wire analog telephone instrument  106  is placed off-hook. In response, the central office emulator  200  notifies the hook switch block  202  of this condition. The hook switch block  202  then notifies the analog line interface  210 , the digital line transceiver  212  and the hybrid line interface  226 . An active one of the analog line interface  210  or the digital line transceiver  212  then sends an appropriate command to the PBX  102  (FIG. 1) so as to emulate the PBX-compatible telephone set  104  (FIG. 1) going off-hook. Once the PBX recognizes that call is to be initiated, the keypad  228  is utilized to dial the telephone number to be called. The DTMF generator  230  then generates dual tones for each digit of a telephone number dialed via the keypad  228 . 
     In an alternate embodiment, the keypad  228  is replaced with a voice recognition block which converts a user&#39;s voice commands into signals appropriate for controlling the DTMF generator block  230 . Such an embodiment could be utilized, for example, so that the user&#39;s hands remain free to perform other tasks, or could be utilized by persons having limited use of their hands. 
     The dual tones are then provided by the DTMF generator  230  to the TX/RX audio block  206  and to the PCM CODEC block  208 . Assuming that the PBX  102  is has an analog interface, the dual tones are passed to the PBX  102  through the analog line interface  210  and learning block  216 . Otherwise, assuming the PCM CODEC block  208  is active, the dual tones are converted according to the protocol required for the particular PBX  102  (FIG. 1) being utilized. Accordingly, the PCM CODEC block  208  is pre-configured for this conversion under control of the learning block  216  and according to data stored in manufacturer specific protocol sets  218 . The appropriately converted dual tones are then passed to the PBX  102  via the digital line transceiver  212 , the digital line interface  214  and the learning block  216 . 
     A second embodiment differs from the first embodiment in that the keypad located on the PBX-compatible telephone set  104  (FIG. 1) is utilized to dial a telephone number to be called. According to the second embodiment, therefore, the keypad  228  (FIG. 2) and DTMF generator (FIG. 2)  230  need not be provided. 
     A third embodiment differs from the first and second embodiments in that a keypad located on the analog telephone instrument  106  or  108  (FIG. 2) can be utilized to dial a telephone number to be called. The interface device  100  (FIG. 1) receives dual-tone, multi-frequency (DTMF) signals which are generated by the analog telephone device  106  or  108  (FIG. 1) as the telephone number is dialed. An active one of the TX/RX audio block  206  (FIG. 2) or PCM CODEC  208  (FIG. 2) then converts these signals into a format appropriate for the PBX  102  (FIG.  1 ). 
     A fourth embodiment differs from the other embodiments in that the interface device  100  (FIG. 1) communicates with the PBX  102  (FIG. 1) so as to notify the PBX  102  that the extension line  116  (FIG. 1) is capable of receiving incoming calls. A keypad located on the analog telephone instrument  106  or  108  (FIG. 1) or the keypad  226  (FIG. 2) can be utilized to dial a telephone number to be called. Thus, in the fourth embodiment, a PBX-compatible telephone set  104  (FIG. 1) is not required to be connected to the extension line  116  along with the interface device  100  (FIG.  1 ). In this embodiment, however, a PBX-compatible telephone set  104  is required for appropriately configuring the interface device  100 . Once the interface device  100  is appropriately configured, the PBX-compatible telephone set  104  can be disconnected from the interface device  100 . 
     When an incoming or outgoing telephone call is complete, the central office emulator  200  (FIG. 2) of the interface device  100  (FIG. 1) detects that current is no longer drawn by the analog telephone instrument  106  or  108 , as occurs when the analog telephone instrument  106  or  108  is returned to its on-hook condition. In response, the central office emulator  200  (FIG.  2 ) recognizes this condition and notifies the hook switch block  202  (FIG.  2 ). The hook switch block  202  then notifies an active one of the digital line transceiver  212  or analog line interface  210 , which then communicates with the PBX  102  so as to emulate the PBX-compatible telephone set  104  returning to its on-hook condition. 
     In order to appropriately configure the interface device  100  (FIG.  1 ), particularly the PCM CODEC block  208 , the digital line transceiver  212 , the TX/RX audio block  206 , the hybrid line interface  226  and the analog line interface block  210  to communicate with the PBX  102  (FIG. 1) according to a communication protocol appropriate to the particular PBX  102  being utilized, the interface device  100  must “learn” the characteristics of the PBX  102 . To accomplish this, the interface device  100  performs a learning algorithm. 
     FIG. 3 illustrates a flow diagram of a learning algorithm which controls operation of the learning block  216  (FIG. 2) according to the present invention. The learning algorithm is initiated for appropriately configuring the interface device  100  (FIG.  1 ). Thus, logic circuitry included in the learning block  216  perform the function of determining whether the telephone switching system  102  communicates voice signals as digital samples or in analog format. In addition, logic circuitry included in the learning block  216 , in conduction with data stored in the manufacturer specific protocol sets  218  (FIG.  2 ), perform the functions of identifying a communication protocol utilized by the telephone switching system  102  and configuring the interface device  100  according to the protocol. It will be apparent, however, that a microprocessor or controller circuit operating according to a stored software program could also perform these same functions. 
     As an example, the learning algorithm determines which one or ones of the digital line interface block  214 , the analog line interface block  210  and the hybrid line interface block  226  is to be active. The learning algorithm can be initiated each time power is supplied to the interface device  100 . Alternately, the learning algorithm is initiated each time a reset control input is applied to the learning block  216  (FIG.  2 ). For example, the control input can be in response to a user pressing a button on the interface device  100 . 
     Upon initiation, the learning algorithm moves from a state  300  to a state  302 . Preferably, the learning algorithm determines whether or not the PBX  102  (FIG. 1) to which the interface device  100  (FIG. 1) is connected communicates voice signals as digital samples, such as an all-digital PBX, or whether the telephone system communicates voice signals in analog form, such as a hybrid PBX or a KTS. Note that a central office of a telephone service provider also communicates voice signals in analog form. Thus, assuming the interface device  100  is connected directly to a central office  110  (FIG. 1) of a telephone service provider, rather than to a PBX  102  (FIG.  1 ), the learning algorithm appropriately configures the interface device  100 . 
     The inventor has observed that the power supply characteristics of the extension lines  116  (FIG. 1) differ for each of these types of telephone switching systems relative to the modular interface terminal locations and the effective DC source resistances as measured via the PBX port  114  (FIG.  1 ). For example, a hybrid switching system generally has more active wires in the extension lines  116  than does either an analog line interface to a PBX or an all-digital interface to a PBX. In addition, a DC source resistance for an analog interface tends to be higher than a DC source resistance for an all-digital interface. 
     Accordingly, in the state  302 , the interface device polls up to eight terminals of the port  114 . This is accomplished by measuring a voltage across selected pairs of the wires included in the extension line  116  via the PBX port  114  (FIG.  1 ). By discovering which of the polled terminals are active, the interface device  100  (FIG. 1) distinguishes between hybrid telephone switching systems and other types of telephone switching systems. Assuming the PBX  102  is a hybrid system, the particular model or manufacturer can generally be identified by discovering which polled terminals are active. 
     Once the step of polling is complete, the learning algorithm moves from the state  302  to the state  304 . Based upon the results of the polling performed in the state  302 , the learning algorithm determines whether or not the PBX is a hybrid PBX. 
     Assuming that the switching system communicates according to a multi-wire hybrid-type format, the learning algorithm moves from the state  304  to a state  306 . In the state  306 , the interface device  100  emulates an off-hook condition. Then, the learning algorithm moves from the state  306  to a state  308 . In response to the emulated off-hook condition, the hybrid PBX is expected to provide a dial tone signal to the receive wires of the extension line  116 . If the interface device  100  does not detect the dial tone signal in the state  308 , this indicates that an erroneous measurement was performed in the state  302 . Therefore, the learning algorithm returns from the state  308  to the state  302  where the measurements are repeated. If the interface device  100  is not appropriately configured after a predetermined number of attempts, then the interface device  100  preferably indicates an error condition. 
     Assuming the interface device  100  detects the dial tone signal in the state  308 , the interface device  100  moves to a state  310 . In the state  310 , the interface device  100  configures itself for a hybrid interface by performing level adjustments for both the receive and transmit signal paths through the TX/RX audio block  206  (FIG.  2 ). The receive signal path is configured using the dial tone signal and the transmit signal path is configured according to a selected set of parameters appropriate to the hybrid PBX from the manufacturer specific protocol sets  218 . This appropriately configures the interface device  100  for providing voice communication between the PBX  102  (FIG. 1) and analog telephone instrument  106  or  108  (FIG.  1 ). Additionally, in the state  310 , the hybrid line interface  226  (FIG. 2) is configured to communicate overhead and commands to the PBX  102  according to parameters stored in the manufacturer specific protocol sets  218 . Once the interface device  100  has been appropriately configured in the state  310 , the learning algorithm moves to a state  312  which signifies that the learning algorithm is complete. Also in the state  312 , the configuration parameters obtained in the state  310  are stored in non-volatile memory, such as a serial EEPROM, so that they will not be lost in the event of a power failure. 
     Assuming that in the state  304  it is determined that the PBX  102  (FIG. 1) is not a hybrid system, then the learning algorithm moves from the state  304  to a state  314 . Because the PBX  102  is not a hybrid PBX, the interface to the PBX  102  can be an analog interface, as in the case of an analog line card or a central office. Alternately the interface to the PBX  102  can be a digital interface, as in the case of an all-digital PBX. In either case, the extension line  116  (FIG. 1) is expected to include only two active wires. 
     The inventor has observed that a difference between these types of telephone systems is in a DC source resistance measured via the two active wires of the extension lines  116  (FIG.  1 ). For example, a central office of a telephone service provider typically provides an unloaded line voltage of 48 volts dc. A source resistance depends upon the distance to the central office, however, 1300 ohms is typical. All-digital PBX&#39;s generally have unloaded line voltages between 14 and 48 volts DC with source resistances between 30 and 60 ohms. It can be seen, therefore, that PBX&#39;s that communicate voice signals as digital samples generally have much lower source resistance than a central office or a PBX that communicates voice signals in analog format. Therefore, the determination of whether or not the PBX  102  (FIG. 1) communicates voice signals as digital samples or in analog form is accomplished by effectively measuring the DC source resistance. 
     FIG. 4 illustrates a schematic diagram of a circuit for measuring a source resistance of the extension lines  116  (FIG. 1) according to the present invention. A DC voltage, Vsource, is provided by the PBX  102  (FIG. 1) via a series resistance Rsource. A DC voltage, Vline, is received by the learning block  216  of the interface device  100  (FIG.  1 ). A switch SW 2  selectively coupled one of three resistive loads across the extension lines  116 . A first load LOAD 1  has a large resistance value (e.g. greater than 20 M ohms or open circuit) so as to leave the extension line essentially unloaded. Second and third loads LOAD 2  and LOAD 3  have alternate values which are lower than the value of LOAD 1  so as to the load the extension lines to varying degrees. For example, the value of LOAD 2  can be comparable to an expected value of the source resistance Rsource for an analog interface (e.g. approximately 1 K ohms), while the value of LOAD 3  can be comparable to an expected value of the source resistance Rsource for an all-digital interface (e.g. approximately 50 ohms), but is preferably a higher resistance to avoid any potentially excessive flow of current. 
     In the state  314 , three DC source resistance measurements are taken for the extension line  116  (FIG. 1) via the active two terminals of the PBX port  114  (FIG.  1 ). A first measurement is an unloaded DC measurement. For this measurement, the switch SW 2  is coupled to the first resistance LOAD 1  and the resultant level of the voltage Vline is detected. Similarly, for the second measurement, the switch SW 2  is coupled to the second resistance LOAD 2  and the resultant level of the voltage Vline is detected. For the third measurement, the switch SW 2  is coupled to the third resistance LOAD 3  and the resultant level of the voltage Vline is detected. For each measurement the value of Vline is influenced by the relative values of Rsource and the resistance value coupled to the switch SW 2  by voltage division. 
     Then the learning algorithm moves from the state  314  to a state  316 . Because the results of the measurements taken in the state  314  are indicative of the values of Rsource and Vsource, in the state  316 , the interface device  100  compares the results of these measurements, or ratios thereof, to pre-stored values to determine whether the telephone switching system is an all-digital system or an analog system. 
     If the comparison made in the state  316  indicates that the PBX  102  (FIG. 1) communicates voice signals as digital samples, a next step in the learning algorithm is to determine the signalling protocol which is utilized for communicating between the PBX  102  and the associated PBX-compatible telephone set  104  (FIG.  1 ). Accordingly, the learning algorithm moves from the state  316  to a state  318 . 
     In the state  318 , the interface device  100  (FIG. 1) momentarily disconnects the PBX-compatible telephone set  104  (FIG. 1) from the extension line  116  (FIG. 1) by momentarily opening the switch SW 1  (FIG. 1) and then closing the switch SW 1 . Then, learning algorithm moves from the state  318  to a state  320 . 
     The PBX  102  (FIG. 1) detects that the PBX-compatible telephone set  104  has been disconnected and, then, reconnected to the extension lines  116 . In response, the PBX  102  communicates with the PBX-compatible telephone set  104  to initialize the PBX-compatible telephone set  104 . These initialization signals differ among the various manufacturers and models of PBX&#39;s. Therefore, they provide indicia (a “signature”) by which the particular PBX manufacturer and model can be recognized. 
     In the state  320 , the interface device  100  (FIG. 1) monitors the indicia provided by these initialization signals communicated between the PBX  102  (FIG. 1) and the PBX-compatible telephone set  104  (FIG. 1) and compares them to pre-stored indicia. Each pre-stored indicia is stored in the manufacturer specific protocol sets  218  in association with a corresponding one of the sets of parameters utilized for appropriately configuring the interface device  100  (FIG.  1 ). Then, the learning algorithm moves to a state  322 . Assuming that the interface device  100  recognizes the indicia (the “signature”) provided by the initialization signals, the interface device  100  configures itself according to the appropriate signalling protocol. Accordingly, the learning algorithm moves from the state  320  to a state  322 . 
     In the state  322 , the learning block  216  selects an appropriate stored set of operational parameters from a plurality of such sets pre-stored in the manufacturer specific protocol sets  218  and appropriately configures the PCM CODEC  208  and digital line transceiver  212  according to the selected set. Then, the learning algorithm moves from the state  324  to a state  326 . 
     In the preferred embodiment, once the interface device  100  (FIG. 1) is appropriately configured for the particular PBX  102  (FIG. 1) being utilized, a verification is performed. Therefore, in the state  326 , the interface device  100  sends a command to the PBX  102  (FIG. 1) which simulates the PBX-compatible telephone set  104  (FIG. 1) going off-hook. Then, the learning algorithm moves from the state  326  to a state  328 . In the state  328 , the interface device  100  determines whether the PBX  102  is providing a dial tone in response to the command sent in the state  324 . Assuming a dial tone is detected, the learning algorithm moves to state  330  which signifies that the learning algorithm is complete. Also in the state  330 , the protocol and system configuration parameters are preferably stored in non-volatile memory, such as a serial EEPROM, so that they will not be lost in the event of a power failure. 
     Alternately, if in the state  328 , a dial tone is not detected, or, if in the state  322 , the interface device  100  does not recognize the indicia (the “signature”) the learning algorithm returns to the state  302 , and learning algorithm begins again. If the interface device  100  is not appropriately configured after a predetermined number of attempts, then the interface device  100  preferably indicates an error condition. 
     Assuming the PBX  102  (FIG. 1) communicates voice signals in an analog format, the interface device  100  (FIG. 1) configures itself accordingly. Thus, if it is determined in the state  316  that the PBX  102  communicates voice signals in an analog format, the learning algorithm moves from the state  316  to a state  332 . In the state  332 , the interface device  100  emulates an off-hook condition. In the preferred embodiment, this is accomplished by placing an appropriate resistance across the extension lines  116  so that the PBX  102  senses a current draw via the extension lines  116 . Then, the learning algorithm moves from the state  332  to a state  334 . 
     In response to the emulated off-hook condition, the PBX  102  (FIG. 1) is expected to provide a dial tone signal to the extension lines  116  (FIG.  1 ). Accordingly, in the state  334 , the interface device  100  determines whether the dial tone is detected. Assuming that the dial tone is detected, the learning algorithm moves from the state  334  to a state  336 . 
     In the state  336  and based upon the level of the dial tone signal, the interface device  100  performs level adjustments for both the receive and transmit signal paths through TX/RX audio block  206  of the interface device  100 . The receive path is appropriately configured first utilizing the dial tone. Then, using side tone characteristics linking the receive and transmit paths, the transmit path is appropriately configured. The transmit path is preferably configured by implementing Transmit Objective Loudness Rating (TOLR) sensitivity levels. 
     Once the transmit and receive paths have been appropriately configured, the learning algorithm moves to state  338  which signifies that the learning algorithm is complete. Also in the state  338 , the protocol and system configuration parameters are preferably stored in non-volatile memory, such as a serial EEPROM, so that they will not be lost in the event of a power failure. 
     The present invention has been described in terms of specific embodiments incorporating details to facilitate the understanding of the principles of construction and operation of the invention. Such reference herein to specific embodiments and details thereof is not intended to limit the scope of the claims appended hereto. It will be apparent to those skilled in the art that modifications may be made in the embodiment chosen for illustration without departing from the spirit and scope of the invention. Specifically, it will be apparent to one of ordinary skill in the art that the method of the present invention could be implemented in several different ways and the apparatus disclosed above is only illustrative of the preferred embodiment of the present invention and is in no way a limitation.