Abstract:
An off-hook management telecommunication system comprising a DTMF detector, an encoder and a microprocessor located within a channel unit. The system allows the user to depress a selected sequence of keys during a line connection which indicate to the microprocessor to hold the line off-hook, via signaling to a central office, despite the fact that the telephone is being returned to an on-hook position. The line is held off-hook for a predetermined amount of time to allow the user to switch telephones.

Description:
FIELD OF THE INVENTION 
   The invention relates generally to a telephone off-hook management system for use with two or more telephones connected to the same telephone line at a subscriber premises. More particularly, the invention provides detection and processing of tones entered via a telephone keypad while the telephone is off-hook to allow placement of the telephone on-hook and subsequent pick up of another telephone without termination of the initial call. 
   BACKGROUND OF THE INVENTION 
   A digital loop carrier (DLC) carries plural voice and/or data channels over a number of lines. A DLC comprises a Central Office Terminal (COT), a remote terminal (RT), and multiple subscriber lines. The RT contains channel units with line cards and serves to connect individual subscriber lines to the COT. The transmit direction of a DLC comprises voice frequency signals entering the channel units from the subscriber premises. These voice frequency signals produce pulse amplitude modulation (PAM) samples at an 8 kHz rate. This sampling of channels is performed in a fixed sequence, producing a stream of pulses. Each PAM pulse is encoded into a pulse code modulation (PCM) word by the channel unit coder/decoder (CODEC) therein. 
   A transmit/receive unit (TRU) at an RT, for example, polls the plurality of channel units therein for PCM words using sequential polling. The TRU time division multiplexes the PCM words into a bitstream and transmits the bitstream to a line interface unit (LIU). The LIU converts the bitstream from the TRU into a T1 signal. This T1 signal is then transmitted to the COT. 
   Regarding the reception of the signals from an RT by the subscriber premises, the LIU performs framing on the T1 input from a line and sends it to the TRU. The TRU then distributes the PCM to the channel units which decode the PCM into corresponding PAM pulses. The voice frequency signals are then reconstructed, and sent over a twisted pair line to the subscriber. 
   In a T1 digital carrier system, for example, a voice signal is sampled, resulting in a PAM signal that is converted to an 8-bit PCM digital signal, and then interleaved with 23 other channels for transmission over a T1 line at a bit stream rate of 1.544 megabits per second (Mbps). T1 signals are processed in a channel bank such as the channel bank  50  depicted in FIG.  1 . In a conventional digital channel bank  50 , twenty-four channels are collectively referred to as a digroup. Channel banks typically comprise two digroups A and B to create a 48-channel framework for transmitting and receiving on two duplex T1 carriers. As shown in  FIG. 1 , the channel bank  50  comprises a chassis  90  having physical card slots  60  into which at least forty-eight channel unit (CU) cards  80  can be inserted, as well as a number of common equipment cards  70 . 
   A disadvantage of current telephone systems is evident when a subscriber receives an incoming call and wishes to switch telephones. The subscriber places the first telephone off-hook and switches to a second telephone connected to the same line, thus taking a second telephone off-hook. In such a situation, if the subscriber does not place the first telephone on-hook after terminating the call, the line will be considered in use and unavailable for subsequent incoming or outgoing calls. Therefore, a need exists to allow for switching telephones, during an incoming or outgoing call, by placing a first telephone on-hook and subsequently taking a second telephone off-hook, yet not losing the initial incoming or outgoing call. 
   Some telephones have the ability to allow a subscriber to enable/disable an on-hold function while the subscriber answers another telephone line or switches to another telephone that is also provided with an on-hold enable/disable function. This on-hold feature is accomplished via components located within the telephone, and is not available to other telephones at the subscriber premises if they lack the internal components to support this function. Therefore, a need exists to allow remote processing (e.g., at a remote terminal or a central office) of a series of dual-tone multi-frequency (DTMF) codes to prevent termination of a call while a first telephone is placed on-hook and, subsequently, a second telephone is placed off-hook. Thus, existing special internal components for the on-hold function, which are not typically available in a telephone, are not needed to take advantage of this feature. 
   Since one RT serves a plurality of subscriber premises, it would be convenient for telephone companies to offer the above mentioned off-hook management to its customers by means of the channel unit. This would require no additional special equipment in subscribers&#39; telephones or homes. 
   SUMMARY OF THE INVENTION 
   The above described disadvantages are overcome and advantages realized by the off-hook management system of the present invention which, in a preferred embodiment, uses a channel unit that is coupled to at least one telephone via its corresponding subscriber line. Located in the channel unit is a DTMF detector, a microprocessor and a decoder which allow a subscriber to place an incoming call on-hold by dialing a sequence of DTMF tones on the telephone. The channel unit is provided with intelligence, by means of a microprocessor, to detect the DTMF tones and maintain the subscriber line in an off-hook status for a selected period of time. Thus, the subscriber is permitted to place the receiver on the cradle and thereafter continue the call using another telephone or facsimile machine. 
   Another aspect of the present invention provides the subscriber the ability to switch from a first telephone to a second telephone, connected to the same line, during an incoming or outgoing call. The subscriber dials a series of DTMF tones for processing by the channel unit or other device to allow placement of the first telephone on-hook, and continuation of the call on the second telephone. The channel unit or other device manages the processing function, thus eliminating the need for special components in either of the two telephones. 
   The foregoing aspects are substantially achieved by a channel unit in a DLC. The channel unit is coupled to at least one telephone via a subscriber line, wherein the telephone is used to generate DTMF tones comprising off-hook control signals. The channel unit comprises a DTMF detector coupled to the subscriber line. The DTMF detector detects the DTMF tones. Additionally, a microprocessor is coupled to the DTMF detector and programmable to receive an input comprising the output of the DTMF detector. Subsequently, the processor evaluates the DTMF tones to determine if it is the off-hook control signal. If the DTMF tones are the off-hook control signal, the microprocessor maintains the subscriber line in an off-hook mode for a selected period of time. Finally, the microprocessor continues to process received DTMF tone sequences. If the microprocessor determines that the DTMF tone sequences do not comprise the off-hook control signal, it continues to process the telephone call in a conventional manner, that is, based on the state of the telephone. 
   In addition, the present invention provides a method for achieving off-hook management by sending an off-hook control signal from a subscriber premises that is processed at a remote location such as a channel unit. The channel unit receives the off-hook control signal, preferably in the form of a selected sequence of DTMF tones, from a telephone via a subscriber line. Subsequently, the method provides the steps of detecting the off-hook control signal on the subscriber line. Initially, if the microprocessor determines that the DTMF tones on the subscriber line are off-hook control signals, then the microprocessor maintains the subscriber line in an off-hook position, for a selected period of time, despite the telephone handset being on-hook. Alternatively, if the microprocessor determines that the DTMF tones generated by the telephone keypad is not the off-hook control signals, then the microprocessor returns the subscriber line to a state dependent upon the status of the telephone and any other telephone or signal receiving device coupled to the subscriber line. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various aspects, advantages and novel features of the present invention will be more readily comprehended from the following detailed description when read in conjunction with the appended drawings, in which: 
       FIG. 1  is a front view of an exemplary and conventional channel bank; 
       FIG. 2  is a block diagram of a conventional DLC; 
       FIG. 3  is a block diagram of a channel unit constructed in accordance with an embodiment of the present invention; and 
       FIG. 4  is a state diagram illustrating the operational states of the channel unit of  FIG. 3  in accordance with an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2  depicts an overview of a DLC  10 . Subscriber lines  17  couple telephones  12 ,  14  and  16 , at the subscriber premises, to channel units  22 . The channel units  22  are housed in a remote terminal  18 , which can be located in a residential neighborhood, for example. The channel units  22  convert the analog signal from the subscriber lines  17  to digital signals. The digital signals are pulse code modulated and carried over T1 lines  20  to a central office terminal (COT)  21 . 
   Components located on the channel unit  22  allow the subscriber to place an incoming call on-hold, as described below. Accordingly, the telephones  12 ,  14 , and  16  at the subscriber premises, are typical residential telephones not requiring any special internal components for implementing an on-hold function. This is a significant advantage for service providers, as they are able to offer a residential off-hook management system as an additional feature to subscribers. Such a system can be marketed similar to call waiting or call forwarding, thus generating additional revenue. Furthermore, existing channel units  22  can be retrofitted with the critical circuitry and software used to implement an embodiment of the present invention and future channel units can be manufactured in accordance with the present invention, as is described below. 
   If a subscriber receives a call on one telephone  12  and wishes to take the call from a second telephone  16  connected to the same subscriber line  17 , an embodiment of the present invention allows the subscriber to enter a sequence of DTMF tones via the telephone  12  (e.g., via the telephone keypad) such as three star (*) keys. The subscriber places the first telephone  12  receiver on the cradle, and subsequently continues the call on a second telephone  16  without losing the off-hook status of the subscriber line  17  and, therefore the initial connection. 
   Referring now to  FIG. 3 , an overview of an embodiment of the present invention comprises the subscriber entering the required DTMF tones, as described above. The DTMF detector  28  implemented at an RT or other remote device (e.g., a channel unit  22 ) detects the sequence of tones from a telephone  12  at the subscriber premises. The decoder  30  decodes a digital input signal from the output of the DTMF detector  28 . The DTMF detector  28  converts the signal from analog to digital format, as described below. The decoder  30  transmits a digital signal to a microcontroller  24  of the channel unit  22  causing the microcontroller to wait for a predetermined period of time (e.g., thirty seconds) for the subscriber to switch telephones. In accordance with an embodiment of the present invention, the microcontroller  24  maintains the line in an electrically off-hook condition by transmitting appropriate signaling to the COT  21 , despite the telephones  12  and  16  being physically in an on-hook condition. 
   The channel unit  22  of  FIG. 3  receives an incoming signal on a twisted pair line  17  through a line resistor network  23 , and a secondary protection circuit  25  which preferably provides protection against lightning strikes. The signals are then sent to a subscriber line interface chip (SLIC)  26 . The SLIC  26  provides a current feed to power the telephone, voice interface, and ringing control. Typically, two subscribers are serviced via the same channel unit  22 . Thus, the shadowed box  31  represents a second set of twisted pair lines, security protection, and additional SLIC. The signals are then sent to a DTMF detector  28  which is preferably a MITEL MT8870C/MT8870C-1 component. Table 1 illustrates the pushbutton layout on a DTMF station equipment pad and the corresponding frequencies. The operation of any pushbutton on a telephone typically generates two frequencies, depending upon the button the subscriber depresses. 
   
     
       
             
             
             
             
             
           
             
             
             
             
             
             
           
         
             
                 
                 
             
             
                 
                 
                 
                 
               1633 Hz 
             
             
                 
               1209 Hz 
               1336 Hz 
               1477 Hz 
               (spare) 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
                 
               697 Hz 
               1 
               ABC 2 
               DEF 3 
               A 
             
             
                 
               770 Hz 
               GHI 4 
               JKL 5 
               MNO 6 
               B 
             
             
                 
               852 Hz 
               PRS 7 
               TUV 8 
               WXY 9 
               C 
             
             
                 
               941 Hz 
               * 
               OPER 0 
               # 
               D 
             
             
                 
                 
             
           
        
       
     
   
   These DTMF tones are capable of transmitting not only address information, but also control signals, and therefore can be used as a method for data communications. The DTMF detector  28  converts these frequencies to four parallel digital bits Q 1 -Q 4  and applies this digital output to a decoder  30 , which then transmits preferably one digital bit to the microcontroller  24 . For example, the decoder  30  uses digital bits Q 1 -Q 4  from the DTNF detector  28  such that, when it receives the bits  1101  corresponding to Q 1 =1, Q 2 =1, Q 3 =0, and Q 4 =1, the decoder  30  generates a single bit “1” indicating an on state for transmission to the microcontroller  24 . However, when any other bit sequence is generated (i.e. not  1101 ), the decoder  30  outputs a “0” bit indicating an off state for transmission to the microcontroller  24 . The input  3 , of the microcontroller  24 , is preferably an interrupt signal from the decoder  30 . Thus, if the decoder  30  sends a “1” bit, then the software provided to the microcontroller  24  in accordance with the present invention causes the microcontroller to send an off-hook sequence to the COT  21  despite the phone being on-hook. If the input  3  of microcontroller  24  receives a “0” bit from the decoder  30 , then the microcontroller  24  is programmed to process signaling on the subscriber line  24  in a conventional manner. 
   The DTMF detector  28  generates digital bits representative of the star key (*), including  1101 . Accordingly, if the star (*) key is depressed three times, the bits sent by the DTMF detector  28  are  1101   1101   1101 , in groups of four bits each. The signal is then decoded by the decoder  30  and interpreted by the microcontroller  24 . If the DTMF detector  28  detects the three star sequence (***), the microcontroller  24  maintains the line off-hook for thirty seconds or a preselected period of time. The microcontroller  24  maintains the line off-hook by sending a signal to the COT  21  via an application-specific integrated circuit (ASIC)  32 . The signal is transmitted from the backplane  34  of the remote terminal  18  to the T1 lines  20  that extend to the COT  21 . 
   With continuing reference to  FIG. 3 , in conjunction with the operation of the DTMF detector  28 , the processing of the voice signal continues. As mentioned above, the voice signal on the twisted pair lines  17  is processed by the secondary protection circuit  25 , and then provided to the SLIC  26 . The voice signals are processed by the coder/decoder (CODEC)  27  which interacts with the SLIC  26  to set return loss and transhybrid loss. The CODEC  27  also determines transmit and receive levels. In addition, the CODEC  27  converts the voice-frequency signals from the SLIC  26  into pulse code modulation (PCM) signals, which are sent to the ASIC  32 . The ASIC  32  combines the voice data with the signaling bits from the microcontroller  24 , and sends that information to the backplane  34  of the channel bank and then to the COT  21 . 
   The above discussion is concerned with the transmit portion of the signal path from the subscriber premises. With respect to the receive portion of the signal path from the COT  21 , it sends the PCM signal to the backplane  34 . The signal is processed by the ASIC  32  and sent to the CODEC  27 . The microprocessor  24  processes signaling information from the backplane  34 . Therefore, once the COT signal enters the backplane  34 , the microcontroller  24  controls the received signal, as opposed to the COT  21 . The CODEC  27  provides the proper amount of gain, converts the digital PCM signal to analog format and conveys the resulting signal to the SLIC  26 . Accordingly, the SLIC  26  transmits the analog signal to the subscriber via the twisted pair lines  17 . As mentioned previously, an additional subscriber is serviced via the components of box  31 . 
   The automatic loss compensation circuit (ALC)  29  determines the DC resistance of the customer drop and equipment. Once the microcontroller  24  has determined the line resistance, it calculates an appropriate gain factor for both the transmit and receive directions. The gain information is then sent to the CODEC  27 . 
   It is to be understood that the present invention can be implemented as a channel unit  22 , as described above in connection with FIG.  3 . In addition, existing channel units can be retrofitted to operate as the channel unit  22  by providing existing channel units with the DTME detector  28  and decoder  30 , as well as program code for operating the microcontroller therein, as described above in connection with microcontroller  24 . In addition to remote terminals, the present invention can be provided to or retrofit into corresponding equipment at the central office. 
     FIG. 4  is a state diagram illustrating the logical functioning of the channel unit  22 , in accordance with an embodiment of the present invention. Initially, the DTMF detector  28  and decoder  30  are in an idle state  36 . While the telephone  12  is on-hook, the microcontroller  24  transmits bits A and B to the COT  21 , thus indicating that the telephone  12  is on-hook. Transmit bits A and B are referred to in  FIG. 4  as T a  and T b , respectively. When a subscriber picks up a telephone receiver, the microcontroller  24  receives information indicating the telephone  12  is off-hook (state  38 ). As is known to those skilled in the art, supervisory signals are used to convey information to the COT  21  as to the status of the DLC. For example, transmit bits A and B are sent by the microcontroller  24  to indicate that telephone  12  is off-hook. In addition, state  38  indicates that the microcontroller  24  initializes the DTMF Counter to 0. 
   State  40  indicates that no DTMF signal has been sent and, further the microcontroller  24  is sending transmit bits A and B to the COT, indicating an off-hook condition. As shown in state  42 , if a DTMF tone comprising the (*) character is detected as a portion of an off-hook control signal, then the DTMF CTR is increased by 1, for example, DTF CTR=DTMF CTR+1, and the microcontroller  24  continues to transmit bits A and B to the COT  21 , indicating an off-hook position of the telephone  12 . The microcontroller software preferably toggles between states  40  and  42 , depending upon whether another DTMF tone is detected. For example, if an additional DTMF character is not sent, as in state  40 , transmit bits A and B continue to be sent which indicate an off-hook position of the telephone  12 . As is mentioned above, if the DTMF detector  28  receives an additional DTMF tone comprising the (*) character, as in state  42 , the DTMF CIR increases by one. State  44  indicates a wait state, whereby the software is monitoring whether the DTMF CTR has reached the maximum of three (*) tones comprising the off-hook control signal, or other preselected number. If the counter is not equal to DTMF CTR=3, the toggling between state  40  and  42  continues. If the counter is equal to DTMF CTR=3, then the delay timer initiates, as indicated by state  46 . Accordingly, if the DTMF CTR=3 then the subscriber is generating the off-hook control signal, via the telephone keypad. State  46  shows the microcontroller  24  continuing to transmit bits A and B to indicate an off-hook state. State  46  preferably starts a 30-second timer, or other preselected time, to allow the subscriber to switch from telephone  12  to telephone  16 , for example. Although the subscriber has replaced the receiver of telephone  12  on the cradle, the microcontroller  24  ignores this on-hook condition and continues to transmit bits A and B to the COT  21  to maintain the line off-hook. State  48  shows the microcontroller  24  continuing to send transmit bits A and B in an off-hook position for the prescribed period of time. After the 30 seconds has elapsed, or other prescribed period of time, the DTMF detector  28  and decoder  30  return to the idle state as depicted in state  36 . 
   Although only several exemplary embodiments of the present invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims.