Abstract:
Method and apparatus for providing a regulated and current limited power source to support bidirectional telephone delivery circuitry and methods for a multiplicity of telephone subscriber lines to a downstream distribution terminal using only two preexisting transmission lines for carrying the regulated and current limited power along with a multiplicity of HDSL TDM communication channels.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to telephone systems and more particularly to methods and apparatus for upgrading an existing tip and ring prior art telephone system which uses a pair of copper conductors for each bidirectional connection between two distribution terminals to a system which requires only two pairs of existing copper (or other metal) conductors for carrying a plurality (such as twenty-four) bidirectional communications between the two distribution terminals. The invention also relates to such methods and apparatus for upgrading a telephone system which, after being upgraded according to the teachings of this invention, is compatible for still further and future upgrades. 
     2. Description of Related Art 
     Over the last twenty years, the demands for “telephone”-type communications has increased at exponential rates. Telephone-type communications, or “telephony,” is no longer simply applicable to telephone voice communications. In fact, the demands on the infrastructure and existing telephone systems from other types of telephony such as faxes, computer modems, etc. and other broadband uses now exceed voice communications in most locations. 
     Consequently, telephone systems in these high-density areas now use many different high speed, broadband transmission techniques including light transmission through optical fibers. Yet, even though there are areas of communications or physical locations demanding this very high-density type of service, there are likely to be other areas of communications or physical locations immediately adjacent where the demands on telephone service has increased only slightly from the demands of twenty years ago. Therefore, methods and equipment must be provided which interface various levels of service demands. 
     It is therefore not unusual to require different upgrade levels of a telephone system depending upon these highly varying needs. In addition, because new and different services which require high-speed and broadband telephone communications seem to be hitting the market everyday, it is also desirable to provide methods and apparatus for upgrading a telephone system which upgraded methods and apparatus will also allow for continuous and much expanded upgrades of service in the future with minimal change to the infrastructure. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the invention to provide methods and apparatus to increase the number of communication channels carried by existing copper wire pairs. 
     It is yet another object of the present invention to provide methods and apparatus for efficient and cost effective upgrading to existing infrastructure copper wire communication pairs. 
     These and other objects can be achieved by the present invention which provides bidirectional telephone delivery circuitry and methods for supporting a multiplicity of telephone subscriber lines at a downstream distribution terminal using only two of a multiplicity of pre-existing transmission line pairs connected between an upstream distribution terminal and the downstream distribution terminal. The methods and apparatus comprise an upstream distribution terminal which may receive and transmit PCM telephone signals to and from one location. These PCM signals are routed to conversion circuitry which includes an HDSL driver and receiving circuit for receiving HDSL TDM signals from a downstream terminal and for transmitting corresponding HDSL TDM signals to the downstream terminal. A pair of HDSL isolation transformers are included and, according to one embodiment, each of the pair has an upstream side coil connected to the HDSL driver and receiver circuit and each also has a downstream side coil connected to one of two conductor pairs such as a pair of copper tip and ring conductors typically used for existing voice telephone service. Each of the downstream coils of the two transformers also includes a center tap. There is included power circuitry for providing a source of regulated and current limited DC power which has a power output terminal connected to the center tap of one of the pair of isolation transformers, and a power return terminal connected to the center tap of the other one of the pair of isolation transformers. The two transmission line pairs will typically be selected from the existing multiplicity of infrastructure, two conductor telephone transmission line pairs used for tip and ring voice telephone service and located between the upstream distribution terminal and the downstream distribution terminal. As mentioned above, one of the transmission line pairs is connected to each side of the downstream coil of one of the isolation transformers and the other one of the transmission line pairs is connected to each side of the downstream coil of the other one of the transmission line pairs. The existing infrastructure at the downstream distribution terminal will typically include a multiplicity of telephone subscriber lines connected between a plurality of user locations such as homes or businesses and the downstream distribution terminal. Thus, the downstream distribution terminal is located between the two conductor transmission line pairs and the selected multiplicity of telephone subscriber lines. The downstream distribution terminal will also include a pair of isolation transformers and an HDSL TDM driver and receiver circuit substantially similar to that in the upstream distribution terminal which connects with the HDSL TDM driver and receiver circuit located in the upstream distribution terminal through the two transmission line pairs. The DC power distribution circuit provides a DC power source in the downstream distribution terminal and includes at least a pair of output terminals and an input power terminal connected to the center tap of one of the isolation transformers connected across one of the transmission line pairs and an input power return terminal connected to the center tap of the other isolation transformers connected across the other one of the transmission pairs so as to receive the source of regulated and current limited DC power from the upstream distribution terminal. 
     The power circuitry located in the upstream distribution terminal provides the regulated and current limited DC power to the downstream distribution terminal on the same two transmission line pairs which also carry HDSL TDM bidirectional telephone signals. The power circuitry comprises a pair of HDSL isolation transformers and the downstream coil of the isolation transformers each include the center taps one each connected to the power output terminal and the other to the power return terminal. The DC regulated power and current limited circuitry comprises an optical isolator, a voltage regulator circuitry, a transistor driver with a shutoff, an over-voltage shutdown, and a three-volt regulator. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features of the present invention will be more fully disclosed when taken in conjunction with the following Detailed Description of the Invention in which like numerals represent like elements and in which: 
     FIG. 1 is a prior art diagram of existing tip and ring telephone circuitry; 
     FIG. 2 shows a block diagram overview of the system circuitry of the present invention utilizing some of the infrastructure of the prior art; 
     FIG. 3 is a block diagram of the system circuitry of the present invention located at an “upstream” distribution terminal; 
     FIG. 3A shows a pair of isolation transformers in the “downstream” distribution terminal used to receive DC regulated power at the center taps. 
     FIG. 4 is a block diagram of the power regulation and current limiting circuitry of the system circuitry shown in FIG. 3; and 
     FIGS. 5A and 5B are an electrical schematic of the power regulation and current limiting circuitry of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, there is shown a portion of the distribution circuitry for a prior art telephone system using a pair of electrical conductors for each channel of communication. As shown in the figure, there is an “upstream” distribution terminal  10  providing a multiplicity “N” of conductor pairs  12 ,  14 ,  16 ,  18 ,  20  and  22  to a “downstream” distribution terminal  24 . The multiplicity “N” of conductor pairs, according to the infrastructure of a typical telephone system, is equal to twenty-four. Downstream distribution terminal  24  provides a like number “N” of conductor pairs to end users or subscribers such as conductor pair  26  to a house  28  having a standard voice telephone connection  30 . The term “upstream” is used herein to indicate a distribution terminal closest to a central office, and the term “downstream” is used to indicate a distribution terminal close to the end user. In this prior art system, it is seen that it was required to have at least as many conductor pairs between the “upstream” distribution terminal and the downstream distribution terminal as it was between the downstream distribution terminal  24  and each of the individual telephone services connected to the distribution terminal  24  as indicated by conductor pairs  26 ,  32 ,  34 ,  36 ,  38  and  40 . 
     Referring now to FIG. 2, there is shown a block diagram of the telephone service of the present invention which uses some of the infrastructure of the prior art system but in a much more efficient way that allows substantial increase in the number of communication channels. For example, the remote distribution terminal  10  may be the same outside steel cabinet as used with the prior art and which has the same number or multiplicity of conductive pairs represented by line pairs  12 ,  14 ,  16 ,  18 ,  20  and  22  which represent pairs  1  through N between the upstream distribution terminal  10  and the downstream distribution terminal  24 . However, according to the present invention, it will only be necessary to use two conductive pairs such as pairs  12  and  14  to service the same number of customers connected to the local or downstream distribution terminal  24  as was serviced by distribution terminal  24  in the prior art circuitry of FIG.  1 . Therefore, if for example there were twenty-four conductive pairs between the upstream distribution terminal  10  and the downstream distribution terminal  24  in the prior art, there will still be twenty-four conductive pairs between the two, but only two of the twenty-four pairs are necessary to provide all of the communication channels between remote or upstream distribution terminal  10  and local or downstream distribution terminal  24 . Thus, the other twenty-two pairs can be used for other purposes as indicated by second downstream ENU (Electrical Network Unit)  42  which may be at a distance from downstream distribution terminal  24 . In addition to being connected to line pairs  12  and  14  from remote or upstream distribution terminal  10 , the ENU  44  in downstream distribution terminal  24  is also connected to the conductive pairs  26 ,  32 ,  34 ,  36 ,  38  and  40  to the end users such as end user  28  on line  26  and end users  46  and  48  on lines  32  and  40  respectively. 
     According to the present invention, remote or upstream distribution terminal  10  will now include new equipment such as the Matrix Distribution Shelf (MDS)  50 , a microprocessor  52  and a power source  54 . According to one embodiment, MDS  50  has sufficient slots to handle up to fourteen separate communication boards, such as communication board  56 . Also, according to the present invention, each of the communication boards can handle four separate distribution circuits according to the teachings of the present invention of the type shown in FIG.  3 . Each of the two circuits are connected to two transmission line pairs such as transmission line pairs  12  and  14 . Thus, according to the present invention, the communications board  56  on the Matrix Distribution Shelf  50  can receive transmission line pairs  12  and  14  connected to ENU  44  and transmission line pairs  16  and  18  connected to the second ENU  42 . As shown in FIG. 3, circuitry included on the communications board  56  includes a field programmable gate array  58 , HDSL signal driver and receiver  60 , Power Protection and Current Limiter Circuitry  62  and surge protection circuitry  64 . In addition to the circuitry located on the communications board  56  as discussed above, the upstream distribution terminal also includes microprocessor  52  and power supply  54  which are also connected in a similar manner to the other thirteen communication boards. 
     Thus, as will become clear hereinafter, by using the new and unique circuitry of this invention and the HDSL transmission format, the two conductor pairs  12  and  14  are now capable of carrying all twenty-four of the communication channels that were carried by the multiplicity or twenty-four conductive pairs required by the prior art circuitry. It should be understood that references to twenty-four communication channels is for convenience only since that is the number in many existing prior art telephone systems. The number can, of course, vary from twenty-four. In addition, it is seen that another multiplicity of twenty-four prior art communication channels can also be routed through downstream distribution terminal  24  to be carried by conductive pairs  16  and  18 . Furthermore, as is indicated by the optical transmission line  66 , completely new and different transmission methods such as light transmission methods through optical fiber  66  can be provided from the remote or upstream distribution terminal  10  to a different downstream distribution terminal  68 . As an example only, downstream distribution terminal  68  may be a completely optical system which receives at least one optical fiber  66  carrying a large multiplicity of communication channels from upstream terminal  10  to the ONU or downstream distribution terminal  68 . ONU  68  may represent a local or downstream distribution terminal for new construction, office buildings, new residential subdivisions, etc. Thus, as shown in the drawing, there is also included at least one communication channel  70  between ONU  68  and subscriber location  72  with a telephone  74 . It will be appreciated that the communication channel between ONU  68  and subscriber location  72  could be by means of an optical fiber or traditional communication pairs. 
     In addition to new equipment provided in the remote distribution cabinet  10 , it will also be necessary to provide new equipment such as ENU  44  in the downstream distribution terminal or cabinet  24 . As shown in the blown-up portion  76  of FIG. 2, ENU  44  will include power distribution circuitry  78 , its own microprocessor  80 , HIU (HDSL Interface Unit)  82  for receiving the HDSL formatted communication data and an EIU or Electrical Interface Unit  84  which interfaces the communication channels carried on the two communication pairs  12  and  14  between upstream terminal  10  and downstream terminal  24  with the individual communication or channel quad cards  86   a  through  86   f . Each of the six channel cards  86   a  through  86   f  can handle up to four communication circuits and thus, the full original twenty-four circuits can receive the twenty-four different communication pairs from the original twenty-four customers. These twenty-four communication signals are then converted to a HDSL format so they can be handled by the two transmission pairs  12  and  14  connected between the upstream terminal  10  and the local or downstream terminal  24  rather than the twenty-four line pairs required by the prior art. 
     As will be appreciated by those skilled in the art, the addition of electronic equipment such as located in ENU  44  at terminal  24  and which allows a significant increase in communication channels without any increase in transmission wire pairs to the cabling infrastructure requires a stable and dependable power source. Therefore, unless each downstream terminal, such as terminal  24 , is connected to a power source and includes its own regulated power supply circuitry for operating the electronic equipment, it is necessary that the stable and dependable power source be provided from the upstream terminal  10  which may be located at a significant distance from downstream terminal  24 . According to the present invention, a regulated and current limiter power source is provided from the upstream distribution terminal  10  to a power distribution panel  78 , which in turn is connected to microprocessor  80 , HIU  82  and the six channel cards  86   a  through  86   f . As was discussed above, regulated power is connected to the center taps  88  and  92  of the two isolation transformers  90  and  94  which transformers are connected across the two transmission line pairs  12  and  14 . As shown in FIG. 3A, two isolation transformers  90 A and  94 A are connected across the same two transmission line pairs which extend between the “upstream” distribution terminal  10  and the “downstream” distribution terminal  24 . Thus, the regulated power provided from the “upstream” terminal is accessed at the “downstream” terminal  24  by center taps  88 A and  92 A. This regulated power is connected to power distribution panel where it is available to the circuitry in “downstream” distribution terminal  24  as discussed above. 
     Referring now to FIG. 4, there is shown a block diagram of the Power Protection and Current Limiter Circuitry  62  included in upstream terminal  10 . Circuitry  62  provides a stable and dependable source of power to one or more downstream distribution terminals located at distances up to 12,000 feet, such as, for example, downstream or local distribution terminal  24 . FIG. 5 is a detailed circuit diagram of the block diagram of FIG.  4 . 
     As shown in FIG. 3, Power Protection and Current Limiter Circuitry  62  provides up to 100 VA from power supply  54  at a −139 volts between the center tap  88  of isolation transformer  90  and the center tap  92  of isolation transformer  94  connected to the return of power supply  54 . Thus, the voltage output of the −139 volt power is carried to downstream distribution terminal  24  by both conductors  95  and  96  of transmission line pair  14  connected to isolation transformer  90  as discussed above. Similarly, the return for the −139 volts is carried between upstream terminal  10  and downstream terminal  24  by both conductors  98  and  100  of transmission line pair  12  by isolation transformer  94 . As will be appreciated by those skilled in the art, the voltage level provided to distribution downstream terminal  24  could be selected to be different than the −139 volts discussed above. However, present day safety standards limit the voltage differential from ground to no more than 140 volts. Therefore, a tap differential value of 139 volts is selected to assure the safety standards are not exceeded, while at the same time providing the maximum possible voltage level. It should also be understood that the actual HDSL TDM communication signals are also carried by these same two transmission pairs  12  and  14 . 
     Referring now to FIGS. 4 and 5, the operation of Power Protection and Current Limiter Circuitry  62  will be discussed. Circuitry  62  provides the 100 VA limit function by limiting the current in the negative lead when it exceeds the predetermined internal fault value. Also, as should be appreciated by those skilled in the art, the limit function of 100 VA is referenced only because that is a convenient and often used value. The invention is not considered to be limited to this value and the actual power limits may vary significantly from that value. As will be discussed, the limiting is provided by a combination of peak current limiting and pulsed on-line control. In normal operation, the unit also provides a controlled voltage drop of about three volts in order to maintain a regulated or constant voltage to the power loop. The Power Protection and Current Limiter Circuitry  62  also will produce a momentary power interruption if the HDSL link is dropped. This allows a line protector to reset which may have activated due to an outside event such as, for example, lightning. The unit will also survive the surges and power fault voltages which manage to make it through a primary protector. 
     More specifically, the Power Protection and Current Limiter Circuitry  62  consists of several functional sections. Transistor  100 , or another similar control element, along with its transistor driver circuitry and shutoff circuitry  102 , provides the necessary current regulation and is the control element for regulation of the three-volt voltage drop. Diode  104  isolates the current limiter circuitry from any negative surges, and Regulator or Integrated Circuit  146  provides the current limit function including sensing and pulsing. Transistors  108 ,  110 ,  112  and  114  as part of the three-volt Regulator circuitry  116  comprise a discrete amplifier used to regulate the voltage drop. Optical isolating circuitry  118  provides an overriding shutdown function from the control FPGA (field programmable gate array) in response to a fault indicator signal from an optical isolator transistor  120 . The Power Protection and Current Limiter Circuitry  62  is powered by the negative voltage provided by the −139 volt supply on line  122  through bias resistor circuit  124  to the 139 volt return on line  126 . 
     In normal operation, integrated circuit  106  contains an internal 9.5V zener diode which provides voltage regulation for Integrated circuit  106  and the transistor driver amplifier  102 . During normal operation, the load current is less than the trip point and the voltage drop across the Limiter Circuitry  62  is controlled to about 3 volts. The three-volt Voltage Regulator  116 , which is comprised of transistors  108 ,  110 ,  112  and  114 , compares a reference voltage generated on the base of transistor  108  with a fraction of the voltage drop appearing on the base of transistor  110 . It then provides gate drive to Control transistor  100 . Control transistor  100  holds the voltage drop at a nominal 3 volts from the input to the output of Limiter Circuit  62 . Diode  128  provides 1.24 volts to the base of transistor  108 . Capacitor  130  and resistor  132  provide compensation to keep the Power Protection and Current Limiter Circuitry  62  regulator loop stable. In the event of a fault or an out of tolerance condition of the circuit discussed above, which fires or activates optical isolator transistor  120 , optical isolation circuitry  118  will turn the current limiter off for a pre-selected time period such as about 90-92 seconds, and thereby provides release for a potentially latched ENU protector. 
     The rather lengthy predetermined time period that the optical isolation circuitry  118  is maintained in an “off” condition also prevents the generation of excessive noise or cross talk to other transmission line pairs which could occur as the Integrated Circuit  106  repeatedly attempts to reset at a very rapid rate when the fault condition still exists. 
     The load current provided to downstream distribution terminal  24  will be interrupted if found to be excessive. More specifically, the load current passes through resistor  134  which develops a voltage on line  136  for current sensing. Integrated circuit  106  is set to have a nominal threshold such as, for example, 50 m volts. When the voltage on line  136  reaches 50 m volts, capacitor  138  begins to charge to a selected value such as, for example, 2.5 volts. Thus, when the voltage on capacitor  138  reaches 2.5 volts, the output on line  140  of integrated circuit  106  will turn off transistor  142  which in turn will turn off control transistor  100 . As will be discussed below, integrated circuit  106  will attempt to turn control transistor  100  back on after a selected time period has expired. While in the trip or off condition, integrated circuitry  106  operates control transistor  100  in a pulse width modulated mode so as to manage the power dissipation in Power Protection and Current Limiter Circuit  62 . The output of integrated circuitry  106  is buffered by transistor  142  to allow for supply voltage differences between integrated circuit  106  and the gate drive of control transistor  100 . Resistor circuit  144  provides load voltage information on line  146  to integrated circuit  106  so that the power dissipation in control transistor  100  can be controlled. 
     If the load is suddenly increased such that the current far exceeds the 100 VA limit (for example, there is a short on the line), integrated circuit  106  provides a current regulation mode that is set to about 5 amps. A divider circuit provided by resistors  148  and  150  are selected to determine this threshold. Thus, in the case of a short in the line between upstream distribution terminal  10  and the downstream distribution terminal  24 , the pulse modulation will begin and limit the power dissipated in the Power Protection and Current Limiter Circuitry  62 . The Power Protection and Current Limiter Circuitry  62  according to one embodiment, is designed to survive surges which may pass through a primary protector. A resistor divider formed by resistors  152  and  154  are arranged to safely handle the anticipated surge voltage using surface mount resistors. During a negative surge, diode  104  blocks the voltage to most of the circuitry. The resistor divider formed by resistors  152  and  154  connected to the base of transistor  110  is prevented from over-driving transistor  110  by clamping diode  156 . Clamping diode  156  limits the voltage swing on the base of transistor  110  to the voltage across one diode. 
     During a positive surge, the same resistor divider formed by resistors  152  and  154  limits over-driving of transistor  110  by the clamping action of the emitter of transistor  158 . It is important to limit the overdrive of transistor  110  to allow for a quick recovery after a surge. If the surge voltage reaches a pre-selected value such as, for example, in excess of 40 volts, over-voltage shutdown circuitry  159 , which circuitry includes the zener diode pair  160  and  162 , conducts and turns on transistor  164 . Transistor  164  lowers the gate drive voltage of control transistor  100  thereby preventing a high voltage/high current situation for control transistor  100 . As control transistor  100  turns off, its collector voltage quickly rises. Control transistor  100  is selected to survive a voltage surge of 1,000 volts. 
     Whereas, the Power Protection and Current Limiter Circuitry  62  controls and limits the power and current provided to downstream distribution terminal  24 , this status is monitored by FPGA control circuitry  58  by means of output  158  from integrated circuitry  106  to determine if and when a fault condition exists. The fault indication on line  158  turns on transistor  150  which in turn switches on the Light Emitting Diode (LED)  160  in optic isolator  120 . The light from LED  160  turns on the light sensitive transistor  162  also in optic isolator  120 . The output of transistor  162  is provided to FPGA control circuitry  58 . As discussed above, if FPGA control circuitry  58  detects a fault condition, it will shut off the power for a pre-selected time period by activating the optical isolator  118  which in turn will turn off control transistor  100 . The FPGA control will try to reapply power after a pre-selected time period such as, for example according to one embodiment, 92 seconds. If the limiter is still indicating an over-current condition, the power will be shut off again and then attempt to reapply after another 92 seconds. 
     The corresponding structures, materials, acts and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material or act for performing the function in combination with other claimed elements as specifically claimed.