Patent Publication Number: US-6671373-B1

Title: Method and apparatus for DC feed control

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to telecommunications, and, more particularly, to a method and apparatus for adjusting DC feed control to a subscriber loop. 
     2. Description of the Related Art 
     In communications systems, particularly telephony, it is common practice to transmit signals between a subscriber station and a central switching office via a two-wire bi-directional communication channel. A line card generally connects the subscriber station to the central switching office. The primary functions of the line card may include supplying talk battery, performing impedance matching, determining whether a telephonic equipment is on-hook or off-hook, and handling ringing signals, voice signals, and testing signals. 
     Subscriber line interface circuits generally include a switch-hook detection circuit that supervises telephone operation by detecting whether a telephone is either on-hook or off-hook. An “off-hook” condition occurs when an end user lifts the handset of a telephone from the cradle, thereby activating the telephone&#39;s hook switch. Conversely, an “on-hook” condition occurs when the handset is placed back in the telephone cradle, thereby terminating the telephone service. Upon detecting the on-hook or off-hook condition, the switch-hook information is passed to the system software of the line card, which then either provides or terminates service. 
     Upon requesting service, a user may establish a connection with a remote user by dialing the telephone number of that user. Telephone dialing may be in the form of dial pulses or tones. Pulse dialing includes generating of a series of electrical interrupts or pulses across the telephone line. One method of generating electrical pulses is by toggling between on-hook and off-hook states such that each transition from an on-hook to off-hook state represents one pulse. The number of pulses generated usually represent the digit that is dialed. 
     The off-hook/on-hook ratio (also referred to as make/break ratio) applied to the loop during pulse dialing is typically 40 to 60; that is, the loop is closed 40 percent of the time and is open 60 percent of the time. The break interval is generally allowed to vary from about 58 percent to 64 percent. However, because of the pulse distortion caused by the loop, the pulse receivers or detectors in the central office must be able to properly respond to a break interval of 55 percent to 65 percent. 
     A telephone system needs DC feed to control the battery feed to the subscriber loop. DC feed delivers enough power for long loop and gradually reduces the power for short loop. A digital signal processing algorithm controls the DC feed curve. During the transition from off-hook to on-hook, the loop impedance changes from low to high and the measured loop voltage between the tip and ring terminals goes from low to high. Generally, the DC feed control is slow to react to sudden changes in the loop impedance. As such, if the loop impedance is high enough during an off-hook to on-hook transition, the DC feed driver may be saturated. During saturation, the measured loop voltage between the tip and ring terminals and the flowing current may not accurately reflect the loop impedance, Z LOOP , a parameter which is generally used for switch hook detection. Thus, an inaccurate measurement of the loop resistance may result in a false detection, thereby causing switch-hook distortion. Additionally, switch-hook distortion may also occur during pulse dialing because of capacitance and inductance present on the subscriber line. 
     The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, a method is provided for adjusting DC feed control of a connection. The method includes receiving a signal from the connection and determining if the change in the signal is greater than a first preselected value. The method further includes adjusting the DC feed in response to determining that the change in the signal is greater than the first preselected value. 
     In another aspect of the present invention, an apparatus is provided for adjusting DC feed control of a connection. The apparatus includes means for receiving a signal from the connection and comparator logic adapted to determine if the change in the signal is greater than a first preselected value. The apparatus further includes means for adjusting the DC feed in response to determining that the change in the signal is greater than the first preselected value. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which: 
     FIG. 1 illustrates a communications system in accordance with the present invention; 
     FIG. 2 depicts a block diagram of an embodiment of an apparatus in accordance with the present invention that may be implemented in the communications system of FIG. 1; 
     FIG. 3 depicts an exemplary DC feed curve that may be employed by the apparatus of FIG. 2; 
     FIG. 4 illustrates a method in accordance with the present invention that may be implemented by the apparatus of FIG. 2; and 
     FIG. 5 depicts a block diagram of an alternative embodiment of an apparatus in accordance with the present invention that may be implemented in the communications system of FIG.  1 . 
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
     Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
     Referring now to the drawings, and in particular to FIG. 1, a communications system  100  in accordance with the present invention is illustrated. The communications system  100  includes a host transceiver  105  and a user transceiver  110  capable of communicating with each other over a connection  115 . The connection  115  may be either a wire-line connection or a wireless connection, depending on the application. An example of a wire-line connection includes a subscriber line, which may comprise a Public Switched Telephone Network (PSTN) line, a Private Branch Exchange (PBX) line, or any other medium capable of transmitting signals. 
     The user transceiver  110  may be a telephonic device capable of supporting pulse dialing. The term “telephonic device,” as utilized herein, includes a telephone, or any other device capable of providing a communication link between at least two users. In one embodiment, the user transceiver  110  may be one of a variety of available conventional telephones, such as cordless telephones, cellular telephones, wired telephones, and the like. In an alternative embodiment, the user transceiver  110  may be any “device” capable of performing a substantially equivalent function of a conventional telephone, which may include, but is not limited to, transmitting and/or receiving voice and data signals. Examples of the user transceiver  110  include a data processing system (DPS) utilizing a modem to perform telephony, a television phone, a wireless local loop, a DPS working in conjunction with a telephone, Internet Protocol (IP) telephony, and the like. IP telephony is a general term for the technologies that use the Internet Protocol&#39;s packet-switched connections to exchange voice, fax, and other forms of information that have traditionally been carried over the dedicated circuit-switched connections of the public switched telephone network (PSTN). One example of IP telephony is an Internet Phone, a software program that runs on a DPS and simulates a conventional phone, allowing an end user to speak through a microphone and hear through DPS speakers. The calls travel over the Internet as packets of data on shared lines, avoiding the tolls of the PSTN. 
     FIG. 2 illustrates an apparatus  210  in accordance with the present invention. In the illustrated embodiment, the host transceiver  105  is a subscriber line audio-processing circuit (SLAC)  215 . Although not necessary, the apparatus  210  of the present invention is implemented within the SLAC  215 . In the interest of clarity and to avoid obscuring the invention, only that portion of the SLAC  215  that is helpful to the understanding of the invention is illustrated. Furthermore, although not shown, those skilled in the art will appreciate that that SLAC  215  may have other components that provide functions analog-to-digital and digital-to-analog conversions, echo suppression, and the like. 
     The SLAC  215  is coupled to the user transceiver  110 , which, in the illustrated embodiment, is a telephonic device  220 . The SLAC  215  interfaces with the telephonic device  220  through a SLIC  221  by the connection  115 , illustrated as a subscriber line  225  in the instant embodiment. Specifically, the connection  115  is a PSTN line that connects the SLAC  215 , which may be located at a central office (CO), to the telephone device  220 , which may be located at a customer premises (CP). The combination of the subscriber line  225  and the telephonic device  220  is commonly referred to as a subscriber loop. The SLIC  221  interfaces with the subscriber line  225  via tip and ring terminals (not shown). The SLIC  221 , which generally includes a driver (not shown) for providing signals to the subscriber loop, may be capable of providing a variety of useful functions, such as battery feed, over-voltage protection, and ringing signal, for interfacing with the telephonic device  220 . 
     The SLAC  215  includes a switch hook detection logic block  230  that is capable of detecting whether the telephonic device  220  is on-hook or off-hook based on the measured electrical parameters, V ab  (voltage between tip and ring terminals) and I mt  (metallic current). For example, the switch hook detection logic block  230  may use equation (1) below for setting a switch-hook threshold to ascertain the switch-hook state of the telephonic device  220 . 
     
       
           V   ab   &lt;I   mt   *T   sh   +V   mk ,  (1) 
       
     
     where T sh  is programmable loop impedance and V mk  is make-voltage parameter utilized to compensate for different phone sets. When V ab  becomes less than I mt *T sh +V mk , an off-hook is detected. Upon detecting the switch-hook state of the telephonic device  220 , the switch hook detection logic block  230  provides the switch-hook information to the system software of the SLAC  215 . 
     The SLIC  221  illustrated in FIG. 2 is a current-feed SLIC in which a current is fed to the subscriber loop and a resulting voltage is measured. It is contemplated that the apparatus  210  of the instant invention may also be employed in conjunction with a voltage-feed SLIC as well, wherein the measured electrical parameter is the loop current after applying a voltage. FIG. 5 illustrates an exemplary voltage-feed SLIC system in accordance with the present invention. 
     Referring back to FIG. 2, the apparatus  210  of the instant invention includes a first comparator logic block  235  that receives the voltage input signal (V ab ) from the tip and ring terminals (not shown) of the SLIC  221 . The V ab  signal is the voltage measured between the trip and ring terminals of the subscriber line  225 . Depending on the value of V ab  or the change in V ab , the first comparator logic block  235  provides a signal to either a low pass filter  240  or a current source  245 . The current source  245  is capable of providing current of a first preselected value. Although not so limited, in the illustrated embodiment, the current source  245  provides 1 mA to the loop. The low pass filter  240  removes substantially all high frequencies and provides a DC signal to a DC feed control  247 . 
     The DC feed control  247  delivers adequate power to the loop by adjusting the current, I dc , in response to the input signal. FIG. 3 illustrates an exemplary DC feed curve that may be adapted for use by the DC feed control  247 . A dashed line  320  provides the upper limits for the electrical power, and a dashed line  325  provides the lower limits the electrical power provided to the subscriber loop. A Y-axis  330  represents voltage, and an X-axis  335  represents current. As can be seen in FIG. 3, although not so limited, the DC feed curve includes an anti-saturation region, a resistance feed region, and current feed region. 
     The DC feed control  247  adjusts the level of the current needed for operation in response to the received voltage, V ab . The DC feed control  247  is not described in detail herein, as it may be implemented in a variety of ways by those of ordinary skill in the art having the benefit of this disclosure. The DC feed control  247  provides I dc  current to a second comparator logic block  250  that determines if I dc  current is less than a preselected threshold value, such as 1 mA, for example. The preselected threshold value may be any value that is sufficient to keep the subscriber loop operational. In the instant embodiment, the preselected threshold value is 1 mA. If I dc  current is less than the preselected threshold value, the current source  245  provides current of the first preselected value ( i.e., 1 mA in the instant embodiment) to the loop. If I dc  current is greater than the preselected threshold value, then the I dc  current is provided to the subscriber loop. 
     FIG. 4 illustrates a method in accordance with the present invention that may be employed by the apparatus  210  of FIG.  2 . The method of FIG. 4 begins at block  400 , where the first comparator logic block  235  receives a signal from subscriber line  225 . In the illustrated embodiment, the first comparator logic block  235  receives the signal V ab  from the subscriber line  225 . At block  410 , the first comparator logic block  235  determines if the change in the V ab  signal is greater than a preselected voltage value, wherein the preselected voltage value is a value that indicates a possible change in the switch-hook state of the telephonic device  220 . In the instant invention, since the SLAC  215  interfaces with the current-feed SLIC  221 , a change in V ab  signal may be utilized to determine a possible change in the switch-hook state of the telephonic device  220 . As such, the preselected voltage value, for example, may be approximately 5 to 10 percent of the battery voltage applied to the subscriber line  225 . Thus, if DC voltage applied to the subscriber line  225  is 50 volts, the preselected voltage value may be 2.5 to 5 volts. In an alternative embodiment, the first comparator logic block  235  may also determine if the V ab  signal is greater than a second preselected voltage value, where the second preselected voltage value is an indication that the telephonic device  220  has gone on-hook. In the current-feed SLAC  215  of FIG. 2, one indication that the telephonic device  220  has gone on-hook is the presence of relatively high voltage level in the signal from the subscriber line  225 . Accordingly, the second preselected voltage value, in one embodiment, may be “V2,” as identified in the DC feed curve of FIG.  3 . The second preselected voltage value may also be a variety of other voltage values, including the voltage range in the anti-saturation region of the DC feed curve. 
     Referring back to the method of FIG. 4, at block  420 , the current source  245  adjusts the DC feed to the subscriber loop in response to determining that the change in the V ab  signal is greater than the first preselected voltage value (5-10% of battery voltage, in the instant embodiment). One way the current source  245  adjusts the DC feed is by providing a relatively low current to the subscriber loop to remove the subscriber loop from a saturation condition. Any of a variety of relatively low current levels can be provided by the current source  245  without departing from the spirit of the instant invention. In the illustrated embodiment, the current source  245  provides about 1 mA current. In an alternative embodiment, the current source  245  adjusts the DC feed to the subscriber loop in response to determining that the change in the V ab  signal is greater than the preselected voltage value (e.g., 5-10% of the battery voltage in the instant embodiment) and in response to determining that V ab  is greater than the second preselected voltage value (e.g., “V2”). A change in V ab , combined with the fact that V ab  is greater than the second preselected voltage value, is an indication that the driver (not shown) of the SLIC  221  may be in saturation. Accordingly, the current source  245  provides a current of a preselected value (e.g., 1 mA current in the instant invention) to the subscriber loop to recover from the saturation condition. 
     FIG. 5 illustrates an alternative embodiment of an apparatus  510  in accordance with the present invention. In the illustrated embodiment, the host transceiver  105  is a SLAC  515  that is coupled to the telephonic device  220  through a current-feed SLIC  516 . The low-pass filter  240  and the switch hook detection logic block  230  are similar to the ones employed in the current-feed SLAC  215  of FIG. 2, as indicated by the like numbering. The apparatus  510  includes a first comparator logic  520  block that receives a current input signal (I mt ) from the subscriber line  225  via the SLIC  516 . Depending on the value of I mt  current, or the change in I mt  current, the first comparator logic block  520  provides a signal to either the low pass filter  240  or a voltage source  525 . The voltage source  525  is capable of providing voltage of a preselected value to the subscriber loop. The low pass filter  240  removes substantially all high frequencies and provides a DC signal to a DC feed control  530 . 
     The DC feed control  530 , based on the input signal, delivers adequate power to the loop by adjusting the voltage provided to the subscriber loop. FIG. 3 illustrates an exemplary DC feed curve that may be adapted for use by the DC feed control  530 . The DC feed control  530  adjusts the level of the voltage needed for operation in response to the received current, I mt . The DC feed control  530  provides a voltage signal to a second comparator logic block  540  that determines if the voltage level is less than a preselected threshold value. The preselected threshold value may be any value that is sufficient to keep the subscriber loop operational. If the voltage level of the signal from the DC feed control  530  is less than the preselected threshold value, the voltage source  525  provides voltage of the preselected value to the loop. If voltage level of the signal from the DC feed control  530  is greater than the preselected value, then the signal from the DC feed control  530  is provided to the subscriber loop. 
     Although the method of FIG. 4 is described with respect to the SLAC  215  of FIG. 2, it is contemplated that the method of the instant mention may be equally applicable to the SLAC  515  that interfaces with the voltage-feed SLIC  516 , as that shown in FIG.  5 . At the block  400 , the first comparator logic block  520  receives a signal (I mt ) from subscriber line  225  via the SLIC  516 . At block  410 , the first comparator logic block  520  determines if the change in the I mt  signal is less than a preselected current value, wherein the preselected current value may be a value that indicates a possible change in the switch-hook state of the telephonic device  220 . Because the SLAC  515  interfaces with a voltage-feed SLIC  516 , a change in the current (I mt ) of the signal from the subscriber line  225  may be utilized to determine a possible change in the switch-hook state of the telephonic device  220 . As such, the preselected current value, for example, may be approximately 5 to 10 percent of the DC current applied to the subscriber line  225 . The first comparator logic block  520  may also, in an alternative embodiment, determine if the I mt  current is greater than a second preselected current value, wherein the second preselected current value is an indication that the telephonic device  220  has gone on-hook. One indication that the telephonic device  220  in FIG. 5 has gone on-hook is a relatively low level of current flowing from the subscriber line. 
     At the block  430 , the source  525  adjusts the DC feed in response to determining that the I mt  change in the signal is greater than the preselected current value. One way the source  525  adjusts the DC feed is by providing a relatively high voltage to the subscriber loop to remove a driver (not shown) of the SLIC  516  from a saturation condition. In alternative embodiment, the current source  525  adjusts the DC feed in response to determining that the change in the I mt  signal is greater than the preselected current value (e.g., 5-10% of applied DC current) and in response to determining that I mt  is smaller than the second preselected value. A change in the I mt , combined with the fact that I mt  is smaller than the second preselected value, is an indication that the driver (not shown) of the SLIC  516  is in saturation. Accordingly, the source  525  provides a voltage of preselected value to the subscriber loop to recover from a saturation condition. 
     The method of FIG. 4 allows the host transceiver  105  to expeditiously recover from a saturation condition (i.e., non-linear state) to a non-saturation condition. Recovering from a saturation condition expeditiously is desirable during switch-hook detection since many telephone systems determine the on-hook or off-hook state of the telephonic device  220  based on the measured loop impedance, which in turn is calculated based on measured voltage and current from the subscriber loop. And since Ohms Law is generally applicable for measuring loop impedance in linear systems, it is desirable to remove the subscriber loop from a saturation condition before an accurate impedance level may be detected. 
     One instance where the subscriber loop goes into saturation mode is during a transition from off-hook to on-hook state. A transition from an off-hook to on-hook state may occur when a user terminates telephone service by engaging the switch hook, or when the telephonic device  220  employs pulse dialing, which generally comprises pulses generated by a sequence of on-hook/off-hook states. The present invention allows for an expedient way of detecting the switch- hook state of the telephonic device  220  by restoring the system to a non-saturation state, such that the loop impedance can be readily detected. 
     Those skilled in the art having the benefit of this disclosure will appreciate that the method of the instant invention may be implemented in software, hardware, or a combination thereof. Additionally, the comparator logic blocks ( 235 ,  250 —see FIG. 2;  520 - 540 —see FIG.  5 ), filter  240 , sources ( 245 —see FIG. 2;  520 —see FIG. 5) and switch hook detection logic  230  may be implemented by those skilled in the art having the benefit of this disclosure using a variety of available components. The SLAC features illustrated in FIG.  2  and FIG. 5 may be implemented within a digital signal processor. 
     The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.