Method and apparatus for ringing

A method and apparatus is provided for ring-trip detection in a line card having an analog-to-digital converter for processing voice signals. The method includes receiving a ringing control signal, transmitting a ringing signal to a subscriber line in response to the ringing control signal, and receiving a portion of the ringing signal from the subscriber line. The method includes converting the portion of the ringing signal to a digital signal using the analog-to-digital converter, and providing a ring-trip indication in response to the digital signal. The apparatus includes first circuitry capable of processing a voice signal, the circuitry including a analog-to-digital converter for processing the voice signal. The apparatus includes a ringing generator, second circuitry, and ring-trip detection logic. The generator is capable providing a ringing signal to a subscriber line in response to receiving a ringing control signal. The second circuitry is capable of delivering the portion of the ringing signal to the analog-to-digital converter of the first circuitry, wherein the analog-to-digital converts the portion of the ringing signal to a digital signal. The ring-trip detection logic is capable of providing a ring-trip indication in response to the digital signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to telecommunications, and, more particularly, to a method and apparatus for ringing a telephonic device.

2. Description of the Related Art

In communications systems, particularly telephony, it is a 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 through a subscriber line. At the subscriber end, a telephonic device may be employed to establish communication with a remote user using the subscriber line. The combination of the telephonic device and the subscriber line is commonly referred to as a subscriber loop.

A line card generally includes at least one subscriber line interface circuit (SLIC) as well as a subscriber line audio-processing circuit (SLAC). The SLIC interfaces with the subscriber loop, and the SLAC interfaces with the SLIC. The SLIC and the SLAC carry out the well-known BORSCHT (Battery feed, Overvoltage protection, Ringing, Supervision, Coding, Hybrid, and Test) functions.

Typically, when an end user initiates a call, the line card provides a ringing AC ringing signal and, often, a DC bias signal, to the subscriber loop to ring the telephonic device. In the United States, the AC ringing signal generally varies from a 16 Hz to 66⅔Hz, although a 20 Hz signal is commonly used. Other countries may employ a ringing signal of a different frequency than that of the ringing signal employed in the United States. For example, in European countries, the ringing signal is 25 Hz. The ringing signal can either be internally or externally generated.

The ringing signals generally tend to be larger signals than the signals utilized for normal voice operations (i.e., during non-ringing mode). For example, during non-ringing mode, the voltage of the signals is generally no more than 50 volts DC, when no current is flowing, In contrast, the ringing signal may be a 80 volt-rms signal that is capable of saturating the voice components of the line card.

SUMMARY OF THE INVENTION

In one aspect of the present invention, a method is provided for ring-trip detection in a line card having an analog-to-digital converter for processing voice signals. The method includes receiving a ringing control signal, transmitting a ringing signal to a subscriber line in response to the ringing control signal, and receiving a portion of the ringing signal from the subscriber line. The method includes converting the portion of the ringing signal to a digital signal using the analog-to-digital converter, and providing a ring-trip indication in response to the digital signal.

In another aspect of the present invention, an apparatus is provided. The apparatus includes first circuitry capable of processing a voice signal, the circuitry including an analog-to-digital converter for processing the voice signal. The apparatus further includes a ringing generator, second circuitry, and ring-trip detection logic. The generator is capable providing a ringing signal to a subscriber line in response to receiving a ringing control signal. The second circuitry is capable of delivering the portion of the ringing signal to the analog-to-digital converter of the first circuitry, wherein the analog-to-digital converts the portion of the ringing signal to a digital signal. The ring-trip detection logic is capable of providing a ring-trip indication in response to the digital signal.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Referring first toFIG. 1, a simplified block diagram of a communications system5in accordance with the present invention is provided. The communications system5includes a line card10that interfaces with a telephonic device12over a subscriber line20. In an actual implementation, the line card10interfaces with a plurality of subscriber lines20, but for clarity and ease of illustration, only one is shown. In accordance with the present invention, the line card10is capable of providing a reliable method of ring-trip detection and AC fault detection based on a received ringing signal and fault-detection signal, respectively. Additionally, the line card10may perform other functions reliably, particularly functions that rely on power calculation.

A subscriber line interface circuit (SLIC)30is coupled to the subscriber line20. Hereinafter, signals received by the line card10over the subscriber line20are referred to as upstream signals, and signals transmitted by the line card10on the subscriber line20are referred to as downstream signals. The SLIC30supplies an analog upstream signal to a coder/decoder (CODEC)40. The CODEC40receives the analog upstream signal from the SLIC30and generates a digital upstream signal that is subsequently passed to a digital signal processor50. The DSP50also provides a digital signal for eventual transmission on the subscriber line20. The CODEC40receives the digital signal, converts it to an analog signal, and provides the analog signal to the SLIC30, which sends the analog signal over the subscriber line20.

In the illustrated embodiment, the line card10, in addition to supporting plain old telephone service (POTS), is adapted to implement an asynchronous digital subscriber line (ADSL) modem for high bandwidth data transfer. The ADSL protocol is described in ANSI T1.413 Issue 2, entitled, “Interface Between Networks and Customer Installation—Asymmetric Digital Subscriber Line (AD SL) Metallic Interface.” The SLIC30of the line card10is capable of performing a variety of functions, such as battery feed, overload protection, polarity reversal, on-hook transmission, and current limiting. Only relevant portions of the SLIC30, CODEC40, and DSP50are described herein, although those of ordinary skill in the art will appreciate that these devices may perform other functions that are not described in this disclosure.

The telephonic device12may comprise a telephone or any other device capable of providing a communication link between at least two users. In one embodiment, the telephonic device12may be one of a variety of available conventional telephones, such as wired telephones and similar devices. In an alternative embodiment, the telephonic device12may 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 telephonic device12include a data processing system (DPS) utilizing a modem to perform telephony, a television phone, 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'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).

FIG. 2illustrates one embodiment of the line card10in accordance with the present invention. Specifically, the line card10includes the SLIC30, which, in the illustrated embodiment, is a voltage-feed SLIC. The line card10also includes the CODEC/DSP40,50, which in the illustrated embodiment are shown as a subscriber line audio-process circuit (SLAC)215that integrates the functions of both the CODEC and DSP40,50. The line card10may be located at a central office or a remote location somewhere between the central office and the telephonic device12(seeFIG. 1). The line card10interfaces with the telephonic device12through tip and ring terminals237,239at the SLIC30. The combination of the telephone device12and the subscriber line20is generally referred to as a subscriber loop.

The impedance of the subscriber line20is herein denoted as ZLOOP, and impedance seen by an incoming signal from the subscriber line20is hereinafter referred to as ZIN. The value of ZLOOP, which is determined by individual telephone authorities in various countries, may be in the range of 600-900 ohms for the POTS band and in the range of 100-135 ohms for the xDSL band. The SLIC30is adapted to be coupled to first and second resistors217,219, which are utilized to define the input impedance.

The line card10, which may be capable of supporting a plurality of subscribers lines20, performs, among other things, two fundamental functions: DC loop supervision and DC feed. The purpose of DC feed is to supply enough power to the telephone device12at the customer end. The purpose of DC loop supervision is to detect changes in DC load, such as on-hook events, off-hook events and rotary dialing, or any other event that causes the DC load to change. In the interest of clarity and to avoid obscuring the invention, only that portion of the line card10that is helpful to an understanding of the invention is illustrated.

The voltage-feed SLIC30is a high voltage bipolar SLIC that drives voltages to the subscriber line20and senses current flow in the subscriber line20. The SLIC30includes first and second differential line drivers230,235that interface with the subscriber line20via tip and ring terminals237,239. The tip terminal237is coupled to a first terminal of a first sensing resistor (Rab)240and to an inverting terminal of the first line driver230. A second terminal of the first sensing resistor240is coupled to an output terminal of the first line driver230. The ring terminal239is coupled to a first terminal of a second sensing resistor (Rbd)242and to an inverting terminal of the second line driver235. A second terminal of the second sensing resistor242is coupled to an output terminal of the second line driver235.

The line card10is adapted to provide external ringing.FIG. 2illustrates a first switch244and second switch245for toggling between internal ringing and external ringing. During external ringing, the first and second switches244,245are in position2, and during normal operation or internal ringing, the switches244,245are in position1. When in position2, the first switch244is coupled to a first terminal of a resistor246, which has a second terminal coupled to a ground node247. The second switch in position2is coupled to a first terminal of a resistor248, which has a second terminal coupled to a first terminal of an external ringing generator249. A second terminal of the external ringing generator249is coupled to the ground node247. For internal ringing, the switches244,245are in position1, and the line card10internally generates a ringing signal and provides it to the subscriber loop20.

The SLIC30includes a sum block250and a current-sensing circuit260. The sum block250includes a first output terminal coupled to a non-inverting terminal of the first line driver230, and a second (inverted) output terminal coupled to a non-inverting terminal of the second line driver235. The sum block250is capable of receiving a DC feed signal (as well as ringing signals) from a DCIN terminal265, a voice signal, a metering signal, and a data signal and is capable of adding one or more of the received signals and providing it to the first and second line drivers230,235. The signals into the SUM block250may be subjected to different levels of gain for optimal performance. The signal from the DCIN terminal265is low-pass filtered.

The current-sensing circuit260produces a current proportional to the current through the current sensing resistors240,242, subtracts a current proportional to a current from a cancellation terminal (CANC)270, a nd provides the resulting (metallic) current to an IMT terminal275of the SLIC30. Although not so limited, in the instant embodiment, the constant of proportionality for the current from the cancellation terminal (CANC)270is unity, and the constant of proportionality for the metallic line current is 0.001. Those skilled in the art will appreciate that only those portions of the SLIC30deemed relevant to the invention are disclosed herein. The SLIC30may employ other circuitry that is not illustrated inFIG. 2.

The SLIC30includes a longitudinal sensing circuit276that provides a current proportional to the current through the current sensing resistors240,242. Specifically, the longitudinal sensing circuit276adds the current flowing through the current sensing resistors240,242, divides the sum by two, and provides the resulting longitudinal current to an ILG terminal277of the SLIC30. Although not so limited, in the instant embodiment, the constant of proportionality for the longitudinal line current is 0.001.

The SLIC30includes a first impedance matching loop278that adjusts a nominal value of the input impedance (ZIN) to substantially match the impedance of the subscriber line20. The first impedance matching loop278includes a nominal Z block279that receives the output signal of the current sensing circuit and provides a selected amount of “fixed” gain and delay to adjust a nominal value of the input impedance, ZIN. In the illustrated embodiment, the nominal Z block279sets the nominal value of the input impedance to a fixed value of 900 ohms, which includes the resistance provided by resistors217,219,240and242.

The SLIC30is connected to the SLAC215as well as to an external resistor280, as well as a capacitor281. In the illustrated embodiment, the resistor280is 100,000 ohms. A first terminal of the resistor280is coupled to the IMT terminal275of the SLIC30, as well as to the VIN terminal285of the SLAC215. A second terminal of the resistor280is coupled to a reference voltage node282, as well as to a terminal of the capacitor281. In one embodiment, the reference voltage282is in the range of about 1.4 volts. The external resistor280and the capacitor281form a single-pole low pass filter283that is capable of filtering at least a portion, if not all, of the signals above the voice band, such as data signals and metering signal. The external resistor280and the capacitor281convert the current flowing from the IMT terminal275to a proportional voltage signal for the SLAC215. Although not necessary, the resistor280is external in the illustrated embodiment because in some embodiments it may be useful for the drive value of the resistor to be relatively precise and because each line card10may require different values.

The ILG terminal277of the SLIC30is connected to a VLG terminal284of the SLAC215as well as to a filter286. The impedance of the filter286converts the current flowing from the ILG terminal277to a proportional voltage signal for the SLAC215. The filter286removes undesirable frequencies such as those above the voice band.

A discrete network288couples the SLIC30to the SLAC215via the CANC terminals270,290. The discrete network288includes a first and second resistor292,294and a capacitor296. A first terminal of the first resistor292is coupled to the CANC terminal270of the SLIC30and a second terminal of the first resistor292is coupled to a first terminal of the second resistor294. The second terminal of the second resistor294is coupled to the CANC terminal290of the SLAC215. The capacitor296is coupled between the second terminal of the first resistor292and the reference voltage node296. The discrete network288acts as a low pass filter and converts the voltage output signal from the SLAC215to a current and provides it to the SLIC30.

The SLAC215interfaces with the telephonic device12through the SLIC30and over the subscriber line20. The SLAC215includes two feedback loops: a DC cancellation loop298and a DC feed loop300. In the illustrated embodiment, the two loops298,300are implemented within a digital signal processor (DSP). Only those portions of the SLAC215deemed relevant to the instant invention are described herein, albeit the SLAC215may perform a variety of other functions that are not illustrated inFIG. 2.

The DC cancellation loop298includes an analog-to-digital converter305, DC cancellation logic315, a current limiter317, and a digital-to-analog converter318, and a switch319. The switch319, during a non-ringing mode, allows an output signal of the digital-to-analog converter318to pass to the CANC215terminal290of the SLAC. In contrast, during a ringing mode, and as is described in more detail below, the switch319couples the VIN and CANC terminals285,290of the SLAC215, thereby disengaging the DC cancellation loop298from the CANC terminal290.

In the illustrated embodiment, to reduce hardwire complexity, the voice and DC components of the input signal from the VIN terminal285share the same analog-to-digital converter305. Additionally, the line card10employs the analog-to-digital converter305for ringing as well as voice processing. The analog-to-digital converter305and digital-to-analog converter318include a decimator and interpolator, respectively. The analog-to-digital converter305in the illustrated embodiment is capable of providing two output signals, the first output signal is sampled at a 4 KHz frequency and provided as a digital signal to the DC cancellation logic315, as well as to a switch hook detection logic320. The second output signal of the analog-to-digital converter305, comprising of voice and/or data (residual) components, is sampled at 32 KHz and provided to a CODEC (not shown). A residual data component may exist at the output of the analog-to-digital converter305since the single-pole low pass filter283may not remove the entire data signal.

During the non-ringing mode, the DC cancellation logic315receives the digital signal from the analog-to-digital converter305, filters high frequencies, and provides substantially a DC signal. The DC signal is provided as an input to the DC feed logic321, as well as to the current limiter317. The output of the current limiter317is converted to an analog signal and then provided back to the SLIC30via the CANC terminal270. The output of the current limiter317is also provided to the switch hook detection logic320for switch hook detection. The current provided to the CANC terminal270of the SLIC30is used to cancel the DC component of the signal from the current sense circuit260. Thus, during a “stable” state (i.e., no transients present), the signal at the VIN terminal285of the SLAC215is essentially DC free.

The DC feed loop300, in addition to the analog-to-digital converter305and DC cancellation logic315, includes DC feed logic321, a switch322, and a digital-to-analog converter325. In the illustrated embodiment, the digital-to-analog converter325may also interpolate. During the non-ringing state, the switch322provides an output signal from the DC feed logic321to the digital-to-analog converter325. However, as will be described in more detail below, during the ringing state, the switch322disengages the output of the DC feed logic321, and, instead, provides a ringing signal generated by a ring generator323to the digital-to-analog converter325. The output from the digital-to-analog converter325is provided to a DCIN terminal265of the SLIC30via VHL terminal326of the SLAC215. The DC feed logic321is capable of providing high DC voltage to the subscriber loop so that sufficient current (20-60 mA) can be driven through a resistance as high as 2K ohms.

When the DC conditions on the subscriber loop change suddenly, the DC feed logic321adapts to the change, thereby allowing normal transmission to continue. Examples of sudden changes in DC conditions include on-hook, off-hook, rotary dialing, and tone signaling. When the telephonic device12goes off-hook, the loop impedance drops almost instantly to a value below 2K ohms. In short subscriber loops, the loop impedance may be less than 200 ohms. For the line card10to function and transmit information properly, the DC conditions on the subscriber loop should be stabilized quickly, and in some cases, within milliseconds.

FIG. 3illustrates an exemplary DC feed curve that may be adapted for use by the DC feed logic321. A dashed line328provides the upper limits for the electrical power, and a dashed line329provides the lower limits for the electrical power provided to the subscriber loop. A Y-axis330represents voltage, and an X-axis335represents current. As can be seen inFIG. 3, although not so limited, the DC feed curve includes an anti-saturation region, a resistance feed region, and a current limit region.

Referring again toFIG. 2, when the line card10is in a “stable” state (i.e., no transients), the signal at the VIN terminal285of the SLAC215comprises primarily a voice signal, although it may include residual metering and data signals that are not removed by the single-pole low pass filter283. This single-pole low pass filter283provides an adequate performance by attenuating the data and metering signals to acceptable levels. Aside from being more cost effective than higher order low-pass filters, the single-pole low pass filter283also provides an added advantage in that it does not make the line card10unstable.

The line card10operates in at least two modes, a non-ringing mode and a ringing mode. A digital interface350, which includes a processor (not shown), controls the operation mode of the line card10. For example, when a remote user places a call to the telephonic device12, the central office instructs the digital interface350to ring the telephonic device12. Accordingly, in response to the request from the central office, the digital interface350provides a ring control signal to switches319,322, as well as to the ring generator323. During the ringing mode, the switch319couples the VIN and CANC terminals285,290of the SLAC215, and the switch322couples the ring generator323to the digital-to-analog converter325, which then converts the ringing signal into a digital signal before it is provided to the subscriber loop30. In contrast, during the non-ring mode, when no ringing control signal is provided, the switches319,322connect the respective DC cancellation and DC feed loops298,300to the respective CANC and VHL terminals290,326of the SLAC215.

In response to receiving the ringing control signal, the ring generator323of the line card10provides an internal ringing signal to the subscriber loop20. Thus, the first and second switches244,245are set to position1. In response to the ringing control signal from the digital interface350, the switch319couples the VIN terminal285to the CANC terminal290of the SLAC215, thereby shielding the DC cancellation loop298from high voltages and currents commonly associated with ringing signals. Typically, for voice and data operation, the current may reach 60 mA DC, which is a current level that may be cancelled by the DC cancellation loop298. In contrast, a ringing signal, which commonly comprises a 20 Hz signal along with a DC offset, may reach a peak current of over 100 mA, a level that is too high for the DC cancellation loop298. Accordingly, connecting the VIN and CANC terminals285,290of the SLAC215aids in lowering the current level to the DC cancellation loop298.

The impedance provided by the discrete network288at the CANC terminal290of the SLAC215is relatively low, approximately 16 to 17 K ohms. So, when the VIN and CANC terminals285,290are shorted, the relatively low impedance of the discrete network288lowers the impedance seen at the VIN terminal285, which is set primarily by the 100,000-ohm resistor280. This is because adding a high and low impedance in parallel has a net effect of lowering the impedance. As a result of lower impedance, the voltage level present at the VIN terminal285during the ringing mode is generally at a lower level. Furthermore, the current sensing circuit260of the SLIC30aids in further reducing the voltage level at the VIN terminal285, perhaps by half. This is because the current flowing from the VIN terminal285to the CANC terminal290is subtracted from the sensed line current in the SLIC30by the current sensing circuit260. By lowering the impedance during the ringing mode, the line card10is able to handle currents of higher level, typically up to 130 mA.

In essence, shorting the VIN and CANC terminals285,290allows the line card10to toggle between a low-current mode to a high-current mode. The low-current mode is the non-ringing mode where the current level is under about 61 mA. The high-current mode is the ringing mode where currents in may range up to 130 mA. Without the VIN and CANC terminals285,290shorted, the line card100can support a current of approximately 61 mA, whereas with the terminals285,290shorted, the line card10can handle a current of approximately 130 mA, which is adequate for ringing.

The SLAC215includes AC fault detection logic355and ring-trip detection logic365. The ring-trip detection logic365is capable of receiving the output signal of the analog-to-digital converter305and providing a ring trip indication to the digital interface350. The ring-trip detection logic365compares its input signal against a threshold value. And if the input signal exceeds the threshold value, the ring-trip detection logic365indicates so to the digital interface365. Upon receiving a ring trip indication from the ring-trip detection logic360, the digital interface350terminates the ringing, and the line card thereafter operates in the non-ringing mode until a next ringing signal is transmitted by the digital interface350. The AC fault detection logic355and ring-trip detection logic360are shown as functional blocks inFIG. 2for illustrative purposes only. It should be appreciated that in actual implementation these blocks are implemented in software within the digital signal processor50(seeFIG. 1).

Referring now toFIG. 4, a method is shown for ring-trip detection in the line card10using a common digital-to-analog converter305for voice processing as well as ring-trip detection. The method begins at block605, where the ringing generator323receives a ringing control signal from the digital interface350. In response to the ringing signal, the switch319shorts the VIN and CANC terminals285,290of the SLAC215. Additionally, the switch connects the ringing generator323to the digital-to-analog converter325. At block610, the ringing generator323transmits a ringing signal to the subscriber line30in response to receiving the ringing control signal.

At block620, the line card10receives a portion of the ringing signal from the subscriber line30. The ringing signal is provided to the VIN terminal285of the SLAC215and then to the digital-to-analog converter305. At block625, the analog-to-digital converter305converts the received ringing signal from the VIN terminal to a digital signal and provides it to the ring-trip detection logic360. The instant invention is able to utilize a common analog-to-digital converter for voice processing as well as for ringing. At block630, the ring-trip detection logic360provides a ring-trip indication in response to the digital signal.