Patent Publication Number: US-7218729-B2

Title: Subscriber line interface circuitry with current drivers for downstream voice and data signals

Description:
FIELD OF THE INVENTION 
   This invention relates to the field of telecommunications. In particular, this invention is drawn to subscriber line interface circuitry. 
   BACKGROUND 
   The plain old telephone system (POTS) was initially architected to carry voice data in analog form from one subscriber to another via configurable switches. Although the telephone network evolved to using a digital transport network (i.e., the Public Switched Telephone Network (PSTN)), communication on the subscriber line connecting subscribers to the central office that serves as the entry point to the PSTN is analog. The “last mile” between the subscriber and the central office was architected for analog communications in the voiceband frequency range. 
   Although modems were developed to enable communicating digital data using the same analog channel used to carry analog voice data, the digital data rates between the subscriber and central office were relatively low due to the constraints of operating exclusively within the voiceband region of the spectrum. Numerous communication protocol standards have since developed to enable using the POTS infrastructure for communicating digital data at higher data rates by utilizing communication bandwidth beyond the voiceband. For example, digital subscriber line (xDSL) services utilize communication bandwidth beyond and exclusive to the voiceband. As a result, xDSL services may co-exist with voiceband communications. Co-location of access to digital networks other than the PSTN at the central office enables leveraging the POTS infrastructure investment in the subscriber line. 
   The central offices may be retrofitted to permit sharing the subscriber line between multiple services such as voice and digital services. Typically this is accomplished by adding digital service specific linecards and a discrete component splitter for each subscriber line so that the subscriber line can be communicatively coupled to both the existing POTS linecard and the added digital service linecard. The discrete component splitters are relatively expensive and represent a significant cost for a large number of subscriber lines. 
   This approach permits leveraging the existing infrastructure at the central office including the existing POTS linecards through the addition of the splitters and digital services linecards rather than the replacement of equipment. However, the additional equipment consumes valuable space within the central office that might otherwise be used for POTS linecards and additional subscriber lines. Thus this approach may not be particularly desirable for new facilities when preservation of existing capital equipment is not an issue. 
   SUMMARY 
   In one embodiment, a subscriber line interface circuit apparatus includes driver circuitry for combining and driving a downstream voice signal and a downstream data signal onto a subscriber line. Receiver circuitry receives and separates an upstream signal from the subscriber line into an upstream voice signal and an upstream data signal. The driver and receiver circuitry reside on a common integrated circuit die. 
   Another subscriber line interface circuit apparatus includes driver circuitry for combining and driving a downstream voice signal, a metering signal and a downstream data signal onto a subscriber line. The apparatus includes receiver circuitry for receiving and separating an upstream signal from the subscriber line into an upstream voice signal and an upstream data signal. The driver and receiver circuitry reside on a common integrated circuit die. 
   In one embodiment, a subscriber line interface circuit apparatus includes first driver circuitry for driving a downstream data signal onto a subscriber line and second driver circuitry for combining and driving a metering signal and a downstream voice signal onto the subscriber line. The apparatus includes receiver circuitry for receiving and separating an upstream signal from the subscriber line into an upstream voice signal and an upstream data signal. The receiver circuitry and the first driver circuitry for driving the downstream data signal reside on the same first integrated circuit die exclusive of the second driver circuitry. 
   In one embodiment, a subscriber line interface circuit apparatus includes first driver circuitry for combining and driving a downstream data signal and a metering signal onto a subscriber line. The apparatus includes second driver circuitry for driving a downstream voice signal onto the subscriber line. The apparatus further includes receiver circuitry for receiving and separating an upstream signal from the subscriber line into an upstream voice signal and an upstream data signal. The receiver circuitry and the first driver circuitry for driving the downstream data signal reside on the same first integrated circuit die exclusive of the second driver circuitry. 
   In one embodiment, a subscriber line driver apparatus includes an impedance bridge coupled to a subscriber line. A first pair of current drivers is coupled to the impedance bridge for driving a voice signal in a first frequency range onto the subscriber line. A second pair of current drivers is coupled to the impedance bridge for driving a data signal in a second frequency range onto the subscriber line. The impedance bridge couples the second pair of current drivers to the subscriber line across an impedance of Z 1 +Z 2  within the second frequency range. 
   Other features and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description that follows below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which: 
       FIG. 1  illustrates one embodiment of a plain old telephone system (POTS) communication architecture. 
       FIG. 2  illustrates one embodiment of a communication spectrum allocation for a subscriber line. 
       FIG. 3  illustrates one embodiment of prior art interface circuitry for supporting digital and voice data on the same subscriber line. 
       FIG. 4A  illustrates separate DSL and POTS linecards coupled to a subscriber line. 
       FIG. 4B  illustrates one embodiment of an integrated POTS and DSL linecard. 
       FIG. 5  illustrates one embodiment of an integrated digital and voice data transceiver architecture. 
       FIG. 6A  illustrates an alternative embodiment of an integrated digital and voice transceiver architecture. 
       FIG. 6B  illustrates one embodiment of an integrated digital and voice data transceiver architecture with capacitive bypass coupling for the receiver circuitry. 
       FIG. 7A  illustrates an alternative embodiment of an integrated digital and voice transceiver architecture. 
       FIG. 7B  illustrates one embodiment of an integrated digital and voice data transceiver architecture with capacitive bypass coupling for the receiver circuitry. 
       FIG. 8  illustrates one embodiment of a current driver for digital and voice transceiver. 
       FIG. 9A  illustrates the circuitry of  FIG. 8  from the perspective of the voiceband driver. 
       FIG. 9B  illustrates the circuitry of  FIG. 8  from the perspective of the DSL driver. 
       FIG. 10  illustrates one embodiment of a SLIC. 
   

   DETAILED DESCRIPTION 
     FIG. 1  illustrates one embodiment of a prior art communications network model supporting voiceband communications associated with plain old telephone services (POTS) telephone system. The network model is divided into three physical domains: network service provider(s)  102 , network access providers  104 , and customer premises  106 . 
   The network service providers (NSP) may have networks that span large geographic areas. Typically, however, the customer premises (CP) must be located within a specified distance of the network access provider (NAP) as a result of electrical specifications on the subscriber line  190 . Thus network access providers typically have a number of central offices (CO) that support customers within a specified radius. Local exchange carriers (LEC) and competitive local exchange carriers (CLEC) are examples of network access providers. 
   In one embodiment, the network access provider is a telephone company. Subscriber equipment (i.e., customer premises equipment such as telephones  170 ,  172 ) is connected to a central office (CO) of the network access provider  104  via a subscriber line  190 . For POTS systems, the subscriber line includes a tip line and a ring line that are typically implemented as an unshielded twisted copper wire pair. 
   The central office has numerous POTS linecards  128  for supporting multiple subscriber lines. Each linecard has at least one subscriber line interface circuit (SLIC)  130  that serves as an interface between a digital switching access network  120  of a local telephone company central office and the subscriber equipment  170 ,  172 . In some embodiments, each linecard has a plurality of SLICs. The access network provides the SLIC and associated subscriber with access to the PSTN  110  for bi-directional communication with other subscribers similarly coupled to the PSTN. 
     FIG. 10  illustrates one embodiment of a SLIC  1010  coupled to subscriber equipment  1070  by a subscriber line comprised of tip line  1096  and ring line  1098 . The tip line  1096 , subscriber equipment  1070 , and ring line  1098  form a subscriber loop  1090 . The POTS standards establish the electrical specifications and communication protocols for voiceband communications carried by the subscriber line. 
   The SLIC receives downstream digital voiceband data from digital network  1020  (e.g., the PSTN) on a downstream data path  1024  for conversion and communication to the subscriber equipment  1070 . The SLIC receives upstream analog voiceband data from subscriber equipment  1070  for conversion and communication to the digital network  1020  on upstream data path  1022 . 
   The SLIC is expected to perform a number of functions often collectively referred to as the BORSCHT requirements. BORSCHT is an acronym for “battery feed,” “overvoltage protection,” “ring,” “supervision,” “codec,” “hybrid,” and “test” (e.g., loop diagnostics). 
   Referring to  FIG. 1 , the SLIC provides power to the subscriber equipment  170 ,  172  using the battery feed function. The overvoltage protection function serves to protect the central office circuitry against voltage transients that may occur on the subscriber line  190 . The ringing function enables the SLIC to signal the subscriber equipment  170 ,  172  (e.g., ringing a telephone). 
   The supervision function enables the SLIC to detect subscriber equipment service requests such as when the caller goes “off-hook”. The supervision function is also used to supervise calls in progress and to detect dialing input signals. 
   The hybrid function provides a conversion from two-wire signaling to four-wire signaling. The transmit path (downstream to subscriber) and receive path (upstream from subscriber) share the same physical lines of the subscriber loop. Given that the upstream signal from the subscriber and the downstream signal from the SLIC share the same subscriber line for communication, the hybrid function typically performs some form of cancellation to remove the downstream signal from the sensed subscriber line in order to distinguish the upstream signal from other signals on the subscriber line. 
   The SLIC includes a codec to convert the upstream analog voiceband data signal into serial digital codes suitable for transmission by the digital switching network  110 . In one embodiment, pulse code modulation is used to encode the voiceband data. The codec also converts the digital downstream voiceband data from serial digital codes to analog signals suitable for downstream transmission on the subscriber line to the subscriber equipment. The SLIC also typically provides a means to test for faults that may exist in the subscriber loop or within the SLIC itself. 
   Historically, the network access providers served to connect customers or subscribers to the PSTN for voiceband communications (communications having an analog bandwidth of approximately 4 kHz or less). Although the PSTN is digital in nature, the connection (subscriber line  190 ) between the customer premises  106  and the network access provider  104  is analog. 
   The subscriber line may be provisioned for additional services by using communication bandwidth beyond the voiceband. Thus, for example, digital subscriber line services may simultaneously co-exist with voiceband communications by using communication bandwidth other than the voiceband. The choice of frequency ranges and line codes for these additional services is the subject of various standards. The International Telecommunication Union (ITU), for example, has set forth a series of recommendations for subscriber line data transmission. These recommendations are directed towards communications using the voiceband portion of the communications spectrum (“V.x” recommendations) as well as communications utilizing frequency spectrum other than the voiceband portion (e.g., “xDSL” recommendations). Various examples of line code standards include quadrature amplitude and phase modulation, discrete multi-tone modulation, carrierless amplitude phase modulation, and two binary one quaternary (2B1Q). 
   Asymmetric digital subscriber line (ADSL) communications represent one variant of xDSL communications. Exemplary ADSL specifications are set forth in “Rec. G.992.1 (06/99)—Asymmetric digital subscriber line (ADSL) transceivers” (also referred to as full rate ADSL), and “Rec. G.992.2 (06/99)—Splitterless asymmetric digital subscriber line (ADSL) transceivers” (also referred to as G.LITE). 
     FIG. 2  illustrates one embodiment of communication spectrum allocation for a subscriber line. Chart  200  compares the portions of the analog channel for voiceband applications (POTS  210 ) as well as digital services (e.g., ADSL  230 ). POTS communications typically use the voiceband range of 300–4000 Hz. One xDSL variant uses frequencies beyond the voiceband in the range of approximately 25–1100 kHz as indicated. A guard band  220  separates the POTS and ADSL ranges. 
   There are multiple line coding variations for xDSL. Carrierless Amplitude Phase (CAP) modulation and Discrete Multi-Tone (DMT) modulation both use the fundamental techniques of quadrature amplitude modulation (QAM). CAP is a single carrier protocol where the carrier is suppressed before transmission and reconstructed at the receiving end. DMT is a multicarrier protocol.  FIG. 2  illustrates DMT line coding. 
   DMT modulation has been established as a standard line code for ADSL communication. The available ADSL bandwidth is divided into 256 sub-channels. Each sub-channel  234  is associated with a carrier. The carriers (also referred to as tones) are spaced 4.3125 KHz apart. Each sub-channel is modulated using quadrature amplitude modulation (QAM) and can carry 0–15 bits/Hz. The actual number of bits is allocated depending upon line conditions. Thus individual sub-channels may be carrying different numbers of bits/Hz. Some sub-channels  236  might not be used at all. 
   ADSL uses some sub-channels  234  for downstream communication and other sub-channels  232  for upstream communication. The upstream and downstream sub-channels may be separated by another guard band  240 . ADSL is named for the asymmetry in bandwidth allocated to upstream compared to the bandwidth allocated to downstream communication. 
   During initialization the signal-to-noise ratio of each DMT sub-channel is measured to determine an appropriate data rate assignment. Generally, greater data rates (i.e., more bits/Hz) are assigned to the lower sub-channels because signals are attenuated more at higher frequencies. DMT implementations may also incorporate rate adaption to monitor the line conditions and dynamically change the data rate for sub-channels. 
   xDSL can be provisioned using the same subscriber line as that used for standard POTS communications thus leveraging existing infrastructure. The availability of xDSL technology permits delivery of additional services to the subscriber. 
     FIG. 3  illustrates an embodiment of a communications network model supporting voice and digital services (e.g., xDSL) on a common subscriber line  390 . Various digital services may utilize different encoding algorithms (e.g., two binary one quaternary (2B1Q)). The POTS subscriber equipment such as telephones  370 ,  372  are connected to a POTS SLIC  330  residing on a POTS linecard  328  via subscriber line  390 . The NAP access network  320  couples the POTS linecard to a voice service provider network  310  such as the PSTN. 
   A digital subscriber line access multiplexer (DSLAM)  342  has a plurality of DSL linecards  340 . The access network  320  enables communication with digital network service providers such as Internet protocol (IP) service providers  312  and asynchronous transfer mode (ATM) service providers  314 . A DSLAM linecard provides a connection from one of the digital networks via access network  320  to the subscriber line  390  through the use of a central office splitter  344 . 
   The splitter  344  serves to route the appropriate portion of the analog channel of the subscriber line  390  to one of the DSL linecard  340  and the POTS linecard  328 . In particular, the splitter filters out the digital portion of the subscriber line communications for the POTS linecard  328 . The splitter filters out the voiceband communications for the DSL linecard  340 . The splitter also protects the DSL linecard from the large transients and control signals associated with the POTS communications on the subscriber line. 
   The CO splitter thus effectively splits upstream communications from the subscriber equipment into at least two spectral ranges: voiceband and non-voiceband. The upstream voiceband range is provided to the POTS linecard and the upstream non-voiceband range is provided to the DSL linecard. The splitter couples the distinctly originating downstream voiceband and downstream non-voiceband communications to a common physical subscriber line  390 . 
   A customer premises equipment splitter  354  may also be required at the customer premises for the POTS subscriber equipment  370 ,  372 . The CPE splitter  354  passes only the voiceband portion of the subscriber line communications to the POTS subscriber equipment. 
   In one embodiment, the CPE splitter provides the DSL communications to a DSL modem  350  that serves as a communications interface for digital subscriber equipment such as computers  360 ,  362 . In one embodiment, the DSL modem includes router functionality. 
   The DSL service overlays the existing POTS service on the same subscriber line. This solution avoids the capital costs of placing dedicated digital subscriber lines and permits utilizing existing POTS linecards. This approach is undesirable, however, for new installations (i.e., in the absence of pre-existing POTS linecard capital costs) because of the significant additional space required for the DSLAM and DSL linecards. Moreover, although some xDSL variants permit the elimination of the CPE splitter with the tradeoff of lower digital communication rates, the approach of  FIG. 3  still require a costly, space consuming discrete component central office splitter  344 . 
   A number of integrated voice and data SLIC architectures are proposed to permit provisioning both the DSL and POTS hybrid functionality from a single linecard without the requirement for an expensive discrete component CO splitter.  FIG. 4A  illustrates the functional components of separate DSL and POTS linecards.  FIG. 4B  illustrates functional components of an integrated DSL and POTS linecard. 
   Referring to  FIG. 4A , elements  442 ,  444 ,  446 , and  448  form an xDSL modem for bi-directional communication of information between the subscriber line  490  and a digital services provider  412 . xDSL digital signal processor  442  provides a digital downstream data signal received from the digital services provider to digital-to-analog converter (DAC)  446  that converts the digital data signal to analog form. DAC  446  provides the resulting analog signal to the xDSL transceiver  448 . The analog downstream data signal from transceiver  448  is combined with the analog downstream voice signal onto the subscriber line using splitter  480 . 
   Splitter  480  provides xDSL transceiver  448  with a subscriber line analog signal that has been filtered to eliminate voiceband spectral components resulting in an analog data signal including the analog upstream data signal. The upstream analog data signal is provided to an analog-to-digital converter (ADC)  444  that converts the analog information to digital form. ADC  444  provides the resulting digital signal to the xDSL digital signal processor (DSP)  442  for upstream communication to the digital services provider. 
   xDSL transceiver  448  is responsible for both driving downstream data signals and receiving upstream data signals from the subscriber line. xDSL transceiver  448  performs the hybrid function of splitting the two-wire bi-directional communication path into an upstream path (xDSL transceiver, ADC, xDSL DSP) and a downstream path (xDSL DSP, DAC, xDSL transceiver). CO splitter  480  provides a high pass filter to prevent POTS related signals (e.g., analog voiceband signals, DC battery feed, ringing, etc.) from reaching the xDSL transceiver. 
   Given that the upstream and downstream xDSL communications share the same transmission line from the subscriber, the DSL linecard may include a cancellation function to eliminate the downstream DSL communications from the upstream path. This cancellation function may be performed within the transceiver itself or by another component such as xDSL DSP  442 . 
   Voice digital signal processor  432  provides digital downstream information received from the voice digital services provider (e.g., PSTN  410 ) to a digital-to-analog converter (DAC)  436  that converts the digital voiceband signals to analog form. DAC  436  provides the resulting analog signal to the POTS transceiver  438  for downstream communication to the subscriber. POTS transceiver  438  receives analog information from the subscriber line and provides it to an analog-to-digital converter (ADC)  434  that converts the analog information to digital form. ADC  434  provides the resulting digital signal to voice DSP  432  for upstream communication to the PSTN  410 . 
   POTS transceiver  438  is responsible for both driving downstream voiceband communications and receiving upstream voiceband communications from the subscriber line. POTS transceiver  438  performs the hybrid function of splitting the two-wire bi-directional communication path into an upstream path (POTS transceiver, ADC, voice DSP) and a downstream path (voice DSP, DAC, POTS transceiver). CO splitter  480  provides a low pass filter to prevent high frequency signals due to the xDSL communications on the subscriber line from reaching the POTS transceiver. 
   Given that the upstream and downstream voiceband communications share the same transmission line from the subscriber, the POTS linecard may include a cancellation function to eliminate the downstream voiceband communications from the upstream path. This cancellation function may be performed within the transceiver itself or by another component such as voice DSP  432 . 
   The POTS transceiver may also be referred to as a subscriber line interface circuit (SLIC). Modern SLICs, however, may include additional functions. For symmetry with the xDSL functional blocks illustrated as DSL linecard  440 , element  438  is referred to as a POTS transceiver. 
     FIG. 4B  illustrates integration of POTS and xDSL services onto a single linecard  452 . Broadband transceiver  450  provides the function of both the xDSL transceiver and the POTS transceiver. 
   xDSL DSP  442  provides downstream digital information from a first digital service network (e.g., IP service provider) to DAC  446  which converts the digital information to analog form for downstream communications using broadband transceiver  450 . Similarly, voice DSP  432  provides downstream digital information from a second digital service network (e.g., PSTN) to DAC  436  that converts the digital information to analog form for downstream communications using broadband transceiver  450 . 
   Broadband transceiver  450  receives analog signals from the subscriber line. Broadband transceiver  450  separates the analog signals into an xDSL upstream data path (high frequency) and a POTS upstream voice path (low frequency). Analog information on the upstream data path is provided to ADC  444  that converts the analog information to digital form for upstream communications to the first digital network using xDSL DSP  442 . Similarly, analog information on the voice path is provided to ADC  434  that converts the analog information to digital form for upstream communication to the second digital network using voice DSP  432 . 
   The broadband transceiver may be embodied as one or more integrated circuits. Each proposed transceiver architecture provides for upstream and downstream voice paths and upstream and downstream data paths. In particular, each proposed architecture incorporates one of the upstream/downstream voice path and upstream/downstream data path onto the same integrated circuit. 
     FIG. 5  illustrates one embodiment of a broadband transceiver wherein the POTS and DSL transceiver circuitry reside on the same integrated circuit die  502 . Due to the symmetry of the transceiver circuitry, only single-ended circuitry is illustrated for clarity. Thus, in these examples, although the subscriber loop still requires a tip and a ring line, the differential signaling architecture is modeled as a single-sided source-to-ground architecture for clarity. Thus the illustrated subscriber line  590  comprises only one of the tip and ring lines because the other line is ground. 
   Integrated circuit die  502  provides for POTS functionality such as metering  560 , downstream voice  570 , and upstream voice  572 . Integrated circuit die  502  also provides for DSL functionality including downstream DSL  580  and upstream DSL  582 . 
   Metering  560  is used primarily for timing toll calls. In one embodiment, the metering signal is approximately a 15-volt peak-to-peak signal with a frequency of 12 kHz or 16 kHz. The voice signals  570 ,  572  are approximately 3 volts peak-to-peak. DSL signaling is approximately 34 volts peak-to-peak downstream and 14 volts peak-to-peak upstream. A DC supply of at least 48 volts is typically required to support the ringing function for long loops. The ringing signal may be 150 volts peak-to-peak. 
   Downstream driver  520  drives the subscriber equipment modeled as R_LOAD  526  with weighted versions of the analog metering signal  560 , analog downstream voice signal  570 , and analog downstream DSL signal  580 . Downstream driver  520  is thus a summing amplifier. Weights W 1 –W 4  control the relative weighting of the input signals to summing amplifier  520 . Downstream driver  520  effectively superimposes the downstream metering, voice, and DSL signals on top of each other in proportions determined by the impedances of weights W 1 –W 4 . 
   In one embodiment, downstream low pass filter  510  is provided to ensure frequencies within the downstream voice signal  570  do not exceed the bandwidth allocated for voiceband communications. Similarly, high pass filter  512  is provided to ensure that downstream DSL signal  580  does not contain spectral components within the bandwidth allocated for voiceband communications. 
   In the illustrated embodiment, the hybrid function for separating the two-wire bi-directional communication path into a 2-wire downstream and a 2-wire upstream path (4-wire total) for each of the voice and data communications is performed in the same integrated circuit. Given that the upstream and downstream POTS communications share the same transmission line, any signals received from the subscriber line inherently include both the upstream signal communicated by the subscriber equipment and the downstream signal transmitted by driver  520 . Accordingly, the downstream signal must be canceled or removed from the received signal to exclusively identify the upstream signal attributable to the subscriber equipment. 
   Referring to  FIG. 4B , this cancellation function may be incorporated into the transceiver  438  or into a functional block further upstream such as voice DSP  432 . The term “upstream” will be used to refer to both the upstream signal before and after the cancellation of any included downstream signal. 
   Integrated circuit die  502  includes receiver circuitry coupled to receive the analog signals on the subscriber line. The receiver circuitry includes upstream voice driver  530  associated with the upstream voice path and upstream data driver  533  associated with the upstream data path. 
   Each of drivers  530 ,  533  is coupled to receive the upstream analog signal from the subscriber line  590  (i.e., at node  523 ). Each upstream driver is also provided with the output of downstream driver  520  on the other side of R_SENSE  522  at node  525  to permit elimination of the downstream data or voice signals from the upstream signal using hybrid filters H 2  and H 1 , respectively. 
   Upstream voice driver  530  is provided with the analog signal appearing on the subscriber line at node  523 . At node  525 , the output of the downstream driver  520  is coupled to the upstream voice driver through hybrid filter H 1  to permit cancellation of the downstream voice signal from the received analog signal. 
   Metering cancellation circuitry (elements  535  and W 5 ) is provided to remove the metering signal  560  from the upstream voice signal. In one embodiment, W 5  is an adaptive finite impulse response (FIR) filter. Upstream low pass filter  532  eliminates substantially all of any high frequency components attributable to the DSL services from the upstream voice signal  572 . Feedback element  536  and weight W 4  provide a mechanism for voiceband impedance synthesis to meet POTS requirements. 
   The upstream DSL driver  533  is provided with the analog signal appearing on the subscriber line at node  523 . At node  525 , the output of the downstream driver  520  is coupled to the upstream DSL driver through hybrid filter H 2  to permit cancellation of the downstream DSL signal from the received analog signal. 
   Upstream high pass filter  534  eliminates substantially all of any low frequency spectral components including the metering signal  560  attributable to voiceband communications. Although hybrid filter H 1  eliminates some of the metering signal, the high pass filter  534  provides greater rejection of the metering signal in the upstream signal. 
   This architecture eliminates the use of an external central office splitter. Moreover, DSL and POTS functionality can be provided with the same line card. One disadvantage of the integrated voice and data (IVD) transceiver of  FIG. 5 , however, is the semiconductor fabrication process required for the manufacture of such an integrated circuit die  502 . In particular, a high voltage process (˜150 volts) is required. The number of semiconductor fabricators capable of manufacturing such an integrated circuit die is extremely limited at this time. 
     FIG. 6A  illustrates an alternative integrated voice and data transceiver architecture. Transceiver circuitry for handling upstream and downstream communications is distributed across more than one integrated circuit die  602 ,  604 . A few discrete components  692  are provided to permit the use of a low voltage integrated circuit die  604  for driving the downstream voice, metering, and ringing signals. The downstream high pass filter  612  for the downstream DSL signal  680  is included in the same integrated circuit die  604 . In one embodiment, integrated circuit die  604  is fabricated with a low voltage process. The DSL driver is located on integrated circuit die  602 . In one embodiment, integrated circuit die  602  is fabricated with a medium voltage process (˜40 volts). 
   Discrete component driver  692  drives the downstream voice, metering, and ringing signals onto subscriber line  690 . Downstream voice  670  passes through downstream low pass filter  610  before being provided to the discrete component driver  692  to ensure that the POTS voice signal does not introduce any high frequency signals that might disrupt the DSL communications. The metering signal  660  and ringing signal  662  are weighted by weights W 1  and W 2 , respectively, before being provided to the discrete component driver  692 . The DSL downstream signal  680  is provided to downstream driver  620  after weighting (W 3 ) and downstream high pass filter  612 . The output of driver  620  is coupled to the subscriber line  690  via transformer  624 . Downstream low pass filter  610 , weights W 1 , W 2 , and downstream high pass filter  612  are located on the same integrated circuit die  604 . 
   The upstream DSL signal  682  is extracted by tapping and filtering the subscriber line  690  on the transceiver side of transformer  624  at  623 . Upstream DSL driver  633  is also provided with the output of downstream driver  620  on the other side of R_SENSE  622  at node  625  to permit elimination of the downstream data or voice signals from the upstream signal using hybrid filter H 2 . Upstream high pass filter  634  eliminates the metering signal  660  and the low frequency spectral components associated with voiceband communications. 
   Upstream voice driver  630  senses the subscriber line across R_SENSE  622  to provide an upstream voice signal. Upstream voice driver  630  is also provided with the output of downstream driver  620  on the other side of R_SENSE  622  at node  625  to permit elimination of the downstream data or voice signals from the upstream signal using hybrid filters H 1 . Metering cancellation circuitry (elements  635 , W 5 ) is provided to at least remove the downstream metering signal from the upstream voice signal  672 . In one embodiment, W 5  is an adaptive finite impulse response (FIR) filter. Upstream low pass filter  632  eliminates substantially all the high frequency components attributable to the DSL services from the upstream voice signal  672 . Feedback element  636  and weight W 4  provide a mechanism for voiceband impedance synthesis to meet POTS requirements. 
   The transceiver components associated with the upstream signal paths for both data and voice signals are formed on the same integrated circuit die  602 . Although the driver circuitry for the downstream data signal resides on integrated circuit die  602 , the downstream high pass filter  612 , weights W 1 , W 2  associated with metering and ringing, and downstream low pass filter  610  reside on a distinct second integrated circuit die  604 . 
   Transformers tend to have poor low frequency response. In various embodiments, the upstream voice, upstream DSL, or both are sensed from the subscriber line via a bypass capacitor.  FIG. 6B  illustrates one embodiment with bypass capacitors C 1  and C 2  for sensing the upstream voice and upstream DSL signals, respectively. In the illustrated embodiment, the upstream voice and upstream DSL signals are sensed on the subscriber line side of transformer  624  rather than at node  623 . Capacitor C 1  permits sensing the upstream DSL signal without degradation due to transformer  624 . Capacitor C 2  permits sensing the upstream DSL signal without degradation due to transformer  624 . The remaining elements function as described with respect to  FIG. 6A . 
     FIG. 7A  illustrates an alternative integrated voice and data transceiver architecture. Transceiver circuitry for handling upstream and downstream communications is distributed across more than one integrated circuit die  702 ,  704 . The transceiver components associated with the upstream signal paths for both data and voice signals are formed on the same integrated circuit die  702 . Although the driver circuitry for the downstream data signal and metering signal resides on integrated circuit die  702 , the downstream high pass filter  712  for the downstream DSL signal resides on integrated circuit die  704 . 
   The primary distinction between the architecture of  FIG. 6A  and that illustrated in  FIG. 7A  is that the weighted metering signal  760  is provided to downstream driver  720  rather than discrete component driver  792 . Weight W 1  for the metering signal also resides within integrated circuit die  702  rather than integrated circuit die  704 . As with the architecture of  FIG. 6A , integrated circuit die  704  may be fabricated with a low voltage process. Integrated circuit die  702  may be fabricated with a medium voltage process (˜40 volts). 
   Downstream driver  720  and discrete component driver  792  drive R_LOAD  726 . The output of driver  720  is coupled to the subscriber line  790  via transformer  724 . The discrete component driver  792  is directly coupled rather than transformer coupled to the subscriber line. 
   Discrete component driver  792  permits the use of a low voltage integrated circuit die  704  for driving the downstream voice signal and the ringing signal onto the subscriber line  790 . Downstream low pass filter  710 , weight W 2  for the ringing signal, and downstream high pass filter  712  for the downstream DSL signal  780  are located on the same integrated circuit die  704 . The downstream driver  720  for the metering signal  760  and the downstream DSL signal  780  are located on integrated circuit die  702 . 
   The downstream voice signal  770  passes through downstream low pass filter  710  before being provided to the discrete component driver  792  to ensure that the POTS voice signal does not introduce any high frequency signals that might disrupt the DSL communications. The ringing signal  762  is weighted by weight W 2  before being provided to the discrete component driver  792 . The DSL downstream signal  780  is provided to downstream driver  720  after weighting (W 3 ) and downstream high pass filter  712 . The metering signal  760  is weighted by W 1  before being provided to the downstream driver  720 . 
   The upstream DSL signal  782  is extracted by tapping and filtering the subscriber line  790  on the transceiver side of transformer  724  at  723 . Upstream DSL driver  733  is also provided with the output of downstream driver  720  on the other side of R_SENSE  722  at node  725  to permit elimination of the downstream data or voice signals from the upstream signal using hybrid filter H 2 . Upstream high pass filter  734  eliminates the metering signal  760  and the low frequency spectral components associated with voiceband communications. 
   Upstream voice driver  730  senses the subscriber line on the transceiver side of transformer  724  at  723 . Upstream voice driver  730  is also provided with the output of downstream driver  720  on the other side of R_SENSE  722  at node  725  to permit elimination of the downstream data or voice signals using hybrid filter H 1 . In one embodiment, metering cancellation circuitry (elements  735 , W 5 ) is required to remove the remainder of any metering signal from the upstream voice signal  772 . In one embodiment, W 5  is an adaptive finite impulse response (FIR) filter. Upstream low pass filter  732  eliminates substantially all the high frequency components attributable to the DSL services from the upstream voice signal  772 . Feedback element  736  and weight W 4  provide a mechanism for voiceband impedance synthesis to meet POTS requirements. 
     FIG. 7B  illustrates an alternative embodiment with bypass capacitors C 1  and C 2  for sensing the upstream voice and the upstream DSL signals, respectively. In particular, the upstream voice and upstream DSL signals are sensed on the subscriber line side of transformer  724  rather than at node  723 . Capacitor C 1  permits sensing the upstream voice signal without degradation due to transformer  724 . Capacitor C 2  permits sensing the upstream DSL signal without degradation due to transformer  724 . In various embodiments, only one of the upstream voice or upstream DSL signals is sensed from the subscriber line via a bypass capacitor. The remaining elements function as described with respect to  FIG. 7A . 
   DSL signaling is traditionally voltage driven in contrast with the current driven POTS model.  FIG. 8 , however, illustrates one embodiment of current-based driver circuitry for both DSL and voice downstream signals. 
   An impedance bridge  858  including Z 1 , Z 2 , and C 1  is coupled to the subscriber line. A first pair of current drivers ( 830 ,  840 ) is coupled to the impedance bridge for driving a voice signal in a first frequency range onto the subscriber line. A second pair of current drivers ( 810 ,  820 ) is coupled to the impedance bridge for driving the DSL signal in a second frequency range onto the subscriber line. 
   Within the first frequency range, the impedance bridge couples the first pair of current drivers to the subscriber line through impedances Z 1  and Z 2 . The output impedance of the first pair of current drivers is controlled by the Z synthesis feedback circuit  836  rather than Z 1  or Z 2 . Within the second frequency range, the impedance bridge couples the second pair of current drivers to the subscriber line across an impedance of Z 1 +Z 2 . 
   In one embodiment, the impedance bridge comprises a first impedance Z 1 , a second impedance Z 2 , and a capacitor C 1 . A tip line and one of the second pair of current drivers is connected to a first terminal  850  of Z 1 . A first terminal of C 1  and one of the first pair of current drivers is connected to a second terminal  852  of Z 1 . The other of the second pair of current drivers is connected to a first terminal  856  of Z 2 . The other of the first pair of current drivers and a second terminal of C 1  are connected to a second terminal  854  of Z 2 . In one embodiment, impedances Z 1 , Z 2  are in a range of 40–60Ω with a nominal impedance of approximately 50Ω. Capacitor C 1  has a value of approximately 5 nF. 
   Capacitor C 1  behaves as an open circuit at low frequencies. Thus  FIG. 9A  illustrates driving the subscriber line at voiceband frequencies from the perspective of voiceband current drivers  930 ,  940 . For longitudinal balance, impedances Z 1  and Z 2  are chosen to have substantially the same value. In one embodiment, Z 1 , Z 2 ≈50Ω. The Z synthesis feedback circuit  936  is used to control the output impedance of the first pair of current drivers. 
   Capacitor C 1  behaves as a short circuit at high frequencies. Thus  FIG. 9B  illustrates driving the subscriber line at xDSL frequencies from the perspective of DSL drivers  910  and  920 . The Z synthesis feedback circuit affects only the output impedance of the voice drivers at low frequencies and does not significantly affect the output impedance for frequencies outside of the voiceband range. Impedances Z 1 , Z 2  are effectively series-coupled from the DSL driver circuitry standpoint. Accordingly, the resulting impedance is Z 1 +Z 2  or approximately 100Ω for Z 1 , Z 2 ≈50Ω within the second frequency range associated with the xDSL communications. 
   Various subscriber line interface circuits having a transceiver integrating or at least partially integrating voice and data paths onto the same integrated circuit. In various embodiments an integrated circuit drives or receives a combination of the upstream/downstream voice and upstream/downstream data signals communicated on a subscriber line. Each combination includes a portion of at least one data path (upstream or downstream or both) and a portion of at least one voice path (upstream or downstream or both). 
   In the preceding detailed description, the invention is described with reference to specific exemplary embodiments thereof. Various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.