Systems and methods for handling crosstalk vectoring failures in multi-card vectoring groups

A communication system comprises a plurality of line cards having transceivers coupled to a plurality of subscriber lines. Each line card has at least one active transceiver within the same vectoring group, and each line card also has vector logic capable of cancelling crosstalk induced by an active transceiver that is a member of the vectoring group. In the event of a vectoring fault that prevents a line card from receiving vectoring information from at least on other line card, the vector logic is configured to disable vectoring for the interferers affected by the error in order to prevent vectoring operations based on obsolete vectoring coefficients from adversely affecting the quality of the communicated signals. The transceivers communicating signals affected by the suspended vectoring operations are also configured to adjust their constellation density profiles, thereby reducing their data rates, to accommodate the increased noise level resulting from the loss of vectoring. By handling the vectoring fault in such manner, communication can continue without requiring a retrain.

RELATED ART

A digital subscriber line access multiplexer (DSLAM) is a device that connects multiple subscriber lines to a high-speed network line using digital subscriber line (DSL) modulation formats across the subscriber lines. In the downstream direction, a DSLAM generally demultiplexes a high-speed data stream from a network across the subscriber lines, and in the upstream direction, a DSLAM generally multiplexes the data streams from the subscriber lines for transmission across the high-speed network line. A DSLAM can be installed at a variety of locations, such as at a network facility (e.g., a central office) or an intermediate point between a central office and one or more customer premises.

A variety of DSL formats have been used for the communication from a DSLAM to a customer premises. Very-high-bit-rate DSL (VDSL) is a solution that is attractive due to the relatively high data rates enabled by VDSL as compared to other DSL solutions. Indeed, first generation VDSL provides data transmission up to about 52 Mega-bits per second (Mbit/s) downstream and about 16 Mbit/s upstream. Second generation VDSL, sometimes referred to as VDSL2, provides up to about 100 Mbit/s simultaneously in the both the upstream and downstream directions. The VDSL line code is discrete multi-tone (DMT) modulation, which uses a sequence of equally spaced frequencies or tones, each of which is modulated using quadrature amplitude modulation (QAM).

Like several other DSL technologies, VDSL suffers from the effects of crosstalk. However, VDSL standards specify vectoring techniques that allow crosstalk cancellation, and such techniques have been employed to cancel the crosstalk among subscriber lines extending from a DSLAM to one or more customer premises in an effort to improve the performance of VDSL signals and allow for longer reaches. However, VDSL vectoring is processing intensive, and as the number of subscriber lines increases, the amount of processing required to cancel crosstalk from the signals carried by the subscriber lines increases exponentially.

In this regard, to cancel crosstalk, vector logic (sometimes referred to as a “vector engine”) maintains a set of vectoring coefficients respectively corresponding to the coupling functions between interfering tones (i.e., tones that induce crosstalk) and victim tones (i.e., tones affected by crosstalk). For each victim tone, the vector engine combines the symbol of the victim tone and each interfering tone with its corresponding vectoring coefficient, and the result becomes the new victim tone. In such manner, the crosstalk channel is effectively inverted, and the crosstalk is cancelled tone-by-tone for all tones. For transmitter-based precoding, the new victim tone is transmitted on the line, and the precoding effectively cancels the crosstalk as the victim tone propagates across the line and the crosstalk couples into the line of the victim tone. For receiver-based cancellation, the new tone is further processed by the receiver but with reduced crosstalk.

A single vector engine can process the coefficients for a limited number of tones, but additional vector engines can be added in order to increase the number of tones subject to the VDSL vectoring. In such case, the vector engines share vectoring information (e.g., tone symbols) so that a larger number of interfering tones can be canceled from each victim tone.

When a vectoring group spans across multiple line cards, the vectoring information should be communicated among the line cards. Reliably communicating vectoring information among multiple line cards can be problematic, particularly for vectoring groups that process a large number of tones thereby generating a large amount of vectoring information.

DETAILED DESCRIPTION

The present disclosure generally pertains to systems and methods for handling crosstalk vectoring failures in multi-card vectoring groups. In one exemplary embodiment, a communication system comprises a plurality of line cards having transceivers coupled to a plurality of subscriber lines. Each line card has at least one active transceiver within the vectoring group, and each line card also has vector logic capable of cancelling crosstalk induced by an active transceiver that is a member of the vectoring group. Further, the line cards are coupled to one another via a ring connection across which vectoring information is passed from one line card to the next. In this regard, the ring connection carries a data stream, referred to hereafter as “vectoring stream,” having a plurality of time slots respectively allocated to the line cards. As the vectoring stream is communicated, each line card inserts vectoring information (e.g., the symbols received by such line card from the subscriber lines or to be transmitted by such line card across the subscriber lines) into the time slots allocated to it. Further, each line card reads the vectoring information of other line cards from the time slots allocated to the other line cards. Accordingly, the vector logic on a given line card has sufficient access to the vectoring information of the other line cards to cancel crosstalk induced by interfering tones communicated across any of the subscriber lines regardless of which line cards receive or transmit the interfering tones.

In some situations, a failure within the system may prevent one or more line cards from receiving the vectoring stream and, hence, vectoring information from other line cards. In such case, vectoring cannot be successfully performed to remove the crosstalk induced by at least some interferers, and it is likely that some transceivers are attempting to communicate at a data rate that cannot be successfully supported. In this regard, vectoring generally reduces the overall noise level for the signals communicated by the transceivers such that the transceivers can communicate at a higher data rate than would otherwise be possible in the absence of vectoring. The loss of vectoring due to a failure of the ring connection effectively increases the overall noise level such that the same data rate used prior to the failure may result in an unacceptably high number of transmission errors after the failure. In such case, the transceivers are configured to reduce their data rates to ones that are suitable for the noise conditions after the failure. In one exemplary embodiment, such a reduced data rate is selected based on training information and/or data indicative of the present line characteristics such that a retrain is not required after the failure. Thus, when a loss of vectoring information is detected, the transceivers automatically revert to a lower data rate without having to perform a retrain, thereby enhancing the robustness and resiliency of the system.

FIG. 1depicts an exemplary embodiment of a communication system10. As shown byFIG. 1, the system10comprises a network12, such as the public switched telephone network (PSTN) or other communication network, configured to communicate with CP transceivers15at one or more customer premises21. In this regard, the network12is coupled to a network access point25via a network line27, such as one or more twisted-wire pairs or optical fibers, and the network access point25is coupled to the CP transceivers15via a plurality of subscriber lines29, such as twisted-wire pairs or optical fibers. For simplicity,FIG. 1depicts four subscriber lines29, but there can be any number of subscriber lines29in other embodiments. Further, the lines29inFIG. 1extend to multiple customer premises21, but it is also possible for the lines29to extend to a single customer premises21.

In a downstream direction, the network access point25receives a high-speed data stream from the network12via the network line27and forwards packets from the high-speed data stream across the plurality of subscriber lines29. In an upstream direction, the network access point25receives data streams from the customer premises21via the subscriber lines29and transmits packets from such data streams across the network line27to the network12.

In one exemplary embodiment, the network line27comprises an optical fiber, and optical modulation formats are used to communicate data across the fiber. In addition, each subscriber line29comprises at least one twisted-wire pair, and digital subscriber line (DSL) modulation formats are used to communicate data across the subscriber lines29. Note that there are a variety of DSL modulation formats that may be used for communicating data across the subscriber lines29, such as asymmetric DSL (ADSL), high-bit-rate DSL (HDSL), very-high-bit-rate DSL (VDSL), and single-pair HDSL (SHDSL). For illustrative purposes, it will be assumed hereafter that the modulation format used for each subscriber line is VDSL, such as first generation VDSL or VDSL2, but it should be emphasized that other DSL and/or non-DSL modulation formats may be used in other embodiments.

FIG. 2depicts an exemplary embodiment of the network access point25. The network access point25comprises a plurality of line cards51-53coupled to the subscriber lines29. In this regard, each line card51-53has at least one transceiver (XCVR)55coupled to at least one subscriber line29. In the exemplary embodiment shown byFIG. 2, each line card51-53has three transceivers55respectively coupled to three subscriber lines29, but any line card51-53may have any number of transceivers55and/or be coupled to any number of subscriber lines29in other embodiments. Further, it is unnecessary for each line card51-53to have the same number of transceivers55and be coupled to the same number of subscriber lines29as the other line cards. As will be described in more detail hereafter, each transceiver55is a member of the same vectoring group, but it is unnecessary for all of the transceivers55to be members of the same vectoring group in other embodiments. In one exemplary embodiment, the components of the same line card reside on a single printed circuit board (PCB), but it is possible for any line card to have more than one PCB, if desired.

As shown byFIG. 2, each line card51-53has a card control element63that is coupled to each transceiver55residing on the same line card51-53. The card control element63controls the general operation of the line card51-53on which it resides, including the forwarding of data packets, as will be described in more detail hereafter. The card control element63may be implemented in hardware, software, firmware, or any combination thereof. Each card control element63is also coupled to a port66, referred to hereafter as a “network-side port,” which is coupled to a switching element71. Further, the switching element71is coupled to a network transceiver74that is coupled to the network line27.

In the downstream direction, assuming that the network line27comprises an optical fiber, the network transceiver74receives an optical data signal from the network line27and converts the optical data signal into an electrical signal comprising data packets. The switching element71is configured to forward the data packets to the line cards51-53based on header information within the data packets using forwarding techniques known in the art. Each data packet received by a line card51-53is received by the card's control element63, which forwards the packet to at least one transceiver55for transmission across at least one subscriber line29. When a transceiver55receives data packets from the card control element63, the transceiver55modulates a carrier signal with the data packets using VDSL or some other desired modulation format in order to form a data signal that is transmitted across the subscriber line29coupled to such transceiver55.

In the upstream direction, the transceivers55receive modulated data signals from the subscriber lines29. For a given signal received by a given transceiver55, the transceiver55demodulates the received signal to recover data packets. The transceiver55forwards the data packets to the card control element63that is residing on the same line card51-53, and the card control element63forwards each packet received from the card's transceivers55to the switching element71. The switching element71combines the packets received from all of the line cards51-53into a high-speed data stream, which is received by the network transceiver74. Assuming that the network line27comprises an optical fiber, the network transceiver74converts the received data stream into an optical data signal for transmission across the network line27to the network12(FIG. 1).

As shown byFIG. 2, each line card51-53comprises vector logic77(e.g., one or more vector engines) configured to perform crosstalk vectoring in order to cancel crosstalk from the tones transmitted and/or received by the card51-53on which it resides. For example, the vector logic77of the line card51maintains vectoring coefficients for the tones communicated (i.e., transmitted or received) by the transceivers55of the card51. Via known techniques, the vector logic77uses such vectoring coefficients to estimate crosstalk that affects the received tones or that will affect tones to be transmitted so that such crosstalk can be cancelled.

For example, the vector logic77of the card51stores sets of vectoring coefficients respectively corresponding to the tones communicated across the subscriber lines29coupled to the card51. For a given tone, referred to in this example as “victim tone,” received by one of the transceivers55from one of the subscriber lines29, the corresponding set of vectoring coefficients includes vectoring coefficients respectively associated with the tones, referred to as “interfering tones,” that interfere with the victim tone. Upon receiving a set of symbols simultaneously communicated across the subscriber lines29, the transceivers55of the card51send such symbols to the vector logic77of the card51. For each interfering tone, the vector logic77combines (e.g., multiplies) the tone's symbol with the associated vectoring coefficient to estimate an amount of crosstalk interference from the interfering tone affecting the victim tone. The vector logic77then cancels such crosstalk interference from the symbol of the victim tone by combining (e.g., subtracting) the estimate with the symbol of the victim tone. The vector logic77performs the same process for the symbols of the other interfering tones in order to cancel, tone-by-tone, crosstalk interference in the victim tone.

After removing the crosstalk interference induced by the interfering tones, the vector logic77sends the symbol of the victim tone back to the transceiver55that originally received it from a subscriber line29. Such transceiver55decodes the symbol and generates an error signal indicating an error estimate for the symbol. The foregoing transceiver55sends the error signal to the vector logic77, which adaptively updates the set of vectoring coefficients corresponding to the victim tone based on the error signal using a known coefficient update algorithm, such as least means square (LMS).

Thus, the vector logic77of the card51cancels, from the symbol of the victim tone, crosstalk induced by each interfering tone received by the card51from the subscriber lines29. In one exemplary embodiment, the vector logic77uses the same techniques to cancel, from the symbol of the victim tone, crosstalk induced by interfering tones received by the other line cards52and53. In this regard, the set of vectoring coefficients corresponding to the victim tone also includes vectoring coefficients associated with the tones received by the cards52and53. As described above for the symbols received by the card51, the vector logic77combines the symbols received by the transceivers55of the line cards52and53with the associated vectoring coefficients to estimate the amount of crosstalk induced by such symbols in the symbol of the victim tone. The vector logic77also combines such estimates with the symbol of the victim tone to cancel, tone-by-tone, the crosstalk induced by the interfering tones received by the line cards52and53.

Further, such vector logic77uses similar techniques to precode the symbols transmitted by the transceivers55of the card51across the subscriber lines29such that crosstalk is cancelled as the symbols propagate across the subscriber lines29. Specifically, the vector logic77receives symbols of interfering tones to be communicated across the lines29at the same time as a symbol of a victim tone to be communicated by the line card51. For each interfering tone, the vector logic77combines (e.g., multiplies) the tone's symbol with the associated vectoring coefficient to estimate an amount of crosstalk interference that will affect the symbol of the victim tone. The vector logic77then combines (e.g., subtracts) the estimate from the symbol of the victim tone in order to precode the symbol so that crosstalk interference from the interfering tone is effectively cancelled during communication as it couples into the line29of the victim tone. The vectoring coefficients used for the precoding are updated based on the error associated with the victim tone, as determined by the CP transceiver15(FIG. 1) that receives the victim tone. Exemplary techniques for performing crosstalk vectoring, including symbol precoding, are described in commonly-assigned U.S. patent application Ser. No. 13/016,680, entitled “Systems and Methods for Cancelling Crosstalk in Satellite Access Devices” and filed on Jan. 28, 2011, which is incorporated herein by references. Note that the vector logics77of the other line cards52and53are configured similar to the vector logic77of the line card51in order to cancel crosstalk affecting the tones communicated by the lines cards52and53, respectively.

Moreover, in one exemplary embodiment, the vector logic77and transceivers55of the line cards51-53form a vectoring group such that crosstalk from any interfering tone communicated across the subscriber lines29can be cancelled from any victim tone communicated across the subscriber lines29. However, in other embodiments, it is unnecessary for each vector logic77and each transceiver55at the network access point25to be a member of the same vectoring group.

In order to enable each line card51-53to cancel crosstalk induced by tones communicated by the other line cards, vectoring information (e.g., tone symbols) is passed from one card to the other. For example, to enable the vector logic77of the line card51to cancel, from a victim tone received by the line card51, crosstalk induced by interfering tones received by the line card53, the line card53transmits symbols of the interfering tones to the line card51. Similarly, to enable the vector logic77of the line card51to precode the symbols transmitted by the transceivers55of the line card51such that crosstalk induced by interfering tones transmit by line card53is cancelled as the symbols propagate across the subscriber lines29, the line card53transmits symbols of the interfering tones to the line card51. In one exemplary, a ring connection80comprising a plurality of segments81-83is configured to carry vectoring information from card-to-card. In this regard, each line card51-53has a plurality of ports87and88, referred to hereafter as “ring ports,” respectively coupled to a plurality of segments of the ring connection80. For example, the ring port87of the line card51is coupled to one end of the segment83, and the other end of the segment83is coupled to the ring port88of the line card52. Further, the ring port87of the line card52is coupled to one end of the segment82, and the other end of the segment82is coupled to the ring port88of the line card53. Also, the ring port87of the line card53is coupled to one end of the segment81, and the other end of the segment81is coupled to the ring port88of the line card51.

As shown byFIG. 2, each ring port87and88of a given line card51-53is coupled to a ring interface94, which is coupled to the card's vector logic77. Thus, the vectoring information carried by the ring connection80is received and processed by the ring interface94. Ultimately, such information is used by the vector logic77to perform crosstalk vectoring operations. Note that the ring interface94may be implemented in hardware, software, firmware, or any combination thereof.

In one exemplary embodiment, the vectoring stream carried by the ring connection80(FIG. 2) has time slots that are respectively allocated to the line cards51-53, and each line card51-53inserts into the slots allocated to it vectoring information for enabling the other line cards to cancel crosstalk. As an example, refer toFIG. 3, which depicts an exemplary embodiment of a vectoring stream125carried by the ring connection80. In this regard, the vectoring stream125comprises a field131allocated to the line card51, a field132allocated to the line card52, and a field133allocated to the line card53. Each line card51-53is configured to insert vectoring information into the slots of the respective field131-133allocated to it, and the fields131-133shall be referred to herein as “vectoring fields.”

For example, the vector logic77of the line card51is configured to transmit to the card's ring interface94the symbols of the tones communicated across the subscriber lines29coupled to the card51. The ring interface94is configured to insert such symbols into the slots of the vectoring field131allocated to the card51. The vector logics77of the other cards52and53are configured to use such symbols to cancel crosstalk. As an example, the vector logic77of the line card52may use the symbols in the vectoring field131to cancel crosstalk induced by the interfering tones received by the line card51from the subscriber lines29coupled to such card51. Further, the vector logic of the line card52may use the symbols in the vectoring field131to precode the symbols transmitted by the transceivers55of the line card52such that crosstalk induced by interfering tones transmitted by the line card51is cancelled as the symbols propagate across the subscriber lines29. Similarly, the other cards52and53are configured to insert symbols into the vectoring fields132and133, respectively, allocated to such cards52and53. Thus, the data carried by the ring connection80provides the symbols communicated across all of the subscriber lines29associated with the vectoring group so that each card51-53can cancel crosstalk induced by an interfering tone communicated by any subscriber line29, regardless of which card51-53actually transmits or receives the interfering tone via the subscriber lines29.

As shown byFIG. 3, the vectoring stream125also comprises a field134, referred to hereafter as “control field,” that may be used by the line cards51-53to transmit control information among the cards51-53, as will be described in more detail hereafter. In one exemplary embodiment, the vectoring stream125is transmitted across the ring connection80(FIG. 2) from card-to-card in the same direction around the ring formed by the ring connection80and cards51-53. For example, referring toFIG. 2, the vectoring stream125may be transmitted from the line card51to the line card52via the segment83, from the line card52to the line card53via the segment82, and from the line card53to the line card51via the segment81. Alternatively, the vectoring stream125may be transmitted from the line card51to the line card53via the segment81, from the line card53to the line card52via the segment82, and from the line card52to the line card51via the segment83. In other embodiments, other techniques for communicating the vectoring stream125are possible. For example, it is possible for the vectoring stream125to be transmitted in both directions, or for one portion of the vectoring stream125to be transmitted in one direction while another portion of the vectoring stream125is transmitted in the opposite direction. For illustrative purposes, it will be assumed hereafter that the vectoring stream125is transmitted in the same direction, clockwise, around the ring unless otherwise noted. Thus, unless otherwise noted, it will be assumed that the vectoring stream125propagates from the segment81to the segment83and then to the segment82.

Exemplary techniques for communicating vectoring information via the ring connection80are described in commonly-assigned U.S. patent application Ser. No. 13/410,674, entitled “Systems and Methods for Communicating Vectoring Information for Multi-Card Vectoring Groups” and filed on even date herewith, which is incorporated herein by reference. In other embodiments, other types of connections between the cards51-53may be used to communicate the vectoring information.

During vectoring, it is possible for a failure to occur on any line card51-53or any segment of the ring connection80. Depending on the type of failure that occurs, it is also possible that the failure may prevent the line card and/or ring connection80from successfully communicating the vectoring stream125to at least one line card51-53. Any failure that prevents at least one line card from receiving vectoring information from at least one other line card shall be referred to herein as a “vectoring fault.” When a vectoring fault occurs, a transceiver55on a line card51-53that stops receiving vectoring information from at least one other line card is likely to be communicating at a data rate that is higher than what is reliable for the current transmission characteristics of the system10.

In this regard, vectoring generally reduces the overall noise level for the signals communicated by the transceivers55such that the transceivers55can communicate at a higher data rate than would otherwise be possible in the absence of vectoring. As an example, by using vectoring to cancel crosstalk induced by interferers communicated by the line cards52and53, a transceiver55on the line card51can communicate at a higher data than would otherwise be possible in the absence of such vector processing. However, if a failure of the ring connection80and/or the line cards52or53prevents the line card51from receiving vectoring information from the line cards52and/or53, then the vector logic77of the line card51is unable to use vectoring to successfully cancel the crosstalk induced by interferers communicated by the line cards52and/or53. In such case, the overall noise levels of the signals communicated by the transceivers55of the line card51increase, and the data rate used by such transceivers55prior to the vectoring failure may result in an unacceptably high number of errors.

To compensate for foregoing effect, the transceivers55of the line card51are configured to automatically reduce their data rates in response to the vectoring fault, thereby reducing the number of transmission errors in the signals communicated by such transceivers55. In one exemplary embodiment, such a reduced data rate is selected based on training information and/or data indicative of the current line characteristics such that a retrain is not required after the failure. Thus, when a loss of vectoring information is detected, the transceivers automatically revert to a lower data rate without having to perform a retrain, thereby enhancing the robustness and resiliency of the system10. Exemplary techniques for selecting data rates will be described in more detail below, but it should be emphasized that, in other embodiments, other techniques are possible.

Prior to communicating data with a CP transceiver15(FIG. 1) in a data phase, each transceiver55(FIG. 2) at the network access point25is configured to enter into a training phase in which the transceiver55and its corresponding CP transceiver15negotiate a data rate to be used in the subsequent data phase. Generally, the transceivers15and55negotiate for the highest data rate that is supported by both transceivers15and55and that results in an acceptable signal quality, as indicated by a particular parameter, such as signal-to-noise ratio (SNR). In negotiating the data rate, the transceivers15and55assign a constellation density to each tone communicated across the subscriber line29. As known in the art, the constellation density for a given tone represents the number of bits carried by each symbol of the tone. A higher constellation density means that each symbol carries more bits resulting in a higher data rate for the tone. Moreover, assigning the constellation density to a tone effectively sets the data carrying capacity for that tone.

Note that the assignment of constellation density (referred to as constellation encoding) is performed tone-by-tone. That is, an SNR measurement is performed for each tone, and the SNR measured for a given tone is used to select the appropriate constellation density for that tone. Thus, different tones may be assigned different constellation densities and, hence, can support different data rates. The aggregate data rate for all of the tones generally represents the overall data rate for the DMT signal.

During training, a given transceiver55defines and stores in memory data166(FIG. 4), referred to hereafter as “constellation density settings166.” Such settings166indicate the constellation densities that are assigned to each tone and are used by the transceiver55in formatting the data signals communicated by it. In this regard, the transceiver55is configured to control the constellation densities of the communicated tones so they are in accordance with the stored constellation density settings166. Over time, as conditions on the line change, the transceiver55may update the constellation density settings166that were originally established during training. In general, such updates are usually gradual and in response to changes in a measured performance parameter, such as SNR. For example, if the SNR for a given tone increases, the constellation density for that tone may be increased, and conversely, if the SNR decreases, the constellation density for that tone may be decreased. As long as the changes in line conditions are gradual, such updates can be successfully performed without requiring a retrain. However, if a line event causes a significant, abrupt change in line conditions, such as the amount of noise affecting the communicated signals, the transceiver55may be unsuccessful in adapting to the changed line conditions requiring a retrain in order to re-establish communication with the far-end CP transceiver15.

In one exemplary embodiment, at least one tone, referred to hereafter as the “control tone,” is used during the data phase as a control channel for enabling the transceivers15and55to communicate control information. The constellation density of the control tone is selected to be low enough such that information may be successfully communicated via the control tone in the absence of crosstalk vectoring. This may be achieved by assigning to the control tone a constellation density during training and then not increasing the constellation density for any signal quality improvement resulting from vectoring. In such case, the control tone should be assigned a sufficiently low constellation density to ensure communication via the control tone even if there is a line card or connection failure that results in the loss of vectoring, though other techniques may be used to achieve a similar effect, if desired.

FIG. 4depicts an exemplary embodiment of a transceiver55. As shown byFIG. 4, the transceiver55comprises a transformer152that is coupled to a subscriber line29(FIG. 2). The transformer152is coupled to a transmitter154and a receiver156through a hybrid159. The transmitter154transmits a DMT signal (e.g., VDSL signal), which is converted by transformer152to a voltage level suitable for communication across the subscriber line29coupled thereto. When a DMT signal is received from the subscriber line29, the transformer152converts the line voltage to a matching signal before passing it to the receiver156. The hybrid159separates the transmitted signals from the received signals in an attempt to keep energy from the transmitter154from affecting the signals received by the receiver156.

As shown byFIG. 4, the transceiver55comprises control logic163for generally controlling the operation of the transceiver55. For example, the control logic163communicates with the vector logic77(FIG. 2) that is on the same line card51-53and also controls (e.g., establishes and updates) constellation density settings166stored in memory169. It should be noted that the control logic163can be implemented in software, hardware, firmware, or any combination thereof. In the exemplary embodiment illustrated inFIG. 4, the control logic163is implemented in software and stored in the memory169.

Note that the control logic163, when implemented in software, can be stored and transported on any computer-readable medium for use by or in connection with an instruction execution apparatus that can fetch and execute instructions. In the context of this document, a “computer-readable medium” can be any means that can contain or store a program for use by or in connection with an instruction execution apparatus.

The exemplary embodiment of the transceiver55depicted byFIG. 4comprises at least one conventional processing element172, such as a digital signal processor (DSP) or a central processing unit (CPU), that communicates with and/or drives the other elements within the transceiver55via a local interface175, which can include at least one bus. Furthermore, the transceiver55comprises a data interface177for enabling the control logic163to communicate with external components and/or devices. As an example, the data interface177is coupled to the control element63(FIG. 2) and vector logic77(FIG. 2) for enabling the control logic163to communicate with such components.

As described above, during communication between a transceiver55at the network access point25and a CP transceiver15at a customer premises21, crosstalk vectoring is employed thereby enabling the transceivers15and55to communicate at a relatively high data rate, referred to hereafter as the “primary data rate,” considering that the crosstalk vectoring reduces the overall noise level. This primary data rate may gradually change over time during the data phase along with gradual changes to the physical characteristics (e.g., noise) of the subscriber line29across which the transceivers15and55are communicating.

After training, the transceiver55at the network access point25and the CP transceiver15at the CP21begin to communicate data signals at the primary data rate, and the vector logic77on the same line card performs vectoring, as described above, to enhance the signal quality of the communicated signals. To perform such vectoring, the vector logic77receives and uses vectoring information, including the symbols communicated by the transceivers55on the same line card as well as the symbols communicated by the transceivers55on the other line cards.

However, at some point, a failure (e.g., a failure of a line card51-53or of the ring connection80) may prevent the vector logic77from receiving vectoring information from at least one other line card. Attempting to perform vectoring using incorrect or obsolete vectoring information may actually introduce noise that adversely affects the quality of the symbols being processed. Thus, when the vector logic77determines that it has stopped receiving vectoring information from at least one other line card, the vector logic77preferably stops performing at least some vectoring operations. Note that if the vector logic77stops receiving the vectoring information for only some interferers, then the vector logic77may be configured to stop performing crosstalk vectoring operations only for such interferers while continuing with vectoring operations regarding the interferers for which the vector logic77is receiving up-to-date vectoring information.

In addition to the vector logic77stopping vectoring operations for at least some interferers, the transceivers55are responsive to a detected vectoring fault for automatically reducing their data rates. In this regard, when the vector logic77on a given line card stops performing vectoring operations for at least some interferers, the vector logic77preferably notifies the control logic163(FIG. 4) of each transceiver55on the same line card. In response, the control logic163automatically updates the constellation density settings166to transition from the primary data rate to a lower data rate, referred to hereafter as a “secondary data rate,” preferably without performing a retrain. Such secondary data rate is sufficiently low such that a given transceiver55may successfully communicate with its corresponding CP transceiver15in the absence of the vectoring operations that have been suspended in response to the vectoring fault while still achieving a desired performance.

In this regard, upon receiving a notification that the vector logic77has stopped crosstalk vectoring operations for at least some interferers, the control logic163of a given transceiver55is configured to cause the transmitter154(FIG. 4) to transmit a command, referred to hereafter as a “rate-switch command,” to its corresponding CP transceiver15via the control tone described above. At this point, the vectoring operations may already be stopped, but communication via the control tone should nevertheless be successful since the control tone has a sufficiently low constellation density to ensure communication in the absence of vectoring, as described above.

The rate-switch command instructs the CP transceiver15to switch from the primary data rate to the secondary data rate. In one exemplary embodiment, the rate-switch command includes data indicative of the updated constellation density settings166so that the CP transceiver15can communicate according to the constellation density profile defined by the these setting166, but other techniques of enabling the CP transceiver15to implement a desired constellation density profile are possible in other embodiments. In response to the rate-switch command, the CP transceiver15updates its constellation density settings (not specifically shown) for communication across the subscriber line29at a lower data rate. Thereafter, the transceivers15and55begin communicating at the secondary data rate. Thus, the transceivers15and55switch from the primary data rate to the secondary data rate in response to a vectoring fault without having to perform a retrain. Further, the network access point25is able to maintain communication with the CP transceivers15, albeit at lower data rates, in the event of a failure that prevents vectoring information from being passed among the line cards51-53.

Note that the constellation density profile indicated by the constellation density settings166, as well as the data of the rate-switch command transmitted to the CP transceiver15, may indicate the constellation densities of both upstream and downstream tones. It is also unnecessary for the upstream data rate to be the same as the downstream data rate. When a transceiver55detects a vectoring fault, data rates in both directions may be reduced since the loss of vectoring at the network access point25affects the quality of both upstream signals and downstream signals. The terms “primary data rate” and “secondary data rate,” as used herein, may refer to rates in either the upstream or downstream direction.

In addition, it is unnecessary to stop all vectoring operations in response to a vectoring fault. For example, the vector operations for interferers communicated by the same line card may be continued, if desired. In this regard, assume that a ring connection failure prevents the line card51from receiving vectoring information from the line cards52and53. In such an example, the vector logic77of the line card51may stop attempting to cancel crosstalk induced by interferers communicated by the line cards52and53. However, the vector logic77may continue to cancel crosstalk induced by interferers communicated by the transceivers55of the same line card51.

There are various techniques that can be used to determine the secondary data rate that is to be used in the event of a vectoring fault. As an example, the secondary data rate may be determined a priori. For example, a safe data rate within a significant margin of error may be selected by a technician, and data indicative of the constellation densities for achieving such a data rate may be stored in the transceivers55and/or15. When a transceiver55is to switch to the secondary data rate, the transceiver55may be configured to update its constellation density settings166to implement the predefined constellation density profile for the secondary data rate. However, in one exemplary embodiment, the secondary data rate is dynamically determined based on training information and/or data indicative of the current line conditions in an attempt to achieve a secondary data rate that is as high as possible within a desired margin of error.

For example, the control logic163of a transceiver55may be configured to update the transceiver's constellation density profile to implement the secondary data rate based on at least (1) the current constellation density settings for the primary data rate prior to the rate switch and (2) the vectoring coefficients used by the vector logic77for vectoring operations. In this regard, the vectoring coefficients are indicative of the amount of crosstalk that is currently affecting communications. Generally, changes to the noise characteristics of a subscriber line29are often gradual. Thus, although the vectoring coefficients used by the vector logic77may become obsolete over time if they are not updated to adapt to the changing line conditions, the vectoring coefficients typically remain valid for a limited time period in the absence of updates. At the time of a vectoring fault, the vectoring coefficients likely indicate within a reasonable margin of error the crosstalk characteristics of the line (e.g., how much crosstalk interference is currently affecting each tone).

Using the vectoring coefficients, the control logic163of a transceiver55can estimate how much noise is being cancelled by the vectoring and, hence, how much each tone will be degraded if vectoring is stopped. Thus, based on the vectoring coefficients and information indicative of the current line conditions, such as the SNR value measured while vectoring was being performed prior to the vectoring failure, the transceiver55can estimate for each tone a respective SNR value (SNR') indicative of the tone's signal quality without the vectoring that has been or is to be stopped in response the vectoring fault. The transceiver55then assigns to the tone a constellation density suitable for the calculated SNR'. That is, the transceiver55assigns to the tone a constellation density assuming that the actual SNR of the tone is equal to the estimate of SNR'. Accordingly, the transceiver55should assign a sufficiently low constellation density such that the tone is successfully communicated despite the loss of vectoring resulting from the vectoring fault and the corresponding increase in the overall noise level.

An exemplary use and operation of a line card51-53for controlling the constellation density profile for a transceiver55will now be described in more detail below with particular reference toFIG. 5.

As shown by block202ofFIG. 5, the control logic163(FIG. 4) of a transceiver55initially establishes the transceiver's constellation density settings166. In this regard, the transceiver55communicates with the far-end CP transceiver15in a training phase in order to assign constellation densities, tone-by-tone, to each tone communicated between the transceivers55and15. The control logic163establishes the settings166such that the assigned constellation densities are appropriately implemented in a subsequent data phase. After training, the transceivers55and15begin communicating according to the constellation density profile indicated by the constellation density settings166thereby achieving a primary data rate.

The control logic163monitors the line conditions, as shown by block205ofFIG. 5, in order to determine whether the constellation density settings166should be changed in response to changes in the line conditions. As an example, the control logic163may determine the SNRs of the communicated tones and then determine in block208whether to update any of the constellation density settings166in response to a change in the SNR for a given tone. If the control logic163does decide to make such a change, the control logic163updates the appropriate constellation density settings166in block211thereby adjusting the primary data rate of the transceiver55.

The control logic163also determines whether a new vectoring fault has been detected, as shown by block214ofFIG. 5. There are various techniques that can be used to detect such an error. In one exemplary embodiment, the control logic163learns of a vectoring fault via the vector logic77(FIG. 2). For illustrative purposes, assume that a ring connection80failure occurs such that the vector logic77of the line card51stops receiving the vectoring stream152(FIG. 3) communicated across the ring connection80. As an example, one of the segments81-83may be inadvertently severed such that the vectoring stream152is unable to propagate across the severed segment.

The vector logic77of the line card51detects the vectoring fault by determining when it stops receiving new vectoring information from the ring interface94of the same card51. In response to the detected vectoring fault, the vector logic77notifies the control logic163(FIG. 4) of the detected error. The vector logic77also disables vectoring for the interferers for which it receives symbols from the ring connection80, as shown by block217ofFIG. 5. In the instant example, the vector logic77of the line card51receives from the ring connection80the symbols of the tones communicated by the other cards52and53. Thus, the vector logic77stops vectoring operations for such tones. That is, the vector logic77stops attempting to cancel, from the tones communicated by the card51, crosstalk induced by interferers communicated by the cards52and53. The vector logic77may, however, continue canceling, from the tones communicated by the line card51, crosstalk induced by interferers communicated by this same card51since the vector logic77has access to the symbols of such interferers. In this regard, such symbols are received by the vector logic77from the transceivers55of the line card51, and the vector logic77, therefore, does not rely on the ring connection80for such symbols.

In response to the detected vectoring fault, the control logic163determines a new constellation density profile that is suitable for the loss of vectoring resulting from the vectoring fault. In one embodiment, the control logic163estimates the effect of the vectoring loss on the signals communicated by the transceiver55based on the current set of vectoring coefficients being used by the vector logic77prior to the vectoring fault. As an example, based on the vectoring coefficients, the control logic163may estimate an increased noise level for the tones affected by the loss of vectoring. The control logic163may then estimate SNR′ for each such tone using the tone's respective noise level, taking into account the estimated noise level increase resulting from the loss of vectoring. The control logic163may then assign to the tone a new constellation density, which is suitable for the tone's SNR'. As shown by block222, the control logic163updates the constellation density settings166as appropriate to implement the constellation density profile for the newly-assigned constellation densities.

As shown by block225ofFIG. 5, the control logic163also transmits to the far-end CP transceiver15, via the control tone, a rate-switch command having data indicative of the updated constellation density settings166so that the CP transceiver15may update its constellation density settings as appropriate to be consistent with those at the transceiver55. Thereafter, the transceivers55and15begin communicating according to the new constellation density profile such that the transceivers55and15transition from the primary data rate to the secondary data rate, which is much lower than the primary data rate to account for the increased noise level resulting from the loss of vectoring.

Once the secondary data rate is established, the control logic163continues to monitor the line conditions to determine whether to update the constellation density settings166for changing the secondary data rate in response to changing line conditions, as described above for the primary data rate, until a “yes” determination is made in block231.

Accordingly, when a vectoring fault occurs, the vector logic77is configured to disable vectoring for the interferers affected by the error in order to prevent vectoring operations based on obsolete vectoring coefficients from affecting the quality of the communicated signals. The transceivers55communicating signals affected by the suspended vectoring operations are also configured to adjust their constellation density profiles, thereby reducing their data rates, to accommodate the increased noise level resulting from the loss of vectoring. By handling the vectoring fault in such manner, communication between the transceivers55and their corresponding CP transceivers15can continue without requiring a retrain. Therefore, the DSL customers should continue to enjoy high-speed data service without interruption.

Upon restoration of a vectoring fault, the vectoring processing will continue to improve the signal quality by joining the previously affected transceivers line by line. The recovery of the vectoring process is similar to the initial training of the vectoring group except some of the lines in the vectoring group, that were not affected by the vectoring fault, can adapt from a partially vectoring to fully vectoring system when the vectoring fault is fully restored.