Method and apparatus for managing XDSL pseudo links

A transceiver having shared and discrete components forming a transmit path and a receive path configured to couple to a subscriber line for establishing a communication channel with a remote transceiver. The transceiver comprises a pseudo link manager coupled between the transmit and receive path components to manage pseudo links on the subscriber line prior to entry into a session with the remote transceiver. The pseudo link manager includes: a line monitor and a disruptor. The line monitor monitors the subscriber line, prior to initiating a session, for a received communication signal indicative of a pre-existing session on the remote transceiver. The disruptor transmits a disruptor signal over the subscriber line to the remote transceiver responsive to the detection by the monitor of the received communication signal; thereby disrupting the remote transmitter's pre-existing session and allowing a new session to be established with the remote transceiver.

BACKGROUND OF THE INVENTION

1. Field of Invention

The field of the present invention relates to multi-tone transceivers.

2. Description of the Related Art

Digital Subscriber Lines (DSL) technology and improvements thereon including: G.Lite, ADSL, ADSL2, ADSL2+, VDSL1, VDSL2, HDSL all of which are broadly identified as X-DSL have been developed to increase the effective bandwidth of existing subscriber line connections to high speed back bone networks developed by telecommunications companies. An X-DSL modem operates at frequencies higher than the voice band frequencies, thus an X-DSL modem may operate simultaneously with a voice band modem or a telephone conversation.

Each new XDSL protocol raises the bandwidth requirements of subscriber lines. As the bandwidth requirements increase so too does the complexity of the modem components. Additionally, because of the enormous variation in loop loss in the individual subscriber lines to which the modem may be coupled the individual components of the modem transmit and receive path must be reconfigurable to match the available bandwidth on a selected subscriber line.

Typically the central office (CO) of the telephone company includes racks of line cards each servicing many subscriber lines. Each line card includes many chips handling the digital and analog portions of the various XDSL communications over the subscriber lines. Each communication channel modulated onto a corresponding one of the digital subscriber lines is subject to crosstalk from communications channels modulated onto remaining ones of the digital subscriber lines in a bundle. This crosstalk degrades the performance of each digital subscriber line in a bundle of subscriber lines and in the extreme may render a given subscriber line connection inoperable.

What is needed is a modem with improved capabilities for responding to varying crosstalk levels in bundled digital subscriber lines.

SUMMARY OF THE INVENTION

A method and apparatus for management of XDSL pseudo links is disclosed. In an embodiment of the invention a transceiver having shared and discrete components forming a transmit path and a receive path configured to couple to a subscriber line for establishing a communication channel with a remote transceiver is disclosed. The transceiver comprises a pseudo link manager. The pseudo link manager is coupled between the transmit and receive path components to manage pseudo links on the subscriber line prior to entry into a session with the remote transceiver. The pseudo link manager includes: a line monitor and a disruptor. The line monitor monitors the subscriber line, prior to initiating a session, for a received communication signal indicative of a pre-existing session on the remote transceiver. The disrupter transmits a disrupter signal over the subscriber line to the remote transceiver responsive to the detection by the monitor of the received communication signal; thereby disrupting the remote transmitter's pre-existing session and allowing a new session to be established with the remote transceiver.

In an alternate embodiment of the invention a communication system comprising a plurality of transceivers each coupled to a corresponding one of a plurality of subscriber lines is disclosed. A selected one of the plurality of transceivers is coupled to the corresponding one of the plurality of subscriber lines for establishing a communication channel with a remote transceiver. The selected one of the plurality of transceivers includes a pseudo link manager. The pseudo link manager is coupled between the transmit and receive path components to manage pseudo links on the corresponding one of the plurality of subscriber lines prior to entry into a session with the remote transceiver. The pseudo link manager includes a line monitor and a disrupter. The line monitor monitors the corresponding one of the plurality of subscriber lines prior to the onset of a session for a received communication signal indicative of a pre-existing session between the remote transceiver and another one of the plurality of transceivers. The disruptor transmits a disruptor signal over the corresponding one of the plurality of subscriber lines responsive to the detection by the monitor of the received communication signal; thereby disrupting the remote transceiver's pre-existing session and allowing a new session to be established between the selected one of the plurality of transceivers and the remote transceiver.

Related means and methods are also disclosed.

DETAILED DESCRIPTION OF THE EMBODIMENTS

A method and apparatus is disclosed for crosstalk channel estimation among a plurality of digital subscriber lines each supporting multi-tone modulation of communications channels thereon. The line cards may be found in a central office, remote access terminal, business or home. The line cards may be coupled directly or indirectly to digital subscriber lines via one or more optical or wireless links. The line cards support communication channels with differing degrees of robustness for multi-tone protocols including: asymmetric digital subscriber line (ADSL); very high bit rate digital subscriber line (VDSL) and other orthogonal frequency division multiplexing (OFDM) plans including but not limited to the following:

FIG. 1is a system diagram of a multi-tone communication system in which individual subscribers are coupled via high speed communication links over a bundle of public service telephone network (PSTN) twisted copper subscriber lines with residential and business customers. The data communication may be modulated using various X-DSL protocols including G.Lite, ADSL and VDSL. Central Office (CO)100is coupled to multiple residential and business subscribers110via a bundle108of subscriber lines102,104,106for example. Each of the CO subscriber connections102-106terminates in the frame room148of the CO. From this room connections are made for each subscriber line via splitters and hybrids to both a Digital Subscriber Line Access Module (DSLAM) controller122and to the voice band racks150. The splitter shunts voice band communications to dedicated line cards, e.g. line card154or to a voice band modem pool (not shown). The splitter shunts higher frequency X-DSL communications on the subscriber line to a selected line card134within DSLAM130. The line cards of the current invention are universal, meaning they can handle any current or evolving standard of X-DSL and may be upgraded on the fly to handle new standards.

Voice band call set up between subscribers on the public switched telephone network (PSTN)118is controlled by a Telco switch matrix152implementing a switching protocol such as the common channel signaling system 7 (SS7) for setting up and tearing down a connection via an associated one of the voice band line cards, e.g. line card154. This makes point-to-point connections to other subscribers for voice band communications. The X-DSL communications may be processed by a universal line card such as line card134. That line card includes a plurality of AFE's e.g.142-146each capable of supporting a plurality of subscriber lines. The AFEs may be coupled directly or as in this embodiment of the invention via a packet based bus140to a DSP136which is also capable of multi-protocol support for all subscriber lines to which the AFE's are coupled. The line card may include more than one DSP.

Pseudo link management for the subscriber lines to which each line card is coupled is, in an embodiment of the invention, handled by a pseudo link management module138, on each line card. Crosstalk between bundled subscriber lines can result in a pseudo link between otherwise unconnected transceiver pairs. In the example shown a pseudo link112, is shown between the CO transceiver associated with AFE142and the subscriber line102to which it is coupled and a subscriber coupled to the remote end of subscriber line104. This pseudo link may result in the establishment of a session between a remote subscriber and a transceiver on the CO to which the subscriber has no physical connection. The only connection is the rather tenuous one facilitated by the inductive coupling and the resultant crosstalk between adjacent subscriber lines, e.g.102&104, in the bundle. The pseudo link not only grossly attenuates data rates associated with the subscriber's ‘pseudo’ session, but also prevents the proper session, e.g. between the CO transceiver to which the subscriber is physically coupled, e.g. via subscriber line104, from taking place. This latter problem results from the fact that the subscriber initiates a session. The subscriber will not send an acknowledge request signal (ACKREQ) to the CO to initiate a session if it is in the training or showtime phase of a ‘pseudo’ session. The pseudo link management module handles this problem by detecting and disrupting a pseudo session, thereby allowing a normal session to be initiated.

Each line card is coupled to a back-plane bus132which may in an embodiment of the invention be capable of offloading and transporting low latency X-DSL traffic between other DSPs for load balancing. Communications between AFE's and DSP(s) are in an embodiment of the invention packet based which allows a distributed architecture such as will be set forth in the followingFIG. 2to be implemented. Each of the DSLAM line cards operates as a multi-tone transceiver under the control of a DSLAM controller122which handles global provisioning, e.g. allocation of subscriber lines to AFE and DSP resources. Once an X-DSL connection is established between the subscriber and a selected one of the DSLAM sub modules, e.g. AFE and DSP, the subscriber will be able to access any network, e.g. the Internet116, to which the DSLAM is connected via server120.

FIG. 2Ais a hardware block diagram of an opposing sets of transceivers coupled to one another by corresponding subscriber lines within a bundle200. Within one of the opposing sets, transceivers220-222are referenced. Within another of the opposing sets transceiver s226,228are referenced. Within the bundle which may include 60 or more subscriber lines, lines202-212are referenced.

FIG. 2Bis a cross-sectional view of the subscriber lines in the bundle shown inFIG. 2A. Subscriber lines202-212are referenced. Any digital communication experiences signal interference, and communication protocols which support multiple sub-channels such as DMT are no exception. Interference can affect both the amplitude and the phase of the sub-channels. Such noise can arise across the time and/or frequency domains. Each of the subscriber lines in a bundle shields or interferes with other lines in the bundle in varying amounts across the time and/or frequency domains. In prior art systems this aggregate behavior is crudely addressed by the requirement of a corresponding fixed PSD mask on any of the subscriber lines which implement multi-tone modulation. All subscriber lines blast out sub-carrier signals at levels up to but not in excess of the corresponding PSD mask regardless of the bit-loading on even the presence of any actual data modulated on any particular carrier. Where the lines are short and crosstalk interference is large, a pseudo link can be established between a requesting subscriber, e.g. via an ACKREQ, and a responding CO transceiver across this crosstalk link. The crosstalk, and the inductive coupling it represents, enables the communication required to establish a session. The pseudo link management module handles this problem by detecting and disrupting a pseudo session, thereby allowing a normal session between physically coupled CO and remote transceivers to be initiated.

FIG. 3is a graph with three axis showing the spectral profile of a multi-tone modulation protocol on a representative one of the subscriber lines over a time interval t0-t3, in which XDSL pseudo link management takes place. Four sub-graphs,300,320,340,360of frequency vs. power are shown at associated times t0, t1, t2, and t3, respectively.

At to the pseudo link management module of a selected CO transceiver begins listening to the subscriber line to which it is coupled. Sub-graph300shows the signal activity on a selected subscriber line, e.g. subscriber line104at time t0. At this point in time, no session is established by the CO transceiver, rather the CO transceiver is listening for an ACKREQ indicating the remote modem to which it is coupled wants to establish a session. What the pseudo link management module of the CO transceiver detects instead is shown in sub-graph300. Specifically a low frequency PSTN signal302, a communication signal304and crosstalk312are shown. The PSTN signal302spans a frequency from 0-4 kHz. The communication signal304spans a frequency from 25 k Hz to 138 kHz associated with an upstream signal spectrum for various XDSL protocols. The power level of the upstream signal304corresponds to that associated with various XDSL standards and is significantly above that of the crosstalk312power level. This indicates that the remote modem is engaged in a pseudo-session.

The response of the pseudo link management module is shown in the sub-graph320at time t1. Specifically, the pseudo link management module injects a disrupter signal328onto the subscriber line it is monitoring, e.g. subscriber line104. The disruptor signal has a power level just above the level of the crosstalk312and below the power level associated with a downstream communication signal proscribed by any of the XDSL standards.

The result of the disruption is shown in sub-graph340at time t2. At time t2the disruptor signal has ceased. The disrupter signal has disrupted the remote transceiver's pseudo session causing the remote transceiver or modem to terminate its pseudo session with a CO transceiver to which it is not physically coupled. The remote transceiver sends an ACKREQ signal346requesting a new session with the CO transceiver to which it is physically coupled. The location and spacing of the one or more sets of tones which carry the ACKREQ signal varies by XDSL standard. In the example shown the tones dedicated to the ACKREQ signal are tones with indexes,9,17, and25. Additional tones may also be utilized for transport of the ACKREQ signal, e.g. tones with indexes944,972,999.

Sub-graph360shows the spectrum of the selected subscriber line at time t3after a new session between the physically coupled selected CO transceiver and remote transceiver is established. The spectral profile of upstream364and downstream368communication signal corresponds in the example shown to an ADSL communication profile.

FIG. 4is a hardware block diagram showing an XDSL transceiver400incorporating components for pseudo link management. The transceiver includes a plurality of shared and or discrete components coupled to one another to form a transmit path410, a receive path460, a local pseudo link management module440and a hybrid front end (HFE)454which couples the transmit and receive paths of the transceiver to subscriber line456.

The receive path460in this embodiment of the invention includes: analog filter462, line amplifier464, analog-to-digital converter (ADC)466, digital filter468, decimator470, cyclic prefix remover472, discrete Fourier transform engine (DFT)474, frequency domain equalizer476, decoder478, tone re-orderer480and deframer482. In operation received data of each communication channel is subject to analog filtration and amplification. The received data is then digitized in the ADC followed by digital filtration, if any. Next received data is subject to any required decimation in the decimator. Next the cyclic prefix or suffix of each data symbol is removed. Each symbol of data is then transformed from the time to the frequency domain in the DFT and subject to equalization in the frequency domain. Each symbol of data is then decoded in decoder and serialized in the tone re-orderer. The demodulated data is then de-framed in the de-framer and transferred to the ATM, Ethernet or other network to which the transceiver is coupled.

The transmit path410includes: framer412, tone orderer414, encoder416, frequency domain equalizer418, inverse discrete Fourier transform engine (IDFT)420, cyclic prefix wrapper422, interpolator424, digital filter426, digital-to-analog converter (DAC)428, line driver430and filter432. In operation transmitted data of each communication channel is framed in the framer, loaded bit by bit into corresponding tone bins by the tone-orderer, converted to a complex number representation of the corresponding point in the symbol constellation and subject to frequency domain equalization. Then each set of resultant tones, a.k.a. a symbol, is transformed from the frequency domain to the time domain in the IDFT. Subsequently any required cyclic suffix or prefix is added and the resultant data in the time domain is subject to interpolation in interpolator. After filtering in the digital filter the interpolated data is passed to the DAC. The DAC converts the digitized data of each communication channel to corresponding analog signals. These analog signals are amplified by the line driver. The amplified output of the line driver is passed to analog filter and then via HFE454to subscriber line456.

In an embodiment of the invention the transceiver also includes a local pseudo link management module440. This module handles the detection and disruption of pseudo links the presence of which indicates a pseudo session between a local and a remote transceiver which are not physically coupled to one another via a subscriber line.

The local pseudo link management module440includes: a controller444, a crosstalk detector446, a disrupter448and a line monitor450. The local pseudo link management module monitors received communications via connection442to the receive path and injects any required disruptor signal into the transmit path via connection452. In operation the pseudo link management module and specifically the controller thereof is responsive to activation of the transceiver generally or specifically to activation by the DSLAM controller122(SeeFIG. 1). The operation of the pseudo link management module takes place before the training phase of operation of the associated transceiver and assures that the remote modem does not enter into a pseudo-session, or if it does, that the pseudo-session is disrupted.

On activation the local pseudo link management module enables monitoring of the subscriber line456via line monitor450, and measurement of crosstalk levels on the subscriber line via crosstalk detector446. The line monitor is listens to the associated subscriber line456for any indication of a pseudo-link and the associated pseudo-session. If a communication signal is detected, e.g. upstream communication signal304(SeeFIG. 3) then the disrupter448is activated for a specified time interval, after which the pseudo-session is aborted by the remote transceiver. Any subsequent ACKREQ signal is received and acknowledged by the correct transceiver, e.g. the CO transceiver400to which the remote transceiver is physically coupled over subscriber line456. Subsequently a normal session is established, free of any pseudo link, and between physically connected transceivers.

The hardware blocks shown in this and the following figures may be alternately implemented in software or firmware. The transmit and receive path described above and the scalable components thereof may be applied with equal advantage in embodiments of the invention where a plurality of XDSL channels are multiplexed on the transmit and receive path of the transceiver. The transmit and receive path components may be coupled to one another either by packet based transfer of successive packetized portions of a communication channel or by dedicated point-to-point coupling between components. In still another embodiment of the invention the pseudo link management processes may be implemented on card in pseudo link management unit,138(SeeFIG. 1).

The components shown inFIG. 4collectively comprise a physical transceiver. In alternate embodiments of the invention the functions performed by the components may be implemented on a logical transceiver implemented on a combined digital signal processor (DSP) and analog front end (AFE) such as that shown on the line card134inFIG. 1.

FIG. 5is a process flow diagram of an embodiment of the processes associated with XDSL pseudo link management. After startup500control passes to process502. In process502DSLAM or transceiver activation of the pseudo link manager results in monitoring of a selected subscriber line for a pseudo link. In process504crosstalk on the selected subscriber line is also measured. Next in decision process506a determination is made as to whether a communication signal indicative of a pseudo session exists on the selected subscriber line. If it does not then control passes to decision process512. If it does control passes to processes508and510for disrupting the pseudo link. In process508the power level of a disruptor signal is set at a level just above the level of the measured crosstalk and well below the power level associated with a downstream power mask of any of the XSL standards. Next in process510the disrupter signal is injected into the transceiver's transmit path for a set interval. Next in decision process512a determination is made as to whether the remote transceiver is sending an ACTREQ signal requesting a normal session. If not control returns to decision process506. If however an ACTREQ signal is received control passes to process514in which the training phase of operation is commenced. Once training is complete the physically coupled CO and remote transceivers enter the showtime phase514of a new session.