Abstract:
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&#39;s pre-existing session and allowing a new session to be established with the remote transceiver.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of prior filed co-pending Provisional Application No. 61/024,331 filed on Jan. 29, 2008 entitled “Method and Apparatus for Correcting the xDSL synch-up Condition over the Cross-Talk of the Twisted-Pairs” which is incorporated herein by reference in its entirety as if fully set forth herein. 
    
    
     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, ADSL 2 , ADSL 2+ , VDSL 1 , VDSL 2 , 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&#39;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&#39;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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the present invention will become more apparent to those skilled in the art from the following detailed description in conjunction with the appended drawings in which: 
         FIG. 1  is a system diagram of a multi-tone communication system in which individual subscribers are coupled via high speed communication links over public service telephone network (PSTN) subscriber lines with residential and business customers; 
         FIG. 2A  is a hardware block diagram of an opposing sets of transceiver s coupled to one another by corresponding subscriber lines within a bundle; 
         FIG. 2B  is a cross-sectional view of the subscriber lines in the bundle shown in  FIG. 2A ; 
         FIG. 3  is 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 in which XDSL pseudo link management takes place; 
         FIG. 4  is a hardware block diagram showing an embodiment of a transceiver of the current invention mounted on a line card in the central office shown in  FIG. 1 ; 
         FIG. 5  is a process flow diagram of an embodiment of the processes associated with XDSL pseudo link management. 
     
    
    
     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: 
     
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Downstream 
                 Upstream 
               
               
                 Standard name 
                 Common name 
                 rate 
                 rate 
               
               
                   
               
             
             
               
                 ANSI T1.413-1998 
                 ADSL 
                 8 Mbit/s 
                 1.0 Mbit/s 
               
               
                 Issue 2 
               
               
                 ITU G.992.1 
                 ADSL (G.DMT) 
                 8 Mbit/s 
                 1.0 Mbit/s 
               
               
                 ITU G.992.1 Annex A 
                 ADSL over POTS 
                 8 Mbit/s 
                 1.0 MBit/s 
               
               
                 ITU G.992.1 Annex B 
                 ADSL over ISDN 
                 8 Mbit/s 
                 1.0 MBit/s 
               
               
                 ITU G.992.2 
                 ADSL Lite (G.Lite) 
                 1.5 Mbit/s   
                 0.5 Mbit/s 
               
               
                 ITU G.992.3/4 
                 ADSL2 
                 12 Mbit/s  
                 1.0 Mbit/s 
               
               
                 ITU G.992.3/4 Annex J 
                 ADSL2 
                 12 Mbit/s  
                 3.5 Mbit/s 
               
               
                 ITU G.992.3/4 Annex L 
                 RE-ADSL2 
                 5 Mbit/s 
                 0.8 Mbit/s 
               
               
                 ITU G.992.5 
                 ADSL2+ 
                 24 Mbit/s  
                 1.0 Mbit/s 
               
               
                 ITU G.992.5 Annex L [1]   
                 RE-ADSL2+ 
                 24 Mbit/s  
                 1.0 Mbit/s 
               
               
                 ITU G.992.5 Annex M 
                 ADSL2+M 
                 24 Mbit/s  
                 3.5 Mbit/s 
               
               
                 ITU G.993.1 
                 VDSL 
               
               
                 ITU G.993.2 
                 VDSL 2 
               
               
                 IEEE 802.16e 
                 WiMax 
               
               
                 IEEE 802.20 
                 Mobile Broadband 
                 1 Mbit/s 
                   1 Mbit/s 
               
               
                   
                 Wireless Access 
               
               
                   
               
             
          
         
       
     
       FIG. 1  is 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)  100  is coupled to multiple residential and business subscribers  110  via a bundle  108  of subscriber lines  102 ,  104 ,  106  for example. Each of the CO subscriber connections  102 - 106  terminates in the frame room  148  of 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) controller  122  and to the voice band racks  150 . The splitter shunts voice band communications to dedicated line cards, e.g. line card  154  or to a voice band modem pool (not shown). The splitter shunts higher frequency X-DSL communications on the subscriber line to a selected line card  134  within DSLAM  130 . 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)  118  is controlled by a Telco switch matrix  152  implementing 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 card  154 . 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 card  134 . That line card includes a plurality of AFE&#39;s e.g.  142 - 146  each 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 bus  140  to a DSP  136  which is also capable of multi-protocol support for all subscriber lines to which the AFE&#39;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 module  138 , 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 link  112 , is shown between the CO transceiver associated with AFE  142  and the subscriber line  102  to which it is coupled and a subscriber coupled to the remote end of subscriber line  104 . 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  &amp;  104 , in the bundle. The pseudo link not only grossly attenuates data rates associated with the subscriber&#39;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 line  104 , 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 bus  132  which 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&#39;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 following  FIG. 2  to be implemented. Each of the DSLAM line cards operates as a multi-tone transceiver under the control of a DSLAM controller  122  which 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 Internet  116 , to which the DSLAM is connected via server  120 . 
       FIG. 2A  is a hardware block diagram of an opposing sets of transceivers coupled to one another by corresponding subscriber lines within a bundle  200 . Within one of the opposing sets, transceivers  220 - 222  are referenced. Within another of the opposing sets transceiver s  226 , 228  are referenced. Within the bundle which may include 60 or more subscriber lines, lines  202 - 212  are referenced. 
       FIG. 2B  is a cross-sectional view of the subscriber lines in the bundle shown in  FIG. 2A . Subscriber lines  202 - 212  are 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. 3  is 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 t 0 -t 3 , in which XDSL pseudo link management takes place. Four sub-graphs,  300 ,  320 ,  340 ,  360  of frequency vs. power are shown at associated times t 0 , t 1 , t 2 , and t 3 , 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-graph  300  shows the signal activity on a selected subscriber line, e.g. subscriber line  104  at time t 0 . 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-graph  300 . Specifically a low frequency PSTN signal  302 , a communication signal  304  and crosstalk  312  are shown. The PSTN signal  302  spans a frequency from 0-4 kHz. The communication signal  304  spans 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 signal  304  corresponds to that associated with various XDSL standards and is significantly above that of the crosstalk  312  power 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-graph  320  at time t 1 . Specifically, the pseudo link management module injects a disrupter signal  328  onto the subscriber line it is monitoring, e.g. subscriber line  104 . The disruptor signal has a power level just above the level of the crosstalk  312  and 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-graph  340  at time t 2 . At time t 2  the disruptor signal has ceased. The disrupter signal has disrupted the remote transceiver&#39;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 signal  346  requesting 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 , and  25 . Additional tones may also be utilized for transport of the ACKREQ signal, e.g. tones with indexes  944 ,  972 ,  999 . 
     Sub-graph  360  shows the spectrum of the selected subscriber line at time t 3  after a new session between the physically coupled selected CO transceiver and remote transceiver is established. The spectral profile of upstream  364  and downstream  368  communication signal corresponds in the example shown to an ADSL communication profile. 
       FIG. 4  is a hardware block diagram showing an XDSL transceiver  400  incorporating components for pseudo link management. The transceiver includes a plurality of shared and or discrete components coupled to one another to form a transmit path  410 , a receive path  460 , a local pseudo link management module  440  and a hybrid front end (HFE)  454  which couples the transmit and receive paths of the transceiver to subscriber line  456 . 
     The receive path  460  in this embodiment of the invention includes: analog filter  462 , line amplifier  464 , analog-to-digital converter (ADC)  466 , digital filter  468 , decimator  470 , cyclic prefix remover  472 , discrete Fourier transform engine (DFT)  474 , frequency domain equalizer  476 , decoder  478 , tone re-orderer  480  and deframer  482 . 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 path  410  includes: framer  412 , tone orderer  414 , encoder  416 , frequency domain equalizer  418 , inverse discrete Fourier transform engine (IDFT)  420 , cyclic prefix wrapper  422 , interpolator  424 , digital filter  426 , digital-to-analog converter (DAC)  428 , line driver  430  and filter  432 . 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 HFE  454  to subscriber line  456 . 
     In an embodiment of the invention the transceiver also includes a local pseudo link management module  440 . 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 module  440  includes: a controller  444 , a crosstalk detector  446 , a disrupter  448  and a line monitor  450 . The local pseudo link management module monitors received communications via connection  442  to the receive path and injects any required disruptor signal into the transmit path via connection  452 . 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 controller  122  (See  FIG. 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 line  456  via line monitor  450 , and measurement of crosstalk levels on the subscriber line via crosstalk detector  446 . The line monitor is listens to the associated subscriber line  456  for any indication of a pseudo-link and the associated pseudo-session. If a communication signal is detected, e.g. upstream communication signal  304  (See  FIG. 3 ) then the disrupter  448  is 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 transceiver  400  to which the remote transceiver is physically coupled over subscriber line  456 . 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  (See  FIG. 1 ). 
     The components shown in  FIG. 4  collectively 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 card  134  in  FIG. 1 . 
       FIG. 5  is a process flow diagram of an embodiment of the processes associated with XDSL pseudo link management. After startup  500  control passes to process  502 . In process  502  DSLAM or transceiver activation of the pseudo link manager results in monitoring of a selected subscriber line for a pseudo link. In process  504  crosstalk on the selected subscriber line is also measured. Next in decision process  506  a 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 process  512 . If it does control passes to processes  508  and  510  for disrupting the pseudo link. In process  508  the 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 process  510  the disrupter signal is injected into the transceiver&#39;s transmit path for a set interval. Next in decision process  512  a determination is made as to whether the remote transceiver is sending an ACTREQ signal requesting a normal session. If not control returns to decision process  506 . If however an ACTREQ signal is received control passes to process  514  in which the training phase of operation is commenced. Once training is complete the physically coupled CO and remote transceivers enter the showtime phase  514  of a new session. 
     The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously many modifications and variations will be apparent to practitioners skilled in this art. It is intended that the scope of the invention be defined by the following claims and their equivalents.