Abstract:
A system and method for reducing transmit carrier wander in a DSL communication system are disclosed. A network timing reference unit provides an automatic embedded solution for synchronizing DSL frames to an external communication system reference clock. The network timing reference unit applies or removes bits to adjust the length of a DSL frame in response to a sliding window state table. A sliding window is selected in response to the relative position of the DSL frame to a system clock reference point over a number of DSL frames. A network timing reference unit in accordance with the present invention may comprise a counter, a network timing latch, a synchronization word detector, a DSL frame latch, a lead/lag comparator, a sliding window buffer, a sliding window state table, a DSL frame state recorder, and a sensitivity buffer. The present invention provides a method for reducing transmit carrier wander in a DSL transceiver. In its broadest terms, the method can be described as: receiving a network clock and a DSL data stream comprising a plurality of frames; identifying a reference point on the network clock signal; identifying a DSL frame reference point; recording the relative position of the DSL frame reference point to the network clock reference point; performing a bit-manipulation responsive to the relative reference positions and a current window position; and adjusting the current window position in response to a consistent relative reference position between the network clock and DSL frame reference points.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of co-pending U.S. provisional patent application, issued Ser. No. 60/171,385, and filed Dec. 22, 1999, which is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to data communications, and more particularly, to a system and method for achieving reduced timing wander in a digital subscriber line (DSL) communication system. 
     BACKGROUND OF THE INVENTION 
     In the field of data communications a transceiver, or modem, is used to convey information from one location to another. Digital subscriber line (DSL) technology now enables DSL transceivers to more rapidly communicate data than previously possible with purely analog modems. DSL transceivers communicate by modulating a baseband signal carrying encoded digital data, converting the modulated digital data signal to an analog signal, and transmitting the analog signal over a conventional copper wire pair using techniques that are known in the art. These known techniques include mapping the information to be transmitted into a multi-dimensional multi-level signal space constellation and slicing the received constellation to recover the transmitted information. 
     The constellation can include both analog and digital information or only digital information. 
     In the above mentioned communications environment, a central office DSL transceiver is located at a telephone company central office location. Connected to the DSL transceiver via a conventional copper wire pair is a suitably configured remote DSL transceiver. The remote transceiver resides at a location commonly referred to as a customer premise. Before the central office transceiver can exchange information with the remote transceiver, clock timing and synchronization between the central office transceiver and a network master clock should be established. 
     Timing and synchronization are fundamental to any digital transmission and switching network. In a digital transmission system, timing is encoded with the transmitted signal using a network master clock, such as a T1 or E1 clock as a reference clock. As such, the central office transceiver must recover system timing and synchronization from this system clock. Once frequency synchronization between the central office transceiver and the network clock is achieved, the central office transceiver can identify frame boundaries of downstream data signals designated for further transmission to the remote transceiver. In addition, the central office transceiver can identify frame boundaries of upstream data signals received from the remote transceiver that may be designated for further transmission to other network connected devices. 
     In the aforementioned communications environment, synchronization is provided in a master-slave relationship such that the network timing (e.g., a T1 clock) is the master allowing it to provide timing information to all the slave data transmission systems connected to the network. Each remote transceiver connected to the network must be synchronized to the network system clock as provided by the central office transceiver. 
     In order to achieve higher data rates with a fixed distance or with a given non-rate adaptive DSL transceiver technology, two or more DSL lines may be combined. By way of example, high-speed DSL (HDSL) technology uses two pairs of twisted copper wire, HDSL transceivers, multiplexers and demultiplexers at each end of a communication link to provide T1 capacity service over two pairs of twisted copper conductors commonly used in local subscriber loops within the PSTN. The European version of HDSL binds three pairs of twisted copper conductors and their related transceivers, multiplexers, and demultiplexers to provide E1 capacity service. 
     The prior art HDSL link illustrated in FIG. 1 is offered by way of example to highlight various interface equipment that may be used to provide a T1 capacity link between a PSTN central office (CO) and a customer premise (CP). In this regard, FIG. 1 illustrates a basic HDSL network link architecture. As illustrated in FIG. 1, a HDSL network link  10  may comprise equipment located within a CO  20 , equipment located within a CP  40 , and HDSL interface equipment  30  as required within each location to transfer data to and from an ATM switch (not shown). More specifically, the central office  20  may comprise a plurality of trunk line interfaces  21 ,  23 , and  25 , herein labeled analog trunk card, digital trunk card, and optical trunk card, respectively; a PSTN digital switch  22 ; and a plurality of HDSL termination units—central office (HTU-C)  24   a ,  24   b ,  24   c , . . ., and  24   x . As illustrated in FIG. 1, each HTU-C  24   a ,  24   b ,  24   c , . . . , and  24   x  may be coupled via two twisted pair telephone transmission lines  31   a ,  31   b  to a dedicated HDSL termination unit—remote (HTU-R)  44  (one shown for simplicity of illustration). 
     As also illustrated in FIG. 1, the combination of the HTU-C  24   c , the two twisted pair telephone transmission lines  31   a ,  31   b , and the HTU-R  44  may comprise the HDSL interface equipment  30 . As further illustrated in FIG. 1, the CP  40  may comprise a customer interface  46  and customer premise equipment  48  which may contain one or more computing devices (not shown). 
     It is significant to note that downstream and upstream data transmissions that are transmitted across the HDSL network link  10  of FIG. 1 must be processed at the HTU-Rs  44  and the HTU-Cs  24  in order to ensure that data transmissions are inverse multiplexed and reconstructed into their original configuration. Each of the HTU-Rs  44  and the HTU-Cs  24  may further comprise a transceiver and a mapper (both not shown). At one end of the HDSL communications network  10 , a first mapper may be used to inverse multiplex or distribute a data transmission across multiple transmit media (i.e., the twisted pair telephone transmission lines  31   a ,  31   b ). At the opposite or receiving end of the HDSL communications network  10 , a second mapper may be used to multiplex or reconstruct the original data transmission. By way of example, a downstream data transmission may be inverse multiplexed such that a portion of the data is transmitted via the HTU-C  24   c  across a first twisted pair telephone transmission line  31   a  with the remaining portion of the data transmission sent via a second twisted pair telephone transmission line  31   b . After the first and second portions of the data transmission are received and reconstructed by the HTU-R  44 , the first and second portions of the original data stream may be multiplexed before being forwarded to the customer interface  46  and the CPE  48 . Often the customer interface  46  is implemented with a router having a port coupled with one or more HTU-Rs  44  and or other network interface devices. 
     A common technique for achieving timing synchronization between the network clock and the central office transceiver is based upon the use of an external framer, which performs a bit-stuffing operation. In this arrangement the aggregate bit stream has a higher data rate than the input data rate from the network. This data rate relationship accommodates the additional stuffing and framing bits. Bits are stuffed (inserted) or deleted (removed) from the incoming data stream until a clock rate derived from the incoming data stream is equal to that of the input data rate from the network. This bit stuffing operation permits the transceiver to derive a local clock with a frequency that tracks the frequency of the network clock. 
     Presently, the add/delete or bit-stuffing mechanism synchronizes a customer interface and a transmit carrier by determining the relative position of a DSL frame reference point to a periodic customer reference point and responding accordingly. When the DSL frame reference point leads the customer reference point, a timing field in the frame is set to 4 bits. Otherwise, the timing field is set to 0 bits. The present bit-stuffing mechanism generates a significant wander in the DSL frame with respect to the customer reference point. The wander is not removable. Accordingly, it is desired to provide a system and method that efficiently and accurately reduces timing reference wander in a DSL based communications system. 
     SUMMARY OF THE INVENTION 
     In light of the foregoing, the present invention uses a sliding window algorithm that may be implemented on a digital signal processor (DSP) to reduce timing reference wander in a DSL communication system. The system and method of the present invention provide for the synchronization of one or more derived clocks to a network system clock without extensive modification or additional external circuit components. 
     The system may be implemented in hardware or with a combination of firmware and software that uses a state table to apply designated stuff/delete bits for each of a plurality of sliding windows. The sliding windows may be controlled by monitoring the relative position of the DSL frame to the network system clock and selecting the active window in response to the relative position over a number of DSL frames. The configuration ensures that after initial acquisition, the locally generated clock and all clock signals derived from the local clock signal dynamically track any frequency and phase variation of the external reference clock. 
     In a preferred embodiment, a network timing reference clock may be configured to drive a counter, which may be used to trigger a first latch upon receiving X clock signal transitions. The first latch may be reset after an appropriate delay. Concurrently, a selected reference point within a DSL frame being processed in a DSL transceiver may be used to trigger a second latch. A comparator may determine from the condition of the first latch at the point the second latch is triggered whether the DSL frame is leading or lagging the network timing reference point. A result from the comparator may be used in conjunction with a sliding window identifier to select an appropriate set of delete or stuffing bits from a sliding window state table. Furthermore, a DSL frame state recorder may be configured to monitor the relative position of the DSL frame reference point with regard to the network timing reference clock over a variable number of frames and responsively select an adjacent sliding window if the DSL frame reference point leads or lags the network timing reference point for M consecutive frames. 
     A network timing reference unit in accordance with the present invention may comprise a counter, a network timing latch, a synchronization word detector, a DSL frame latch, a lead/lag comparator, a sliding window buffer, a sliding window state table, a DSL frame state recorder, and a sensitivity buffer. The present invention provides a method for reducing transmit carrier wander in a DSL transceiver. In its broadest terms, the method can be described as: receiving a network clock and a DSL data stream comprising a plurality of frames; identifying a reference point on the network clock signal; identifying a DSL frame reference point; recording the relative position of the DSL frame reference point to the network clock reference point; performing a bit-manipulation responsive to the relative reference positions and a current window position; and adjusting the current window position in response to a consistent relative reference position over time. 
     Other features and advantages of the present invention will become apparent to one skilled in the art upon examination of the following drawings and detailed description. It is intended that all such additional features and advantages be included herein within the scope of the present invention, as defined in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention, and together with the description serve to explain the principles of the invention. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. In the drawings: 
     FIG. 1 is a schematic diagram illustrating a prior art HDSL communication system. 
     FIG. 2 is a schematic diagram illustrating the standard HDSL frame structure for data streams communicated across the HDSL communication system of FIG.  1 . 
     FIG. 3 is a functional block diagram illustrating an improved HDSL transmission unit in accordance with the present invention. 
     FIG. 4 is a functional block diagram illustrating a network timing reference unit of the improved HDSL transmission unit of FIG.  3 . 
     FIGS. 5A and 5B are a flowchart illustrating a method for reducing transmit carrier wander as may be practiced by the network timing reference unit of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Having summarized the invention above, reference is now made in detail to the description of the invention as illustrated in the drawings. While the invention will be described in connection with these drawings, there is no intent to limit it to the embodiment or embodiments disclosed therein. On the contrary, the intent is to cover all alternatives, modifications and equivalents included within the spirit and scope of the invention as defined by the appended claims. 
     The present invention can be implemented in software, hardware, or a combination thereof. In the preferred embodiment, the elements of the present invention are implemented in hardware within the various circuit components of an application specific integrated circuit (ASIC) such as a suitably configured digital signal processor (DSP). In an alternative embodiment, a network timing reference unit in accordance with the present invention may be implemented in software that is stored in a memory and that configures and drives a suitable DSP. However, the foregoing software can be stored on any computer-readable medium for transport or for use by or in connection with any suitable computer-related system or method. In the context of this document, a computer-readable medium is an electronic, magnetic, optical, or other physical device or means that can contain or store a computer program for use by or in connection with a computer-related system or method. 
     While the foregoing preferred embodiment illustrates the invention in the context of a high-bit-rate DSL (HDSL) remote transceiver, the features of the present invention are applicable to DSL central office transceivers and like devices configured to support HDSL2, SDSL, G.SHDSL, and SHDSL2 communication protocols. For that matter, the features of the present invention are applicable to any master-slave timing application where a device synchronizes one or more data streams using a protocol that permits bit-stuffing to match a data stream to a master clock. Those skilled in the art will appreciate that the system and method for reducing transmit carrier wander in accordance with the present invention is applicable and preferable in both the central office and remote transceiver equipment. 
     Before presenting the teachings and concepts of a network timing reference unit in accordance with the present invention, FIG. 2 is presented to illustrate a standard protocol for transmitting data in a synchronous DSL communication system. In this regard, reference is directed to FIG. 2, which illustrates a HDSL frame structure as defined by the European Telecommunications Standards Institute (ETSI) in technical specification 101 135 V1.5.1 (1998-11) entitled,  Transmission and Multiplexing  ( TM );  High bit - rate Digital Subscriber Line  ( HDSL )  transmission systems on metallic local lines; HDSL core specification and applications for combined ISDN - BA  and 2,048 kbit/s  transmission . In this regard, FIG. 2 illustrates the HDSL frame structure composed of quaternary symbols (quats) and the mapping of the core frame bytes to the frame structure. The frame is divided into four groups. The first group of the frame starts with a synchronization word herein labeled, “SYNCH WORD.” As illustrated in FIG. 2, the synchronization word has a length of seven symbols followed by one HDSL overhead quat, “HOH,” and 12 blocks of HDSL payload data, “B 01  to B 12 .” Each of the blocks of payload data consists of 72.5 quats or 145 bits. As shown in FIG. 2, the payload blocks comprise an overhead bit, Z mn , and 18 bytes of the core frame. The overhead bits, Z mn , identify the transmission pair and the HDSL payload block as follows: m=1, 2 indicates one of the two-pairs; n=1 to 48 is the number of the HDSL payload block in the frame. As further illustrated in FIG. 2, the odd bytes are designated for transmission via pair one (i.e., the twisted-pair telephone transmission line  31   a ), whereas the even bytes are designated for transmission via pair two. 
     As shown in FIG. 2, the Z mn -bits, provide an additional overhead channel of 48 bits/frame for each HDSL transceiver system  10  (FIG. 1) at a data rate of 8 kbit/s. The first eight Z-bits, Z m1  to Z m8 , are reserved for core applications. Bits Z m1 , Z m2  are used for pair identification, whereas bits Z m3  to Z m8  are reserved for future use and are presently set to one. The Z-bits 9 to 48, Z m9  to Z m48 , are application specific and are transparently transported through the HDSL core. 
     The three groups following the first group have an equivalent structure. Each of the groups consists of five HDSL overhead quats (HOH) and 12 HDSL payload blocks (B 01 -B 12 , B 13 -B 24 , B 25 -B 36 , and B 37 -B 48 ) as described above. So one HDSL frame  100  consists of a synchronization word, 16 HDSL overhead quats, 48 Z-bits and 864 bytes of the core frame. 
     At the end of the frame, two placeholders, “SQ 1 ” and “SQ 2 ,” are available for stuffing quats. The stuffing quats are used together; this means that either none or two of the stuffing quats are inserted into the frame, depending on relative timing with an external reference. Each stuffing quat may contain a sign bit and a magnitude bit. In accordance with the standard, the values of the stuffing quats are left as a choice to individual vendors. 
     As a result of the variability provided by the stuffing quats, the length of the HDSL frame is either 3,505 quats, which corresponds to 6+1/584 ms for the nominal HDSL clock frequency, or 3,503 quats, which corresponds to 6−1/584 ms. Over time the average will tend to 3,504 quats or 6 ms. It will be appreciated that the HDSL transceivers  22 ,  44  in a HDSL communication system  10  (FIG. 1) may evaluate the length of an incoming frame by detecting the synchronization word in the following frame and reacting accordingly. 
     It will be appreciated by those skilled in the art that transmit carrier wander will vary depending on the specific variety of DSL used. For example, if a three twisted-pair configuration is used, the HDSL frame will vary from 6−1/392 ms to 6+1/392 ms. For the case where one twisted-pair telephone transmission line  31  (FIG. 1) is used, the frame will vary from 6−1/1,160 ms to 6+1/1,160 ms. 
     Having described the HDSL frame structure and the origin of the expected wander of a transmit carrier in a HDSL communication system, reference is now directed to the remaining figures which highlight the concepts and teachings of a network timing reference unit in accordance with the present invention. In this regard, reference is now directed to FIG. 3, which illustrates a functional block diagram of an improved HDSL transmission unit in accordance with the present invention. As illustrated in FIG. 3, a customer premise located HTU-R  244  in accordance with the present invention may be integrated with a customer interface  46  to complete a HDSL communication link between a CO  20  and CPE  48  at a CP  40  (FIG.  1 ). As shown in FIG. 3, the improved HTU-R  244  in accordance with the present invention may receive and transmit digitally encoded data transmissions that may be formatted in accordance with the HDSL frame of FIG.  2 . As previously described in regard to FIG. 1, the data transmissions may be sent and received along a pair of twisted-pair telephone transmission lines  31   a ,  31   b . As illustrated in FIG. 3, the twisted-pair telephone transmission lines  31   a ,  31   b  may be communicatively coupled to the improved HTU-R  244 . As also illustrated in FIG. 3, the improved HTU-R  244  may be communicatively coupled via at least one bi-directional data bus  260  with a customer interface  46 . The customer interface  46  may be further configured with at least one bi-directional data interface  270  to complete a data communications link between the improved HTU-R  244  and the CPE  48  (FIG.  1 ). As is further illustrated in FIG. 3, the customer interface  46  may receive a timing reference signal  275 . It will be appreciated that the timing reference signal  275  may take the form of a T1 or E1 clock. 
     As illustrated in FIG. 3, the improved HTU-R  244  in accordance with the present invention may also comprise a memory device  230 , and a DSP  220 , in addition to the PSTN interface  210 . The DSP  220  may be in communication with the PSTN interface  210  via at least one PSTN bi-directional interface bus  240 . As shown in FIG. 3, the DSP  220  may be communicatively coupled to the memory device  230  via at least one memory bus  250 . Those skilled in the art will appreciate that the DSP  220  may be configured along with the memory device  230  to provide a plurality of functions to coordinate the transfer of data between the CPE  48  (FIG. 1) and various computing devices interconnected to the PSTN via a CO located HTU-C  24   c  (FIG.  1 ). 
     In accordance with a preferred embodiment, the DSP  220  within the improved HTU-R  244  may comprise a network timing reference unit  300  and at least one sliding window state table  222 ,  224 . As will be explained below in regard to FIG. 4, the network timing reference unit  300  may receive the timing reference signal  275  as well as a series of HDSL frames. The network timing reference unit  300  may be configured to apply the delete and stuffing bits D 1 , D 2 , S 1 , and S 2  as indicated in at least one of the sliding window state tables  222 ,  224 , respectively. Those skilled in the art will appreciate that in an alternative embodiment, the sliding window state table(s)  222 ,  224  may be stored in the memory device  230  for retrieval and application as required by the DSP  220 . As shown in FIG. 3, the first sliding window state table  222  may be appropriate for application with HDSL communication systems configured to apply CAP modulation. 
     Whereas, the second sliding window state table  224 , may be appropriate for application with HDSL communication systems configured to apply 2B1Q data modulation (e.g., the exemplary HDSL frame structure of FIG.  2 ). Having introduced and described an improved HTU-R  244  in accordance with the present invention with regard to FIG. 3, reference is now directed to FIG. 4, which illustrates a functional block diagram of a network timing reference unit  300  of the improved HTU-R  244  (FIG.  3 ). As illustrated in FIG. 4, a network timing reference unit (NTRU)  300  may receive a sliding window register input  365 , a sensitivity buffer input  399 , a timing reference signal  275  (e.g., the T1/E1 clock), and a HDSL data input  240 . As also illustrated in FIG. 4, the NTRU  300  may generate a stuff/delete control output signal  225 . As shown in FIG. 4, the NTRU  300  may comprise a counter  310 , a reference clock latch  320 , a lead/lag comparator  330 , a synchronization word detector  340 , a DSL frame reference latch  350 , a sliding window register  360 , a sensitivity buffer  390 , a DSL frame state recorder  380 , and the sliding window state table  222 ,  224  (see FIG.  3 ). Those skilled in the art will appreciate that alternatively, the sliding window state table  222 ,  224  may be stored in the memory  230  rather than within the NTRU  300 . 
     As also shown in FIG. 4, the sliding window register input  365  may be coupled to the sliding window register  360  to select an initial sliding window (i.e., window  1 ,  2 ,  3 , or  4 ) for the NTRU  300 . Similarly, the sensitivity buffer input  399  may be coupled to the sensitivity buffer  390  in order to store a sensitivity threshold, M. As will be explained later with regard to flowchart of FIGS. 5A and 5B, the sensitivity threshold, M, may represent the number of consecutive DSL frames that must have the same lead/lag state before a new sliding window (e.g., an adjacent window) is selected for directing the application of stuff/delete bits to DSL frames. As illustrated in FIG. 4, the timing reference signal  275  may be coupled to an input of the counter  310 . In accordance with a preferred embodiment, the counter  310  may be configured to trigger a reference clock latch input signal  315  upon receipt of the X th  clock transition. Having received an indication that the X th  clock transition has occurred, the reference clock latch  320  may be configured to indicate the same via a first lead/lag comparator input  325 . As also illustrated in FIG. 4, the HDSL data input  240  may be coupled to an input of the synchronization word detector  340 . The synchronization word detector  340  may be configured to trigger a DSL frame latch input  345  upon receiving a synchronization word within the DSL data stream. Having received an indication that the DSL frame. synchronization word for the next DSL frame has been processed, the DSL frame latch  350  may be configured to indicate the same via a second lead/lag comparator input  355 . In turn, the lead/lag comparator  330  may receive the first and second lead/lag comparator inputs  325 ,  355 , respectively, and may be configured to provide an output signal  335  that indicates whether the DSL frame synchronization word is leading or lagging the timing reference signal  275 . 
     As illustrated in FIG. 4, the sliding window state table  222 ,  224  may receive a sliding window register input signal  375  indicative of the current sliding window ( 1  through  4 ) that is to be applied for selecting the stuff/delete bits. In addition, the sliding window state table  222 ,  224  may be configured to receive the lead/lag comparator output signal  335 . Together, the lead/lag comparator output signal  335  and the sliding window register input signal may identify the appropriate stuff/delete bits to be applied to the DSL frame to correct the relative timing of the DSL frame to the timing reference signal  275 . As shown in the schematic of FIG. 4, the sliding window state table  222 ,  224  may be configured to supply the stuff/delete bits via the stuff/delete control signal  225 . It will be appreciated by those skilled in the art that the counter  310 , the reference clock latch  320 , the lead/lag comparator  330 , the synchronization word detector  340  and the DSL frame reference latch  350  may be reset at any time after the sliding window table  222 ,  224  has sent the stuff/delete control output signal  225 . 
     As further illustrated in the schematic of FIG. 4, the sensitivity buffer  390  may be configured to apply an indicator of a desired sliding window control sensitivity threshold via output  395  to the DSL frame state recorder  380 . As illustrated in the schematic of FIG. 4, the DSL frame state recorder  380  may also be configured to receive the lead/lag comparator output signal  335  from the lead/lag comparator  330 . In accordance with a preferred embodiment, the DSL frame state recorder  380  of the NTRU  300  may be configured to wait until it receives M consecutive lead or conversely M consecutive lag signals before sending an indicator of a new desired sliding window to the sliding window register  360  via the DSL frame state recorder output  385 . As will become apparent during the description of the flowchart of FIGS. 5A and 5B below, if the DSL frame state recorder  380  indicates that the DSL frame is lagging behind the timing reference signal  275  for M consecutive frames, the sliding window will be incremented (i.e., the sliding window may transition from sliding window “2” to sliding window “3.”) Conversely, if the DSL frame state recorder  380  indicates that the DSL frame is leading the timing reference signal  275  for M consecutive frames, the sliding window will be decremented (i.e., the sliding window may transition from sliding window “2” to sliding window “1.”) 
     Having introduced and described a network timing reference unit  300  that may be implemented within an improved HTU-R  244  with regard to FIG. 4, reference is now directed to FIGS. 5A and 5B, which present a flowchart illustrating a method for reducing transmit carrier wander that may be performed by the network timing reference unit  300  of FIG.  4 . In this regard, a method for reducing transmit carrier wander  400  may begin with step  405 , herein designated as “Start.” Next, in step  410 , the method for reducing transmit carrier wander  400  may be configured to initialize at least two variables herein designated, “WIN_NUM” and “CURRENT_STATE,” respectively. In addition, a frame state recorder  380  (FIG. 4) may be cleared, a sliding window control sensitivity threshold, M, may be set, and a counter may be reset to 0. The method for reducing transmit carrier wander  400  may proceed by waiting for a DSL frame reference point as indicated in step  415 . As described above with regard to the NTRU  300  of FIG. 4, a synchronization word (see FIG. 2) or other readily identifiable portion of the DSL data frame may serve as the DSL frame reference point. Once the DSL frame reference point has been received, the method for reducing transmit carrier wander  400  may record the relative position of a timing reference signal to the DSL frame reference point as illustrated in step  420 . 
     Having recorded the relative position of the timing reference signal to the DSL frame reference point, the method for reducing transmit carrier wander may be configured to determine if the DSL frame reference point leads the timing reference signal, as illustrated in the query of step  425 . If the query of step  425  indicates that the DSL frame reference leads the timing reference signal, as shown by the affirmative branch from step  425 , the method for reducing transmit carrier wander  400  may be configured to apply the delete bits in accordance with the sliding window state table  222 ,  224  of FIG. 3 to the HDSL frame  100  of FIG. 2 as shown in step  430 . Otherwise, if the query of step  425  indicates that the DSL frame reference lags the timing reference signal, as shown by the negative branch from step  425 , the method for reducing transmit carrier wander  400  may be configured to insert the stuffing bits in accordance with the sliding window state table  222 ,  224  of FIG. 3 to the HDSL frame  100  of FIG. 2, as illustrated in step  435 . 
     Next, the method for reducing transmit carrier wander  400  may be configured to send the current state of the relative position of the DSL frame reference to the timing reference to a DSL frame state recorder  380  (FIG. 4) as indicated in step  440 . As illustrated in step  445  of the flowchart of FIG. 5A, the method for reducing transmit carrier wander  400  may proceed by making a determination if the DSL frame state recorder  380  (FIG. 4) indicates that the DSL frame reference point has remained at the same state for two consecutive DSL frames. If the query of step  445  is negative, the method for reducing transmit carrier wander  400  may perform step  450 , where the DSL frame state recorder  380  (FIG. 4) is reset. If the determination in the query of step  445  is affirmative, the method for reducing transmit carrier wander  400  may be configured to perform step  455 , where a query may be performed to determine is the DSL frame reference point is lagging behind the timing reference signal. As illustrated in the flowchart of FIG. 5A, if the determination in the query of step  455  is affirmative, the method for reducing transmit carrier wander  400  may continue with flowchart connector, “A” on FIG.  5 B. Otherwise, if the determination in the query of step  455  is negative, the method for reducing transmit carrier wander  400  may continue with flowchart connector “B” on FIG.  5 B. 
     As illustrated in the flowchart of FIG. 5B, if the method flow from FIG. 5A arrives at flowchart connector “A,” the method for reducing transmit carrier wander  400  may proceed by checking if the frame state counter has reached the sliding window control sensitivity threshold, M, as illustrated in step  460 . If the determination in the query of step  460  is affirmative, the method for reducing transmit carrier wander  400  may be configured to increment the sliding window number as indicated in step  465 . Next, the method for reducing transmit carrier wander  400  may be configured to clear the DSL frame state recorder  380  (FIG. 4) and reset the frame state counter as indicated in step  470 . Otherwise, if the determination in the query of step  460  is negative, the method for reducing transmit carrier wander  400  may be configured to perform step  475 , where the frame state counter may be incremented by one. Having determined that the DSL frame reference point is lagging behind the timing reference signal and having reacted appropriately, the method for reducing transmit carrier wander  400  may proceed by continuing at flowchart connector “C” on FIG.  5 A. As revealed in the flowchart of FIGS. 5A and 5B, steps  415  through  475  may be repeated as previously described. 
     As illustrated in the flowchart of FIG. 5B, if the method flow from FIG. 5A arrives at flowchart connector “B,” the method for reducing transmit carrier wander  400  may proceed by checking if the frame state counter has reached the sliding window control sensitivity threshold, M, as illustrated in step  480 . If the determination in the query of step  480  is affirmative, the method for reducing transmit carrier wander  400  may be configured to decrement the sliding window number as indicated in step  485 . Next, the method for reducing transmit carrier wander  400  may be configured to clear the frame state recorder and reset the frame state counter as indicated in step  490 . Otherwise, if the determination in the query of step  480  is negative, the method for reducing transmit carrier wander  400  may be configured to perform step  495 , where the frame state counter may be incremented by one. Having determined that the DSL frame reference point is leading the timing reference signal and having reacted appropriately, the method for reducing transmit carrier wander  400  may proceed by continuing at flowchart connector “C” on FIG.  5 A. As illustrated in the flowchart of FIGS. 5A and 5B, steps  415  through  455  and  480  through  495  may be repeated as previously described. 
     Any process descriptions or blocks in the flowchart of FIGS. 5A and 5B should be understood to represent modules, segments, or portions of code which include one or more executable instructions for implementing specific logical functions or steps in the process for reducing transmit carrier wander in a DSL transceiver. Alternate implementations are included within the scope of the preferred embodiment of the present invention in which functions may be executed out of order from that shown or discussed, including substantially, concurrently, or in reverse order, depending on the functionality involved, as would be understood by those reasonably skilled in the art of the present invention. 
     It will be appreciated that the method for reducing transmit carrier wander  400  in accordance with the present invention may comprise an ordered listing of executable instructions for implementing logical functions and can be embodied in any computer-readable medium for use by or in connection with an instruction execution system, apparatus, or device, such as a computer-based system, processor-containing system, or other system that can fetch the instructions from the instruction execution system, apparatus, or device and execute the instructions. In the context of this document, a “computer-readable medium” can be any means that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. The computer readable medium can be, for example but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, device, or propagation medium. More specific examples (a non-exhaustive list) of the computer-readable medium would include the following: an electrical connection (electronic) having one or more wires, a portable computer diskette (magnetic), a random access memory (RAM) (electronic), a read-only memory (ROM) (electronic), an erasable programmable read-only memory (EPROM or Flash memory) (electronic), an optical fiber (optical), and a portable compact disc read-only memory (CDROM) (optical). Note that the computer-readable medium could even be paper or another suitable medium upon which the program is printed, as the program can be electronically captured, via for instance optical scanning of the paper or other medium, then compiled, interpreted or otherwise processed in a suitable manner if necessary, and then stored in a computer memory. 
     It should be emphasized that the above-described embodiments of the present invention, particularly, any “preferred” embodiments, are merely possible examples of implementations, merely set forth for a clear understanding of the principles of the invention. Many variations and modifications may be made to the above-described embodiment(s) of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of the present invention and protected by the following claims.