WO2005036919A2 - Optimizing performance of a digital subscriber line - Google Patents

Abstract

Method and apparatus for optimizing the performance of a DSL communications link. The method can include automatically evaluating (506, 508, 510, 512, 514, 606) one or more parameters defining a performance related condition of a DSL transmission line (103). In response to the evaluating step, reflected harmonic energy {206, 208) appearing in a receive band (203) of the DSL transceiver (104) can be selectively reduced by automatically making certain adjustments. Specifically, these adjustments can include adjusting a transmit power (202, 212) of the DSL transceiver (104), adjusting a width of a transmit band (201) of the DSL transceiver, or both.

Description

OPTIMIZING PERFORMANCE OF A DIGITAL SUBSCRIBER LINE

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This Application claims the priority of United States provisional patent application No. 60/511 ,605 filed October 14, 2003.

BACKGROUND OF THE INVENTION Field of the Invention

[0002] The present invention relates to telephone networks and more particularly to performance optimization of a digital subscriber line.

Description of Related Art

[0003] Digital Subscriber Line ("DSL") is a telecommunications technology that permits broadband communications over ordinary copper telephone wires. It is well known that the connection speed available with DSL is directly affected by the distance between the network operator and the user, Connection speed goes down with increases in length of the connection. However, there are additional internal and external factors that also affect DSL transmission rate and reach. External factors can include line impairments such as bridged taps (one or more wire pairs spliced off of a main line for other telephones), different wire sizes, telephone loading coils, and so on. External factors can also include EMI and other sources of interference. Internal factors can include the noise floor for the system and its dynamic range.

[0004] Asymmetric DSL (ADSL) is one of the more common of DSL services, particularly among small business users. Many ADSL systems use a modulation technique consisting of multiple channels or sub-carriers called discrete multitone (DMT). Several standards exist for DMT based DSL including but not limited to ANSI T1 .413, ITU G.992.1 (G.DMT), ITU G.992.3 (ADSL2), ITU G.992.5 (ADSL2+), and G.993.1 (VDSL). The G.DMT standard divides data into 256 separate channels (sub- carriers), each 4 kHz wide, with some carriers unused. Digital data can be transmitted on each of the separate channels. In some overlapped echo cancelled modes of operation, frequencies are bi-directional, allowing data to flow upstream and downstream, but typically frequency division modulation is used so each channel is assigned to either transmit or receive data. From a customer standpoint, lower frequency channels are used to transmit data while higher frequency channels are used to receive data. In typical G.DMT ADSL Annex A, the transmit band occupies the lower frequencies from 30 kHz to 140 kHz and the receive band occupies 160 kHz to 1.1 MHz.

[0005] Two pieces of equipment required to implement a DSL link include an ADSL transceiver unit (ATU) at the remote terminal (ATU-R) and at the central office terminal (ATU-C). Before any valid exchange of data can occur between the ATU-R and the ATU-C, the ADSL link must be activated. The handshake process for establishing this link is defined in ITU standard G.994.1 . Establishing synchronization between the ATU- C and ATU-R involves a process of handshake and channel analysis of the ADSL link to determine the bit capacity and power levels for each DMT subcarrier, as well as the forward error correction (FEC) parameters. These various DSL link characteristics are often adjusted in response to problems caused by line impairments.

[0006] The DMT training begins with a transmit request and several test patterns to verify that the channel is operational. Subsequently, additional test patterns are exchanged so that timing and synchronization of the ATUs can be established. Pseudorandom sequences are transmitted across the channel to estimate the channel transfer function and the spectral noise distribution. During this training process, the noise spectrum can also be estimated utilizing techniques that are known in the art.

[0007] Once the channel transfer function and noise distribution are determined, the bit power allocations are computed. These bit power allocations are exchanged between the ATU-R and ATU-C. The time domain and the frequency domain equalizers in each ATU are also calculated. Thereafter, test patterns are transmitted to train the echo cancellers in each transceiver. Echo refers to harmonic signal components that appear at the ATU as reflected energy produced by non-ideal impedance matching in the system. Each ATU transmitter transmits a test sequence for evaluating the echo path transfer function in the frequency domain. Using information obtained from the test patterns, the configuration of the echo cancellers is determined.

[0008] Based on information obtained from the training session, a profile is generated by each of the ATU-R and ATU-C. The profile typically contains bit allocation and gain tables, FEC parameters, and interleaver depth. After the training cycle is complete, frame synchronization is established and the ADSL channel session can begin.

[0009] One limitation of the echo canceller calibration process described above is that it optimizes the echo canceller circuit but does nothing to mitigate the underlying factors that are causing the echo to occur. This fact is significant because any remaining echo signal and harmonics from the ATU-R transmission in the upstream direction can substantially impair the signal to noise and data rate of data transfer in the downstream direction. Various approaches have been suggested for enhancing the performance of echo canceling circuitry. For example, U.S. Patent No. 6,477,250 to Sheets discloses a system that utilizes an electronic hybrid and a multitone calibration signal for improved dial tone performance of an echo cancellation circuit. Still, none of these systems address the detection and mitigation of the self interfering echo problem. SUMMARY OF THE INVENTION

[0010] The invention concerns a programmable method and apparatus for optimizing the performance of a DSL communications link. The method can begin by automatically evaluating one or more parameters defining a performance related condition of a DSL transmission line. In response to the evaluating step, reflected harmonic energy appearing in a receive band of a DSL transceiver can be selectively reduced by automatically making certain adjustments. Specifically, these adjustments can include adjusting a transmit power of the DSL transceiver, adjusting power levels of individual transmit sub-carriers, or turning individual sub-carriers on or off, or combinations of these adjustments. The parameter and/or the results of the evaluating step described herein can be recorded in a data store to create a status log for the DSL communications link.

[001 1] The method can include performing a DMT training sequence prior to the evaluating step in order to obtain one or more of the parameter values that are to be evaluated. The parameter can be selected to include one or more of the following data that can be obtained directly or indirectly as a result of the DMT training process: a minimum downstream data rate, a minimum upstream data rate, a line attenuation, a ratio of the downstream data rate and the upstream data rate, and a status indication of a failed DMT training attempt. The process, including DMT training, can be repeated after the DSL transceiver is adjusted to reduce reflected harmonic energy appearing in the receive band of the DSL transceiver.

[0012] As an alternative to relying on the DMT training data, a special test can be performed on the DSL transmission line to measure the reflected harmonic energy appearing in the receive band of the DSL transceiver. For example, the test can include transmitting a signal on at least one DMT channel exclusively within the transmit band assigned to the DSL transceiver, and measuring the reflected harmonic energy appearing in the receive band of the DSL transceiver. According to one aspect of the invention, this step can include transmitting the signal on all of the DMT channels exclusively within the transmit band assigned to the DSL transceiver. In either case, a value associated with the resulting measured reflected harmonic energy can be compared to a predetermined threshold value. If the results of this evaluation indicate excess levels of reflected harmonic energy, the DSL transmit power can be adjusted to reduce reflected harmonic energy appearing in a receive band of the DSL transceiver. Thereafter, the test to measure the reflected harmonic energy appearing in the receive band of the DSL transceiver can be repeated.

[0013] Regardless of the specific process or parameters used to evaluate the amount of reflected harmonic energy, the step of automatically adjusting the transmit power of the DSL transceiver can include reducing the transmit power on all of the DMT channels within the transmit band, reducing the power on individual channels, or turning specific channels off. The step of automatically adjusting the width of the transmit band of the DSL transceiver can include turning off one or more DMT channels at a highest frequency end of the transmit band to shape the transmit band.

[0014] The invention can also include a DSL transceiver that is capable of providing optimized communication performance over a DSL communications link. The DSL transceiver can include a processor programmed for automatically evaluating at least one parameter defining a performance related condition of a DSL transmission line. The transceiver can also include a DSL modem responsive to the processor. The processor can be programmed to selectively cause the DSL modem to reduce reflected harmonic energy appearing in a receive band of the DSL transceiver. For example, this can be accomplished by automatically adjusting transmit power of one or more channels or turning off specific channels in order to reduce the width of a transmit band of the DSL transceiver. [0015] According to one aspect of the invention, the DSL modem can be a DMT modem that performs a DMT training sequence to obtain the parameter values. In that case, the performance parameters can be determined as part of the DMT training sequence. As with the method described above, the parameters can include one or more of a minimum downstream data rate, a minimum upstream data rate, line attenuation, a ratio of the downstream data rate and the upstream data rate, and a status indication of a failed DMT training attempt. The DMT modem can repeat the DMT training sequence after the DSL transceiver is configured to reduce reflected harmonic energy appearing in the receive band of the DSL transceiver as described herein.

[0016] In an alternative embodiment of the invention, the processor can automatically determine the parameter value to be evaluated by causing the DSL modem to perform a test on the DSL transmission line exclusive of the DMT training sequence. The test can measure the reflected harmonic energy appearing in the receive band of the DSL transceiver. For example the test can include transmitting a signal on at least one DMT channel exclusively within the transmit band assigned to the DSL transceiver, and measuring the reflected harmonic energy appearing in the receive band of the DSL transceiver.

[0017] According to one aspect of the invention, the test can include transmitting the signal on all of the DMT channels exclusively within the transmit band assigned to the DSL transceiver. In any case, the processor can compare a value representing the measured reflected harmonic energy to a predetermined threshold value. If the reflected harmonic energy is deemed excessive, the DSL transceiver can take specific measures to reduce it as described herein.

[0018] Regardless of the specific approach that is used for evaluating the DSL transmission line, the processor can automatically adjust reflected harmonic energy by reducing the transmit power of all the DMT channels within the transmit band assigned to the DSL transceiver. Alternatively, or in addition thereto, the processor can automatically adjust the width of the transmit band of the DSL transceiver by turning off one or more the DMT channels at a highest frequency end of the transmit band. [0019] According to yet another aspect of the invention, a processor associated with the DSL transceiver can measure a line attenuation at a plurality of sub-carrier frequencies. Using this information, the processor can compute an attenuation slope from low to high frequencies. Thereafter, the processor can compare an actual line attenuation at each sub-carrier frequency to an expected value of line attenuation at each frequency as indicated by the attenuation slope. Finally, processor can generate a log entry indicating the presence of an impairment when the actual line attenuation deviates from the expected value of line attenuation by a predetermined amount. [0020] The processor can also measure a quiet line noise at each sub-carrier frequency channel. Using this information, the processor can compute a minimum noise level from low to high frequencies, and can compare the minimum noise level to the actual measured noise level for each sub-carrier channel. A log entry can be generated by the processor if a difference between the minimum noise level and an actual measured noise level exceeds a predetermined threshold value.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG. 1 is a block diagram that is useful for understanding the invention.

[0022] FIG. 2 is a plot of signal level versus frequency that is useful for understanding the effects of a reduction in ATU-R transmit power level.

[0023] FIG. 3 is a plot of signal level versus frequency that is useful for understanding the effects of a reduction in ATU-R transmit power level and bandwidth.

[0024] FIG. 4 is a flowchart that is useful for understanding a conceptual process for optimizing DSL performance.

[0025] FIG. 5 is a flowchart that illustrates a first embodiment of the invention.

[0026] FIG. 6 is a flowchart that illustrates a second embodiment of the invention.

[0027] FIG. 7 is a graphical representation of line attenuation versus DMT tone number in a DSL communication system.

[0028] FIG, 8 is a graphical representation of line noise versus DMT tone number in a DSL communication system.

DETAILED DESCRIPTION [0029] Conventional approaches to solving the DSL performance in the presence of impairments have primarily focused on improvements to the echo canceling circuitry while failing to address the underlying causes of the problem. The present invention takes a different approach which is intended to dynamically reduce the occurrence of echo produced on the telephone lines. This is accomplished by utilizing an advanced training process that analyzes line conditions and uses information gained from such analysis for selective control over transmitted frequencies and power levels. The result is substantially improved data rates in the downstream direction from the central office to the customer premises.

[0030] FIG. 1 is a block diagram showing a typical DSL communication system 100. The system includes an ATU-C 102 located at a central office and a remotely located ATU-R 104. Typically, the ATU-R will be located at the customer premises. The architecture and operation of the ATU-R is generally known in the art. The ATU-R will commonly include several major component subsystems. A hybrid circuit 106 is provided to match the impedance of telephone line 103 and increase echo rejection. Line driver/receiver circuit 108 is provided for buffering transmitted and received data signals. The analog front end 110 provides analog to digital and digital to analog conversion, analog filtering, and so on.

[0031 ] The ATU-R contains a DMT modem 1 12 that will typically include a Digital Signal Processor 116 as well as suitable hardware and software for equalization, error correction, interleaving, modulation and demodulation. DSP 1 16 can assist the DMT function with digital signal processing tasks. The ATU-R 104 is generally controlled by a system controller 118. The system controller 118 will generally have associated therewith a memory 120. System controller 118 can be any suitable microcontroller or processor programmed with a set of instructions. Conventional system controllers usually are provided with one or more application programming interfaces (APIs) for facilitating operation of the ATU-R. Interface circuitry 1 14 can be provided for communications between the ATU-R and a computer system.

[0032] Those skilled in the art will appreciate that the ATU-R block diagram shown in FIG. 1 is merely intended as one possible example illustrating conventional ATU-R architecture and the invention is not intended to be limited in this regard. Instead, any other system architecture can be used provided that it is capable of performing the functions described herein.

[0033] As illustrated in FIG. 1 , the telephone line can include one or more impairments. A bridged tap 122 impairment is illustrated in FIG. 1. However, many other types of telephone line impairments can exist. These impairments can adversely affect DSL data transmissions by creating what are commonly referred to as echoes. The echoes are actually reflected harmonic energy from signals that have been transmitted on the telephone line 103. This concept can be better understood with reference to FIGS. 2 and 3, which illustrate relative upstream and downstream spectrum of signals for an ATU-R in an ADSL link implemented with DMT.

[0034] The DMT signaling standard for ADSL provides for a transmit band 201 of frequencies for ATU-R transmitted data. In FIG. 2, transmitted energy spectrum 202 is comprised of signals in the transmit band. A receive band 203 of frequencies is assigned for receiving data at ATU-R that is communicated from ATU-C. Received energy spectrum 204 is comprised of signals from the ATU-C in the receive band. In conventional ADSL systems using DMT, the transmit band occupies the lower frequencies from 30 kHz to 140 kHz and the receive band occupies 160 kHz to 1.1 MHz. The bands are collectively divided into 256 discrete multitone carriers with some carriers unused.

[0035] Some of the harmonic energy from transmitted energy spectrum 202 will be reflected from various line impairments. These echoes can appear in the receive band 203 of the ATU-R. The reflected harmonic energy 206, 208 is shown in FIG. 2. In some cases, the reflected harmonic energy can exceed the noise floor 210, resulting in a low or negative signal to noise ratio and prevent data from being loaded on some channels and causing a reduction in downstream data rates. In FIG. 2, reflected harmonic energy 206 exceeds the noise floor.

[0036] Notably, the signal level of the reflected harmonic energy 206, 208 will be determined, at least in part, by the power level of the transmitted energy spectrum 202. Accordingly, selected adjustments to the power or signal level of the transmitted energy can mitigate the reflected harmonic energy. According to an embodiment of the invention, ATU-R can automatically evaluate the condition of the telephone line 103, including any impairment, to determine if the transmitted energy spectrum 202 at a particular transmitted power level will result in reflected harmonic energy 206, 208 exceeding the noise floor 210. In response to this condition being detected, the ATU-R can automatically control the power level of the transmitted energy spectrum 202 so that the power level of the reflected harmonic energy 206, 208 is reduced to a more acceptable level. For example, the power level of the transmitted energy spectrum 202 can be reduced to transmitted energy spectrum 212. In this way, the resulting reflected harmonic energy 216, 218 can be closer to or below the noise level 210.

[0037] In addition, or as an alternative to, reducing the power level of transmitted energy spectrum 202, the bandwidth of the transmitted energy spectrum 202 can be reduced. This concept is illustrated in FIG. 3, which shows a transmitted energy spectrum 302 that has a reduced bandwidth and power level. The reduction in bandwidth for the transmitted energy spectrum 202 can be accomplished by disabling one or more of the discrete multitone carriers. For example, these disabled carriers can be chosen so that they are near the upper end of the frequency spectrum assigned for the transmitted upstream data. The result, as shown in FIG. 3, is a further reduction in harmonic energy introduced into the receive band occupied by received energy spectrum 204. Specifically, the reflected harmonic energy 306, 308 that is produced by transmitted energy spectrum 302 can be reduced in power and bandwidth, thereby causing less intrusion into the receive band. The result is additional signal area usable above the noise and reflected energy which will result in improved downstream data rates from the ATU-C to the ATU-R.

[0038] FIG. 4 shows a flowchart 400 that is useful for understanding a method for reducing reflected harmonic signals received by the ATU-R that are produced by upstream transmission of data to the ATU-C. The method can begin in step 401 by automatically acquiring one or more parameters that are useful for evaluating a performance related condition of a DSL data transmission system. As used herein, performance related condition means any characteristic of the DSL transmission system that has at least some affect on its ability to transmit data. For example, the parameters can be specifically selected for identifying the existence of line impairments and reflected harmonic energy in the receive band of the ATU-R caused by such line impairments.

[0039] In step 402, an evaluation can be made of the one or more parameters to determine if impairments are present and whether optimization is needed. If optimization is needed, then levels of reflected harmonic energy that appear in a receive band of the DSL transceiver can be selectively reduced in step 403 by automatically making certain adjustments. Specifically, these adjustments can include adjusting a transmit power of the DSL transceiver, adjusting a width of a transmit spectrum of the DSL transceiver, or both. In step 404, the parameter and/or the results of the evaluating step 402 described herein can be recorded in a data store to create a status log for the DSL communications link. Note that the line analysis results can be used for diagnosis and troubleshooting of problems that cannot be corrected automatically. [0040] The process illustrated in Fig. 4 can be implemented in at least two ways. In a first embodiment shown in Fig. 5, the one or more parameters used for evaluating the system are obtained based on information obtained as part of a standard DMT training sequence. In a second embodiment shown in Fig. 6, the parameter(s) used for evaluation are obtained using a separate test specifically designed to measure the occurrence of reflected harmonic energy in a receive band caused by line impairments. This embodiment involves a special test sequence, not part of normal DMT training, to generate modulated data on transmit spectrum while measuring the power level on the receive spectrum when the ATU-C is quiet. This is done while the ATU-C is quiet to eliminate false readings and ensure that the energy measured is from reflected transmit power.

[0041] Referring now to Fig. 5, the process can begin in step 501 by initializing certain optimization parameters. These optimization parameters can include Minimum Upstream Rate, Minimum Downstream Rate, Maximum Line Attenuation, Downstream to Upstream (DS-US) Ratio, TX Gain Adjustment, and TX Carrier. Examples of typical default values for these optimization parameters can be selected as follows:

Minimum Upstream Rate = 32 Kbps

Minimum Downstream Rate = 384 Kbps

Maximum Line Attenuation = 120 dB

Downstream to Upstream (DS-US) Ratio = 2.0

TX Gain Adjustment = -3 dB

TX Carrier = Tone 23 - 30 turned off In steps 502 the DSL modem can be initialized. Thereafter, in step 504 the standard DSL training sequence can be initiated utilizing a conventional DSL initialization and training sequence (including DMT training) as is known in the art.

[0042] The DSL training sequence will produce data that can be stored for later evaluating line impairments. The data from the DSL training sequence can include upstream and downstream data rates, and line attenuation figures. The information can also include status information that shows whether the training attempt was successful. In addition, the information collected can include quiet noise levels, signal to noise levels, and attenuation on a per sub-carrier basis.

[0043] In step 505, a determination is made as to whether DSL optimization is enabled. If so, the process continues to step 506 where a determination is made as to whether the DSL training attempt failed. If the DSL training attempt failed, the process goes directly to step 515 for DSL optimization. Otherwise, the process continues to steps 507, 508, 510, and 512 where the stored data from the DSL training sequence is evaluated.

[0044] The process for optimizing the downstream data rate can, in some cases, adversely affect the upstream data rate. Accordingly, in step 507, a preliminary determination can be made as to the condition of the upstream data rate. If the upstream data rate is already low, then further optimization of the downstream data rate can be undesirable. In step 507, a comparison is made to determine if the upstream data rate obtained from the DSL training sequence is less than or equal to the Minimum Upstream Rate. If the actual upstream data rate is less than or equal to the required minimum upstream data rate threshold value, then the downstream optimization process can be terminated.

[0045] If the upstream data rate is determined to be higher than the required minimum in step 507, then the process continues on with the optimization process in step 508. A determination can be made in step 508 as to whether the actual downstream data rate from the DMT training sequence is less than or equal to the Minimum Downstream Rate. If it is determined in step 508 that the actual downstream data rate is less than the Minimum Downstream Rate, then the process continues on to step 515 - 519. Otherwise the process continues evaluating performance parameters in step 510. [0046] In step 510, a determination is made as to whether the actual downstream line attenuation is greater than or equal to the Maximum Line Attenuation. If it is determined in step 510 that the actual downstream line attenuation is greater than or equal to the maximum line attenuation, then the process continues on with steps 515 and 519. Otherwise, the process continues evaluating performance parameter in step 512.

[0047] In step 512 a determination is made as to whether the last measured downstream data rate is less than or equal to a calculated value. The calculated value D is computed as follows: D = (desired downstream data rate to upstream data ratio) x (last upstream data rate)

If the last measured downstream data rate is less than or equal to the calculated value D, then the process continues on with steps 515 - 519. Otherwise, the process terminates.

As will be apparent from the foregoing description, if any of the evaluations performed in steps 506, 508, 510, or 512 yields an affirmative result, then the process continues on to steps 515 - 519 to reduce reflected harmonic energy in the receive band of the DSL transceiver. In order to achieve this result, the DSL training sequence is dropped in step 515. In step 516, the transmit gain of one or more transmit sub-carriers can be reduced. For example, a gain of all DMT carriers in an upstream transmit band can be reduced. The precise amount of gain reduction can be selected to be any value. However, a TX Gain Adjustment setting of -3 dB has been found to yield acceptable results.

[0048] In addition, or as an alternative to, the reduction in gain for the transmitted carriers described in step 516, the transmit band 201 that is allocated to the upstream transmitted carriers can be reduced in step 518. For example, in the case of a DMT based ADSL system, this can be accomplished by simply turning off selected DMT carriers. DMT carriers at the high end of the transmit band or the low frequency end of the transmit band can be turned off. However, it can be advantageous to turn off those DMT carriers at the highest frequency end of the transmit band 201 as reflected harmonic energy from these higher frequency carriers is the most likely to appear in the receive band 203. For example, tones 23-30 can be turned off . [0049] In addition to the performance optimization methods previously mentioned, data collected by the previous DSL train cycle can be used to diagnose problems. For example, attenuation over a line typically increases linearly as the frequency increases. However, the presence of an impairment such as a bridged tap or loading coil will cause the measured attenuation at each DMT sub-carrier frequency to deviate from this linear relationship. This effect is illustrated in Fig. 7, which shows actual measured line attenuation 701 plotted versus DMT tone number corresponding to various sub-carrier frequencies. The actual measured line attenuation data can be collected in the DMT train cycle described above.

[0050] In Fig. 7, the linear slope defined by the attenuation from low sub-carriers to high sub-carriers in an ideal system is represented by the line 702. The attenuation slope can vary from this relationship in the presence of an impairment. For example, line 703 overlaid on a portion of the attenuation data shows a slope that deviates substantially from that illustrated by line 702.

[0051 ] Using the data collected from the DMT train cycle, the slope of line 702 can be calculated. Thereafter, the attenuation at each sub-carrier can be compared to an expected value indicated by values corresponding to line 702. Alternatively, or in addition thereto, the attenuation at each sub-carrier can be compared to adjacent sub- carriers. In either case, if the difference between the actual attenuation and the expected attenuation for a particular sub-carrier varies by more than some predetermined amount, then it can be concluded that an impairment is present. Accordingly, a diagnostic message can be logged to indicate that an impairment is present on the line. This diagnostic can trigger further investigation by service personnel to correct the impairment.

[0052] In addition to the diagnostic information and performance optimization methods previously mentioned, quiet line noise collected by the previous DSL train cycle can be used to diagnose problems. For example, quiet line noise on a line is typically flat over the spectrum. This typical quiet line noise level is illustrated in Fig. 8 by dashed line 802. However, if an impairment such as external EMI is present then measured noise at each DMT sub-carrier frequency will not be flat. This is illustrated in Fig. 8 by the curve 804 that shows increased quiet line noise levels at certain sub- carrier frequencies. This variation in quiet line noise level indicates the presence of EMI line impairments.

[0053] In order specifically identify which sub-carrier may be affected by the foregoing EMI line impairments, the minimum noise level from low sub-carriers to high sub-carriers is calculated. Thereafter, the noise level at each sub-carrier can be compared to the other sub-carriers. If a particular sub-carrier is determined to have a quiet line noise level that exceeds an expected value by some predetermined amount, then EMI is detected and a diagnostic message is logged to indicate that an impairment is present on the line. EMI at certain frequencies can be used to match known EMI sources such as T1/E1 disturbers. This diagnostic can trigger further investigation by service personnel to correct the impairment.

[0054] An alternative embodiment of the invention is illustrated in flowchart 600 shown in Fig. 6. Instead of relying on data acquired during a conventional DMT training sequence, a specific test can be performed to evaluate the amount of reflected harmonic energy produced in the receive band 203 of a DSL transceiver as a result of its own transmitted signals in the transmit band 201 . The process in Fig. 6 can begin in step 602 with a conventional initialization of the DMT modem 1 12. The initialization process may or may not include the DMT training sequence. In step 604, a test can be performed to measure reflected harmonic energy produced by the DSL transceiver's own transmitted signals.

[0055] A variety of tests can be performed for this purpose. For example, the DSL transceiver can transmit a test signal on one or more carrier channels in the transmit band. At the same time, the DSL transceiver can monitor the receive band to detect reflected harmonic energy produced by the transmitted signals. According to one embodiment of the invention, the DSL transceiver can transmit a test signal on all of the transmit carrier channels simultaneously. However, the invention is not limited in this regard.

[0056] In step 606, the reflected harmonic energy measured at the DSL transceiver in the receive band can be compared to a threshold value. The measured reflected harmonic energy value can be an average and/or normalized power level of reflected harmonic energy appearing in the receive band as a result of the transmitted test signals. The reflected harmonic energy can be compared to at least one threshold value to determine whether the reflected harmonic energy should be reduced.

[0057] If the reflected harmonic energy value exceeds the predetermined threshold value, then the process can continue on to step 608 to reduce the reflected energy in the receive band. This can be accomplished in step 608 by reducing the gain of the signal transmitted from the DSL transceiver. For example, in the case of an ADSL link utilizing a conventional DMT communications protocol, the gain can be reduced for one or more upstream DMT transmit carriers. According to one embodiment, the gain can be reduced for all DMT transmit carriers.

[0058] In addition, or as an alternative to, the reduction in gain for the transmitted carriers described in step 608, the transmit band 201 that is allocated to the upstream transmitted carriers can be reduced in step 610. For example, in the case of a DMT based ADSL system, this can be accomplished by simply turning off selected DMT carriers. DMT carriers at the high end of the transmit band or the low frequency end of the transmit band can be turned off. However, it can be advantageous to turn off those DMT carriers at the highest frequency end of the transmit band 201 as reflected harmonic energy from these higher frequency carriers is the most likely to appear in the receive band 203.

[0059] The invention described and claimed herein is not to be limited in scope by the preferred embodiments herein disclosed, since these embodiments are intended as illustrations of several aspects of the invention. Any equivalent embodiments are intended to be within the scope of this invention. Indeed, various modifications of the invention in addition to those shown and described herein will become apparent to those skilled in the art from the foregoing description. Such modifications are also intended to fall within the scope of the appended claims.

Claims

1 . A method for optimizing the performance of a DSL communications link, comprising: automatically evaluating at least one parameter defining a performance related condition of a DSL transmission line; and responsive to said evaluating step, selectively reducing reflected harmonic energy appearing in a receive band of a DSL transceiver by automatically adjusting at least one of a transmit power and a width of a transmit band of said DSL transceiver connected to said DSL transmission line.

2. The method according to claim 1 , further comprising performing a DMT training sequence prior to said evaluating step to obtain a value for said parameter.

3. The method according to claim 2, further comprising: selecting said parameter to include at least one of a minimum downstream data rate, a minimum upstream data rate, a line attenuation, a ratio of said downstream data rate and said upstream data rate, and a status indication of a failed DMT training attempt.

4. The method according to claim 2, further comprising determining said parameter as part of said DMT training sequence.

5. The method according to claim 2, further comprising repeating said DMT training sequence after said step of selectively reducing reflected harmonic energy appearing in said receive band of said DSL transceiver.

6. The method according to claim 1 , further comprising determining said value of said parameter by automatically performing a test on said DSL transmission line to measure said reflected harmonic energy appearing in said receive band of said DSL transceiver.

7. The method according to claim 6, further comprising selecting said test to include transmitting a signal on at least one DMT channel exclusively within said transmit band assigned to said DSL transceiver, and measuring said reflected harmonic energy appearing in said receive band of said DSL transceiver.

8. The method according to claim 7, further comprising selecting said test to include transmitting said signal on all of said DMT channels exclusively within said transmit band assigned to said DSL transceiver.

9. The method according to claim 6, further comprising comparing said reflected harmonic energy to a predetermined threshold value.

10. The method according to claim 6, further comprising repeating said test after said step of selectively reducing reflected harmonic energy appearing in a receive band of said DSL transceiver.

1 1 . The method according to claim 1 , further comprising recording at least one of said parameter and a result of said evaluating step in a data store to create a status log for said DSL communications link.

12. The method according to claim 1 , further comprising selecting said step of automatically adjusting said transmit power to include reducing said transmit power on all of said DMT channels within said transmit band assigned to said DSL transceiver.

13. The method according to claim 1 , further comprising selecting said step of automatically adjusting said width of said transmit band of said DSL transceiver to include turning off at least one DMT channel at a highest frequency end of said transmit band.

14. A DSL transceiver for providing optimized communication performance over a DSL communications link, comprising: a processor programmed for automatically evaluating at least one parameter defining a performance related condition of a DSL transmission line; and a DSL modem responsive to said processor, wherein said processor selectively causes said DSL modem to reduce reflected harmonic energy appearing in a receive band of said DSL transceiver by automatically adjusting at least one of a transmit power and a width of a transmit band of said DSL transceiver.

15. The DSL transceiver according to claim 14, wherein said DSL modem is a DMT modem that performs a DMT training sequence to obtain a value of said parameter.

16. The DSL transceiver according to claim 15, wherein said parameter includes at least one of a minimum downstream data rate, a minimum upstream data rate, a line attenuation, a ratio of said downstream data rate and said upstream data rate, and a status indication of a failed DMT training attempt.

17. The DSL transceiver according to claim 15, wherein said parameter is determined as part of said DMT training sequence.

18. The DSL transceiver according to claim 15, wherein said DMT modem repeats said DMT training sequence after reducing reflected harmonic energy appearing in said receive band of said DSL transceiver.

19. The DSL transceiver according to claim 14, wherein said processor automatically determines said value for said parameter by causing said DSL modem to perform a test on said DSL transmission line to measure said reflected harmonic energy appearing in said receive band of said DSL transceiver.

20. The DSL transceiver according to claim 19, wherein said test includes transmitting a signal on at least one DMT channel exclusively within said transmit band assigned to said DSL transceiver, and measuring said reflected harmonic energy appearing in said receive band of said DSL transceiver.

21 . The DSL transceiver according to claim 20, further comprising wherein said test includes transmitting said signal on all of said DMT channels exclusively within said transmit band assigned to said DSL transceiver.

22. The DSL transceiver according to claim 19, wherein said processor compares said reflected harmonic energy to a predetermined threshold value.

23. The DSL transceiver according to claim 19, wherein said processor automatically repeats said test after selectively reducing reflected harmonic energy appearing in a receive band of said DSL transceiver.

24. The DSL transceiver according to claim 14, wherein said processor records at least one of said parameter and a result of said evaluation in a data store to create a status log for said DSL communications link.

25. The DSL transceiver according to claim 14, wherein said processor automatically adjusts said transmit power by reducing said transmit power on all of said DMT channels within said transmit band assigned to said DSL transceiver.

26. The DSL transceiver according to claim 14, wherein said processor automatically adjusts said width of said transmit band of said DSL transceiver by turning off at least one DMT channel at a highest frequency end of said transmit band.

27. The DSL transceiver according to claim 14 wherein said processor measures a line attenuation at a plurality of sub-carrier frequencies and computes an attenuation slope from low to high frequencies.

28. The DSL transceiver according to claim 27, wherein said processor compares an actual line attenuation at each sub-carrier frequency to an expected value of line attenuation at each frequency as indicated by the attenuation slope.

29. The DSL transceiver according to claim 28, wherein said processor generates a log entry indicating the presence of an impairment when said actual line attenuation deviates from said expected value of line attenuation by a predetermined amount.

30. The DSL transceiver according to claim 14, wherein said processor measures a quiet line noise at each sub-carrier frequency channel, computes a minimum noise level from low to. high frequencies, and compares the minimum noise level to the actual measured noise level for each sub-carrier channel.

31 . The method according to claim 30, wherein a log entry is generated by said processor if a difference between said minimum noise level and an actual measured noise level exceeds a predetermined threshold value.