Abstract:
A system estimates impairment contributions for upstream communications in a cable television system. The system receives equalization coefficients used by end devices in the cable television system. The equalization coefficients are used by equalizers to mitigate distortion in upstream channels for the end devices. The system analyzes the coefficients based on impairment thresholds to determine whether impairment problems exist and to identify the types of impairment problems that exist.

Description:
BACKGROUND 
     Cable television networks, including community antenna television (CATV), hybrid fiber-coaxial (HFC), and fiber networks, have been in widespread use for many years and are extensive. The extensive and complex cable networks are often difficult for a cable operator to manage and monitor. A typical cable network generally contains a headend which is usually connected to several nodes which provide bi-directional content to a cable modem termination system (CMTS). In many instances, several nodes may serve a particular area of a town or city. The CMTS contains several receivers, and each receiver connects to several modems of many subscribers. For instance, a single receiver may be connected to hundreds of modems at customer premises. Data may be transmitted downstream to the modems on different frequency bands. The modems communicate to the CMTS via upstream communications on a dedicated frequency band, referred to as a return band. 
     Cable networks are also increasingly carrying signals, which require a high quality and reliability of service, such as Voice over IP (VoIP) communications. Any disruption of voice or data traffic is a great inconvenience and often unacceptable to a customer. Various factors may affect the quality of service, including the quality of the upstream channels. One factor that affects the quality of upstream communications is the presence of up-stream channel impairments, such as micro-reflections (MRs) of communication signals, group delay variation (GDV), and amplitude distortion (AD). 
     AD is an undesirable variation in the channel&#39;s amplitude response. Common forms of AD include tilt, ripple, and roll-off. A common cause of AD is upper return band-edge carriers, aggravated by long reaches of a cable network plant. The long reaches accumulate diplex filters from devices including amplifiers and in-line equalizers. While individually contributing small attenuation versus frequency, the accumulated diplex filters can create appreciable response variation. In a QAM constellation, the amplitude roll-off causes the symbols to spread in a pattern similar in appearance to Additive White Gaussian Noise (AWGN) and causes received symbols to cross decision boundaries, resulting in errors. 
     GDV is an undesirable variation in the communication channel&#39;s phase response, resulting in distortion of the digital signal phase, or a variation in the propagation of frequency components of the signal across the channel. As is the case for AD, one major cause of GDV in the plant is upper-band-edge operation, combined with long reaches of cable network plant. The reasoning is the same as in the AD case. Note that filtering functions typically induce nonlinear phase responses as the band edges are approached, so the combination of AD and GDV in the same band region is perfectly expected, with the understanding that diplex filtering is the cause. Different filter functions induce different GDV responses, in a similar manner that different filter functions induce different stop-band characteristics. It is typical that the sharper the roll-off, such as would be the case for long cascades, the worse the GDV will be. In a QAM constellation, GDV causes the symbols to spread in a pattern similar to AWGN and AD and causes received symbols to cross decision boundaries, resulting in errors. 16-QAM is less sensitive to GDV than 64-QAM because of reduced decision boundary size of 64-QAM. 
     As seen by a receiver, a MR is a copy of the transmitted signal, arriving late and with reduced amplitude. The result of the additional copy is the typically seen by end users as image ghosting in analog video reception, whereas for digital communications the result is inter-symbol interference (ISI). MR sources are composed of pairs of hybrid fiber-coaxial (HFC) components separated by a distance of cable. The HFC components, also referred to as cable network components, facilitate the propagation of signal copies in a variety of ways including return loss, isolation, mixing, and combining. For instance, the MR may arise if a length of cable separates two devices with poor return loss, acting as signal reflectors. The reflector return loss and the loss between the reflectors determine the amplitude of the MR. Any HFC component, for instance a cable modem (CM), has the potential to act as a signal reflector. Note that the CM typically has as a design limit of 6 dB return loss, meaning it may reflect up to 25% of its incident power. In the cable network plant, components other then the CM typically reflect a lower percentage of incident power because the design limits are typically significantly better. However, as the cable network plant ages and elements that contribute to good RF matching degrade, for instance connectors, cable, splitters, and interfaces on printed circuit boards (PCBs), the reflected percentage of incident power increases. 
     These upstream channel impairments are known to be mitigated by the fundamental digital communications receiver function of equalization. During equalization, an equalizer generates coefficient information that is used to create an equalizing filter, with an inverse channel response, canceling distortion in the channel caused by the upstream channel impairments. The equalization coefficients in Data Over Cable Service Interface Specification (DOCSIS) 2.0 and DOCSIS 3.0 are 24 symbol-spaced coefficients (also referred to as taps). Equalization is part of virtually all modern telecommunications platforms, and is instrumental in proper return operation for all DOCSIS systems. 
     In order to offer higher data rates to subscribers in the competitive world of high-speed data and Internet access, operators must take advantage of the throughput benefits gained from leveraging more complex digital modulation schemes, such as 32-QAM and 64-QAM. Use of 32-QAM allows, for example, a 20 Mbps 16-QAM upstream to become a 25 Mbps upstream. On the other hand, for 64-QAM, it allows a 16-QAM, 20 Mbps upstream channel to become a 30 Mbps channel. This represents a 25-50% throughput improvement. Unfortunately, channels using these digital modulation schemes are also considerably more sensitive to digital communication channel impairments, including the upstream impairments described above, than the 16-QAM channels they are often replacing in the return band. 
     Given the potential problems that can be caused by the upstream impairments, upstream channels are one of the most challenging digital communication channels to manage and fully exploit. Operators prefer to ensure that capacity associated with the upstream channel, or as much of the capacity as possible, is realized for services and revenue. To do so requires a thorough understanding of a diverse set of HFC and digital communications variables. More importantly, variables that did not matter very much for 16-QAM operation now become not just relevant, but critical to understand for successful deployment of 64-QAM, and to a lesser extent, 32-QAM. Accurately diagnosing upstream issues typically requires technicians or engineers to be at multiple locations within a HFC plant and simultaneously inject test signals at the suspected device locations. This diagnostic process requires extensive manual effort, often requiring rolling trucks to remote locations within a plant or specialized test equipment. The diagnostic process is also time consuming and costly. 
     SUMMARY 
     According to an embodiment, a system estimates impairment contributions for upstream communications in a cable television system. The system receives equalization coefficients used by end devices in the cable television system. The equalization coefficients are used by equalizers to mitigate distortion in upstream channels for the end devices. The system analyzes the coefficients based on impairment thresholds to determine whether impairment problems exist in the upstream channels and to identify the types of impairment problems that exist. Other embodiments include computer-implemented methods estimating impairment contributions for upstream communications based on received equalization coefficients and impairment thresholds. 
     Embodiments interpret equalization coefficients for end devices and identify potential impairments of upstream channels for the end devices based on an analysis of the equalization coefficients. Also, a particular type of impairment problem can be identified based on the analysis of equalization coefficients. Determination of the type of impairment can be coupled with additional information, such as location of the end device or tap, to determine suspect cable network components that may be causing the impairment. Thus, identification of an impairment problem and potential solutions can be determined before a customer problem is experienced and without dispatching technicians to diagnose the problem. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features of the present invention will become apparent to those skilled in the art from the following description with reference to the figures, in which: 
         FIG. 1  illustrates a block diagram of a cable network, according to an embodiment of the invention; 
         FIG. 2  illustrates a CMTS architecture, according to an embodiment of the invention; 
         FIG. 3  illustrates a device for estimating impairment contributions and isolating defective network components, according to an embodiment of the invention; and 
         FIG. 4  illustrates a device for estimating impairment contributions and isolating defective network components, according to an embodiment of the invention; 
         FIG. 5  illustrates a method for estimating impairment contributions and isolating defective network components using a plurality of end devices, according to an embodiment of the invention; and 
         FIG. 6  shows a block diagram of a computer system that may be used for estimating impairment contributions and isolating defective network components, according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     For simplicity and illustrative purposes, the present invention is described by referring mainly to exemplary embodiments thereof. In the following description, numerous specific details are set forth to provide a thorough understanding of the present invention. However, it will be apparent to one of ordinary skill in the art that the present invention may be practiced without limitation to these specific details. In other instances, well known methods and structures have not been described in detail to avoid unnecessarily obscuring the present invention. 
     The abbreviation “decibels relative to a carrier (dBc)” refers to a measure of the power ratio of a signal to a carrier signal, and is expressed in decibels. Note “dB” refers to a decibel, “ns” refers to a nanosecond, and “MHz” refers to a megahertz. 
     The term “equalization coefficient” refers to complex tap values used to create an equalizing filter with an inverse channel response. 
     The term “impairment contribution” refers to causes of impairment in an upstream hybrid fiber coaxial (HFC) plant. 
     The term “micro-reflection (MR)” refers to an impairment contribution wherein a copy of a communication signal is reflected back onto itself, with a time delay. Significant MRs can degrade upstream HFC plant performance. 
     The term “group delay variation (GDV)” refers to an impairment contribution wherein different frequency components of a signal propagate through a network component with different time delays. 
     The term “cable network plant components” refers to any component that may cause impairment in an upstream channel in the cable network. The components may be components of an HFC network, and may be active or passive components. The upstream channel may be a channel between a modem and a CMTS or another upstream channel in the cable network. 
       FIG. 1  illustrates a network  100 , such as an HFC network, including end devices  102 . The end device  102  may be DOCSIS Terminal devices, such as cable modems (CMs), modem terminal adapters, MTAs, and embedded cable modems of DOCSIS set-top gateways (eCMs of DSGs), or any other like devices. The end devices  102  are connected to a headend  104  of the network  100  via nodes  106  and an RF cascade  103  comprised of multiple amplifiers and passive devices including cabling, taps, splitters, and in-line equalizers. A network tap is a hardware device providing access to data within the network  100 . The network tap provides the ability to monitor data between two points, for instance components, in the network  100 . An impairment contribution estimator  200 , shown in  FIG. 3 , may be connected to the network  100  through any network access point including a tap. The headend  104  connects to an IP (Internet Protocol) and/or PSTN (Public Switched Telephone Network) network  108 . Data, such as TV programs, audio, video and other data, which may be from the network  108 , is sent from the headend  104  to the end devices  102 . In addition, the end devices  102  may send data upstream towards the headend  104 . Although not shown, each of the nodes  106  may be connected to multiple end devices. 
     As illustrated in  FIG. 1 , the headend  104  includes a CMTS  110  and optical transceivers  112  which provide optical communications to and from the CMTS  110  through optical fiber to the nodes  106 . Typically, the nodes  106  connect to the headend  104 , and the headend  104  contains a plurality of CMTS units  110 . Each CMTS  110  contains a plurality of transceivers, which communicate with the plurality of end devices  102 . For example, each CMTS  110  may have eight or more receivers (e.g., for DOCSIS 2.0), and each receiver may communicate with hundreds of end devices  102 . The CMTS may have more than eight receivers (e.g., DOCSIS 3.0 may use 48 receivers). 
       FIG. 2  illustrates an architecture of the CMTS  110 , according to an embodiment. As illustrated, the CMTS  110  includes a processing unit  114  having a microprocessor  116  that receives information, such as instructions and data, from a RAM  118  and a ROM  120 . The processing unit  114  controls the operation of the CMTS  110  and RF communication signals to be sent by the end devices  102  to the CMTS  110 . The processing unit  114  is connected to a display  122 , which may display status information such as whether station maintenance (SM) is being performed, or a receiver is in need of load balancing. An input keypad  124  may also be connected to the processing unit  114  to permit an operator to provide instructions and process requests. 
     The CMTS  110  also includes an RF transceiver (transmitter/receiver) unit  126  having transmitters  128  and receivers  130  providing bi-directional communication capability with the end devices  102  through optical transceivers  112 , nodes  106  and an RF cascade  103  comprised of multiple amplifiers and passive devices including cabling, taps, splitters, and in-line equalizers. The CMTS  110  may contain a plurality of RF receivers  130 , such as eight RF receivers and a spare RF receiver. Each of the RF receivers  130  may provide support for a hundred or more end devices  102 . 
     By way of example, the receivers  130  can be BROADCOM 3140 receivers that each includes a demodulator unit  132  and an equalizer  134  to which received RF signals are provided, for instance, for purposes of acquiring equalizer values and burst modulation error ratio (MER) measurements, packet error rate (PER) and bit error rate (BER). The equalizer  134  can be a multiple tap linear equalizer (e.g. a twenty-four tap linear equalizer), which also is known as a feed forward equalizer (FFE). The equalizer  134  can be integrally contained in the RF receiver, or alternatively, may be provided as a separate device. The communication characteristics of each receiver  130  may be stored on ROM  120  or RAM  118 , or may be provided from an external source. Note that the equalizer  134  is in the upstream path, for example, from the end devices  102  towards the network  108 . 
     The RF transceiver unit  126  also includes a modulator  136 , which provides the modulated signals to RF transmitters  128 . The modulator  136  and demodulator  132  are capable of modulation schemes of various levels of complexity. For example, some upstream DOCSIS 2.0 modulation schemes that may be used in order of level of complexity include, but are not limited to 16 QAM, 32 QAM, 64 QAM and 128 QAM. The microprocessor  116  may provide instructions to the end devices  102  as to which modulation scheme is to be used during communication. 
     The CMTS  110  also provides instructions for the end devices  102  using a transmit pre-equalization (PRE-EQ) feature in order to compensate for upstream channel impairments. The CMTS  110  receives an incoming signal from each of the end devices  102  and compares the incoming signal with an expected signal, which is an ideal response. If the incoming signal received by the CMTS  110  differs from the expected signal, the microprocessor  116  or other processing device performing a PRE-EQ function then determines a set of equalization coefficients (alternately referred to as transmit pre-equalization coefficients) for each of the end devices  102  and instructs the end devices  102  to set their transmit equalization coefficients to the transmit pre-equalization coefficients determined by the PRE-EQ function. The end devices  102  apply the pre-equalization coefficients and then continue to transmit. The CMTS  110  thereafter continues to monitor and compare the incoming signal against the expected signal. 
       FIG. 3  illustrates an architecture of an impairment contribution estimator  200 . The impairment contribution estimator  200  may be connected to the network  100  through any network access point, for instance through a network access terminal. The impairment contribution estimator  200  is configured for estimating impairment contributions and isolating defective network components in the system  100  according to the method  300  below. As such, the impairment contribution estimator  200  includes a data storage device  201 , and a testing module  202 . The testing module  202  includes an equalization coefficient receiving (ECRC) module  203 , an equalization coefficient resolution (ECRS) module  204 , an impairment level determination (ILD) module  205 , and a cable network plant components isolation (CPCI) module  206 . The testing module  201  may also include a modulation configuration (MC) module (not shown). 
     The data storage device  201  is configured to store an impairment threshold for at least one impairment contribution. The ECRC module  203  is configured to receive equalization coefficients from the end devices  102 . The equalization coefficients are thereafter stored in the data storage device  201 . The ECRS module  204  is configured to resolve the equalization coefficients into the at least one impairment contribution. The ILD module  205  is configured to determine whether each of the end devices  102  exceeds the impairment threshold and to group each of the end devices  102  into sets that exceed impairment thresholds as impaired sets or sets that do not exceed impairment thresholds as unimpaired sets. The CPCI module  206  is configured to identify cable network plant components associated with each of the ILD sorted sets wherein the cable network plant components are designated as suspect components. cable network plant components are correlated with each set of end devices, for example, based on whether they are used in an upstream or downstream path for an end device. 
     The components  202 - 206  are configured to perform the method  300  described with respect to  FIG. 5 . The components  202 - 206  may comprise software modules, hardware modules, and a combination of software and hardware modules. Thus, in one embodiment, one or more of the modules  202 - 206  comprise circuit components. In another embodiment, one or more of the modules  202 - 206  comprise software code stored on a computer readable storage medium, which is executable by a processor. It should be understood that the impairment contribution estimator  200  depicted in  FIG. 3  may include additional components and that some of the components described herein may be removed and/or modified without departing from a scope of the impairment contribution estimator  200 . According to an embodiment, the impairment contribution estimator  200  comprises a part of a network device such as a RF-Sentry application. According to another embodiment, the impairment contribution estimator  200  comprises a part an edge router, such as a part of the advanced spectrum management function of the BSR64000 edge router. 
       FIG. 4  illustrates an embodiment of one of the end devices  102  (shown as  102   a ), such as a cable modem. The end device  102   a  contains a processor  181  which communicates with a RAM  182  and ROM  183  and which controls the general operation of the end device  102 , including applying the pre-equalization coefficients and controlling preamble lengths of communications sent by the end device  102   a  in accordance with instructions from the CMTS  110 . The end device  102   a  also contains a transceiver  186  which provides bidirectional RF communication with the CMTS  110 . A demodulator  185  demodulates signals received by the transceiver  186 , and an equalizer  187  biases communications transmitted to the CMTS  110 . For example, the equalizer  187  is connected in the upstream path between a transmitter in the transceiver  186  and the CMTS  110 . The microprocessor  181  configures the equalizer  187  using the coefficients received from the CMTS  110  to compensate for upstream impairments. The end device  102   a  also contains a modulator  188 , which modulates signals to be transmitted upstream to the CMTS  110  according to a modulation scheme, which the end device  102   a  has been instructed to use by the CMTS  110 . In addition, the end device  102   a  has an attenuator  189  controlled by microprocessor  181  to attenuate signals to be transmitted by the RF transmitter to be within a desired power level. Those of skill in the art will appreciate that the components of end device  102   a  have been illustrated separately only for discussion purposes and that various components may be combined in practice. 
     By way of example, the end device  102   a  may be a DOCSIS network element, such as a cable modem, to generate a variety of test signals. Accordingly, the test signals may be implemented using one of the available upstream DOCSIS bandwidths, e.g. 200 kHz, 400 kHz, 800 kHz, 1600 kHz, 3200 kHz or 6400 kHz. 
     Accurate knowledge of the available and/or optimum modulation schemes of the network  100  enables the operator to utilize available resources of their network more efficiently, such as by adding additional end devices to improve portions of the network with the least complex modulation schemes so that those portions may be able to use more complex modulation schemes. 
     It will be apparent that the system  100  may include additional elements not shown and that some of the elements described herein may be removed, substituted and/or modified without departing from the scope of the system  100 . It should also be apparent that one or more of the elements described in the embodiment of  FIG. 1  may be optional. 
     An example of a method in which the system  100  and the impairment contribution estimator  200  may be employed for estimating impairment contributions and isolating defective network components using the end devices  102  will now be described with respect to the following flow diagram of the method  300  depicted in  FIG. 5 . It should be apparent to those of ordinary skill in the art that the method  300  represents a generalized illustration and that other steps may be added or existing steps may be removed, modified or rearranged without departing from the scopes of the method  300 . In addition, the method  300  is described with respect to the system  100  by way of example and not limitation, and the method  300  may be used in other systems. 
     Some or all of the operations set forth in the method  300  may be contained as one or more computer programs stored in any desired computer readable medium and executed by a processor on a computer system. Exemplary computer readable media that may be used to store software operable to implement the present invention include but are not limited to conventional computer system RAM, ROM, EPROM, EEPROM, hard disks, or other data storage devices. 
     At step  301 , as shown in  FIG. 5 , the impairment contribution estimator  200  retrieves at least one impairment threshold corresponding to an impairment contribution from the data storage  201 . The impairment contribution may be selected from the group including GDV, AD, MR and any impairment contribution that may be isolated by analysis of the coefficients as described in detail at step  303  below. At least one impairment threshold corresponding to the impairment contribution may be selected from the group comprising an industry standard specification, a customer preferred limit, a PRE-EQ failure limit, a PRE-EQ failure limit less acceptable system margin and, where applicable, a function of signaling characteristics. Signaling characteristics include, for instance, RF frequency, QAM modulation level, bandwidth, symbol rate, forward error correction (FEC) settings, and other properties related to signaling. 
     To illustrate, where the impairment contribution is GDV, the impairment threshold may be selected from the group comprising the industry standard specification (for GDV), the customer preferred limit, the PRE-EQ failure limit, the PRE-EQ failure limit less acceptable system margin and a function of a radio frequency (RF) cascade. The function of the RF cascade may comprise a to-be-determined (TBD) value in ns/MHz per RF Amplifier. For instance, a DOCSIS assumption for GDV is 200 ns/MHz. 
     Next, where the impairment contribution is AD, the impairment threshold may be any of the industry standard specification. For instance any of a DOCSIS assumption for amplitude ripple of ≦0.5 dB per MHz, the customer preferred limit, the PRE-EQ failure limit, the PRE-EQ failure limit less acceptable system margin and a function of RF frequency and RF amplifier cascade length. 
     Similarly, where the impairment contribution is MR, the impairment threshold may be any of the industry standard specification. For instance a DOCSIS assumption of −10 dBc@&lt;=0.5 μsec (alternately −20 dBc@&lt;=1.0 μsec, or −30 dBc@&gt;1.0 μsec) for a single dominant MR, the customer preferred limit, the PRE-EQ failure limit, the PRE-EQ failure limit less acceptable system margin, and a function of RF frequency. Simulation and tests may be performed to determine the highest MR impairment level that is correctable using DOCSIS 2.0/3.0 PRE-EQ. The results of these simulations may be used to define the PRE-EQ failure limit. 
     Further, the impairment contribution may comprise any impairment contribution that may be isolated by analysis of the coefficients, for instance as described at step  303  below. After the impairment contribution has been isolated, the at least one impairment threshold corresponding to the impairment contribution may be thereafter selected in a similar manner as described above with regard to the impairment threshold for AD, MR, and GDV. 
     At step  302 , the impairment contribution estimator  200  determines the equalization coefficients currently being used by the end devices  102  for upstream communication. The equalization coefficients may be received from the end devices  102  or the CMTS  110 . The end devices  102  may comprise at least one of the group comprising DOCSIS terminal devices, including cable modems (CMs), modem terminal adapters, (MTAs), and embedded cable modems of DOCSIS set-top gateways, (eCMs of DSGs). The resolution of the 24-tap equalizer of DOCSIS 2.0 more effectively identifies impairments, compared to the 8-tap equalizer of DOCSIS 1.1. In a current HFC plant, in order to more effectively identify impairments, the majority of the end devices  102  are required to support at least DOCSIS 2.0 with the pre-equalization feature enabled. 
     The ECRC module  203  may be configured to query the end devices  102  (preferably a DOCSIS 2.0 CM population) using a simple network management protocol (SNMP) query tool such as a modem PRE-EQ response tool. The modem PRE-EQ response tool, developed by MOTOROLA, is operable to query multiple DOCSIS terminal devices based on an Internet protocol (IP) address list. The modem PRE-EQ response tool is operable to conduct periodic polls of coefficient values and other relevant physical layer (PHY) metrics and to subsequently display the results of the periodic polls and/or to store the results of the periodic polls into a log file for post processing. The modem PRE-EQ response tool also provides users with a graphical view of the impulse response or alternately the amplitude response for each CM poll. The modem PRE-EQ response tool is operable to establish a baseline of performance, and may be used to identify defective network components based on CM IP addresses of the plurality of end devices. 
     At step  303 , the impairment contribution estimator  200  determines whether an impairment problem exists for upstream communications from the end devices  102 . The determination is based on an analysis of the coefficients determined from step  302  and may be based on the impairment thresholds determined from step  301 . There may be multiple techniques for determining whether an impairment problem exists. In one embodiment, the coefficients are analyzed to determine whether any of the impairment thresholds are exceeded. For example, based on experience, certain coefficient values are associated with certain impairment problems and exceeded impairment thresholds. A table may be stored that includes sets of coefficient values (e.g., impairment coefficient signatures) and the type of impairment problem associated with each set of values. This tables of signatures is compared against each of the coefficients determined at step  302 . If an impairment coefficient signature is found in coefficients determined at step  302 , then the end device using those coefficients is determined to have the particular type of impairment associated with the signature as indicated in the table. Thus, at least two determinations may be made. One determination is whether an impairment problem exists, such as unsatisfactory GDV, AR, MR, etc. Then, if an impairment problem exists, at step  304 , a second determination is made which identifies the type of impairment. 
     According to an embodiment, at step  303 , to determine if an impairment problem exists, the ECRS module  204  performs a Fast Fourier Transform (FFT) function on the equalization coefficients for the end devices  102  (e.g. a set of 24 complex coefficients in DOCSIS 2.0), and determines frequency domain information, including a frequency response. For instance, the ECRS module  204  may use a 1024-point FFT to arrange the equalization coefficients for the PRE-EQ baseline and determine the optimal translation of the equalization coefficients. The frequency domain information may be interpreted in multiple ways including in terms of magnitude versus frequency, phase versus frequency, and group delay versus frequency. Based on these magnitudes, a determination is made as to the type of impairment problem that exists, if any exists. For example, negligible amplitude correction but increased correction for phase and group delay is indicative of a GDV impairment. Similarly, other types of impairments can be determined. For example, the end devices  102  are sorted into sets, on increasing levels that sum the DOCSIS PRE-EQ regions for each of the end devices  102 , according to the impairment that the ECRS module  204  is configured to determine. For example, the ECRS module  204  may determine which of the end devices  102  experiences the greatest amount of MR impairment contribution by sorting on the levels which result from summing the taps located in the post-tap region of each tap of the 24-tap equalizer of DOCSIS 2.0. 
     At step  304 , the ILD module  205  determines the type of impairment for each end device, for example, if the impairment threshold is exceeded for the end device. In one embodiment, the ILD module  205  groups each of the end devices  102  into impairment level determined (ILD) sets. The ILD sets include impaired sets comprising end devices that exceed impairment thresholds and unimpaired sets comprised of end devices that do not exceed impairment thresholds as unimpaired sets. Furthermore, the impairment sets may include sets by type of impairment and may indicate the level of impairment for each end device. In one embodiment, the ILD module  205  determines the relation of the measured impairment contribution to the impairment threshold for each of the impairment contributions. If the impairment contribution exceeds the impairment threshold, the upstream impairments may be at a level at which a customer problem is experienced. Alternately, if the impairment threshold has an acceptable system margin, the ILD module  205  is configured to provide information so that an end user may perform preventive maintenance. The ILD module  205  may also be configured to determine a dominant impairment contribution. For instance, the ILD module  205  may analyze the translation of the equalization coefficients of the end devices  102 , and an expected translation of the equalization coefficients for each of the impairment contributions in order to determine the dominant impairment contribution. The expected translation of the equalization coefficients for each of the impairment contributions comprises a translation of equalization coefficients for a channel with a single impairment, for instance AD. 
     The operation of the ILD module  205  may be enhanced by application of an increased understanding of the different impairment contributions and how they originate in HFC plant. For example, although an MR source has been discussed in the preceding section regarding MRs, combining an understanding of other probable permutations of MR sources with the location of the ILD sets increases the probability of successful isolation of the MR sources. The understanding of probable sources may be used to eliminate possible sources of the impairment contribution and to therefore isolate the source of the impairment contribution. The results may be used to define what impairment levels will likely result in service calls, and thereafter impairment thresholds as defined at step  301  may be determined. Further, the ILD module  205  may be configured to prioritize the impaired sets or prioritize end devices in each set according to level of impairment. 
     At step  305 , suspect cable network components are identified that are probable causes for the type of impairment being experienced by an end device. Identification of the suspect components may be based on experience or historical analysis of past impairments and their fixes. For example, the CPCI module  206  identifies cable network plant components associated with each of the impaired sets. This process of identification may be enhanced by consulting data regarding the end devices  102  and network components between each of the plurality of end devices and the CMTS  110 . The CPCI module  206  identifies the cable network components associated with impaired sets as suspect components. The CPCI module  206  then leverages the end devices  102  to isolate those experiencing an impairment problem related to a specific impairment contribution. For example, a query of the end devices  102  may reveal that all of the end devices  102  located off a particular node are reporting a MR impairment contribution above the impairment threshold for MR, while the other end devices  102 , unimpaired sets are not reporting a problem. 
     At step  306 , corrective action is taken. For example, the operator physically inspects all suspect components isolated at step  305  and repairs and replaces as necessary the defective components. The impairment contribution estimator  200  may provide guidance helping cable operators decide the significance of the information that they are analyzing. The impairment contribution estimator  200  may contain a checklist of possible sources of the impairment contribution, preferably sorted in order of probability. For instance, inspection of the suspect components may show that the MR impairment contribution source is a combination of tap-to-output port isolation loss and an improperly terminated cable splice at the end of a feed amplifier. By properly terminating the splice, the operator may reduce the MR to negligible amplitudes. Alternately, the impairment contribution estimator  200  may sort the impaired sets into a less impaired set of devices and a more impaired set of devices according to a level of impairment and route traffic to another channel that the impairment contribution estimator  200  indicates is less impaired. 
     The steps of the method  300  may be repeated periodically and for each of the end devices  102  or groups of end devices to detect future impairment problems and to ensure that detected impairment problems are eliminated and the improvements are sustainable. If the operator is preparing to upgrade the network  100  to a higher modulation scheme, for instance upgrading from 16-QAM to 64-QAM, the operator may perform the method  300  in order to determine potential problem components. In order to test the network  100 , the operator may configure the network at the higher modulation scheme. Thereafter, the operator may perform the testing process of the method  300 , designating the suspect components as potential upgrade components. 
     Although described specifically throughout the entirety of the instant disclosure, representative embodiments of the present invention have utility over a wide range of applications, and the above discussion is not intended and should not be construed to be limiting, but is offered as an illustrative discussion of aspects of the invention. 
     Embodiments of the present invention interpret equalization coefficients for end devices and identify potential impairments of upstream channels for the end devices based on an analysis of the equalization coefficients. Also, a particular type of impairment problem can be identified based on the analysis of equalization coefficients. Determination of the type of impairment can be coupled with additional information, such as location of the end device or tap, to determine suspect cable network components that may be causing the impairment. Thus, identification of an impairment problem and potential solutions can be determined before a customer problem is experienced and without dispatching technicians to diagnose the problem. 
     What has been described and illustrated herein are embodiments of the invention along with some of their variations. The terms, descriptions and figures used herein are set forth by way of illustration only and are not meant as limitations. Those skilled in the art will recognize that many variations are possible within the spirit and scope of the invention, wherein the invention is intended to be defined by the following claims—and their equivalents—in which all terms are meant in their broadest reasonable sense unless otherwise indicated.