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 . fig1 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 fig3 , 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 fig1 , 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 ). fig2 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 . fig3 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 fig5 . 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 fig3 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 . fig4 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 fig1 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 fig5 . 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 fig5 , 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 .