Abstract:
A method of analyzing the performance of a modem connection. The method includes connecting a line interface to a communication link carrying signals of a modem connection, between a pair of end modems, collecting signals passing on the communication link, between the end modems, through the line interface, determining quality or transmission characteristics regarding the modem connection, responsive to signals collected through the line interface, and displaying information on the determined characteristics.

Description:
RELATED APPLICATIONS 
     The present application is a U.S. national application of PCT Application No. PCT/IL2003/000088, filed on Feb. 4, 2003. 
     FIELD OF THE INVENTION 
     The present invention relates to communication systems and in particular to modem communication systems. 
     BACKGROUND OF THE INVENTION 
     Digital networks are used to transfer computer data and other signals between computers and other end apparatus. In connecting home users to digital networks, analog lines are commonly used. In transmitting data over analog lines, modems on opposite sides of the analog lines modulate the transmitted signals on one side and demodulate the transmitted signals on the other side. 
     In order to allow the end modems to communicate, protocols define the precise signals to be transmitted by the modems. These protocols include, among others, voice band modem (VBM) protocols which use the frequency band allocated to voice signals and DSL protocols (e.g., ADSL) which use-much larger portions of the frequency band of the analog line. 
     Some of the defined protocols are very complex, so as to achieve maximal utilization of the available bandwidth. Due to the complexity of the modem protocols, there are cases in which different vendors implement the protocols differently, leading in some cases to problems in communications. 
     Modem emulators are sometimes used to test the operation of modems. A modem emulator is connected on an opposite side of a communication line to which the tested modem is connected, transmits testing signal to the tested modem and provides a report on the operation of the tested modem. Another existing tool for testing modems is a pair of scope modems which are connected in the middle of a tested analog line connecting opposite end modems. Each modem in the pair of scope modems connects to a different one of the opposite end modems, forming a double connection. The pair of scope modems mirror data between the connections and provide the transferred data to an upper layer for analysis. 
     For security reasons it may be desired to listen to communication lines of suspected individuals. When the communication lines carry modem signals, the signals are generally demodulated in order to determine their content. The demodulation is a relatively hard task, especially if the demodulation is performed by a passive unit, which cannot request retransmissions and cannot adjust the transmission levels according to the noise level in the vicinity of the eavesdropping apparatus. Therefore, when possible, eavesdropping is performed on signals which are not modulated (e.g., already demodulated). Most VBM connections are established with an Internet Service provider (ISP) and eavesdropping can be performed by the ISP. Modem connections, however, can be established between two private modems. In such connections, it is not always known, for a monitored modem, where the other end modem is and it is not always possible to access that modem. 
     The signals of ADSL connections, on the other hand, are always demodulated by a modem at a local line-card of a telephone service company, and when necessary can be monitored at the line-card. 
     SUMMARY OF THE INVENTION 
     An aspect of some embodiments of the present invention relates to a performance analyzer for modem connections, optionally full-duplex modem connections, which analyzes the operation of a modem connection between a pair of modems. The performance analyzer does not replace the modems of the connection, but rather listens to the communications on the modem connection. In some embodiments of the invention, the modem performance analyzer passively listens to the signals transmitted between a pair of modems without sending acknowledgment signals or any other modem tangible signals to either of the modems. Optionally, the performance analyzer may inject noise on the communication line of the connection, at specific times, in order to cause a retrain of the connection. The performance of a retrain may aid in better understanding the signals passing on the connection and/or in identifying connection parameters. 
     The performance analyzer optionally provides information on at least one parameter of the physical operation of the connection, such as the power spectrum density (PSD), discrete multi-tone (DMT) mapping, noise parameters of the line and line quality. Alternatively or additionally, the performance analyzer provides information on non-data signals passing on the connection, such as negotiation signal content, and/or signaling signals passing in parallel to the data transferred on the line, for example AOC (ADSL overhead control) signals, embedded operating channel (EOC) signals and/or indicator bits (IB). 
     In some embodiments of the invention, the performance analyzer provides information on the modem connection in different operation layers and shows cross-references between the different layers. Optionally, the performance analyzer provides information on possible connections between data retransmissions or connection retrains and noise effects or other modem acts which caused the retransmissions or retrains. 
     An aspect of some embodiments of the invention relates to third-party apparatus adapted to force a retrain on an xDSL connection of a twisted pair line, without obstructing telephone service on the twisted pair line. Optionally, the apparatus does not participate in the xDSL connection. In some embodiments of the invention, the apparatus comprises a performance analyzer and/or an eavesdropper which passively listens to the xDSL connection. Forcing a retrain is optionally used to determine parameters of the xDSL connection, required for proper interpretation of the signals passing on the connection. 
     An aspect of some embodiments of the invention relates to an ADSL eavesdropping system which passively listens to signals passing on the connection. 
     There is therefore provided in accordance with an exemplary embodiment of the invention, a method of analyzing the performance of a modem connection, comprising connecting a line interface to a communication link carrying signals of a modem connection, between a pair of end modems, collecting signals passing on the communication link, between the end modems, through the line interface, determining quality or transmission characteristics regarding the modem connection, responsive to signals collected through the line interface, and displaying information on the determined characteristics. 
     Optionally, the modem connection comprises a full-duplex modem connection. Optionally, the modem connection comprises an ADSL modem connection. Optionally, connecting the line interface to the communication line comprises connecting at a point at least two times closer to one of the modems than the other modem. Optionally, connecting the line interface to the communication line comprises connecting at a point at most two times closer to one of the modems than to the other modem. Optionally, collecting signals passing on the communication link comprises collecting without sending to either of the modems acknowledgment signals or any other modem tangible signals. Optionally, displaying information on the determined characteristics comprises displaying the contents of one or more modem negotiation signals. 
     Optionally, displaying information on the determined characteristics comprises providing information on noise levels on the connection. Optionally, providing information on noise levels on the connection comprises suggesting possible sources of the noise. Optionally, displaying information on the determined characteristics comprises providing information on effects in upper layers caused by the noise levels on the connection. 
     Optionally, displaying information on the determined characteristics comprises providing information on the symbol mapping used by the connection. Optionally, displaying information on the determined characteristics comprises displaying information on signaling signals transmitted in parallel to data transmission. Optionally, the method includes performing signal tests on test signals transmitted on the connection and comparing the results of the tests to negotiation signals reporting test results from one of the modems. Optionally, the method includes injecting by the performance analyzer noise which forces a retrain of the modem connection. 
     Optionally, injecting the noise comprises injecting noise in a manner which does not substantially interfere with a different connection passing on the communication link. Optionally, injecting the noise comprises connecting a low impedance circuit, for at least some of the frequency bands carrying signals, to the communication link. Optionally, the modem connection comprises a DSL connection. Optionally, the injected noise does not interfere with voice frequency bands of the communication link. Optionally, the modem connection comprises a voice band modem connection. Optionally, the method includes identifying changes in the operation of the modem connection and providing suggested causes of the changes. 
     Optionally, identifying changes comprises identifying a retrain and/or a bit swap. Optionally, providing suggested causes of the changes comprises identifying, for at least one change, a noise that caused the change. Optionally, the method includes identifying data retransmissions and providing suggested causes of the data retransmissions. Optionally, displaying information on the determined characteristics comprises displaying a raw bit content of signals transmitted on the modem connection. Optionally, displaying information on the determined characteristics comprises providing a warning on a possible tapping of the communication link. Optionally, the method includes extracting the data transmitted on the modem connection. 
     There is further provided in accordance with an exemplary embodiment of the invention, a modem connection performance analyzer, comprising a line interface adapted to collect signals of a modem connection passing on a communication link, between two end modems connected to the link, a processor adapted to determine one or more quality or transmission characteristics regarding the modem connection, responsive to the collected signals, and a human interface adapted to provide information on the determined characteristics. Optionally, the performance analyzer includes a low impedance shorting circuit adapted to short at least some of the frequencies of the communication link, responsive to a command from the processor. 
     There is further provided in accordance with an exemplary embodiment of the invention, a method of monitoring an xDSL modem connection, comprising connecting a line interface to a communication link carrying signals of an xDSL modem connection, between a pair of end modems separate from the line interface, collecting signals passing between the end modems on the communication link, through the line interface, and providing information on the modem connection, responsive to the collected signals. 
     Optionally, providing information on the modem connection comprises providing information on the operation of the connection and/or providing data passing on the connection. 
     There is further provided in accordance with an exemplary embodiment of the invention, a method of forcing a retrain on a modem connection, comprising determining at least one first frequency band to be disrupted, and connecting to a communication line carrying the modem connection, between two end modems, a circuit which disrupts transmission of signals on the at least one first frequency band. Optionally, determining the at least one first frequency band to be disrupted comprises determining a frequency band including a pilot tone frequency band of the modem connection. Optionally, the circuit disrupts the first frequency band substantially without interfering with signals of a second frequency band. Optionally, the second frequency band comprises a frequency band of voice signals. Optionally, connecting the disruption circuit comprises connecting a circuit which shorts the at least one first frequency band without shorting the second frequency band. Optionally, connecting the disruption circuit comprises connecting a circuit which injects noise at the at least one first frequency band. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
       Particular non-limiting embodiments of the invention will be described with reference to the following description of embodiments in conjunction with the figures. Identical structures, elements or parts which appear in more than one figure are preferably labeled with a same or similar number in all the figures in which they appear, in which: 
         FIG. 1  is a schematic illustration of a performance analyzer connected to an ADSL connection, in accordance with an exemplary embodiment of the invention; 
         FIG. 2  is a schematic block diagram of a performance analyzer, in accordance with an exemplary embodiment of the invention; 
         FIG. 3  is a schematic block diagram illustrating the operation of a channel analyzer, in accordance with an exemplary embodiment of the invention; 
         FIGS. 4A-4D  are schematic screen views provided by a graphic user interface, in accordance with an exemplary embodiment of the invention; and 
         FIGS. 5A and 5B  are schematic block diagrams of channel separators, in accordance with exemplary embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIG. 1  is a schematic illustration of a performance analyzer  102  connected to an ADSL connection  100 , in accordance with an exemplary embodiment of the invention. ADSL connection  100  is typically established over a twin pair line  104 , which generally connects a client location  120  to a line-card  122  of a public telephone network. A client modem  110  connects to twin pair line  104  at a first end, in client location  120 , and a server modem  112  (referred to also as a DSLAM), connects to twin pair line  104  at a second end, in the location of line-card  122 . In some embodiments of the invention, ADSL connection  100  operates according to a protocol which uses different frequency bands for upstream and downstream transmission. Alternatively, as described below with reference to  FIG. 5B , ADSL connection  100  may operate according to a protocol including overlapping frequency bands, and time separation, for upstream and downstream signals. 
       FIG. 2  is a schematic block diagram of performance analyzer  102 , in accordance with an exemplary embodiment of the invention. Performance analyzer  102  optionally includes a line interface  202  which connects to twin pair line  104  and senses the signals thereon. Line interface  202  optionally includes a high-impedance low-noise interface suitable for sensing signals from line  104  without interfering with the signals transmitted on the line. The sensed signals from line interface  202  are optionally provided to an A/D converter and channel separation unit  204 , which separates the sensed signals into upstream and downstream signals and converts the sensed analog signals into digital samples. The upstream and downstream digital samples are provided, respectively, to an upstream analyzer  206  and a downstream analyzer  210 , which analyze the data of their respective channel, as described in detail below. A controller  220  optionally receives information from both upstream analyzer  206  and downstream analyzer  210  and generates combined analyzed data. Examples of the combined analyzed data are described below. In some embodiments of the invention, controller  220  additionally controls the operation of upstream analyzer  206 , downstream analyzer  210  and/or A/D converter and channel separation unit  204 . 
     Optionally, the samples and/or analysis results are stored on a storage media  208 , for further reference and/or for comparison to later received signals. A graphic user interface (GUI)  212  optionally displays the raw digital samples and/or the analyzed data. Optionally, a user may select which data is to be stored and/or displayed. Alternatively or additionally, the user may control other functions of performance analyzer  102 , such as when alerts are provided and/or when retrains are to be initiated. Optionally, performance analyzer  102  may be configured to monitor only some of the signals passing on the connection. In an exemplary embodiment of the invention, performance analyzer  102  may be configured only to monitor retrains, only to monitor show-time transmission, or to perform continuous monitoring. In some embodiments of the invention, the raw signals and/or analyzed data are transmitted to a remote unit for further analysis and/or for display. 
     In some embodiments of the invention, an interruption generator  214  is adapted to provide a noise signal on line  104 , so as to induce a retrain of the modem connection. Optionally, interruption generator  214  provides a noise which does not affect low frequencies of the channel, which are dedicated for carrying regular telephone connections. Optionally, interruption generator  214  provides a noise on one or more specific frequencies, which noise disrupts the entire ADSL connection. In some embodiments of the invention, interruption generator  214  provides a noise-on the pilot tone frequency of DSL connections, e.g., 276 kHz. 
     Alternatively or additionally, noise is provided in a plurality of frequencies so that a retrain is forced regardless of the type of DSL connection used and/or the exact frequencies currently used for data transmission. For example, the provided noise may include a wide band noise with a relatively high amplitude. 
     Alternatively or additionally, interruption generator  214  shorts line  104  for high frequencies, by connecting a low impedance connection for high frequencies, to line  104 . The use of a low impedance short in order to initiate a retrain makes it harder to identify the source of the retrain. In addition, the short does not include actively generated noise which may interfere with neighboring lines to which the short is not directly connected. 
       FIG. 3  is a schematic block diagram illustrating the operation of an analyzer  300 , which may be used in implementing upstream analyzer  206  and/or downstream analyzer  210 , in accordance with an exemplary embodiment of the invention. A state machine  302  optionally keeps track of the state of the modem connection, for example whether the connection is carrying data or initialization and activation signals. State machine  302  optionally further keeps track of the frequency bands used for transferring data, the bit rates used in each frequency band and/or the signal mappings used. State machine  302  optionally receives data from other units of analyzer  300  (for simplicity connections are not shown) and accordingly keeps track of the state of the connection. A demodulator  304  optionally receives the signals of the connection from channel separation unit  204  and converts them into bits, according to the current state of the modem connection as tracked by state machine  302 . In some embodiments of the invention, a noise determination unit  306  determines the noise spectrum on the connection, based on a comparison of a theoretical re-modulation of the converted bits to the actually received signals. Optionally, a noise characterizer  308  attempts to determine the type of the noise according to the noise spectrum, for example by comparing the determined noise spectrum to predetermined known spectrums. 
     During initialization and activation, an initialization analyzer  310  optionally receives the demodulated bits and forms therefrom a map of the initialization process. Accordingly, state machine  302  is optionally updated. 
     In some embodiments of the invention, when known training signals are transmitted from one of the modems, to allow the other modem to evaluate the line, a channel testing unit  320  analyzes the training signals to determine the channel quality, in a manner similar to that performed by the end modems of the connection. The results of the channel testing are optionally compared to the results transmitted by the end modem ( 110  or  112  in  FIG. 1 ) receiving the testing signals. According to the comparison, testing-signal analyzer  310  optionally determines the relative positioning of performance analyzer  102  on twin pair line  104 , and/or evaluates the operation of the end modem. 
     During data transfer (also referred to as show time), a signaling analyzer  312  optionally receives the demodulated bits and extracts from them signaling bits which are transmitted along with the data. Signaling analyzer  312  optionally generates a map of the signaling signals and accordingly updates state machine  302 , when appropriate. Alternatively or additionally, signaling analyzer  312  directly updates demodulator  304  on changes which relate to the demodulation, when necessary. In this alternative, state machine  302  may be omitted from analyzer  300  or may be included in demodulator  304  or controller  330 . For example, the signaling bits may indicate a change in the utilization of the channel frequencies, and this data is required for the demodulation. 
     The signaling bits transferred during the show time stage (e.g., AOC, EOC and/or indicator bits) from analyzer  312  are optionally provided to controller  220  for display to the user and/or for verifying correct performance of the end modems. In some embodiments of the invention, when the signaling bits indicate a change in the transmission, such as a bit swap, controller  220  suggests possible causes of the change. 
     In some embodiments of the invention, a data analyzer  314  determines the data content of the received data bits, performing one or more higher layer tasks. Alternatively, the data content is not determined, so that performance analyzer  102  can be used without affecting the privacy of the users of the modem. 
     A controller  330  optionally controls the transfer of signals from demodulator  304  to the other elements of analyzer  300 , for example according to the state of state machine  302 . 
     Referring in more detail to demodulator  304 , in some embodiments of the invention, the demodulation includes a trellis operation, during data transfer. Optionally, demodulator  304  keeps track of the operation of the trellis decoding and provides indications on the trellis decoding to controller  220  for display to the user. For example, in some cases when iterative decoding is used, demodulator  304  may track the number of trellis rounds required for correct demodulation and/or the number of samples having values corrected during traversing the trellis. In some embodiments of the invention, demodulator  304  performs trellis decoding using different methods and reports which decoding methods were sufficient to demodulate the signals correctly and which methods were not able to achieve a correct result. In other embodiments of the invention, no information is provided to the user from the trellis decoding. Optionally, when the protocol requires a trellis decoding or allows a trellis decoding, performance analyzer  102  includes a processor suitable for performing the trellis decoding efficiently and quickly. 
     Alternatively or additionally, demodulator  304  keeps track of other demodulation parameters, such as filter coefficients, mapper points and/or Reed Solomon performance. In some embodiments of the invention, demodulator  304  is adapted to identify retrain signals and accordingly to update state machine  302 . In some embodiments of the invention, demodulator  304  also provides data on the power spectrum density of the signals. 
     Referring in more detail to noise determination unit  306 , in some embodiments of the invention, noise determination unit  306  accumulates noise values for data blocks of a predetermined size and for each data block determines noise statistics, for example noise levels for each frequency band. Optionally, noise determination unit  306  is adapted to identify unusual noise levels and/or events and characterize the noises, for example as impulse noises, cross-talk noises or attenuations. Noise characterizer  308  optionally compares the determined noise statistics to predetermined noise spectrums of known events. In an exemplary embodiment of the invention, noise characterizer  308  stores noise spectrums of interference events, such as from wireless sources, adjacent wire communication lines, radio sets and electric appliances. Alternatively or additionally, noise characterizer  308  is pre-configured with expected affects of temperature changes on the noise level and/or attenuation causes. In some embodiments of the invention, noise characterizer  308  is configured with expected spectrums of purposely added noise, for example noise added by a line tapping unit. Optionally, controller  220  provides a warning on possible tapping units on line  104 . 
     In some embodiments of the invention, when a possible tapping is identified, performance analyzer  102  provides indications on the probable location of the tapping unit, for example based on the timing and/or amplitude of the tapping noise. Alternatively or additionally, when performance analyzer  102  identifies a first sign of a possible tapping unit, the performance analyzer keeps track of additional signs in the received signals which indicate presence of the tapping unit. 
     In some embodiments of the invention, performance analyzer  102  actively transmits test signals on the line to identify whether a tapping unit is connected to line  104 . Alternatively or additionally, performance analyzer  102  may be configured to prevent any transmissions on lines  104  when a tapping is identified, in order to prevent leakage of secret information. 
     Optionally, for each block having an irregular noise pattern, noise characterizer  308  provides controller  220  with one or more possible reasons for the irregularity in the noise. 
     Noise determination unit  306  is optionally adapted to determine the attenuation level caused by lines  104 , for example by comparing the level of the received signals to the known transmitted level. Alternatively or additionally, noise determination unit  306  determines the signal to noise ratio of the signals. 
     In some embodiments of the invention, controller  220  provides warnings on incorrect behavior of the end modems. Optionally, controller  220  follows transmission threads of the modems and provides a warning when one of the modems performs differently than required by the governing protocol. For example, when a modem is requested to change the frequency bands it uses for data transmission, controller  220  verifies that the modem moved at the correct time to the new band division. In another example, controller  220  follows the negotiation signals transmitted between the modems and provides a warning when the signals are different than prescribed by the governing protocol. 
     Optionally, controller  220  also searches for lower layer causes of problematic higher layer effects. For example, when a retransmission of data is requested, controller  220  optionally searches to see if, at the time the original data was transmitted, there was an interference on the line, such as an unusual noise level, a low signal to noise ratio (SNR), a high attenuation level and/or if the modem suffered from skew or other problematic effects in some embodiments of the invention, suggested causes are provided for other identified higher layer problems, such as CRC errors, corrupted bits, and/or low performance of Reed Solomon decoding. In some embodiments of the invention, when one of the modems requests a change in the bit allocation of the frequency band, controller  220  determines the cause of the request and whether the request was necessary. 
     Alternatively or additionally, controller  220  indicates higher layer effects caused by lower layer problems, such as impulse noises and/or cross-talk noises (e.g., from other ADSL lines, from wireless transmissions and/or from regular telephone use). In some embodiments of the invention, controller  220  provides statistics on the stability of the connection. For example, controller  220  may provide indication of the percentage of cases in which an impulse noise causes higher layer trouble and/or the average rate at which higher layer problematic events occur. 
     In some embodiments of the invention, controller  220  is adapted to compare signal exchanges of one connection with signal exchanges of previous connections. For example, a test session may include connecting a plurality of different modems to a tested modem and comparing the training sessions of the different connections. 
     Optionally, a user may request controller  220  to display data from previous connections, optionally showing an entire connection. 
       FIGS. 4A-4D  are schematic screen views provided by GUI  212 , in accordance with an exemplary embodiment of the invention. In  FIG. 4A , a screen  520  shows a general high level overview of a monitored connection. Screen  520  lists the stages (phases) of the connection and optionally the protocols governing each of the stages. For each operation stage (phase) of the connection, a user may optionally request additional details. For example, responsive to a request for additional details about a negotiation phase, a screen  500  ( FIG. 4B ) which shows a portion of an ADSL initialization stage, is optionally shown. Screen  500  optionally includes a downstream portion  502  and an upstream portion  504 . The name  506  of each signal transmitted on the connection is optionally displayed with its beginning  508  and ending  510  transmission times and/or the number  512  of samples it includes. Optionally, the signals of the upstream and downstream are displayed with time correlation, such that a viewer can easily determine the signal time line of the connection. In some embodiments of the invention, a user may click on any of the signal names to view more details about the signal. 
     A screen  540  ( FIG. 4C ) optionally displays the spectrum, magnitude  544  and phase  546  of the transmitted signals. Optionally, a user may select whether to view upstream and/or downstream signals  548 , the type of view (e.g., spectrum)  541  and/or the processing stage  549  at which the spectrum signals are viewed. In some embodiments of the invention, the user may select whether to view the spectrum before they undergo time domain equalization (TEQ), fast Fourier transformation (FFT), frequency domain equalization (FEQ) or after the FEQ. Optionally, the user may select the time duration for which the spectrum may be viewed. The user may select one or more signals for which the spectrum is to be displayed from bar  543  and/or may select one or more time blocks from bar  542 . 
     In some embodiments of the invention, a screen  560  ( FIG. 4D ) lists the fields of each signal and the bit content of each field with an explanation of the value of each field. Optionally, a screen  570  shows the properties of line  104 . Optionally, an upstream graph  571  and a downstream graph  507  (shown only partially) show the transfer function  572  and the noise form  574  of the communication line. Graphs  575  and  577  optionally show the respective phases. 
     In some embodiments of the invention, a screen  580  shows the position of performance analyzer  102  along line  104 . The position is optionally estimated based on the signals received by performance analyzer  102 . 
       FIG. 5A  is a schematic block diagram of A/D converter and channel separation unit  204 , in accordance with an exemplary embodiment of the invention. Unit  204  optionally includes an upstream processing path  404  and a down stream processing path  406 . At the entrance to upstream path  404 , an up stream filter  410  filters the sensed signals allowing only signals belonging to the upstream band to pass (in exemplary protocols known in the art, signals between 27-138 kHz). Optionally, upstream filter  410  allows additional signals at marginal frequencies to pass through the filter so as not to loose any data. The upstream signals are passed through an automatic gain controller (AGC)  414  and an analog to digital converter (A/D)  416 , which converts the signals into digital samples. The digital samples are optionally passed to an estimator  418  which removes affects of the downstream on the upstream signals, using methods known in the art. 
     In parallel, the sensed signals are optionally passed to a downstream filter  412  which passes only signals that belong to the downstream channel, optionally with additional margins. The downstream signals are transferred through an AGC  424  and an A/D  426 , which converts the downstream signals into digital samples. The digital samples of the downstream are passed to an estimator  428  which removes affects of the upstream on the downstream signals, using methods known in the art. 
     In some embodiments of the invention, if necessary according to the protocol governing the transmission of signals on lines  104 , the bands of filters  410  and  412  are adjustable according to the current transmission conditions on the lines. 
     Optionally, for additional accuracy, estimators  430  are also included in unit  204  to remove residue cross-talk between the upstream and downstream channels, as is known in the art. The cleaned upstream and downstream signals are optionally passed to their respective signal analysis units  206  and  210  ( FIG. 2 ). 
       FIG. 5B  is a schematic block diagram of an overlap cancellation unit  450  to be used instead of A/D converter and channel separation unit  204 , in accordance with an exemplary embodiment of the invention. Overlap cancellation unit  450  is optionally used when ADSL connection  100  carries signals according to a protocol including overlapping frequency bands for upstream and downstream signals. Overlap cancellation unit  450  optionally includes an analog circuit  452 , which converts the signals from line interface  202  into digital samples. The digital samples are optionally provided to separate upstream and downstream processors  454  and  456  which estimate the signals of their respective channel, for example according to the different amplitudes of the channels. The estimations from the processors  454  and  456  and optionally provided to cross-talk cancellers  458  which reduce the estimations of the opposite channel from the signals provided to the processors. 
     In some embodiments of the invention, the estimation performed by processors  454  and  456  are repeated several times, to achieve a good channel separation. Optionally, the estimation is repeated until an error rate below a predetermined value is achieved and/or until an achieved correction between two consecutive iterations is smaller than a predetermined level. Alternatively or additionally, a predetermined number of iterations is performed. 
     Alternatively or additionally, to differentiating between the signals in the upstream and downstream according to their amplitude, the signals are separated according to the timings of the signals. Optionally, overlap cancellation unit  450  keeps track of and/or receives data from other units of performance analyzer  102  and accordingly the upstream and downstream signals are separated. For example, when some of the frequency bands do not overlap, the signals in overlapping frequencies are separated according to which of the upstream or downstream bands have signals at the same time. Alternatively or additionally, the signals are separated according to the stream to which the signals immediately preceding or following in time, belong. 
     In some embodiments of the invention, controller  220  operates independently from unit  204  or  450 . Alternatively, one or more parameters determined by unit  204  or  450 , such as the AGC gain level is provided to controller  220  and is used in evaluating the connection and/or is displayed by GUI  212 . 
     Performance analyzer  102  is optionally positioned close to client modem  110  or close to server modem  112 . Positioning performance analyzer  102  close to one of the modems may allow simpler separation of the upstream and downstream signals. Alternatively, performance analyzer  102  is positioned at a middle point between the modems so that neither of the upstream and downstream signals in maximally attenuated. It is noted that since, in some embodiments of the invention, performance analyzer  102  is passive and, for example, does not request retransmissions, avoidance of high attenuation is required to a greater extent than for the end modems. 
     Although the above description relates to an ADSL connection, the invention is not limited to any specific modem connection, and may be used with other DSL connections as well as other modem connections, such as VBM connections. 
     In some embodiments of the invention, performance analyzer  102  is used in testing modems for standard compliance and/or in testing a pair of modems for interoperability assurance. Alternatively or additionally, performance analyzer  102  is used in modem testing and/or debugging. Further alternatively or additionally, performance analyzer  102  is used in troubleshooting, quality of service (QoS) monitoring, modem qualification and/or rating, communication system maintenance and/or for general performance analysis. 
     The elements of performance analyzer may be implemented using dedicated hardware or may be implemented by a combination of software and hardware. For example, analyzer  300  may be implemented in software on a single general purpose computer or on a plurality of dedicated or general purpose processors. Each element of analyzer  300  may be implemented by a separate software module or some or all of the elements of analyzer  300  may be implemented by a single software module. Alternatively or additionally, different task divisions than those described above may be used. 
     It will be appreciated that the above described methods may be varied in many ways, including, changing the order of steps, and/or performing a plurality of steps concurrently. It should also be appreciated that the above described description of methods and apparatus are to be interpreted as including apparatus for carrying out the methods, and methods of using the apparatus. The screen shots shown in  FIGS. 4A-4D  were presented by way of example and many other views may be used in accordance with the present invention. The present invention has been described using non-limiting detailed descriptions of embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. It should be understood that features and/or steps described with respect to one embodiment may be used with other embodiments and that not all embodiments of the invention have all of the features and/or steps shown in a particular figure or described with respect to one of the embodiments. Variations of embodiments described will occur to persons of the art. Furthermore, the terms “comprise,” “include,” “have” and their conjugates, shall mean, when used in the claims, “including but not necessarily limited to.” 
     It is noted that some of the above described embodiments may describe the best mode contemplated by the inventors and therefore may include structure, acts or details of structures and acts that may not be essential to the invention and which are described as examples. Structure and acts described herein are replaceable by equivalents which perform the same function, even if the structure or acts are different, as known in the art. Therefore, the scope of the invention is limited only by the elements and limitations as used in the claims.