Abstract:
Methods and apparatus to perform and/or analyze communication channels are described. In one embodiment, a test signal may be generated and transferred using a first signal route to a line-under-test in response to line qualification mode. The first signal route may include a compensation device to modify the test signal. In some embodiments, a reflected signal from the line-under-test may be received in response to the test signal. A second signal route may be utilized to route the reflected signal in response to line qualification mode.

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATION 
   The present application is related to U.S. patent application Ser. No. 10/457,092, filed Jun. 6, 2003. 
   FIELD 
   The subject matter disclosed herein generally relates to techniques to test signal propagation media. 
   DESCRIPTION OF RELATED ART 
   Line qualification can determine whether a signal propagation medium is capable of providing communications in accordance with a particular communications standard. For example, line qualification can be performed to determine whether a signal propagation medium can be used with DSL standards and variations thereof (including but not limited to ADSL, SHDSL, and VDSL) (DSL standards and variations thereof hereafter are referred to as xDSL). For a description of xDSL standards, see, for example, ITU-T G.991.1, High bit rate Digital Subscriber Line T transceivers (1998); ITU-T G.991.2, Single-pair high-speed Digital Subscriber Line transceivers (2001); ITU-T G.992.1 ADSL standard G.dmt (1999); and related standards. Current DSL and ADSL line qualification techniques typically use dedicated and very expensive hardware and can involve an expensive procedure of dispatching of a trained technician to the client modem site. Currently, it is impractical to build line qualification systems inside cost-sensitive DSL and ADSL mass production modems. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with features and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings in which: 
       FIG. 1  depicts in block diagram format a communications system in accordance with an embodiment of the present invention; 
       FIG. 2  depicts one possible implementation of a modem in accordance with an embodiment of the present invention; and 
       FIG. 3  depicts an example of a compensation device in accordance with an embodiment of the present invention. 
   

   Note that use of the same reference numbers in different figures indicates the same or like elements. 
   DETAILED DESCRIPTION 
     FIG. 1  depicts in block diagram format a communications system  5 . Modem  10  may provide communications between a personal computer (PC)  20  and a central office modem  30  using a communications medium such as a twisted pair telephone line. Modem  10  may provide communications capabilities in accordance, for example, with xDSL and/or other protocols. 
   Modem  10  may communicate with PC  20  using a cable or bus compliant, for example, with Peripheral Component Interconnect (PCI), Universal Serial Bus (USB), Ethernet (e.g., IEEE 802.3), and/or IEEE 1394, although other techniques may be used such as wireless techniques described, for example, in IEEE 802.11 (and related standards). 
   In accordance with an embodiment of the present invention, a modem may perform line qualification of a line, such as one or a combination of a twisted pair telephone line, coaxial cable, or other signal propagation medium, to determine whether the line is capable of providing xDSL or other communications services. The modem may at least perform line qualification of lines of different lengths and having one or more bridge taps. For example, a bridge tap may represent a juncture in which another modem or device may access the line. To perform line qualification, the modem may transmit test signals to the line and process signals reflected by the line in response to the test signals. 
     FIG. 2  depicts one possible implementation of a modem  200  in accordance with an embodiment of the present invention, although other implementations may be used. One embodiment of modem  200  may include signal processor  230 , interface  240 , transmitter  210 , compensation device  215 , receiver  220 , line driver  245 , hybrid  260 , and switches  252 A- 252 D and  254 A- 254 C, although other implementations may be used. Modem  200  may be implemented as any or a combination of: hardwired logic, software stored by a memory device and executed by a microprocessor, firmware, an application specific integrated circuit (ASIC), and/or a field programmable gate array (FPGA). In one implementation, one advantage, although not a necessary feature, is that line qualification capabilities may be manufactured using cost-effective analog front end devices. 
   Modem  200  may communicate with a far end modem or other device using line  250 . Modem  200  may operate in line qualification mode and modem mode. In line qualification mode, modem  200  may operate in “transmit” and “receive” modes. In “transmit” and “receive” line qualification modes, switches  252 A- 252 D may be set to close (i.e., transfer signals) and switches  254 A- 254 C may be set to open (i.e., to not transfer signals). During “transmit” line qualification mode, test signals transmitted by signal processor  230  to line  250  may traverse interface  240 , transmitter  210 , closed switch  252 A, compensation device  215 , closed switch  252 B, line driver  245 , and closed switch  252 C. Such test signals may bypass hybrid  260  and may avoid introducing board echo attributable to the hybrid  260 . 
   In “receive” line qualification mode, a reflection signal from line  250  based on a test signal transmitted in “transmit” line qualification mode may be transferred by closed switch  252 D to amplifier  227 . Accordingly, modem  200  may route reflection signals during “receive” line qualification mode to bypass the hybrid  260  to avoid introducing board echo attributable to the hybrid  260 . 
   In one implementation, if line qualification passes, then modem  200  may operate in normal modem mode. When the modem  200  operates in modem mode, switches  254 A- 254 C may be set to close (i.e., transfer signals) and switches  252 A- 252 D may be set to open (i.e., not transfer signals). To transmit signals in modem mode, transmitter  210  may transmit signals to line  250  through closed switch  254 A, line driver  245 , closed switch  254 B, and hybrid  260 . To receive signals in modem mode, amplifier  227  of receiver  220  may receive signals from line  250  through hybrid  260  and closed switch  254 C. Additional description of receiving signals during modem mode is provided with respect to description of receiver  220 . In one implementation, a controller (such as signal processor  230 ) may control the open/close states of switches  252 A- 252 D and  254 A- 254 C during line qualification and modem modes. 
   Signal processor  230  may perform modulation/demodulation, encoding/decoding of signals in accordance with xDSL. For example, signal processor  230  may operate in compliance with xDSL standards such as ITU-T G.991.1, ITU-T G.991.2, and ITU-T G.992.1. Signal processor  230  may determine and indicate whether line  250  passes or fails line qualification for xDSL or another service. For example, signal processor  230  may control the amplitude and duration of test signal pulses. Based on signals reflected by the line  250  in response to the test signals, signal processor  230  may modify the duration and/or amplitude of the test signals. Based on signals reflected by line  250  in response to test signals, signal processor  230  may determine characteristics of the line (such as the length of the line, whether a bridge tap exists in the line, and cross talk noise level) and/or whether the line passes line qualification for xDSL or other services. 
   For example, signal processor  230  may use time domain reflectometry (TDR) techniques to determine the length of line  250 . The length of the line may be an important factor in whether the line passes line qualification for xDSL service although other factors may be considered such as the existence and distance of any bridge taps in the line and cross talk noise level. Signal processor  230  may be implemented as any or a combination of: hardwired logic, software stored by a memory device and executed by a microprocessor (for example, software executed for example by PC  20  or a central processing unit of modem  200  (not depicted)), firmware, an application specific integrated circuit (ASIC), and/or a field programmable gate array (FPGA). 
   Compensation device  215  may reduce distortions in test signals provided through transmitter  210 . In one implementation, compensation device  215  may provide a gain boost, high-pass filtering, and phase modification of test signals.  FIG. 3  depicts one possible implementation of compensation device  215 , in accordance with an embodiment of the present invention. Compensation device  215  may include gain stage  310 , filter  320 , and phase modifier  330 . Gain stage  310  may provide a power gain boost of the test signal of approximately 50 dB. Filter  320  may high-pass filter the gain boosted test signal from gain stage  310 . For example, filter  320  may transfer the gain boosted test signal for frequencies above approximately 138 kHz. Phase modifier  330  may provide a one-hundred-eighty (180) degree phase shift of the signal transferred from filter  320 . 
   In order to estimate the length of line  250 , the attenuated amplitude of the reflected signal must be reliably detected so that the time interval between the test signal and peak of the reflected signal can be accurately calculated. In some implementations, low-pass filtering of the test signal (e.g., by LPF  234 ) may spread the width of the test signal and generate ripples in the test signal that could bury the reflected signal thereby making TDR measurement impossible. Compensation device  215  may reduce the stop band signal attenuation caused by low-pass filtering. The time interval between the compensated test signal and loop echo may then be accurately calculated to accurately determine the length of the line  250  or other characteristics of the line  250 . 
   Interface  240  may transfer signals to transmitter  210  and/or receive signals from receiver  220 . For example, interface  240  may transfer signals to and from a signal processor  230 . Interface  240  may transfer signals to and from a personal computer (PC) (not depicted). 
   Transmitter  210  may process signals for transmission to a far end modem or central office in compliance, for example, with xDSL standards, although other standards may be complied with. One implementation of transmitter  210  may include digital-to-analog converter (DAC)  233 , low pass filter (LPF)  234 , and line driver  235 , although other implementations can be used. DAC  233  may receive signals from interface  240 . DAC  233  may be implemented as a conventional digital-to-analog converter. LPF  234  may receive signals from DAC  233 . LPF  234  may be implemented as a filter having a pass band from approximately DC to 138 kHz. Line driver  235  may receive signals from LPF  234 . Line driver  235  may provide a voltage gain of approximately 15.7 dB. In one implementation, a controller (such as signal processor  230 ) may program the characteristics (e.g., gain, pass band, on/off state) of each of the components of transmitter  210 . 
   Receiver  220  may process received signals in compliance, for example, with xDSL standards, although other standards may be complied with. Received signals may be transmitted by a far end modem or central office via a bridge tap or be a reflection of a transmitted test signal. One implementation of receiver  220  may include amplifier  227 , low pass filter (LPF)  228 , and receiver analog-to-digital converter (ADC)  229 , although other implementations may be used. 
   During “receive” modem mode, amplifier  227  may receive signals from line  250  through hybrid  260  and closed switch  254 C, but during “receive” line qualification mode, amplifier  227  may receive reflected signals from line  250  through switch  252 D. Amplifier  227  may provide a voltage gain in the range of approximately 0 to 9 dB. LPF  228  may be implemented as a low pass filter having a pass band of approximately DC to approximately 552 or 1104 kHz. LPF  228  may provide signals to ADC  229 . ADC  229  may convert signals from analog to digital formats. ADC  229  may provide digital format signals to interface  240 . 
   MODIFICATIONS 
   The drawings and the forgoing description gave examples of the present invention. The scope of the present invention, however, is by no means limited by these specific examples. Numerous variations, whether explicitly given in the specification or not, such as differences in structure, dimension, and use of material, are possible. The scope of the invention is at least as broad as given by the following claims.