Active link cable diagnostics

Abstract

A novel apparatus for and method of estimating the cable length of an active network link. The cable diagnostics mechanism of the invention is particularly suited for use in estimating the length of Ethernet network between two edges when the link is active, i.e. data is being transmitted in both directions simultaneously and the transmission of test pulses is not possible. The cable length estimation mechanism of the present invention is based on a well-known property of the spectrum of the insertion loss of the cable, namely, the linear relationship between the attenuation of the cable at a given frequency in decibels and the cable length. Information characterizing this relationship is extracted and used to determine the length of the cable.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:

FIG. 1 is a block diagram illustrating an example cable modem system incorporating the active link cable diagnostic mechanism of the present invention;

FIG. 2 is a block diagram illustrating an example Ethernet PHY scheme incorporating the cable diagnostic mechanism of the present invention;

FIG. 3 is a flow diagram illustrating the cable length estimation method of the present invention;

FIG. 4 is a graph illustrating the linear relationship between the spectrum slope and the cable length;

FIG. 5 is a flow diagram illustrating an exemplary DFT length estimation validation algorithm of the present invention;

FIG. 6 is a block diagram illustrating an example fixed point signal flow of the present invention;

FIG. 7 is a block diagram illustrating an example table implementation scheme of the present invention;

FIG. 8 is a diagram illustrating the comparison of example results for real and estimated lengths;

FIG. 9 is a diagram illustrating simulation results of a DFT length estimation for a single segment;

FIG. 10 is a diagram illustrating the slope of the simulated signal spectrum;

FIG. 11 is a diagram illustrating the error of the simulation of FIG. 10;

FIG. 12 is a diagram illustrating the best linear fit to the signal spectrum slope; and

FIG. 13 is a diagram illustrating the comparison of results for real and estimated lengths.

Claims

1. A method of estimating the cable length of an active link in a network, said method comprising the steps of: determining a first channel spectrum magnitude at a first frequency and a second channel spectrum magnitude at a second frequency;calculating a channel spectrum slope from said first channel spectrum magnitude and said second channel spectrum magnitude; andcalculating said cable length estimate from said channel spectrum slope.

2. The method according to claim 1, wherein said step of determining comprises estimating said first channel spectrum magnitude and said second channel spectrum magnitude by performing discrete Fourier transform (DFT) operations on a plurality of equalizer coefficients.

3. The method according to claim 2, wherein said plurality of equalizer coefficients comprise feed forward equalizer (FFE) and decision feedback equalizer (DFE) coefficients.

4. The method according to claim 1, wherein said step of determining comprises estimating said first channel spectrum magnitude and said second channel spectrum magnitude by performing a plurality of discrete Fourier transform (DFT) operations on a signal received over said link.

5. The method according to claim 4, further comprising the step of taking an average of the results of said DFT operations to determine the spectrum magnitude at said first frequency and said second frequency.

6. The method according to claim 1, wherein said first frequency is approximately 1.46 MHz and said second frequency is approximately 10.6 MHz.

7. The method according to claim 1, wherein said first frequency is approximately 7 MHz and said second frequency is approximately 43 MHz.

8. The method according to claim 1, wherein said step of calculating said cable length estimate comprises the step of calibrating a plurality of linear constants for each particular cable topology.

9. The method according to claim 1, wherein said network comprises an Ethernet based network.

10. An apparatus for estimating the cable length of an active link in a network, comprising: means for measuring a first attenuation over said link at a first frequency and a second attenuation over said link at a second frequency;means for calculating slope as a function of said first attenuation and said second attenuation measurements; andmeans for calculating said cable length estimate as a function of said slope and linear calibration constants K1 and K2.

11. The apparatus according to claim 10, wherein said means for measuring comprises means for performing discrete Fourier transform (DFT) operations on a plurality of equalizer coefficients.

12. The apparatus according to claim 11, wherein said plurality of equalizer coefficients comprise feed forward equalizer (FFE) and decision feedback equalizer (DFE) coefficients.

13. The apparatus according to claim 10, wherein said means for measuring comprises means for performing a plurality of discrete Fourier transform (DFT) operations on a signal received over said link

14. The apparatus according to claim 13, wherein said means for performing DFT operations comprises means for taking an average of the results of said DFT operations to determine the spectrum magnitude at said first frequency and said second frequency for use in calculating said slope.

15. The apparatus according to claim 10, wherein said first frequency is approximately 1.46 MHz and said second frequency is approximately 10.6 MHz.

16. The apparatus according to claim 10, wherein said first frequency is approximately 7 MHz and said second frequency is approximately 43 MHz.

17. The apparatus according to claim 10, wherein said linear calibration constants K1 and K2 are calculated in accordance with a particular cable topology.

18. The apparatus according to claim 10, wherein said network comprises an Ethernet based network.

19. A cable modem, comprising: a memory;one or more interface ports;a tuner coupled to a CATV cable having a plurality of channels, said tuner operative to tune a received broadband signal in accordance with a tune command;a PHY circuit coupled to said tuner and operative to generate a baseband signal from the output of said tuner;a cable diagnostics module for estimating the slope of the linear relationship between cable attenuation and length for an active link in a network, said cable diagnostics module comprising: means for measuring a first attenuation over said link at a first frequency and a second attenuation over said link at a second frequency;means for calculating slope as a function of said first attenuation and said second attenuation measurements; anda processor coupled to said memory, said one or more interface ports, said tuner, said cable diagnostics module and said PHY circuit, said processor comprising means for calculating said cable length estimate as a function of said slope and linear calibration constants K1 and K2.

20. The cable modem according to claim 19, wherein said linear calibration constants K1 and K2 are calculated in accordance with a particular cable topology.

21. The cable modem according to claim 19, wherein said network comprises an Ethernet based network.

22. The cable modem according to claim 19, wherein said means for measuring comprises means for performing discrete Fourier transform (DFT) operations on a plurality of equalizer coefficients.

23. The cable modem according to claim 22, wherein said plurality of equalizer coefficients comprise feed forward equalizer (FFE) and decision feedback equalizer (DFE) coefficients.

24. The cable modem according to claim 19, wherein said means for measuring comprises means for performing a plurality of discrete Fourier transform (DFT) operations on a signal received over said link

25. The cable modem according to claim 24, wherein said means for performing DFT operations comprises means for taking an average of the results of said DFT operations to determine the spectrum magnitude at said first frequency and said second frequency for use in calculating said slope.