Detection of duplicate participants in a two-way modem environment
View Patent ↗Duplicate participants (e.g., cloned subscriber units) ( 274 ) are detected in a communication network, such as a hybrid fiber/coax (HFC) cable television network or the like, by monitoring the physical layer of the network to detect transmission differences between such units. The subscriber units ( 14, 16, 18, 274, 276 ) may be cable modems that transmit upstream signals with associated identifiers to a headend ( 10, 200 ), e.g., to access the Internet or for telephony. Measured characteristics of the upstream signals can include: (1) propagation time ( 235 ), (2) frequency ( 240 ), (3) power ( 245 ), and (4) spectral characteristics ( 250 ). For propagation time, the reception time of the upstream message can be compared to a headend clock ( 215 ) and also to other messages with the same modem ID. For spectral characteristics, adjustment coefficients can be provided to the units to normalize the spectrum of the upstream signal to a baseline spectrum. When discrepancies are detected for an upstream signal that indicate duplicate modems are operating in the network, the account of the corresponding unit can be terminated.
1. A method for detecting a clone subscriber unit in a communication network, comprising the steps of:
recording a transmission characteristic of a signal from an original subscriber unit that is authorized for use in said network;
measuring a comparable transmission characteristic of a signal from a subscriber unit on said network alleging to be said original subscriber unit; and
determining whether there is a difference between the measured transmission characteristic and the recorded transmission characteristic;
wherein any such difference is indicative that the alleging subscriber unit is a clone subscriber unit.
2. The method of claim 1 , wherein:
said recording, measuring and determining steps occur at a headend of the network.
3. The method of claim 1 , wherein:
the measured transmission characteristic is associated with a physical layer of said network.
4. The method of claim 1 , wherein:
said network is a hybrid fiber/coax cable television network.
5. The method of claim 4 , wherein:
said original and alleging subscriber units are cable modems.
6. The method of claim 4 , wherein:
said original and alleging subscriber units comprise hybrid fiber/coax consumer premises equipment.
7. The method of claim 1 , wherein:
the recorded transmission characteristic comprises at least one of a propagation time and a propagation time offset for the signal of the original subscriber unit.
8. The method of claim 7 , wherein:
the propagation time offset is determined by comparing an assigned propagation time of the signal of the original subscriber unit to a receive time thereof.
9. The method of claim 7 , comprising the further steps of:
providing adjustment data to adjust the propagation time offset to a desired value; and
communicating the adjustment data to any subscriber unit in the network that uses an identifier associated with the original subscriber unit for use in adjusting a propagation time offset thereof.
10. The method of claim 9 , wherein:
the adjustment data is provided by a headend of the network.
11. The method of claim 9 , comprising the further steps of:
determining the propagation time offset by sampling the signal of the original subscriber unit using a clock having a clock rate corresponding to a first resolution; and
providing the adjustment data at a second, coarser resolution.
12. The method of claim 11 , wherein:
the clock rate is increased from a nominal level corresponding to said second resolution for recovering upstream transmissions from subscriber units in the network, to a higher level to achieve the first resolution for determining the propagation time offset.
13. The method of claim 11 , wherein:
the clock rate operates at the first resolution for initially providing the adjustment data; and
the adjustment data is provided at the second resolution by omitting at least one least significant bit thereof.
14. The method of claim 11 , wherein:
the clock rate operates at the first resolution for initially providing the adjustment data; and
the adjustment data is provided at the second resolution by randomizing at least one least significant bit thereof.
15. The method of claim 1 , wherein:
the recorded transmission characteristic comprises at least one of a frequency and a frequency offset.
16. The method of claim 15 , comprising the further steps of:
providing adjustment data to adjust the frequency offset to a desired value; and
communicating the adjustment data to any subscriber unit in the network that uses an identifier associated with the original subscriber unit for use in adjusting a frequency thereof.
17. The method of claim 1 , wherein:
the recorded transmission characteristic comprises at least one of a power and a power offset.
18. The method of claim 17 , comprising the further steps of:
providing adjustment data to adjust the power offset to a desired value; and
communicating the adjustment data to any subscriber unit in the network that uses an identifier associated with the original subscriber unit for use in adjusting a power thereof.
19. The method of claim 1 , wherein:
the recorded transmission characteristic comprises a spectral characteristic.
20. The method of claim 19 , wherein:
the spectral characteristic comprises at least one of a power spectrum and a power spectrum offset.
21. The method of claim 20 , comprising the further steps of:
providing adjustment data to adjust the power spectrum offset to a desired value; and
communicating the adjustment data to any subscriber unit in the network that uses an identifier associated with the original subscriber unit for use in adjusting a power spectrum thereof.
22. The method of claim 21 , wherein:
the adjustment data comprises filter coefficient data.
23. The method of claim 1 , wherein:
the recorded transmission characteristic is obtained from a measurement of the signal of the original subscriber unit.
24. An apparatus for detecting a clone subscriber unit in a communication network, comprising:
means for recording a transmission characteristic of a signal from an original subscriber unit that is authorized for use in said network;
means for measuring a comparable transmission characteristic of a signal from a subscriber unit on said network alleging to be said original subscriber unit; and
means determining whether there is a difference between the measured transmission characteristic and the recorded transmission characteristic;
wherein any such difference is indicative that the alleging subscriber unit is a clone subscriber unit.
25. An apparatus for detecting a clone subscriber unit in a communication network, comprising:
means for recording a transmission characteristic of an original subscriber unit authorized for use in said network; and
means for comparing said recorded transmission characteristic to a comparable transmission characteristic of a subscriber unit on said network alleging to be said original subscriber unit;
wherein a difference between the compared transmission characteristics is indicative that the alleging subscriber unit is a clone subscriber unit.