Method and apparatus for time synchronization in a communication system

• US 6,449,291 B1
• Filed: 11/24/1998
• Issued: 09/10/2002
• Est. Priority Date: 11/24/1998
• Status: Expired due to Fees

• Current Assignee: Hewlett-Packard Development Company, L.P. (HP Inc.)
• Original Assignee: 3Com Corporation (HP Inc.)
• Inventors: Boyd, Edward, Burns, Lawrence M.
• Primary Examiner(s): Kizou, Hassan
• Assistant Examiner(s): Yin, Lu
• Application Number:
  US09/199,030
• Time in Patent Office: 1,386 Days
• Field of Search: 370/94.2, 370/503, 370/79, 370/519, 370/520, 375/354, 375/222, 375/371, 375/356, 375/362, 390/516, 725/185, 725/138
• US Class Current: 370/516
• CPC Class Codes: H04J 3/0655 using timestamps
• CPC Class Codes: H04L 12/2801 Broadband local area networks
• CPC Class Codes: H04L 47/28 in relation to timing considerations

Abstract

A method and apparatus for time synchronization in a communications system such as a system including cable modems is provided. The synchronization method comprises the steps of receiving a first timestamp from a headend unit with the cable modem, and generating a first cable modem time reference with the cable modem in response to the receipt of the first timestamp. The cable modem then receives a second timestamp from the headend unit and generates a second cable modem time reference in response to the receipt of the second timestamp. A headend difference time comprising the difference of the first and second timestamps is then generated as is a cable modem difference time comprising the difference of the first and second cable modem time references. A clock error time that comprises the difference of the headend difference time and the cable modem difference time is then generated as well as a correction factor in response to the clock error time. The local clock of the cable modem is then synchronized with the master clock of the headend unit through the adjusting of the output of the local clock output in response to the correction factor. The correction factor in one instance comprises an offset value representative of the amount of clock error per local clock pulse. This offset value is added to an accumulator on each local clock pulse, and the local clock output is adjusted when the accumulator rolls over. The synchronized clock signal can then be utilized to determine a time slot in which the cable modem can transmit an upstream signal to the headend without colliding the upstream signal with upstream signals from other modems in the system.

Claims

1. In a communications system comprising a control unit that includes a master clock and a plurality of transceiver devices coupled to the control unit wherein each transceiver device includes a local clock, a method for synchronizing the local clock of at least one of the transceiver devices with the master clock, the method comprising the steps of:

receiving a first and second timestamp with the transceiver, wherein the first and second timestamp comprise signals generated by the control unit in response to the value of the master clock at the time the first and second timestamp signals are generated, respectively;

generating a first and second transceiver time reference with the transceiver device in response to the receipt of the first and second timestamp signals, respectively;

generating a correction factor in response to the first and second timestamp and the first and second transceiver time reference;

providing the correction factor to an accumulator device; and

generating a synchronized downstream reference signal by adjusting an output clock signal of the local clock in response to the accumulator device.

2. The method of synchronizing the local clock with the master clock of claim 1 further comprising generating a synchronized upstream reference signal wherein the synchronized upstream reference signal comprises a time signal utilized by the transceiver to determine when the transceiver can transmit a first output signal to the control unit such that the first output signal does not collide with a second output signal transmitted by a different transceiver in the plurality of transceivers.

3. The method of synchronizing the local clock with the master clock of claim 2 wherein generating the upstream reference signal comprises one of adding and subtracting a delay offset time from the downstream reference time, wherein the delay offset time comprises the amount of time required for a ranging time signal to be received by the transceiver after the ranging time signal is transmitted by the control unit.

4. The method of synchronizing the local clock with the master clock of claim 1 wherein the transceiver device comprises a cable modem and the control unit comprises a headend unit.

5. The method of synchronizing the local clock with the master clock of claim 1 wherein the steps of generating a correction factor and generating a synchronized downstream reference signal are performed by digital logic.

6. The method of synchronizing the local clock with the master clock of claim 1 wherein the generation of the correction factor is responsive to a clock error time, the method further comprising:

generating a control unit difference comprising the difference of the first and second timestamp;

generating a transceiver difference time comprising the difference of the first and second transceiver time reference; and

generating the clock error time wherein the clock error time comprises the difference of the control unit difference time and the transceiver difference time.

7. The method of synchronizing the local clock with the master clock of claim 6 wherein the first transceiver time reference comprises a time signal generated by the transceiver device in response to the value of the local clock at the time the first timestamp is received, and wherein the second transceiver time reference comprises a time signal generated by the transceiver device in response to the value of the local clock at the time the second timestamp is received.

8. The method of synchronizing the local clock with the master clock of claim 7 wherein the correction factor comprises an offset signal corresponding to the clock error time divided by the transceiver difference time.

9. The method synchronizing the local clock with the master clock of claim 8 wherein generating the synchronized downstream reference signal comprises:

adding the offset signal into the accumulator device on each pulse of the local clock; and

adjusting the output clock signal when the accumulator device rolls over.

10. The method of synchronizing the local clock with the master clock of claim 9 wherein the correction factor comprises an addsub signal that indicates if the local clock is running one of faster and slower than the master clock, and wherein the output clock signal is adjusted in response to the addsub signal.

11. The method of synchronizing the local clock with the master clock of claim 6 further comprising generating a synchronized upstream reference signal wherein the synchronized upstream reference signal comprises a time signal utilized by the transceiver to determine when the transceiver can transmit a first output signal to the control unit such that the first output signal does not collide with a second output signal transmitted by a different transceiver in the plurality of transceivers.

12. The method of synchronizing the local clock with the master clock of claim 11 wherein generating the upstream reference signal comprises one of adding and subtracting a delay offset time from the downstream reference time, wherein the delay offset time comprises the amount of time required for a ranging time signal to be received by the transceiver after the ranging time signal is transmitted by the control unit.

13. In a cable modem communications system comprising a headend unit that includes a master clock and plurality of cable modems coupled of the headend unit wherein each cable modem includes a local clock, a method for synchronizing the local clock of at least one of the cable modems, the method comprising the steps of:

receiving a first timestamp with the cable modem, wherein the first timestamp comprises a signal generated by the headend unit in response to the value of the master clock at the time the first timestamp signal is generated;

generating a first cable modem time reference with the cable modem in response to the receipt of the first timestamp;

receiving a second timestamp with the cable modem, wherein the second timestamp comprises a signal generated by the headend unit in response to the value of the master clock at the time the second timestamp signal is generated;

generating a second cable modem time reference with the cable modem in response to the receipt of the second timestamp;

generating a headend difference time comprising the difference of the first and second timestamps;

generating a cable modem difference time comprising the difference of the first and second cable modem time references;

generating a clock error time that comprises the difference of the headend difference time and the cable modem difference time;

generating a correction factor in response to the clock error time;

providing the correction factor to an accumulator device; and

generating a synchronized downstream reference signal by adjusting an output clock signal of the local clock in response to the accumulator device.

14. The method of synchronizing the local clock with the master clock of claim 13 further comprising generating a synchronized upstream reference signal wherein the synchronized upstream reference signal comprises a time signal utilized by the cable modem to determine when the cable modem can transmit a first output signal to the headend unit such that the first output signal does not collide with a second output signal transmitted by a different cable modem in the plurality of cable modems.

15. The method of synchronizing the local clock with the master clock of claim 14 wherein generating the upstream reference signal comprises one of adding and subtracting a delay offset time from the downstream reference signal, wherein the delay offset time comprises the amount of time required for a ranging time signal to be received by the cable modem after the ranging time signal is transmitted by the headend unit.

16. The method of synchronizing the local clock with the master clock of claim 13 wherein the steps of generating a correction factor and generating a synchronized downstream reference signal are performed by digital logic.

17. The method of synchronizing the local clock with the master clock of claim 13 wherein the first cable modem time reference comprises a time signal generated by the cable modem in response to the value of the local clock at the time the first timestamp is received, and wherein the second cable modem time reference comprises a time signal generated by the cable modem in response to the value of the local clock at the time the second timestamp is received.

18. The method of synchronizing the local clock with the master clock of claim 17 wherein the correction factor comprises an offset signal corresponding to the clock error time divided by the transceiver difference time.

19. The method of synchronizing the local clock with the master clock of claim 18 wherein generating the synchronized downstream reference signal comprises:

adding the offset signal into the accumulator device on each pulse of the local clock; and

adjusting the output clock signal when the accumulator device rolls over.

20. The method of synchronizing the local clock with the master clock of claim 19 wherein the correction factor comprises an addsub signal that indicates if the local clock is running one of faster and slower than the master clock, and wherein the output clock signal is adjusted in response to the addsub signal.

21. In a communications system comprising a headend unit including a master clock and a plurality of cable modems coupled to the headend unit, a cable modem apparatus comprising:

an input/output node coupled to the headend unit;

a receiver, coupled to the input/output node, that receives a first and second timestamp generated by the headend unit in response to the value of the master clock at the time the first and second timestamp signals are generated, respectively;

a local clock; and

a synchronization counter, coupled to the receiver, that generates a first and second cable modem time reference in response to the receipt of the first and second timestamp, respectively, wherein the synchronization counter further includes;

a correction signal generator that generates a correction factor in response to the first and second timestamp and the first and second cable modem time reference; and

a correction circuit, including an accumulator device responsive to the correction factor, wherein the correction circuit generates a synchronized downstream reference signal by adjusting an output clock signal of the local clock in response to the accumulator device.

22. The cable modem apparatus of claim 21 wherein the correction circuit further generates a synchronized upstream reference signal wherein the synchronized upstream reference signal comprises a time signal utilized by the cable modem to determine when the cable modem can transmit a first output signal to the headend unit such that the first output signal does not collide with a second output signal transmitted by a different cable modem in the plurality of cable modems.

23. The cable modem apparatus of claim 22 wherein the correction circuit generates the upstream reference signal by one of adding and subtracting a delay offset time from the downstream reference signal and wherein the delay offset time comprises the amount of time required for a ranging time signal to be received by the cable modem after the ranging time signal is transmitted by the headend.

24. The cable modem apparatus of claim 21 wherein the correction signal generator and the correction circuit comprise digital logic.

25. The cable modem apparatus of claim 21 wherein the correction signal generator generates the correction factor in response to the difference of a headend difference time and a cable modem difference time, wherein the headend difference time comprises the difference of the first and second timestamp, and the cable modem difference time comprises the difference of the first and second cable modem time reference.

26. The cable modem apparatus of claim 25 wherein the first cable modem time reference comprises a time signal generated by the cable modem in response to the value of the local clock at the time the first timestamp is received, and wherein the second cable modem time reference comprises a time signal generated by the cable modem in response to the value of the local clock at the time the second timestamp is received.

27. The cable modem apparatus of claim 26 wherein the correction factor comprises an offset signal corresponding to a clock error time divided by the cable modem difference time, wherein the clock error time comprises the difference of the headend difference time and the cable modem difference time.

28. The cable modem apparatus of claim 27 wherein the synchronized downstream reference signal comprises a signal generated by:

adding the offset signal to the accumulator device on each pulse of the local clock; and

adjusting the output clock signal when the accumulator device rolls over.

29. The cable modem apparatus of claim 28 wherein the correction factor comprises an addsub signal that indicates if the local clock is running one of faster and slower than the master clock, and wherein the output clock signal is adjusted in response to the addsub signal.

30. The cable modem apparatus of claim 25 wherein the correction circuit further generates a synchronized upstream reference signal wherein the synchronized upstream reference signal comprises a time signal utilized by the cable modem to determine when the cable modem can transmit a first output signal to the headend unit such that the first output signal does not collide with a second output signal transmitted by a different cable modem in the plurality of cable modems.

31. The cable modem apparatus of claim 30 wherein the correction circuit generates the upstream reference signal by one of adding and subtracting a delay offset time from the downstream reference signal and wherein the delay offset time comprises the amount of time required for a ranging time signal to be received by the cable modem after the ranging time signal is transmitted by the headend.

32. In a communications system comprising a headend unit including a master clock and a plurality of cable modems coupled to the headend unit, a cable modem apparatus comprising:

an input/output node coupled to the headend unit;

a receiver, coupled to the input/output node, that receives a first timestamp generated by the headend unit in response to the value of the master clock at the time the first timestamp signal is generated, and that receives a second timestamp generated by the headend unit in response to the value of the master clock at the time the second timestamp signal is generated;

a local clock; and

a synchronization counter, coupled to the receiver, that generates a first cable modem time reference in response to the receipt of the first timestamp, and that generates a second cable modem time reference in response to the receipt of the second timestamp, wherein the synchronization counter further includes;

a correction signal generator that generates a correction factor in response to the difference of a headend difference time and a cable modem difference time, wherein the headend difference time comprises the difference of the first and second timestamps, and the cable modem difference time comprises the difference of the first and second cable modem time references; and

a correction circuit, including an accumulator device responsive to the correction factor, wherein the correction circuit generates a synchronized downstream reference signal by adjusting an output signal of the local clock in response to the accumulator device.

33. The cable modem apparatus of claim 32 wherein the correction signal generator and the correction circuit comprise digital logic.

34. The cable modem apparatus of claim 32 wherein the first cable modem time reference comprises a time signal generated by the cable modem in response to the value of the local clock at the time the first timestamp is received, and wherein the second cable modem time reference comprises a time signal generated by the cable modem in response to the value of the local clock at the time the second timestamp is received.

35. The cable modem apparatus of claim 34 wherein the correction factor comprises an offset signal corresponding to a clock error time divided by the cable modem difference time, wherein the clock error time comprises the difference of the headend difference time and the cable modem difference time.

36. The cable modem apparatus of claim 35 wherein the synchronized downstream reference signal comprises a signal generated by:

adding the offset signal into the accumulator device on each pulse of the local clock; and

adjusting the output clock when the accumulator device rolls over.

37. The cable modem apparatus of claim 36 wherein the correction factor comprises an addsub signal that indicates if the local clock is running one of faster and slower than the master clock, and wherein the output clock signal is adjusted in response to the addsub signal.

38. The cable modem apparatus of claim 37 wherein the correction circuit further generates a synchronized upstream reference signal wherein the synchronized upstream reference signal comprises a time signal utilized by the cable modem to determine when the cable modem can transmit a first output signal to the headend unit such that the first output signal does not collide with a second output signal transmitted by a different cable modem in the plurality of cable modems.

39. The cable modem apparatus of claim 38 wherein the correction circuit generates the upstream reference signal by one of adding and subtracting a delay offset time from the downstream reference signal and wherein the delay offset time comprises the amount of time required for a ranging time signal to be received by the cable modem after the ranging time signal is transmitted by the headend.

40. In a communications system comprising a control unit that includes a master clock and a plurality of transceiver devices coupled to the control unit wherein each transceiver device includes a local clock, an apparatus for synchronizing the local clock of at least one of the transceiver devices, the apparatus comprising:

a means for receiving a first and second timestamp, wherein the first and second timestamp comprise signals generated by the control unit in response to the value of the master clock at the time the first and second timestamp are generated, respectively;

a means for generating a first and second transceiver time reference in response to the receipt of the first and second timestamp, respectively;

a means for generating a correction factor in response to the first and second timestamp and the first and second transceiver time reference;

a means for providing the correction factor to an accumulator device; and

a means for generating a synchronized downstream reference signal that comprises an output clock signal of the local clock adjusted in response to the accumulator device.

41. The apparatus for synchronizing the local clock with the master clock of claim 40 further comprising a means for generating a synchronized upstream reference signal wherein the synchronized upstream reference signal comprises a time signal utilized by the transceiver to determine when the transceiver can transmit a first output signal to the control unit such that the first output signal does not collide with a second output signal transmitted by a different transceiver in the plurality of transceivers.

42. The apparatus for synchronizing the local clock with the master clock of claim 41 wherein generating the upstream reference signal comprises one of adding and subtracting a delay offset time from the downstream reference time, wherein the delay offset time comprises the amount of time required for a ranging time signal to be received by the transceiver after the ranging time signal is transmitted by the control unit.

43. The apparatus for synchronizing the local clock with the master clock of claim 40 wherein the transceiver device comprises a cable modem and the control unit comprises a headend unit.

44. The apparatus for synchronizing the local clock with the master clock of claim 40 wherein the means for generating a correction factor and the means for generating a synchronized downstream reference signal comprise digital logic.

45. The apparatus for synchronizing the local clock with the master clock of claim 40 wherein the generation of the correction factor is responsive to a clock error time, the apparatus further comprising:

a means for generating a control unit difference time comprising the difference of the first and second timestamp;

a means for generating a transceiver difference time comprising the difference of the first and second transceiver time reference; and

a means for generating the clock error time wherein the clock error time comprises the difference of the control unit difference time and the transceiver difference time.

46. The apparatus for synchronizing the local clock with the master clock of claim 45 wherein the first transceiver time reference comprises a time signal generated in response to the value of the local clock at the time the first timestamp is received, and wherein the second transceiver time reference comprises a time signal generated in response to the value of the local clock at the time the second timestamp is received.

47. The apparatus for synchronizing the local clock with the master clock of claim 46 wherein the correction factor comprises an offset signal corresponding to the clock error time divided by the transceiver difference time.

48. The apparatus for synchronizing the local clock with the master clock of claim 40 wherein the means for generating the synchronized downstream reference signal comprises:

a means for adding the offset signal into the accumulator device on each pulse of the local clock; and

a means for adjusting the output clock signal when the accumulator device rolls over.

49. The apparatus for synchronizing the local clock with the master clock of claim 48 wherein the correction factor comprises an addsub signal that indicates if the local clock is running one of faster and slower than the master clock, and wherein the means for generating the synchronized downstream reference signal adjusts the output clock signal in response to the addsub signal.

50. The apparatus for synchronizing the local clock with the master clock of claim 45 further comprising a means for generating a synchronized upstream reference signal wherein the synchronized upstream reference signal comprises a time signal utilized by the transceiver to determine when the transceiver can transmit a first output signal to the control unit such that the first output signal does not collide with a second output signal transmitted by a different transceiver in the plurality of transceivers.

51. The apparatus for synchronizing the local clock with the master clock of claim 50 wherein generating the upstream reference signal comprises one of adding and subtracting a delay offset time from the downstream reference time, wherein the delay offset time comprises the amount of time required for a ranging time signal to be received by the transceiver after the ranging time signal is transmitted by the control unit.

52. In a communications system comprising (i) a control unit that includes a master clock and a headend unit and (ii) a plurality of transceiver devices coupled to the control unit wherein each transceiver device includes a local clock and a cable modem, a method for synchronizing the local clock of at least one of the transceiver devices with the master clock, the method comprising the steps of:

receiving a first and second timestamp with the transceiver, wherein the first and second timestamp comprise signals generated by the control unit in response to the value of the master clock at the time the first and second timestamp signals are generated, respectively;

generating a first transceiver time reference and a second transceiver time reference with the transceiver device in response to the receipt of the first and second timestamp signals, respectively;

generating a correction factor in response to the first and second timestamp and the first and second transceiver time reference;

providing the correction factor to an accumulator device; and

generating a synchronized downstream reference signal by adjusting an output clock signal of the local clock in response to the accumulator device.

53. The method of synchronizing the local clock with the master clock of claim 52, wherein the correction factor comprises an offset signal corresponding to the clock error time divided by the transceiver difference time.

54. The method of synchronizing the local clock with the master clock of claim 53, wherein generating the synchronized downstream reference signal comprises:

adding the offset signal into the accumulator device on each pulse of the local clock; and

adjusting the output clock signal when the accumulator device rolls over.