Synchronizing separated upstream and downstream channels of cable modem termination systems

• US 7,583,704 B1
• Filed: 06/10/2003
• Issued: 09/01/2009
• Est. Priority Date: 06/10/2003
• Status: Active Grant

• Current Assignee: Cisco Technology, Inc. (Cisco Systems, Inc.)
• Original Assignee: Cisco Technology, Inc. (Cisco Systems, Inc.)
• Inventors: Fox, David, Healy, Michael, Walker, Carl, Matevossian, Achot
• Primary Examiner(s): Vu; Huy D.
• Assistant Examiner(s): Chery; Dady
• Application Number:
  US10/459,136
• Time in Patent Office: 2,275 Days
• Field of Search: 370/503, 370/352, 370/350, 370/353, 370/354, 370/356, 375/222, 375/257, 725/63, 725/64, 725/65, 725/66, 725/67, 725/702, 725/71, 725/111, 725/114, 725/144
• US Class Current: 370/503
• CPC Class Codes: H04J 3/0644 External master-clock
• CPC Class Codes: H04J 3/0664 unidirectional timestamps
• CPC Class Codes: H04L 7/0008 Synchronisation information channels, e.g. clock distribution lines

Abstract

Methods and devices are provided for allowing upstream and downstream cable modem termination systems to be located in different physical locations. According to some embodiments of the invention, a downstream CMTS is located at a head end of a cable network and an upstream CMTS is located in each of a plurality of hubs in a cable network. Such a configuration provides more flexibility than prior art systems, especially when much of the traffic passing through the downstream CMTS is broadcast traffic, such as IP video, that will be passed to multiple hubs. Such configurations are more economical because they do not require every hub to have its own downstream CMTS. In preferred embodiments, the upstream and downstream functions are synchronized using a Global Positioning System (“GPS”) reference source.

Claims

1. A system, comprising:

a Cable Modem Termination System (CMTS) component operating a Data Over Cable Service Interface Specifications (DOCSIS) Media Access Control (MAC) framer, the CMTS component exchanging data between a first access network and a second cable network, the DOCSIS MAC framer configured to frame communications received over the first access network into a DOCSIS format before communicating those DOCSIS framed communications over a variable delay packet switched portion of the second cable network;

wherein the DOCSIS MAC framer is configured to insert DOCSIS timestamps obtained from a local counter of the CMTS component in conjunction with the framing, the DOCSIS timestamps usable for frequency locking downstream cable modems with the CMTS component;

synchronization circuitry on the CMTS component, the synchronization circuitry configured to extract a time value from a Global Positioning System (GPS) message, to process the extracted time value to generate a DOCSIS timestamp, and to synchronize the local counter to the generated DOCSIS timestamp according to a GPS pulse associated with the serial GPS message; and

a hub containing a Quadrature Amplitude Modulator (QAM) and being located remotely from the CMTS component and separated from the CMTS component by the variable delay packet switched portion of the second cable network, the hub configured to receive the DOCSIS framed communications over the variable delay packet switched portion of the second cable network, the hub configured to quadrature amplitude modulate the DOCSIS framed data for sending on a constant delay portion of the second cable network;

wherein the hub is configured to replace the DOCSIS timestamps as the DOCSIS framed communications are introduced into the constant delay portion of the second cable network according to a local counter of the hub, wherein said local counter of the hub is synchronized according to a GPS input at the GPS pulse, and wherein said DOCSIS timestamp replacement by the hub maintains the frequency lock between the CMTS component and the downstream cable modems regardless of the variability and length of delays associated with transmitting the DOCSIS framed communications between the CMTS component and the hub.

2. The system of claim 1, wherein the hub is further configured to:

remove the DOCSIS timestamps included in the DOCSIS framed communications; and

replace the DOCSIS timestamps with new DOCSIS timestamps obtained according to the local counter of the hub.

3. The system of claim 1, wherein the counters are synchronized independently of any synchronization messages exchanged over the packet switched portion of the second cable network.

4. The system of claim 3, wherein the synchronization independently of the packet switched portion of the second cable network allows the hub and the CMTS component to be separated by any number of packet switching devices and any physical distance without disruption of the frequency lock between the CMTS component and the cable modems.

5. The system of claim 1, wherein the packet switched portion of the second cable network is an Internet Protocol (IP) network, and the system further comprises switches or routers operating between the CMTS component and the hub.

6. The system of claim 1, wherein the synchronization circuitry is further configured to:

determine an elapsed time difference between the extracted time value and a previous GPS time value;

multiply the elapsed time difference by 10.24; and

synchronize the local counter of the CMTS component according to the product.

7. The system of claim 1, wherein synchronization of the local counters at the same GPS pulse allows the CMTS component and the cable modems to decode and process, respectively, upstream and downstream modulated data independently of the physical proximity of the hub and the CMTS component.

8. A system, comprising:

a hub containing a Quadrature Amplitude Modulator (QAM) and Being located remotely from a Cable Modem Termination System (CMTS) component and separated from the CMTS component by a variable delay packet switched portion of a cable network, the hub configured to receive Data Over Cable Service Interface Specifications (DOCSIS) framed communications from the CMTS over the variable delay packet switched portion of the cable network, the hub configured to quadrature amplitude modulate the DOCSIS framed data for sending on a constant delay portion of the cable network to a plurality of downstream cable modems;

wherein the hub is configured to replace DOCSIS timestamps in the DOCSIS framed communications with new values obtained from a local counter prior to introducing the data onto the constant delay portion of the second cable network; and

wherein said local counter of the hub is synchronized according to a same reference Global Positioning System (GPS) time value that is used to synchronize a counter on the remote CMTS component.

9. The system of claim 8, wherein said DOCSIS timestamp replacement with the new values maintains a frequency lock between the CMTS component and the downstream cable modems regardless of the variability and length of delays associated with transmitting the DOCSIS framed communications from the remote CMTS component to the hub.

10. The system of claim 8, further comprising:

a DOCSIS Media Access Control (MAC) framer located in the CMTS component, the DOCSIS MAC framer configured to frame communications received over an access network into a DOCSIS format before communicating those DOCSIS framed communications over the variable delay packet switched portion of the cable network.

11. The system of claim 10, wherein the DOCSIS MAC framer is configured to insert time values obtained from the CMTS component'"'"'s counter in conjunction with the framing, the time values usable for frequency locking downstream cable modems with the CMTS component.

12. The system of claim 11, further comprising:

synchronization circuitry on the CMTS component, the synchronization circuitry configured to extract a time value from a Global Positioning System (GPS) message, to process the extracted time value to generate a reference DOCSIS timestamp, and to synchronize the CMTS component'"'"'s counter to the generated reference DOCSIS timestamp according to a GPS pulse associated with the GPS message;

wherein the CMTS component'"'"'s counter is synchronized in unison with the hub'"'"'s counter at the GPS pulse.

13. The system of claim 8, wherein the counters are synchronized in unison to the same reference GPS time value independently of any synchronization messages exchanged over the packet switched portion of the cable network.

14. The system of claim 13, wherein the synchronization independently of the packet switched portion of the cable network allows the hub and the CMTS component to be separated by any number of packet switching accruements and any physical distance without disruption of the frequency lock between the CMTS component and the cable modems.

15. The system of claim 8, wherein the packet switched portion of the cable network is an Internet Protocol (IP) network, and the system further comprises switches or routers operating between the CMTS component and the hub.

16. The system of claim 8, wherein the CMTS component is configured to:

determine an elapsed time difference between the reference GPS time value and a previously received reference GPS time value;

multiply the elapsed time difference by 10.24; and

synchronize the local counter of the CMTS component according to the product.

17. A system, comprising:

a Cable Modem Termination System (CMTS) component operating a Data Over Cable Service Interface Specification (DOCSIS) Media Access Control (MAC) framer, the CMTS component exchanging data between a first access network and a second cable network, the DOCSIS MAC framer configured to frame communications received over the first access network into a DOCSIS format before communicating those DOCSIS framed communications over a variable delay packet switched portion of the second cable network;

wherein the CMTS component is configured to send the DOCSIS framed communications over the packet switched portion of the second cable network for remote quadrature amplitude modulation, and wherein the processes of DOCSIS framing and quadrature amplitude modulating the DOCSIS framed communications are distributed across the packet switched portion of the second cable network; and

synchronization circuitry on the CMTS component, the synchronization circuitry configured to extract a reference time value from a Global Positioning System (GPS) message, to process the extracted time value to generate a reference DOCSIS timestamp, and to synchronize a local counter to the generated DOCSIS timestamp according to a GPS pulse associated with the GPS message;

wherein the CMTS component is configured to insert the DOCSIS timestamps obtained from the local counter of the CMTS component in conjunction with the framing, the DOCSIS timestamps usable for frequency locking downstream cable modems with the CMTS component.

20. The system of claim 17, wherein the local counter is synchronized at the GPS pulse in unison with a remote counter located on a remote hub configured to perform the quadrature amplitude modulation.

21. The system of claim 17, wherein the local counter is synchronized at the GPS pulse in unison with a remote counter located on a remote upstream CMTS component.

18. The system of 17, further comprising:

a hub containing a Quadrature Amplitude Modulator (QAM) and being located remotely from the CMTS component and separated from the CMTS component by the variable delay packet switched portion of the second cable network, the hub configured to receive DOCSIS framed communications over the variable delay packet switched portion of the second cable network, the hub configured to quadrature amplitude modulate the DOCSIS framed data for sending on a constant delay portion of the second cable network;

wherein the hub is configured to replace the DOCSIS timestamps as the DOCSIS framed communications are introduced into the constant delay portion of the second cable network according to a local counter of the hub, wherein said local counter of the hub is synchronized in unison with the local counter of the CMTS component at the GPS pulse.

19. The system of claim 18, wherein said DOCSIS timestamp replacement by the hub maintains the frequency lock between the CMTS component and the downstream cable modems regardless of the variability and length of delays associated with transmitting the DOCSIS framed communications between the CMTS component and the hub.

22. A system, comprising:

a Cable Modem Termination System (CMTS) component operating a Data Over Cable Service Interface Specification (DOCSIS) Media Access Control (MAC) framer, the DOCSIS MAC framer configured to frame communications received over a first access network into a DOCSIS format before communicating those DOCSIS framed communications over a variable delay packet switched portion of a second cable network;

wherein the CMTS component is configured to send the DOCSIS framed communications over the packet switched portion of the second cable network for remote quadrature amplitude modulation; and

synchronization circuitry on the CMTS component, the synchronization circuitry configured to synchronize a counter used for DOCSIS time-stamping the framed communications with another counter used to replace the DOCSIS timestamps with new values after transmission over the packet switched portion of the second cable network, and wherein the counters are synchronized according to a Global Positioning System (GPS) input.

23. The system of claim 22 wherein the counters are synchronized to a same DOCSIS time value.

24. The system of claim 22, further comprising a remote hub configured to replace the DOCSIS timestamp values according to the another counter and quadrature amplitude modulate the DOCSIS framed communications.

25. The system of claim 24, wherein the remote hub contains an upstream CMTS.

26. The system of claim 22, wherein the synchronization of the counters is based on out of band communications such that variable delays associated with the packet switched portion of the second cable network do not disrupt synchronization, and wherein the out of band communications include GPS messages transmitted to the CMTS component and a remote hub over a different path than a path used for sending the DOCSIS framed communications from the CMTS component to the remote hub.