US 7,583,704 B1Granted

Synchronizing separated upstream and downstream channels of cable modem termination systems

OwnerCarl Walker, David Fox +2inventors

InventorsCarl Walker, David Fox, Achot Matevossian, Michael Healy

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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.

Claims26

Dependent claims

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.

3 more elements

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.

1 more element

Dependencies are inferred from the claim text.