Network data transmission synchronization system and method
First Claim
1. A cable modem connected to a cable transmission system to communicate with a cable modem termination system that has a master clock operating at a system frequency, the cable modem comprising:
- a frequency controllable oscillator having an oscillator frequency representative of a local frequency;
a demodulator for receiving messages representative of the system frequency;
a comparator for generating an error signal representative of the difference between the local frequency represented by the oscillator frequency and the system frequency represented by the messages;
a loop filter having an input to which the error signal is applied and an output that is applied to the oscillator to control the oscillator frequency, the loop filter having initial coefficients that define a first bandwidth; and
a controller that adjusts the loop filter to have coefficients that define a second bandwidth that is smaller than the first bandwidth when the error signal drops below a threshold level.
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Accused Products
Abstract
A number of features for enhancing the performance of a cable transmission system in which data is transmitted between a cable modem termination system at a headend and a plurality of cable modems located different distances from the headend. The power transmission level, slot timing, and equalization of the cable modems are set by a ranging process. Data is transmitted by the modems in fragmented form. Various measures are taken to make transmission from the cable modems robust. The upstream data transmission is controlled to permit multiple access from the cable modems.
81 Citations
24 Claims
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1. A cable modem connected to a cable transmission system to communicate with a cable modem termination system that has a master clock operating at a system frequency, the cable modem comprising:
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a frequency controllable oscillator having an oscillator frequency representative of a local frequency;
a demodulator for receiving messages representative of the system frequency;
a comparator for generating an error signal representative of the difference between the local frequency represented by the oscillator frequency and the system frequency represented by the messages;
a loop filter having an input to which the error signal is applied and an output that is applied to the oscillator to control the oscillator frequency, the loop filter having initial coefficients that define a first bandwidth; and
a controller that adjusts the loop filter to have coefficients that define a second bandwidth that is smaller than the first bandwidth when the error signal drops below a threshold level. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9)
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10. A cable modem connected to a cable transmission system to communicate with a cable modem termination system (CMTS) that has a master clock operating at a system frequency, the cable modem comprising:
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a frequency controllable oscillator synchronized to the master clock;
a counter operated by the oscillator to generate a time stamp message representative of the frequency of the master clock;
a timing of offset detector for generating a timing offset message representative of the transmission time delay between the cable modem and the CMTS;
a summer for adding the time stamp message and the timing offset message to identify time slots on an upstream channel;
a downstream receiver for processing MAP messages that specify time slots assigned to the cable modem on the upstream channel;
an upstream data queue for storing data to be transmitted upstream; and
an upstream transmitter for transmitting data in the queue on the upstream channel when the time stamp message and the timing offset message identify time slots that match the time slots specified by the MAP messages. - View Dependent Claims (11, 12)
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13. A cable modem termination system (CMTS) comprising:
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a time stamp generator that generates time stamp messages for synchronizing cable modems to each other;
a downstream data queue that has frame boundaries between portions of the data in the queue;
a downstream processor that formats data from the queue into a data stream;
a time stamp send generator for issuing time stamp send commands; and
a downstream transmitter connected to the downstream processor to send the formatted data, connected to the time stamp generator to receive the time stamp messages, and connected to the time stamp send generator to insert time stamp messages from the time stamp generator into the data stream responsive to the time stamp send commands. - View Dependent Claims (14, 15, 16, 17)
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18. A method for coordinating the transmission of data to a cable headend on an upstream channel of a cable system from one of a plurality of cable subscriber stations located different distances from the headend, the method comprising the steps of:
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synchronizing a frequency controllable local clock at the subscriber station to a master clock at the headend;
generating a time stamp message from the local clock representative of the frequency of the master clock;
receiving at the subscriber station a timing correction message generated at the headend, the timing correction message representing the transmission time delay between the one subscriber station and the headend relative to the other subscriber stations;
adding the time stamp message and the timing correction message to define time slots for upstream transmissions from the subscriber stations; and
transmitting data from the one subscriber station to the headend in the time slots. - View Dependent Claims (19, 20, 21)
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22. In a cable transmission system having a headend, a plurality of cable modems located different distances from the headend, and a cable network that connects the headend to the cable modems, a combination comprising:
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a master clock at the headend;
a local clock at each cable modem;
means for synchronizing each local clock to the master clock;
means for sending from the headend to each cable modem a command to transmit a timing offset signal at a prescribed time referenced to the local clock, there being for each cable modem an expected time of arrival at the headend based on the distance from the headend;
means for transmitting the timing offset signal from each cable modem at the prescribed time referenced to the local clock;
means for receiving the timing offset signal from each cable modem at the headend;
means for generating at the headend for each cable modem a timing correction message, the timing correction message representing the difference between the actual time of arrival of the timing offset signal at the headend and the expected time of arrival referenced to the headend clock;
means for transmitting the timing correction messages to the respective cable modems;
means responsive to the local clock at each cable modem for generating a continuously incremented local clock timing signal;
means at each cable modem for adding the timing correction message to the local clock timing signal to produce a time slot defining signal corrected for the particular cable modem;
means for transmitting data from each cable modem to the headend in time slots allocated from the headend responsive to the time slot defining signal so as to place the data from the cable modems in the allocated time slots. - View Dependent Claims (23, 24)
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Specification