Source: http://www.google.co.uk/patents/US6965616
Timestamp: 2015-09-05 14:19:10
Document Index: 196402694

Matched Legal Cases: ['Application No. 60', 'Application No. 60', 'Application No. 60', 'Application No. 60', 'Application No. 60', 'Application No. 60']

Patent US6965616 - Network data transmission synchronization system and method - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inAdvanced Patent SearchPatentsA 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...http://www.google.co.uk/patents/US6965616?utm_source=gb-gplus-sharePatent US6965616 - Network data transmission synchronization system and methodAdvanced Patent SearchPublication numberUS6965616 B1Publication typeGrantApplication numberUS 09/684,712Publication date15 Nov 2005Filing date6 Oct 2000Priority date30 Oct 1998Fee statusPaidAlso published asUS6961314, US7120123Publication number09684712, 684712, US 6965616 B1, US 6965616B1, US-B1-6965616, US6965616 B1, US6965616B1InventorsThomas J. Quigley, Jonathan S. Min, Lisa V. Denney, Henry Samueli, Sean F. Nazareth, Feng Chen, Fang Lu, Christopher R. JonesOriginal AssigneeBroadcom CorporationExport CitationBiBTeX, EndNote, RefManPatent Citations (20), Non-Patent Citations (4), Referenced by (43), Classifications (25), Legal Events (4) External Links: USPTO, USPTO Assignment, EspacenetNetwork data transmission synchronization system and method
US 6965616 B1Abstract
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.
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. 2. The cable modem of claim 1, wherein the loop filter comprises a linear loop having linear coefficients and an integral loop having integral coefficients and the controller adjusts both the linear coefficients and the integral coefficients when the error signal drops below a threshold level.
3. The cable modem of claim 2, additionally comprising a time stamp generating counter connected to the oscillator to generate time stamps that are representative of the oscillator frequency, the messages received by the demodulator are time stamps, and the error signal generated by the comparator represents the difference between the time stamps.
4. The cable modem of claim 3, wherein the demodulator receives messages representative of a slot timing offset of the cable modem relative to the cable modem termination system and the slot timing offset messages are added to the time stamps generated by the counter.
5. The cable modem of claim 4, wherein the resolution of the timing offset messages is equal to or finer than the resolution of the time stamps generated by the counter.
6. The cable modem of claim 3, additionally comprising an acquisition counter connected to the demodulator to count the number of time stamps received by the demodulator, the controller measures the error signal after the acquisition counter reaches a threshold value, and the controller adjusts the linear and integral coefficients if the measured error signal drops below the threshold level after the acquisition counter reaches the threshold value.
7. The cable modem of claim 6, wherein the controller resets the acquisition counter if the measured error signal does not drop below the threshold level after the acquisition counter reaches the threshold value.
8. The cable modem of claim 7, wherein the controller compares the state of the acquisition counter with the threshold value, advances the state of the acquisition counter if the state is below the threshold value, measures the error signal if the state is above the threshold value, and adjusts the coefficients if the measured error signal drops below the threshold level.
9. The cable modem of claim 1, additionally comprising a controller for averaging the interarrival time between messages and selecting the coefficients from a table of filter coefficients depending on the average interarrival time.
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:
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. 11. The cable modem of claim 10, wherein the downstream receiver recovers time stamp messages sent downstream by the CMTS responsive to the master clock, and the cable modem additionally comprises a control loop that adjusts the frequency of the oscillator responsive to the difference between the time stamp messages sent downstream by the CMTS and time stamp messages generated by the counter to synchronize the oscillator to the master clock.
12. The cable modem of claim 10, wherein the downstream receiver recovers a ranging signal sent downstream by the CMTS and the timing offset detector generates the timing offset message from the ranging signal.
13. A cable modem termination system (CMTS) comprising:
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. 14. The CMTS of claim 13, wherein the time stamp generator comprises a master clock and a counter operated by the master clock to generate time stamp messages that represent the current state of the counter, the downstream processor being connected to the counter to insert time stamp messages from the counter into the data stream responsive to the time stamp send commands.
15. The CMTS of claim 14, wherein the time stamp send generator comprise a counter operated by the master clock.
16. The CMTS of claim 15, wherein the counter is configured to generate a time stamp send command after the counter reaches a predetermined count.
17. The CMTS of claim 16, wherein the predetermined count is programmable.
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:
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. 19. The method of claim 18, wherein the timing correction message is referenced to the master clock.
20. The method of claim 18, wherein the local clock is an oscillator, the step of generating a time stamp message at the subscriber station comprises counting cycles of the oscillator, and the step of synchronizing the local clock to the master clock comprises generating an error signal representative of the difference between the time stamp message generated at the subscriber station and a time stamp message generated at the headend by the master clock and controlling the frequency of the local clock to reduce the difference.
21. The method of claim 20, wherein the step of receiving at the subscriber station a timing correction message comprises:
sending a command from the headend to the subscriber station to transmit a timing offset signal at a prescribed time referenced to the subscriber's local clock, there being for each subscriber station an expected time of arrival at the headend based on the distance from the headend; transmitting the timing offset signal from the subscriber station at the prescribed time referenced to the subscriber's local clock; receiving the timing offset signal from the subscriber station at the headend; generating at the headend for the subscriber station 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 master clock; and transmitting the timing correction message for receipt by the subscriber station. 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:
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. 23. The combination of claim 22, additionally comprising means responsive to the master clock for generating at the headend a continuously incremented master clock timing signal, means for generating MAP messages to allocate time slots to the cable modems based on the master clock timing signal and means for transmitting the MAP messages to the cable modems.
24. The combination of claim 23, wherein the synchronizing means comprises means for transmitting the master clock timing signal to each cable modem, means for comparing the master clock timing signal and the local clock timing signal to generate an error signal, and means for correcting the frequency of the local clock responsive to the error signal.
This application is a continuation of U.S. application Ser. No. 09/574,558, filed on May 19, 2000, which is a continuation-in-part of U.S. regular application Ser. No. 09/430,821, filed on Oct. 29, 1999, which claimed the benefit of the filing date of U.S. Provisional Patent Application No. 60/106,264, filed Oct. 30, 1998 and entitled HEADEND UPSTREAM MAC/PHY INTERFACE; U.S. Provisional Patent Application No. 60/106,427, filed Oct. 30, 1998 and entitled ROBUST TECHNIQUE FOR OPTIMAL UPSTREAM COMMUNICATION BETWEEN CABLE MODEM SUBSCRIBER AND A HEADEND; U.S. Provisional Patent Application No. 60/106,438, filed Oct. 30, 1998 and entitled SYSTEM FOR, AND METHOD OF, FRAGMENTING DATA PACKETS IN A CABLE MODEM SYSTEM; U.S. Provisional Patent Application No. 60/106,439, filed Oct. 30, 1998 and entitled CABLE MODEM SYSTEM; U.S. Provisional Patent Application No. 60/106,440, filed Oct. 30, 1998 and entitled NETWORK DATA TRANSMISSION SYNCHRONIZATION SYSTEM AND METHOD; U.S. Provisional Patent Application No. 60/106,441, filed Oct. 30, 1998 and entitled BURST RECEIVER SYSTEM, the entire contents of all of which are hereby expressly incorporated by reference.
The present invention relates generally to communication systems. The present invention relates more particularly to a cable modem system wherein information is communicated between a plurality of cable modems and a cable modem termination system.
According to an aspect of the invention a modem communicates with a cable transmission system. The modem requests an amount of bandwidth on the cable system to transmit data. The modem receives a grant of an amount of bandwidth to transmit data in response to the request. The modem compares the requested amount of bandwidth with the granted amount of bandwidth. Responsive to the comparison, the modem fragments the data to be transmitted into a plurality of segments if the requested amount is larger than the granted amount. One of the segments is no larger than the granted amount. Responsive to the grant, the modem transmits all the data to be transmitted to the cable system if the requested amount is no larger than the granted amount and transmits only the one segment to the cable system if the requested amount is larger than the granted amount.
In another aspect, the present invention includes a method for facilitating communications between a plurality of cable modems and a cable modem termination system. The method comprises transmitting a request from the cable modem to the cable modem termination system. The request is a request to transmit a data packet from the cable modem to the cable modem termination system. A time slot for transmitting the data packet from the cable modem to the cable modem termination system is allocated, optionally by the cable modem termination system. Information representative of the time slot is then transmitted, optionally by the cable modem termination system, to the cable modem. The cable modem then transmits the data packet from the cable modem to the cable modem termination system within the allocated time slot.
Transmitting the data packet from the cable modem to the cable modem termination system within the time slot mitigates collisions between data packets which are transmitted by different cable modems to the cable modem termination system at the same time and upon the same frequency channel.
In another aspect, the present invention includes an apparatus and method for synchronizing upstream communications between a plurality of cable modems and a cable modem termination system (CMTS). A first message representative of a frequency of a clock of the cable modem termination system is generated, particularly by the cable modem termination system. Similarly, a second message representative of a slot timing offset of a selected one of the cable modems with respect to the clock of the cable modem termination system is generated, again particularly by the cable modem termination system. The first and second messages are transmitted to the selected cable modem. Frequency synchronization of the clock of the selected cable modem is enhanced with respect to the cable modem termination system utilizing the first message. The slot timing offset of the clock of the cable modem with respect to the clock of the cable modem termination system is compensated for using the second message. This process is repeated, as necessary, for each cable modem which is to communicate with the cable modem termination system.
In another aspect, the present invention includes an apparatus and method for rapidly acquiring data packets in a receiver such as that of a cable modem termination system. The method generally comprises determining fractional symbol timing correction by a feedback loop process, determining carrier phase correction by a feedback loop process and determining fine amplitude by a conventional coherent estimation process.
Optionally, the amplitude of the input to the fractional symbol timing correction circuit and/or the input gain to the carrier phase correction circuit may be adjusted so as to be within the optimal operational range of the fractional symbol timing correction circuit and/or the carrier phase correction circuit via the use of phase detector gain boosting logic, wherein a sensor senses the amplitude of the signal input to the fractional symbol timing correction circuit and/or the carrier phase correction circuit and the amplitude of the signal is then modified so as to be within the optimal range.
Optionally, timing recovery may be accelerated by utilizing two out-of-phase, e.g., 180 degree out-of-phase, sampling timing signals such that one of the two sampling timing signals will always be located sufficiently distant from a null point in a binary input sequence as to enhance timing recovery.
Another aspect of the present invention is a method for communicating modulated information from a plurality of cable modems to a cable modem termination system in a way that enhances the robustness of the upstream channels. One way is to monitor upstream channels for at least one modulation parameter which is indicative of channel quality and adjusting the modulation in response to the channel quality parameter.
Another way is to vary the symbol rate of information transmitted over a channel in a near-continuous manner by varying at least one of the upper and lower bounds.
Another way is to monitor the upstream channels and to use fine carrier frequency agility and/or variable symbol rate to facilitate dynamic channel allocation so as to communicate only within channels having quality which is above a predetermined threshold.
According to another aspect of the present invention, slot timing information and/or data-type information is transmitted from a MAC to a burst receiver so as to facilitate processing of upstream data packets by the burst receiver in an efficient and reliable manner. Channel status/error information is transmitted from the burst receiver to the MAC. The error information is used by the MAC to facilitate spectrum management which enhances the data rate and/or reliability of upstream communications.
In an aspect of the invention, noise is abated in a cable system. The cable system has a headend and a plurality of user terminals interconnected by a cable that has a single downstream transmission path from the headend to each of the user terminals and a plurality of upstream transmission paths from each of the user terminals to the headend. A notch filter at the headend is adjusted to reject one or more bands of common noise. A feedback equalizer (FBE) at the headend compensates for the distortion caused by the notch filter. Afterwards, a ranging signal is received at the headend from each user terminal and feedforward equalizer coefficients corresponding to the transmit equalizer coefficients for each user terminal are generated from the ranging signals. The feedforward equalizer (FFE) coefficients are transmitted to the respective user terminals, where they are applied to a transmit equalizer filter to compensate for individual noise at the respective user terminals.
FIG. 5A is a schematics block diagram showing the interconnections of the burst receiver, medium access control (MAC) and transmitter downstream modulator within a cable modem termination system;
FIG. 23 is a block diagram of a phase detector gain boosting logic circuit wherein the amplitude of a signal input to a phase detector is monitored by a sensor and the amplitude of the signal to the low pass filter of the loop is controlled b