Rapid rate adaptation in NBASE-T ethernet

1. A method for fast link recovery for an Ethernet link, the method comprising:

operating the Ethernet link at a first data rate using an adaptive filter having first filter coefficients based on the first data rate;

detecting a drop in link quality on the Ethernet link;

performing a first fast retrain sequence to improve the link quality; and

if the first fast retrain sequence fails to recover the link quality, reducing the first data rate to a reduced data rate, and performing a second fast retrain sequence including low-pass filtering the first filter coefficients to obtain filtered coefficients and subsampling the filtered coefficients to generate updated filter coefficients for the adaptive filter.

2. The method according to claim 1 , wherein reducing the first data rate comprises:

reducing a first symbol rate associated with the first data rate along the Ethernet link; and

maintaining a modulation scheme associated with the first data rate.

3. The method according to claim 2 , wherein the low-pass filtering comprises:

receiving filter coefficient information from a link partner;

low-pass filtering the filter coefficient information.

4. The method according to claim 2 , wherein the subsampling comprises:

subsampling at a rate that matches the reduced first symbol rate along the Ethernet link.

5. The method according to claim 2 , wherein:

operating the Ethernet link at a first data rate comprises operating the Ethernet link at a rate of 5 Gbps;

reducing a first symbol rate comprises reducing the first symbol rate from 200 MHz to 100 MHz; and

maintaining the modulation scheme comprises maintaining a PAM-16 modulation.

6. The method according to claim 1 , wherein reducing the first data rate comprises:

maintaining a constant symbol rate; and

changing a first modulation scheme associated with the first data rate to a different modulation scheme that includes a sparser constellation.

7. The method according to claim 6 , wherein:

operating the Ethernet link at a first data rate comprises operating the Ethernet link at a rate of 5 Gbps;

maintaining the constant symbol rate at 200 MHz; and

changing the first modulation scheme to generate symbols having fewer data bits.

8. The method according to claim 7 , wherein changing the first modulation scheme comprises:

changing the first modulation scheme from PAM-16 to PAM-4.

9. The method according to claim 1 , wherein reducing the first data rate comprises:

communicating between link partners via an InfoField Exchange protocol; and

agreeing to reduce the first data rate through a symbol rate change and/or a data modulation change.

10. The method according to claim 1 , wherein detecting the drop in link quality comprises analyzing a power spectral density (PSD) characteristic.

11. An integrated circuit (IC) Ethernet transceiver chip comprising:

adaptive filter circuitry including filter taps having first filter coefficients based on a first data rate;

modulation logic to modulate transmit data into symbols and to demodulate received symbols into data;

symbol rate logic to establish a symbol rate with a link partner and to transceive data symbols with the link partner at a selectable symbol rate;

detection circuitry to measure a drop in link quality; and

training logic to, when a drop in link quality is detected, perform a first fast retrain sequence to improve the link quality and if the first fast retrain sequence fails to recover the link quality, reduce a data rate associated with the link from the first data rate to a reduced data rate that is less than the first data rate, the training logic to perform a second fast retrain sequence including low-pass filtering the first filter coefficients to obtain filtered coefficients and subsampling the filtered coefficients to generate updated filter coefficients for the adaptive filter.

12. The IC Ethernet transceiver chip according to claim 11 , wherein the adaptive filter circuitry includes at least one filter from the group including echo filter circuitry, near-end crosstalk (NEXT) filter circuitry and far-end crosstalk (FEXT) circuitry.

13. The IC Ethernet transceiver chip according to claim 11 , wherein the training logic further comprises:

low-pass filter circuitry to receive filter coefficient information from the link partner during an exchange process at the reduced data rate; and

subsampling circuitry to subsample the filtered coefficient information.

14. The IC Ethernet transceiver chip according to claim 13 , wherein the symbol rate logic is responsive to the training logic to reduce the data rate from the first data rate to the reduced data rate by reducing a symbol rate.

15. The IC Ethernet transceiver chip according to claim 13 , wherein the modulation logic is responsive to the training logic to reduce the data rate from the first data rate to the reduced data rate by changing a data modulation scheme.

16. The IC Ethernet transceiver chip according to claim 11 , wherein the detection circuitry is configured to detect at least one of signal-to-noise ratio (SNR) and power spectral density (PSD).

17. The method according to claim 1 , wherein the performing the first fast retrain sequence comprises training the Ethernet link without first performing an autonegotiation sequence.

18. The IC Ethernet transceiver chip according to claim 11 , wherein the training logic is configured to perform the first fast retrain sequence by training the link without first performing an autonegotiation sequence.