Link training with multiple hops

1 . A method for performing link training over a plurality of hops between host network devices, the method comprising:

performing, by a first intermediate network device among one or more intermediate network devices, link training on a first network link, the first network link extending between the first intermediate network device and a first host network device;

performing, by a second intermediate network device among the one or more intermediate network devices, link training on a second network link, the second network link between the second intermediate network device and a second host network device;

performing, by the first intermediate network device, link training on a third link between the first intermediate network device and the second intermediate network device prior to i) the first intermediate network device performing link training on the first network link with the first host network device and ii) the second intermediate network device performing link training on the second network link;

determining, by the one or more intermediate network devices, when link training of the first network link and the second network link has been completed; and

transmitting, by the one or more intermediate network devices, respective link training completion indications to the first host network device and to the second host network device, including delaying transmission of the respective link training completion indications until link training of both the first network link and the second network link has been completed.

2 . The method of claim 1 , wherein determining when link training of the first network link has been completed includes receiving, at the one or more intermediate network devices, i) a first indication from the first host network device, the first indication indicating that link training has been completed with the first host network device and ii) a second indication from the second host network device, the second indication indicating that link training has been completed with the second host network device.

3 . The method of claim 1 , further comprising:

suppressing both i) link training on the first network link with the first host network device and ii) link training on the second network link with the second host network device until link training on the third link is completed.

4 . The method of claim 3 , further comprising, when link training on the third link has been completed,

transmitting respective first predetermined pseudorandom binary sequence (PRBS) signals between the first intermediate network device and the second intermediate network device to maintain a lock between the first intermediate network device and the second intermediate network device, and

initiating i) the link training on the first network link, and ii) the link training on the second network link.

5 . The method of claim 4 , wherein i) the training on the first network link includes synchronizing a first transmit clock at the first intermediate network device with a transmit clock of the first host network device and ii) the link training on the second network link includes synchronizing a first transmit clock at the second intermediate network device with a transmit clock of the second host network device.

6 . The method of claim 5 , further comprising

switching, by the first intermediate network device, transmitting the first predetermined PRBS to the second intermediate network device to transmitting a second predetermined PRBS to the second intermediate network device to signal to the second intermediate network device that the first transmit clock at the first intermediate network device has been synchronized with the transmit clock of the first host network device, and

switching, by the second intermediate network device, transmitting the first predetermined PRBS to the first intermediate network device to transmitting the second predetermined PRBS to the first intermediate network device to signal to the first intermediate network device that the first transmit clock at the second intermediate network device has been synchronized with the transmit clock of the second host network device.

7 . The method of claim 6 , further comprising:

transmitting a first link training completion indication from the first intermediate network device to the second intermediate network device when link training between the first intermediate network device and the first host network device has been completed, and

transmitting a second link training completion indication from the second intermediate network device to the first intermediate network device when link training between the second intermediate network device and the second host network device has been completed.

8 . The method of claim 7 , wherein:

transmitting the first link training completion indication comprises switching, by the first intermediate network device, transmitting the second predetermined PRBS to the second intermediate network device to transmitting a third predetermined PRBS to the second intermediate network device to signal to the second intermediate network device that link training between the first intermediate network device and the first host network device has been completed, and

transmitting the second link training completion indication comprises switching, by the second intermediate network device, transmitting the second predetermined PRBS to the first intermediate network device to transmitting the third predetermined PRBS to the first intermediate network device to signal to the first intermediate network device that link training between the second intermediate network device and the second host network device has been completed.

9 . A method for performing link training over a plurality of hops between host network devices, the method comprising:

performing, by a first intermediate network device among one or more intermediate network devices, link training on a first network link, the first network link extending between the first intermediate network device and a first host network device, the performing of the link training on the first network link including synchronizing respective one or more first transmit clocks at the one or more intermediate network devices with a transmit clock of the first host network device;

performing, by the first intermediate network device or by a second intermediate network device among the one or more intermediate network devices, link training on a second network link, the second network link extending between i) either the first intermediate network device or the second intermediate network device and ii) a second host network device, the performing of the link training on the second network link including synchronizing respective one or more second transmit clocks at the one or more intermediate network devices with a transmit clock of the second host network device;

transmitting, by the one or more intermediate network devices, respective link training completion indications to the first host network device and the second host network device, including delaying one or more of the respective link training completion indications until both i) the respective one or more first transmit clocks are synchronized with the transmit clock of the first host network device and i) the respective one or more second transmit clocks are synchronized with the transmit clock of the second host network device.

10 . An active cable assembly, comprising:

a first intermediate network device having a first interface configured to couple with a first host network device via a first network link that extends between the first intermediate network device and the first host network device, the first intermediate network device including a first link training controller configured to perform link training on the first network link with the first host device;

a second intermediate network device having a second interface configured to couple with a second host network device via a second network link that extends between the second intermediate network device and the second host network device, the second intermediate network device including a second link training controller configured to perform link training on the second network link; and

a third network link between the first intermediate network device and second the intermediate network device;

the first link training controller of the first intermediate network device being further configured to:

perform link training on the third network link prior to i) the first link training controller performing link training on the first network link and i) the second link training controller performing link training on the second network link, and

determine when link training of the first network link and the second network link has been completed,

transmit a first link training completion indication to the first host network device, and

delay transmission of the first link training completion indication until link training of both the first network link and the second network link has been completed; and

the second link training controller of the second intermediate network device being further configured to:

determine when link training of the first network link and the second network link has been completed,

transmit a second link training completion indication to the second host network device; and

delay transmission of the second link training completion indication until link training of both the first network link and the second network link has been completed.

11 . The active cable assembly of claim 10 , wherein the one or more link training controllers are configured to determine when link training of the first network link has been completed in response to receiving i) a first indication from the first host network device, the first indication indicating that link training has been completed with the first host network device and ii) a second indication from the second host network device, the second indication indicating that link training has been completed with the second host network device.

12 . The active cable assembly of claim 10 , wherein:

the first link training controller is configured to suppress link training on the first network link with the first host network device until link training on the third link is completed, and

the second link training controller is configured to suppress link training on the second network link with the second host network device until link training on the third link is completed.

13 . The active cable assembly of claim 12 , wherein the first link training controller and the second link training controller are further configured to, when link training on the third network link has been completed:

transmit respective first predetermined pseudorandom binary sequence (PRBS) signals between the first intermediate network device and the second intermediate network device to maintain a lock between the first intermediate network device and the second intermediate network device, and

initiate i) the link training on the first network link, and ii) the link training on the second network link.

14 . The active cable assembly of claim 13 , wherein:

the first intermediate network device is configured to, during the link training on the first network link, synchronize a first transmit clock at the first intermediate network device with a transmit clock of the first host network device,

the second intermediate network device is configured to, during the link training on the second network link, synchronize a second transmit clock at the second intermediate network device with a transmit clock of the second host network device.

15 . The active cable assembly of claim 14 , wherein:

the first intermediate network device is further configured to switch transmitting the first predetermined PRBS to the second intermediate network device, to transmitting a second predetermined PRBS to the second intermediate network device to signal to the second intermediate network device that the first transmit clock at the first intermediate network device has been synchronized with the transmit clock of the first host network device, and

the second intermediate network device is further configured to switch transmitting the first predetermined PRBS to the first intermediate network device to transmitting the second predetermined PRBS to the first intermediate network device to signal to the first intermediate network device that the first transmit clock at the second intermediate network device has been synchronized with the transmit clock of the second host network device.

16 . The active cable assembly of claim 15 , wherein:

the first link training controller of the first intermediate network device is further configured to transmit a first link training completion indication to the second intermediate network device when link training between the first intermediate network device and the first host network device has been completed, and

the second link training controller of the second intermediate network device is further configured to transmit a second link training completion to the first intermediate network device when link training between the second intermediate network device and the second host network device has been completed.

17 . The active cable assembly of claim 16 , wherein:

the first link training controller of the first intermediate network device is configured to switch transmitting the second predetermined PRBS to the second intermediate network device to transmitting a third predetermined PRBS to the second intermediate network device to signal to the second intermediate network device that link training between the first intermediate network device and the first host network device has been completed, and

the second link training controller of the second intermediate network device is configured to switch transmitting the second predetermined PRBS to the second intermediate network device to transmitting the third predetermined PRBS to the first intermediate network device to signal to the first intermediate network device that link training between the second intermediate network device and the second host network device has been completed.

18 . An active cable assembly, comprising:

one or more intermediate network devices, wherein a respective intermediate network device among the one or more intermediate network devices includes i) a first interface configured to couple with a first host network device directly or via one or more upstream intermediate network devices and i) a second interface configured to couple with a second host network device directly or via one or more downstream intermediate network devices; and

one or more link training controllers implemented by the one or more intermediate network devices, the one or more link training controllers configured to:

perform link training on a first network link, the first network link extending between a first intermediate network device and a first host network device,

during link training on the first network link, synchronize respective one or more first transmit clocks at the one or more intermediate network devices with a transmit clock of the first host network device,

perform link training on a second network link, the second network link extending between i) either the first intermediate network device or a second intermediate network device and ii) a second host network device,

during link training on the second network link, synchronize respective one or more second transmit clocks at the one or more intermediate network devices with a transmit clock of the second host network device,

determine when link training of the first network link and the second network link has been completed,

transmit respective link training completion indications to the first host network device and to the second host network device, and

delay one or more of the respective link training completion indications until both i) the respective one or more first transmit clocks are synchronized with the transmit clock of the first host network device and i) the respective one or more second transmit clocks are synchronized with the transmit clock of the second host network device.