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DS100BR111: When configured as a 10G-KR repeater, does the DS100BR111 monitor the link and modify its own transmit and receive equalizer coefficients?

Lee Moreau
Prodigy 90 points
Part Number: DS100BR111
Other Parts Discussed in Thread: DS125BR111

The description of this device has this statement:

“When configured as a 10G-KR repeater, the DS100BR111 allows the KR host and the end point to optimize the full link by adjusting transmit and receive equalizer coefficients using back-channel communication techniques specified by the 802.3ap Ethernet standard.”

 

I want to confirm, when the link is configuring the transmit strength and receive equalization for the endpoints of the link, is the DS100BR111 functioning as a part of this process?

Please see the diagram below:

 

 

When the endpoints are configuring their transmit and receive equalizer coefficients, will the DS100BR111 also monitor the link and modify its equalizer coefficients as seen in the first possibility identified in the diagram above.

For example, say Endpoint B says that it requires a higher transmit from the link, will DS100 b increase its transmit strength from this request, or will that request propagate down to Endpoint A and Endpoint A will increase its transmit coefficients (conceptually seen as the second possibility identified in the diagram above - as if the repeaters were not present)?

  • 0
    TI__Mastermind 29655 points
    Hi Lee,

    The DS100BR111 is a repeater that only provides CTLE EQ and transmit VOD gain and de-emphasis. The setting is programmed manually and does not actively change while the host and end-point undergo auto-negotiation and link-training. From the perspective of the host and end-point, the DS100BR111 is invisible and is meant only as a way to improve signal integrity when the link is running at speed.

    I am unable to see the diagram you posted, but I think I understand the gist of your question. When the endpoints negotiate their transmit and receive equalizer coefficients, they are requesting updates from the corresponding endpoint, not the DS100BR111. To the endpoints, the DS100BR111 are transparent and should not affect the ability for the endpoints to optimize their coefficients.

    Thanks,

    Michael
  • 0 in reply to Michael Lu (Santa Clara)
    Prodigy 90 points

    Hi Michael,

    The purpose of the backchannel in 802.3ap is to establish a method for the receiver to provide feedback to the transmitter for adaptive TX settings (swing, de-emphasis etc...). Since the TX  settings of the DS100BR111 are manually configured (VOD = 1.1 to 1.3 for 10GBase-KR, and manual de-emphasis level) I do not understand how the back channel coefficients updates with a DS100BR111 will have any impact at the receiver. If link partner B wants link partner A to increase its amplitude or change de-emph, this is because of the signal that is being received from the DS100BR111, when this is communicated over the back channel A will increase amplitude and the DS100BR111 will receive an increased signal but link partner B still receives the original signal (amp and de-emph) from the DS100BR111 based on the manual settings. I could understand swing being compensated for if it was a gain setting instead of a VOD = 1.1 to 1.3, but see no way other TX parameters could be compensated for in this behavior.

    Thanks,

    Lee

  • 0 in reply to Lee Moreau
    TI__Mastermind 29655 points
    Hi Lee,

    The scenario you described is valid if the redriver's Tx driver is a limiting driver, where the Tx parameters (such as precursor and postcursor) are not passed through after the redriver. In this case, link partner B will not be able to tell any difference from when changes are made to link partner A's transmit output coefficients.

    To preserve the incoming precursor and postcursor through the redriver, we recommend operating the DS100BR111 in 10G-KR mode. By doing this, the DS100BR111 driver operates in a more linear mode. By doing this, the precursor and postcursor can be maintained through the DS100BR111 so that the link partner B can see the difference in the signal it receives from link partner A.

    Thanks,

    Michael
  • 0 in reply to Michael Lu (Santa Clara)
    Prodigy 90 points
    Hi Michael,

    I could see how this would work well in a balanced system with the repeater having very similar channels on each side, but if this were an unbalanced channel I would expect this to cause problems. We need to keep our backplane as simple as possible and have the repeater on line card itself. Depending on the specific slot used the channel from the PHY to the repeater (on the line card) will vary significantly from the repeater out through the backplane to the link partner (which may well be the same line card with the DS100BR111). In an extreme case would our line card’s PHY essentially saturate the receiver of the repeater due to the more lossy channel on the other side?

    Thanks,

    Lee
  • 0 in reply to Lee Moreau
    TI__Mastermind 29655 points

    Hi Lee,

    There is a possibility that if if there is a very short channel before the repeater, there may be an over-EQ scenario where the residual boost from the smallest CTLE setting. I'm guessing that this is the saturation you are referring to. However, given a long output channel from the repeater and the linear operation of the DS100BR111,  the over-EQ applied by the repeater's Rx will act indirectly to help the transmit EQ alongside the repeater's de-emphasis. However, as you mentioned, in an ideal scenario the DS100BR111 would have similar channel loss on either side.

    If linearity for link training is a key concern, alongside placing the repeater in an unbalanced system, I suggest taking a look at our "advanced linear" repeater, the DS125BR111. This device is pin-compatible with the DS100BR111 and has a higher degree of flexibility regarding where it can be placed due to its improved linearity compared to the DS100BR111.

    Michael