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Spirent TestCenter: What is PFC Cable delay feature and how does it work?

  • STC 4.89 release
  • PFC
  • Cable Delay
We will use this KB article to document about PFC Cable Delay feature under Advance tab, a new feature added around 2019 in 4.89 release

  • What is this used for?

PFC Cable Delay emulate delay in PFC operation caused by long cables. Cables up to 1 kilometer. User would be able to enable/disable this delay. As well as specify how much of a delay to emulate, so Spirent's response will be delayed by the estimated amount of time shown. This delay is configured per-port.

  • How does this feature work?
FPGA will handle the vast majority of this feature. It will handle buffering packets to emulate the requested delay. FPGA will provide a single 32-bit register. FPGA will implicitly validate the configured delay. Any values exceeding maximum supported value will be ignored. A value of zero in the configuration register will disable the feature.
  • Can it fit for a customer that wants Spirent ports add a delay before handling PFC frame? Let it say a customer wants to add some nano sec delay before pause traffic, to simulate an specific Cable length? 

Yes, this feature will makes sense to the customer.


  • Which module supports this and which version? 
  • DX3-400G and PX3-400G modules Starting in 4.89 release (just at 400G and 200G speed (Not available at 100G or 50G)
    • This is not currently docummented on 4.89 and around release notes, we probably missed out on the Release Notes back then, but starting in 4.89 this feature is available ony on 400G and 200G, not 100G/50G
  • FX3/MX3-QSFP28-X - Starting in 5.11 release (As per 5.11 Release notes)
  • PX3-QSFP28-12 - Starting in 5.13 release (As per 5.13 Release notes)
  • PX3-100GQ-T12 - Tested in 5.17 release
    • There is no current indication on any relese that this card support PFC delay feautre, or at least not documented, however it looks like is working on 5.17 (Not tested on other releases yet)
  • What is the cable range supported? 
Cable lenght supported is up to 1km


Refer to the document attached for detailed information about Description / Design and Restrictions and Limitations.

Product : Spirent TestCenter