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Spirent TestCenter: Dynamic results view shows Frame Loss Duration negative for multicast traffic


When testing a multicast (1:N) where the listeners are placed into a different port each one, and there is some frame lost, Frame Loss Duration (us) under a dynamic result view shows a negative value.

  • Multicast
  • 1:N
  • Frame Loss Duration (us)
  • Negative frame loss duration
  • Dropped Frame Duration (us)
There are two options available to get this to work on multicast test:

enlightened NOTE: It is important to mention you need to stop the traffic to see real/accurate Dropped/Lost Frame Duration
1.- Use Dropped Frame Duration (us) instead:

While Frame Loss is calculated using (TX-RX) it is always better to use (if the module supports it) Dropped Frame counter instead (Which uses advance sequencing)

So, Dropped Frame duration is calculated using Dropped Frames from all Ports counters individually, which gives a correct value even when testing multicast:


2.- If the module you are using does not support Dropped Frames counter, you must keep using Frame Loss and Frame Loss Duration, but for multicast tests you can get an accurate frame loss duration for every multicast listener/receiver individually using the next option:
  • Check “Disable auto-grouping” feature under Customize Result View – Grouping/Sorting tab.
So you can see stats for each port (Multicast receiver/listener) and hence accurate frame loss duration:

Root Cause

Spirent Test Center calculates frames lost duration as follows:


TX Count    –    Rx Count   =   # of Frames Lost 

# of Frames Lost  X   148 Bytes*   X   8 bits   =    # of bits lost
*148 Bytes since Frames size is 128 and there are 20 Bytes of preamble/SFD

# of bits lost    X    .01 us*    =    Frame Lost Duration in us (microseconds**) 
*since port speed was 100 Mbps the bit time is .01 us 
**multiply by 1000 to convert to ms (milliseconds)

Since this particular multicast test has multiple listeners/receivers along with multiple ports, there are more RX frames than TX on multicast streamblock, so, the first calculation gives a negative value (TX – RX), hence the total calculation of Frame Loss duration gives a negative value
  • As per our testing Frame Loss duration on the dynamic results view (Which uses Tx count and Rx count to make the maths) is NOT using port counters directly but uses TX and RX DRV counters that are collected directly on the dynamic view
  • ​So, since these counters are grouped (showing all multicast traffic – either TX and RX frames) that is why we are getting negative values.

Product : Spirent TestCenter,Multicast