Knowledge base
How to: Link Speed Calculation
Posted by Mathieu Strubbe, Last modified by Dries Decock on 03 April 2018 02:08 PM

On the Link Layer, each Frame is surrounded by a number of extra bytes. The table below shows the complete Ethernet Frame.

 
Preamble Start of Frame Delimiter (SFD) Frame (as displayed in the Frame View) Frame Check Sequence (32 bit CRC) Interframe Gap (Pause)
7 Bytes 1 Byte 60 - 10000 Bytes 4 Bytes 12 Bytes

For every Frame, an extra 24 Bytes need to be taken into account.  This knowledge is crucial, for example when determining the maximum throughput on an ethernet link.

For example, when sending Frames with a length of 60 Bytes, the actual amount of bytes sent is 84. This means that only 60/84 = 71.42% of the transmitted data consists of the bytes displayed in the Frame View. So, to achieve Line Rate on a 1 Gbps link, the Frame Rate is calculated as follows :

  • Frame Rate = 1 000 000 000 bps / ( ( 1B + 7B + 60B + 4B + 12B ) * 8b/B ) = 1 488 095.24 fps
  • Ethernet Bitrate (without overhead) = 1 488 095.24 fps * 60B * 8b/B = 714 285 714,29 bps = 714 Mbps

 

Tip : When sending bigger Frames, the relative overhead becomes smaller.

In the Preferences of the ByteBlower GUI under Project>BitRate, you can specify what you want to be included in the Layer 2 Speed calculation. A screencapture of this view can be found at the bottom of this article.

  • Frame (as displayed in the Frame View)
  • Frame and FCS (includes the CRC, so each Frame gets 4 Bytes extra)
  • Frame, FCS, Preamble, SFD and Pause (each Frame gets 24 Bytes extra)

This affects the calculated rates displayed in the Frame Blasting Flow Templates, and in the Reports.

Configuring the reported bitrate

 

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