Printer FriendlyEmail Article Link

Spirent TestCenter: How do I calculate Layer 1 and Layer 2 bits per second and verify with the reported results?

Environment/Versions
  • Spirent TestCenter
    • PGA
Answer
  •  Layer 1 and Layer 2 bits per second calculation 
    • When Spirent TestCenter presents the data and results, the received L1 bits include everything that is put on the line, including the inter-frame gap minimum of 12 bytes and preamble of 8 bytes, for a total of 20 bytes per frame.
    • The Rx bit count, on the other hand, is basically the L2 count and includes the entire frame starting from the Destination MAC address through the FCS .
    • Header fields are not counted as overhead but as part of the received L2 frame.
    • The same happens with the rates. Notice that TestCenter identifies the L2 data simply as Rx or Tx, while L1 data is specifically identified as such.  
  • Example:
    • Frame Size = 128
    • Bandwidth of the module = 100M
    • Frame Part               Minimum Size Frame
    •  Inter Frame Gap min(9.6µs) 12 Bytes
    •  MAC Preamble (+ SFD)     8 Bytes
    •  MAC Destination Address 6 Bytes
    •  MAC Source Address       6 Bytes
    •  MAC Type (or Length)     2 Bytes
    •  Payload (Network PDU) 110 Bytes
    •  Check Sequence (CRC)     4 Bytes
  • Therefore:
    • Total Frame L2 Size = 128 (Payload + MAC + FCS)
    •  Total Frame Physical (L1) Size = 148 Bytes (L2 frame + IFG min + Preamble)
  • Also:
    • Total Tx rate fps = Ethernet Data Rate (bits per second) / Total Frame Physical Size (bits)
    • Since bandwidth of the module is 100 Meg then:
      • Tx L1 rate fps = 1 000 000 00 / (148 x 8 )
      •                                    = 84460 Frames per Seconds
      • L2 bps = Frame per Seconds * (Frame size * 8)
      • Total Tx L2 Rate bps = 84460 * (128*8)
      •                                     = 84460 * 1024
      •                                     = 86,487,040 bps
  • Example 2
  • From the above  screen capture:
    • Rx L1 bits: 853,731,096
    • Rx bits (L2): 656,341,976
    • L1 – L2 bits: 853,731,096 - 656,341,976 = 197,389,120
    • Rx Frames (L2): 1,233,682
    • (L1-L2)/Rx Frames: 197,389,120/1,233,682 = 160 bits
    • 160 bits/8 bits/octet = 20
  • Hence, the difference between L1 and L2 received bits is exactly 20, equivalent to the inter-frame gap min and preamble!
  • Also Please see the attached PDF file

Product : Spirent TestCenter