Transmission Time Test Results

NT100A01 4 × 10/1 Gbit/s Reducing Transmit Path Delay Variations
Platform
Napatech SmartNIC
Content Type
Application Note
Capture Software Version
Link™ Capture Software 12.7

Test setup

As shown in the figure below, a loopback cable is used on port 3 of an NT100A01 SmartNIC. The test was done using an NT100A01 SmartNIC running on the test and measurement image, 9560-44-10. The pktgen and latency tools are used for the test.

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The pktgen tool

Packets are transmitted using the pktgen tool with time stamp injection applied (see DN-0449). See a pktgen command and output example below.
/opt/napatech3/bin/pktgen -p 3 -i -r 90%
pktgen (v. 3.25.0.0)
==============================================================================
Options for use with the latency tool
-------------------------------------
Link speed:                   1000
Timestamp offset inject mode: SOF
Timestamp method:             EOF

Options for latency: -i SOF -t EOF -o 2 -l 1000

The latency tool

The latency tool calculates the time it takes for packets to be transmitted from a port and received at another port (possibly the same port) (see DN-0449). The measured time includes transmit path delay, cable delay and receive path delay. See a latency command and output example below. 
/opt/napatech3/bin/latency -p 3 -r 5 -u 5 -i SOF -t EOF -o 2 -l 1000
latency (v. 3.25.0.0)
==============================================================================
Assigning traffic to stream 0
>>> : Assign[streamid=0]=Port==3
>>> : NT_NTPL returned id 2
--------------------+--------------------------------------------------+----------------+
                    |                  5 sec sampling                  |      Last      |
                    |       Max      |      Min       |     Average    |    Roundtrip   |
--------------------+----------------+----------------+----------------+----------------+
Round trip in nSec  |            444 |            438 |            441 |            444 | 
--------------------+----------------+----------------+----------------+----------------+
Resetting best, worst and average sessions
--------------------+--------------------------------------------------+----------------+
                    |                  5 sec sampling                  |      Last      |
                    |       Max      |      Min       |     Average    |    Roundtrip   |
--------------------+----------------+----------------+----------------+----------------+
Round trip in nSec  |            444 |            438 |            441 |            444 |

Test results

The table below shows transmission time test results with an NT100A01 running at 4 × 1 Gbit/s.
1G Transmission time (ns) Transmission time jitter during test (ns)
Max Min Average Max Min Average
90% of line rate 447 438 441 5 0 1
99% of line rate 447 438 442 6 0 1
100% of line rate with TX rate limit 447 439 442 5 0 0
100% of line rate without TX rate limit 3519 2671 3077 676 331 537
The table below shows transmission time test results with an NT100A01 running at 4 × 10 Gbit/s.
10G Transmission time (ns) Transmission time jitter during test (ns)
Max Min Average Max Min Average
90% of line rate 6 1 4 5 0 0
99% of line rate 16 1 13 14 0 1
100% of line rate with TX rate limit 16 1 12 14 0 3
100% of line rate without TX rate limit 246 1 186 244 0 45
As shown in the above test results, the transmission time is not stable and jitter is increased when packets are transmitted at 100% of line rate without the transmit rate limit configuration. The test results also show that jitter at 100% of line rate is very low if the transmit rate limit is configured to the full line rate.