Spirent SmartBits System, Reference Manual

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Chapter 9: Ethernet Cards
Gigabit Ethernet Testing, Clock Tolerance
SmartBits System Reference 133
switching is per port pair. Therefore, a 24-port switch would have 12 port pairs and would
have to store 228,564 bytes per second (12 x 19,047), assuming it is able to process at wire
speed. This is a lot of data to store for long bursts of traffic.
Sample Test Scenarios and Interpretations
If you have a simple setup of a two-port switch and two test analyzer ports, what happens?
The following example runs a SmartApplications (Back-to-Back Test) and relates the
clock tolerance to the test results.
A frequency counter tested the clock tolerances of the ports on the SmartBits ports and the
DUT, and showed the following values:
•
SmartBits port 1: minus 14ppm
•
SmartBits port 2: plus 10.16ppm
•
DUT port 1: minus 4.024ppm
•
DUT port 2: plus 2.64ppm
SmartApplications Test Test Results and Related Clock
Tolerance
In test 1, transmitted unidirectional traffic from
port 1 of the test analyzer into port 1 of the DUT
(device under test) and received test packets
back on port 2 of the test analyzer from port 2 of
the DUT. The DUT was able to keep up for 500
seconds without any data loss when:
input = minus 14ppm
output = plus 2.64ppm
(output faster by 16.64ppm)
In test 2, reversed the direction of the
unidirectional traffic, leaving the ports hooked
up the same way. The test failed in 5.9 seconds when:
input = plus 10.16ppm
output = minus 4.024ppm
(output slower by 14.184ppm)
In test 3, transmitted unidirectional traffic from
port 1 of the test analyzer into port 2 of the DUT
and received data back on port 2 of the test
analyzer from port 1 of the DUT. The test ran for 500 seconds without loss
when:
input = minus 14ppm
output = minus 4.024ppm
(output faster by 9.976ppm)
In test 4, reversed the direction of unidirectional
traffic without changing the cable connections. The test failed in 10.5 seconds when:
input = plus 10.16ppm
output = plus 2.64ppm
(output slower by 7.52ppm)