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Spirent TestCenter: What does it mean when we set the ppm to any range between -100 to +100?

Environment/Versions
Spirent TestCenter
Answer
The IEEE 802.3 specification defines Ethernet as an asynchronous technology, with each
interface having its own clock source. This differs from synchronous WAN technologies, where
all interfaces use a common clock source.

To accommodate timing variations among various transceivers, sections 22 and 36 of the IEEE
802.3 specification state that a transmitter must run at a given frequency, ±100 parts per million
(which at 10G Ethernet rates equates to ±1,488 fps with minimal-sized 64-byte frames). Over
time, differences between transmit and receive rates can lead to buffering and increased delay.
In production networks, it’s a virtual certainty that different Ethernet interfaces will run at
slightly different rates. In lab settings, it may be desirable to have all test interfaces run at
exactly the same rate; for example, a fabric designer may wish to set all transmitting interfaces
ABOUT TIME: LATENCY AND RELATED TIME-BASED METRICS
to the maximum allowable rate – exactly +100 ppm – to verify a fabric will forward all traffic
without frame loss.

A test instrument should offer both capabilities – slightly different clock rates on each interface,
or uniform clock rates on all interfaces. Spirent TestCenter allows PPM adjustments on
individual interfaces (see Figure 6). If “Internal” clocking or a 0-ppm setting is used, the test port
will use Ethernet’s nominal line rate.
There is one unscrupulous use of PPM settings: Some testers have been known to reduce the
PPM setting so that a slightly slow DUT will forward all traffic at the newly redefined line rate
without loss, and also show reduced latency. This is a form of cheating, and should be avoided.
Essentially, it redefines line rate downward simply to accommodate a slow DUT.

Network equipment manufacturers (NEMs) cannot know in advance what exact rates their
customers’ Ethernet devices will use, and thus should always assume a PPM default of 0 and/or
randomly selected PPM values between -100 and +100. 

Product : Spirent TestCenter