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Spirent KB Article
Doc ID: FAQ17354
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Spirent TestCenter: What does it mean when we set the ppm to any range between -100 to +100?
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
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