IEEE 1588 Analysis
243
9.3.2. Wander (MTIE / TDEV) Test
Wander in a digital signal is defined as a slow phase fluctuation that degrades the
quality of the clock embedded in the signal. The upper limit to the wander frequency
band is usually defined to be 10 Hz but wander could have periods of hours or even
days. Wander is propagated through the network and because it is not absorbed or
filtered, it is accumulated impairing the timing signal. This concept of phase fluctuation
could be translated without relevant modifications to synchronization protocols for
packet switched networks such as PTP / IEEE 1588v2.
Measuring wander to assess the long term stability of a timing signal involves the use
of an accurate clock reference due to the inability to filter out wander components from
the signal under test. The phase of the reference clock is then compared with the phase
corresponding to the signal under test. Performance metrics are then derived from the
Low frequency Low frequency Time Error. This result expresses TE compo-
nents that are immune to filtering. Such TE components are
the result of, for example, asymmetry in the transmission
medium between network elements or asymmetries within
network elements. The low frequency TE spans the fre-
quency band between 0 and 0.1 Hz. Constant (0 Hz) TE
could theoretically be compensated through a static setting
in the slave equipment, but slowly varying TE with periods of
hours or days is both difficult to compensate or filter and it
therefore should be avoided as much as possible.
Current, Minimum and Maximum figures are computed for
the low frequency TE. The minimum and maximum values
are referred to the whole test duration.
High frequency High frequency Time error. High frequency component of the
TE. It is related to random noise accumulation due to packet-
delay variation experienced by the timing signal packets or
due to any other phase noise source. The power spectrum of
the high frequency TE is spread out over the frequencies
larger than 0.1 Hz and the power can be reduced, to some
extent, through low-pass filtering.
Current, Minimum and Maximum figures are computed for
the dTE. The minimum and maximum values are referred to
the whole test duration.
Table 9.9: TE Statistics
Result Description
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