Chapter 3
High Accuracy Timing
Common test applications
T-BERD/MTS/SC Timing Expansion Module User Manual
July 2017
22112315, Rev. 004
Page 27
PTP Delay Symmetry
To accurately measure PTP delay symmetry, you must either use two precisely
synchronized Viavi Ethernet test instruments that are optioned and configured for PTP
testing, or one test instrument synchronized to a PTP Master (for example, a Grand-
master or Boundary Clock). When using two synchronized test instruments, one instru-
ment would be configured as a PTP slave, and the other would be configured as the
PTP Master.
PTP Time Errors
Finally, using a single 5800v2 with a connected TEM Module configured as a PTP
slave, you can measure PTP time errors (the difference in the timestamps provided by
the instrument and those provided by a third party PTP Grandmaster clock).
Instructions for measuring PTP time errors, asymmetry, and delay are provided in the
Ethernet, IP, TCP/UDP, Fibre Channel, VoIP, and Video Testing Manual
that shipped
with your instrument or upgrade.
One way delay measurements
One way delay measurements are measurements of delay
in a single direction
(from a
source node to a destination node on an Ethernet link). They differ from round trip delay
measurements because they help you determine whether a problem exists in the
upstream or the downstream direction, and they do not include the cumulative network
delays associated with inbound and outbound traffic.
To accurately measure one way delay, you must use two precisely synchronized Viavi
Ethernet test instruments that are optioned and configured for one way delay testing.
TEM Modules can be used to synchronize near and far end 5800v2 test instruments to
a GNSS synchronization source before measuring one way delay. The synchronized
instruments will tag outgoing Acterna Test Packets (ATP) with timestamps. Both instru-
ments will recognize received ATPs, and use the timestamps carried within the packets
to measure delay.
Instructions for measuring one way delay are provided in the
Ethernet, IP, TCP/UDP,
Fibre Channel, VoIP, and Video Testing Manual
that shipped with your instrument or
upgrade.
Wander and jitter analysis
Slow, periodic and non-periodic phase changes in the 0 Hz to 10 Hz frequency range
are known as
wander
. Wander can also be described as the phase difference between
a very precise reference clock and the signal under test. The phase difference is
sampled over time and is expressed in nanoseconds.
Jitter is defined as any phase modulation with a frequency above 10 Hz in a digital
signal. This unwanted phase modulation is always present in devices, systems and
networks.
