AES67 Practical Guide
Page 9 of 28
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DELAY REQUEST intervals: no need to deviate from the default values (
2^0
) either (unless
you know what you are doing). Keep the delay measurement mode configured to end-to-end
(E2E) delay measurement.
2.1.5.2
BMCA parameters
For best synchronization results, you want to make sure that the best available master clock on the
network is actually taking this role. If you have a dedicated Grandmaster device, all settings are
usually in place by default to allow this device to become Grandmaster.
However, if this is not the case or if no dedicated Grandmaster device is present, you may have to dig
a bit further into the BMCA parameter configuration in order to resolve any problems or make sure
that only those devices qualify for BMCA competition which exhibit a decent PTP Grandmaster
functionality by design (usually a device with a very precise and stable internal clock circuitry or which
can be connected to an external reference signal, i.e. a word clock or a black-burst input).
The BMCA is an exactly specified algorithm that each device has to follow to come to the same
conclusion on the best available master clock on the network; any failure to fully and correctly
implement the BMCA (even in end nodes which can never become Grandmaster) may result in
improper synchronization results (yes, we have seen this). The BMCA relies on the ANNOUNCE
messages being distributed in the network. The ANNOUNCE messages contain certain parameters
about the clock quality which are compared in certain precedence:
1.
Priority 1 Field
: This is a user-settable value. The lowest number wins. Normally this is set at
128 for master-capable devices and 255 for slave-only devices. However, if you want to
overrule the normal selection criteria you can change Priority 1 and create any pecking order
you wish.
2.
Clock Class
: This is an enumerated list of clock states. For example, a clock with a GPS
receiver locked to Universal Coordinated Time (UTC) has a higher rating than one which is
free running and set by hand to your wrist watch. There are also states for various levels of
holdover when a clock which has a GPS receiver loses the connection.
3.
Clock Accuracy
: This is an enumerated list of ranges of accuracy to UTC, for example 25-100
ns.
4.
Clock Variance
: This is a complicated log-scaled statistic which represents the jitter and
wander of the clocks oscillator over a SYNC message interval
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.
5.
Priority 2 Field
: You guessed it, another user-settable field. The main purpose at this low end
of the decision tree is to allow system integrators to identify primary and backup clocks
among identical redundant Grandmasters.
6.
Source Port ID: This is a number which is required to be unique, and is usually set to the
Ethernet MAC address. Essentially this is a coin toss to break a tie.
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Quote from D. Arnold, Meinberg: “
In fact it is so complicated that if you can accurately determine it for a clock
then you get three credits toward a degree in mathematics.
” – also see
http://blog.meinbergglobal.com/2013/11/14/makes-master-best/
for more in-depth information on
BMCA