PRISM MPI2-25 and MPX2-25 Media Analysis Platform User Manual
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PTP operational overview
This section provides an overview of PTP network operation. Not all of the
functionality described is currently available in the PRISM monitor.
PTP introduction
PTP master selection
In a PTP network, all the masters on the network are evaluated by the Best Master
Clock Algorithm (BMCA). The BMCA runs on all devices, and chooses a master
based on several parameters. Some of the parameters relate to clock quality;
conversely the Priority 1 and Priority 2 parameters are set by the user to influence
the choice of a master:
NOTE.
The PRISM monitor has no role in setting the priority parameters or in
providing information to the BMCA.
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Priority 1. This parameter defines which clocks are allowed to be
considered as masters. To be considered as a master, the value should be
set to 128 or less. Priority 1 is the first criteria in the BMCA, so if Priority
1 is set to a lower value than other devices on the system, then it will be
chosen even if the clock quality is poor. Typically, all the masters in a
domain should have the same value for the Priority 1 parameter.
•
Priority 2. This parameter is used to break the tie between masters that have
the same clock quality. Several values may be used to define a hierarchy of
devices. Most profiles use 128 as a default, so a value of 127 or lower would
indicate a preferred master.
•
The final tie-breaker in the BMCA is the clock ID. This is usually the MAC
address so it provides a unique value by which the BMCA can choose.
Network topology
In a typical IEEE1588 PTP network, many devices may be either masters or
slaves. If the current master stops working, all the devices that are capable of
being a master broadcast their clock quality and a new master is selected.
For video networks it may make sense to depart from the telecom-style PTP
topology and dedicate devices to be only masters or slaves. This is the approach
supported by the PRISM monitor, which allows for a more traditional primary
and backup master structure controlling a range of slave devices. The BMCA is
still used to select the active master and the network still takes the place of the
ECO in the legacy Black-burst/Tri-level network. The PRISM monitor is a
slave-only device.
