Telestream PRISM MPI, User Manual

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PTP Operational Overview
PTP Introduction
PRISM MPI-MPX User Manual
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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 is 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.
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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.
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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.