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PTP Operational Overview
Multicast, Unicast, and Mixed Communication Modes
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PRISM MPI-MPX User Manual
Multicast, Unicast, and Mixed Communication
Modes
There are three basic message modes for PTP: Multicast, Unicast, and mixed Multicast
and Unicast. For full Multicast or Unicast modes, all of the PTP messages are sent in the
selected mode type.
For some profiles, such as some telecom profiles and the SMPTE ST2059 profile, a
mixture of Multicast and Unicast are allowed. On the SMPTE profile unique mixed
mode, the Announce and Sync messages are sent as multicast. However, the Delay
Request and Delay Response messages are sent as Unicast.
Some points to understand about communication modes:
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Whatever mode is chosen, the master and slave must match or be compatible.
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Multicast and Mixed mode may need IGMP joins and leaves.
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Full Unicast must have the master address in all slave AMTs.
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Unicast without negotiation does not allow master to regulate load.
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Two masters can be used on different domains to serve slaves on different
communication modes.
All of the master and slave devices on a given domain must use compatible
communication modes. For most profiles, this means the master and slave
communication modes need to match exactly. On the SMPTE profile, the Multicast and
Mixed modes are compatible, so a master in any of these modes should work with a
slave in any of those modes.
Multicast System Messages
Multicast messages are broadcast and so they can be received without knowing the
address of the sender. This allows a slave to hook up to the network, receive the
announce messages and discover the identity of the master. Multicast in some
networks requires the devices to use IGMP to join and leave the multicast group.
In Multicast PTP systems, the master sets the rate of the announce and sync messages.
The master also sends the maximum allowed delay request rate in the delay response
message. Ideally, the slave uses that value to set the delay request rate although some
slaves set this rate independently. The delay request rate is typically the same as the
sync rate but may be higher or lower.
Unicast System Messages
Unicast messages require the address of the master be entered into the Acceptable
Master Table (AMT) in each slave. If there are multiple masters, the IP address for each
master must be entered in the AMTs of all the slaves.
In Unicast PTP systems, the slave must have the address of the master in its acceptable
master table because the slave initiates the communication with the master. The slave