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RT430/434
PTP Synchronization
90
The second revision of the standard also specifies the form of unicast
communication where the clock master has to send time synchronization packets for
each slave device connected to the network, which requires the master clock to have
greater processing power and causes the network traffic to be more overloaded.
Through use of the BMC algorithm, PTP elects a master source of time for an
IEEE1588 domain and for each network segment in the domain. Clocks determine the
offset between themselves and their master. Let the variable represent physical time.
For a given slave device, the offset ��(��) at a time �� is defined by:
where ��(��) represents the time measured at the clock at physical time t, and ��(��)
represents the time measured at the master at physical time t.
The master clock periodically broadcasts the current time as a message to the other
clocks. Under IEEE1588-2088, broadcasts are up to 10 messages per second.
Each broadcast begins at time T1 which is a Sync multicast message sent by the
master to all the clocks in the domain. A clock receiving this message takes note of
the local time T1' when this message is received. The master may subsequently send
a multicast Follow Up with accurate timestamp. Not all masters have ability to
present an accurate timestamp in the Sync message. It is only after the transmission
is complete that they are able to retrieve an accurate timestamp for the Sync
transmission from their network hardware. Masters with this limitations use the
Follow Up message to convey T1. Masters with PTP capabilities built into their
network hardware are able to present an accurate timestamp in the Sync message
and do not need to send Follow Up messages.
In order to accurately synchronize to their master, clocks must individually determine
the network transit time of the Sync messages. The transit time is determined
indirectly by measuring round-trip time from each clock to its master. The clocks
initiate an exchange with their master designed to measure the transit time d. The
exchange begins with a clock sending a Delay Req message at time T2 to the master.
The master receives and time stamps the Delay Req at time T2' and responds with a
Delay Resp message. The master includes the time stamp T2' in the Delay Resp
message. Through these exchanges, a clock learns T1, T1', T2 and T2'.
If d is the transit time for the Sync message, and �� ̃ is the constant offset between
master and slave clocks, then:
Combining the above two equations, we find that:
The clock now knows the offset õ during this transaction and can correct itself by this
amount to bring it into agreement with their other master.
��(��) = ��(��) − ��(��)
��1 − ��1' = �� ̃ + ��
��2 − ��2' = −�� ̃ + ��
(��1' − ��1 − ��2' + ��2)
�� ̃ =
2
Chapter 10 - Appendixes
RT430/434
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