GE Reason RT430 Technical Manual page 91

Delay measurement mechanism
ccording to IEEE1588 a slave clock is capable of measuring the delay of message propagation that
represents the time that a message takes to cross the master-slave path. The measurement of this delay is
necessary to perform a time correction of the time of receipt of the message in relation to the time it was
sent. The delay measurement is performed by sending messages containing the timestamp of the time of
receipt to the master clock which sends a reply with information of the delay.
The second review of IEEE1588 standard, in 2008, specifies two ways of compensating delay: End-to-end
and Peer-to-peer:

End-to-end: measurement of delay across the network between master and slave clocks.

Peer-to-peer: measurement of delay only between master and slave clocks as neighbors.
The advantage of P2P is that the accuracy of synchronism is immune to change in the network topology,
since the delay between each master-slave connection is calculated for each packet sent. However, the P2P
solution is possible only when all devices in the network are transparent, i.e., they can perform delay
measurement between one point and another. In network applications where the network comprises devices
without measurement of delay, it is necessary to use the E2E mode, which calculates the delay in a general
way between the two ends of the network.
RT430 is capable of measuring the time a synchronism message takes to cross a master-slave path, using
E2E and P2P mechanisms.
Master, Slave e Grandmaster clocks
In PTP protocol, master clocks send message packets with synchronism information, slave clocks receive
and process the synchronism messages, and grandmaster clocks that are the source of synchronism for the
entire network.
IEEE1588 standard specifies the Best Master Clock algorithm (BMC) which selects the best candidate to be
elected the master of the network, used as synchronism source. The selection is performed from the
attributes and attributed priorities to the possible candidate. The algorithm establishes a search order, and
from the results, it determines which one will be the clock used as time source.
RT430 is pre-configured to operate as master of the network and it can be configured to operate as slave.
PTP Messages
In PTP protocol, the messages containing synchronism information and timestamp are sent to the entire
network in multicast mode, in which a PTP message sent by a network port can be received by all other
ports connected to the same network. The advantage of multicast mode network is that the master clock
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