GE Reason RT430 Technical Manual page 92

GPS Grandmaster Clock
sends only one packet containing synchronism information to the network and this packet is received by all
slave devices connected to this network.
Among the messages specified by IEEE1588 standard, the ones that stand out are related to synchronism
information, timestamp and propagation delay.
The Announce messages are used to inform the devices connected to the network about the existence of a
master clock, available to send synchronism packets. The message includes a packet of values that
indicates the time accuracy of the clock. From the information obtained by the Announce message, it is
possible to execute the BMC algorithm to decide which of the available clocks will be used as master. The
speed the Announce messages are sent influences directly the frequency the slave will perform the BMC
algorithm. Many announce messages can be transmitted at the same time through the network and the slave
clock is responsible to process these messages. All devices connected to the network that are able to be
synchronism source, i.e., operate as master, should periodically send Announce messages to the network,
becoming candidates to be master of the network.
The clock connected to the network selected as master by the BMC algorithm, should send Sync messages
with synchronism information, and in case it is a two-step clock, it should also send a Follow Up message,
containing a timestamp. The sending interval of the messages is configurable and its standard value,
specified by IEEE1588 standard, is one-message-per-second. This interval specifies the frequency the slave
devices receive synchronism information, allowing to adjust its internal clocks to use the master clock as time
reference. In the interval between two synchronism messages the slave devices operate free from
synchronism sources and the time stability in this period is determined by its internal time base, that can be,
for example a crystal oscillator. By choosing a frequency to send synchronism messages through the master
clock, it is important to consider the accuracy of the internal clocks of the slave devices that will be
synchronized by it, and also the bandwidth, because the higher the frequency to send messages is, the
higher the network traffic is.
The delay measurement of messages passing through devices is important to reach the accuracy required
by IEEE1588 standard. Especially in E2E networks, the propagation delay measurement is crucial for the
synchronization. In networks with E2E delay measurement, the frequency the slave devices should measure
delay, which results in sending and receiving messages, should be according to the network stability
regarding the variation of this information.
92 RT430-TM-EN-5