Figure 67: PTP protocol mechanism
At a cyclic period, the GMC sends sync messages throughout the network in a
multicast mechanism. Typical cyclic is 1 second, that is, each second the network will
receive a sync message from the GMC. The messages will then flow until the slave
clock receives it.
Each path will exchange information to calculate the path delay time. Thus, at each
transparent clock there will be a time which will be considered before sending the
message to the slave, minimizing the error from the GMC to the slave clock. At the
end, all path delay and forwarding process delay at the TC will be corrected, and
then the slave clock will receive the timestamp that the GMC clock has really sent.
The protocol allows choosing the mechanism of one-step or two-step mode delay
mechanisms. If the hardware of all of the equipment involved is one-step aware, that
is, the time stamping process happens directly at the hardware, the equipment will
operate without Follow-up messages. Besides, if the time stamping process is done
at the software level, the equipment will then operate as a two-step mode. Thus, in
two-step mode, the TC will store the time stamp of the received Sync frame and will
forward it without correction. After the Sync message, the TC will do a sum with the
correction of the Sync message ingress, its own forwarding delay and the path delay
previously calculated. After that sum, the TC will then send a Follow-up message with
the correction that should be made by other clocks in the network.
GE Reason Switches do the time stamping process at software level. Thus, only two-step delay mechanism mode is allowed to
be used.
If there is more than one master in the network, the master-capable equipment will
do an election to determine which clock has the best accuracy at the network, based
on an Announce message. Thus, PTP network with more than one master will have a
failover mechanism directly at the protocol, to allow reliability in the network
synchronization.
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