PTP software
(application layer)
oS
MAC
hardware assist
PhY
network Cable
network
Figure 3. one-step operation.
t1
t4
Master
Delay Response
Sync
Delay Request
Slave
t2
t3
Timestamps
known by slave
t2
t1, t2, t3
t1, t2, t3, t4
culate the offset between the master
and slave described earlier are used by
a two-step clock. A one-step clock uses
specialized network hardware not
only to timestamp when a PTP Sync
message leaves the device, but also to
modify the outgoing Sync message’s
t1 value with the actual departure time
(see Figure 3).
This value is normally sent in the
Follow-up message, but because the
hardware makes it available in the
Sync message, the Follow-up is re-
dundant and therefore not needed.
A slave device must also understand
that its master is operating as a one-
step clock. It can determine this by
reading a bit field in the PTP message
headers sent by the master. A one-step
clock helps minimize network traffic
while maintaining high synchroniza-
tion performance and is therefore a
requirement for PTP applications in
certain industries.
