GE Reason RT430 Technical Manual page 89

RT430
The first revision of the IEEE1588 standard specifies only multicast network where a PTP message sent by a
network port can be received by all other ports connected to the same network. The great advantage of the
multicast network is that the master clock sends only one packet of time synchronism to the network, and it is
received by all slave devices connected to that network.
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.
PTP Synchronization
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 $o(t)$ at a time $t$ is defined by:
where s(t) represents the time measured at the clock at physical time t, and m(t) 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 a Sync multcast message sent by the master to all the clocks in the
domain. A clock receiving this message takes not 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 Synch 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 Synch 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 Synch message, and is the constant offset between master and slave clocks,
then:
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