14.3
Transparent clock switch synchronization
The operation of PTP relies on a measurement of the communication path delay
between the time source, referred to as a master, and the receiver, referred to
as a slave. This process involves a message transaction between the master
and slave where the precise moments of transmit and receive are measured
-
preferably at the hardware level. Messages containing current time information
are adjusted to account for their path delay, therefore providing a more accurate
representation of the time information conveyed. The path delay measurement
process of PTP involves the precision timing of two messages
-
a sync message
and a delay request. The average path delay of the two messages gives the
one
-
way delay. This, however, assumes that the communication path is
completely symmetric. This assumption does not hold in a switched network,
however, largely due to the buffering process within Ethernet switches. PTP
allows transparent clocks to measure and account for this delay in a time
-
interval field within timing packets, thus making the switches temporary
transparent to master and slave nodes. Transparent clocks must perform this
operation very accurately and at the communication speed without introducing
more delays. The End
-
to
-
End transparent clock forwards all messages just as
a normal switch does.
14.3.1
Message
-
based synchronization
PTP is based upon the transfer of network datagrams to determine system
properties and to convey time information. A delay measurement principle is
used to determine path delay, which is then accounted for in the adjustment of
local clocks. At start
-
up, a master/slave hierarchy is created using what is called
the Best Master Clock (BMC) algorithm to determine which clock has the best
source of time. The BMC algorithm is then run continuously to quickly adjust
for changes in network configuration. Synchronization is achieved using a
series of message transactions between master and slaves. There are five
message types
-
Sync, Delay Request, Follow Up, Delay Response and
Management
-
which are used for all aspects of the protocol. A sequence of
message transactions takes place to synchronize a pair of clocks as shown in
Figure 14.3.
GEN3i
324
I3763-3.1 en HBM: public