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GEN7iB

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GEN series SYNCHRONIZATION METHODS

14.2.5 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eldwithintimingpackets,thusmakingtheswitchestemporarytranspar-

ent 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.

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conguration.Syn-

chronization is achieved using a series of message transactions between master and

slaves.Therearevemessagetypes-Sync,DelayRequest,FollowUp,DelayResponse

and Management - which are used for all aspects of the protocol. A sequence of mes-

sage transactions takes place to synchronize a pair of clocks as shown in Fig. 14.9.

HBK GEN7iB GEN Series User Manual

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