EXFO FTB-800 Series for FTB-1 - Manual (Page 552)

Glossary
542 FTB-700G/800 Series
1588 PTP
The following diagram shows the sequence of message exchange
between a Client Clock and a Master Clock. First, the Client request Unicast
Announce message service. When this is granted and the first Announce
message is received, the Client can check the QL value conveyed in the
clockClass field of the message. From there the Client Clock can request
service for Sync and Delay Request messages. Service requests are
reissued at the end of the Announce duration interval to ensure continuity
of service.