1588 uses the below five messages while communicating between the Packet Master and the Packet Slave.
a. SYNC Message
b. FOLLOW UP Message
c. DELAY REQUEST Message
d. DELAY RESPONSE Message
e. ANNOUNCE Message
Among these SYNC and DELAY REQUEST Messages are called Event Messages as they carry a valid time stamp. Others belong to the category of General Messages.
Let us briefly see what these messages are.
SYNC Message is transmitted by the Packet Master at a well-defined (configurable) rate to acceptable Packet Slaves. It contains the origin time stamp (T1) of the message in nanoseconds measured from PTP epoch time (1st January 1970). The arrival time of the SYNC Message as recorded by the Packet Slave in nanosecond is T2.
FOLLOW UP Message is transmitted by the Packet Master followed by SYNC Message (In case of Two Step Clock), which contains the precise or true origin of time stamp (T1), while the message leaves the Master egress PHY. This message is not used in case of Single Step Clock.
DELAY REQUEST Message is transmitted by the packet slave towards the acceptable Master. It contains the origin time stamp (T3) of the message in nanoseconds. This message helps in the computation of one way delay between Packet Master and Packet Slave.
DELAY RESPONSE Message is transmitted by the Packet Master in response to DELAY REQUEST message from Packet Slave. It contains the arrival time of DELAY REQUEST message (T4) at the Packet Master.
ANNOUNCE Message is transmitted by the Packet Master carrying the data sets of the Master. The Packet Slave uses these messages for Best Master Clock (BMC) Algorithms to elect the best available Master.
These messages are represented in the following diagram.
It is evident from the above diagram that Packet Slave upon receiving the DELAY RESPONSE Message, has all the four Time Stamps (T1, T2, T3 and T4) for its computation.
In addition to the above messages, there are signaling requests and response messages between Packet Master and Slave as mentioned below. We will discuss these in detail in a separate thread.
a. Unicast Request Announce Message
b. Unicast Request Sync Message
c. Unicast Request Delay Response Message
d. Unicast Grant Announce Message
e. Unicast Grant Sync Message
f. Unicast Grant Delay Response Message
Also the following messages are used by the Transparent Clocks to measure delays in the PTP network so that they can be compensated for by the system. We will discuss these in a different thread.
a. Path Delay Request
b. Path Delay Response
c. Path Delay Response Follow Up