Tektronix SPG8000A, Руководство пользователя

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158 SPG8000A User Manual
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For video networks it may make sense to depart from the telecom - style PTP
topology and dedicate devices to be only masters or slaves. This is the approach
supported by the SPG8000A, which allows for a more traditional primary and
backup master structure controlling a range of slave devices. The BMCA is still
used to select the active master and the network still takes the place of the ECO in
the legacy Black - burst/Tri - level network.
The five basic PTP timing
messages
There are five basic timing messages in a PTP system: Announce, Sync, Follow -up,
Delay request, and Delay response. Other messages are present in some instances.
For example, grant requests and responses appear in unicast systems, and Type
Length Value (TLV) management messages appear in SMPTE ST2059 profile
systems.
Announce message: The Announce message is sent by the master to advertise its
capability. This message contains the clock quality and priority settings needed for
the BMCA to evaluate which device is the best master.
Sync message:
The Sync message is sent by the master and is used to measure the
propagation delay from the master to slave. The sync message may contain the
timestamp indicating when it was sent, or that time may be in the Follow -up
message. The receiver must timestamp the sync message upon receipt. These two
timestamps are usually called “t1” and “t2” and provide the first delay
measurement.
Follow-up message: The Follow - up message is sent only in cases where the PTP
network hardware is not capable of inserting the timestamp into the Sync message.
(See One-step and two-step operation .)
Delay request message:
The Delay request is sent by the slave. The time at which it
is sent is note d by the slave, but is not included in the message. This time is usually
called “t3.” When the delay request is received by the master, the master
timestamps the receive time. This timestamp is usually called “t4.”
Delay response message: The delay response is sent from the master to the slave.
The response contains the t4 time stamp from when the master received the delay
request. When the slave receives the delay response, it has the second pair of
timestamps needed to calculate the second delay measurement of the slave to
master delay.