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Features and Configuration Options
Calibir GXM640 IR Camera User's Manual
Precision Time Protocol
The Calibir GXM640
supports IEEE 1588 Precise Time Protocol (PTP) which provides a
method for synchronizing devices over a Local Area Network (LAN). PTP is capable of
synchronizing multiple clocks to microsecond accuracy on a network specifically designed
for IEEE-1588. One device clock is considered the master, and multiple slave devices
synchronize their clocks to the master; this allows timestamps to be consistent across
the network.
Hardware Considerations
Ethernet switches can be categorized as standard Ethernet switches and IEEE-1588
enabled Ethernet switches. A standard Ethernet switch temporarily stores packets before
sending them out. The storing time of the packet is non-deterministic and depends on
network load, resulting in packet delay variation. The packet delay variation is the
primary reason that standard Ethernet switches result in poor time synchronization even
when the master and slave clock support hardware timestamping.
An IEEE-1588 enabled switch is a transparent clock. Using a transparent clock improves
synchronization between the master and slave and ensures that the master and slave
are not impacted by the effects of packet delay variation.
PTP Synchronization
When more than one device on a LAN is PTP enabled, the IEEE 1588 protocol uses the
Best Master Clock algorithm (refer to the protocol documentation for more information)
to automatically determine which clock in the network is the most precise. It becomes
the PTP master clock. All other clocks become PTP slaves and synchronize their clocks
with the PTP master.
To synchronize, the PTP master clock periodically broadcasts synchronization messages
that the PTP slaves use to correct their local clocks. Message timestamps are used to
precisely determine the time at which a message was sent by the PTP master, and the
time at which it was received by the PTP slave. The time difference is a combination of
the clock offset (between PTP master and PTP slave) and network transmission delay.
When a PTP slave receives a synchronization message from the PTP master, it updates
its local clock in two ways:
It adjusts its clock offset to compensate to past errors;
It adjusts its own clock speed, to reduce future errors. When the PTP slave detects
that its clock is consistently lagging behind or accelerating ahead of the PTP
master’s clock, it will adjust its own clock speed to match the PTP master’s clock
speed.
