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Chapter 10 - Appendixes
Hierarchical Topology
Multicast and Unicast Networks
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serve as the source of time, i.e., be a master clock, and may synchronize to
another clock, i.e., be a slave clock.
Ordinary Clock: According to IEEE1588 standard, an ordinary clock has a single
PTP port in a domain and maintains the timescale used in the domain. It may
serve as a source of time, i.e., be a master clock, or may synchronize to another
clock, i.e., be a slave clock.
Transparent Clock: According to IEEE1588, a transparent clock is a device that
measures the time take from a PTP event message to transit the device and
provides this information to clocks receiving this PTP event message.
End-to-end Transparent Clock: According to IEEE1588 standard, it is a
transparent clock that supports the use of the end-to-end delay measurement
mechanism between slave clocks and master clock.
Peer-to-peer Transparent Clock: According to IEEE1588 standard, it is a
transparent clock that, in addition to providing PTP event transit time
information, also provides corrections for the propagation delay of the link
connected to the port receiving the PTP event message.
One-step Clock: According to IEEE1588 standard, it is a clock that provides time
information using a single event message.
Two-step Clock: According to IEEE1588 standard, it is a clock that provides time
information using the combination of an event message and subsequent
general message.
Accuracy: According to IEEE1588 standard, the mean of the time or frequency
error between the clock under test and a perfect reference clock, over an
ensemble of measurements. Stability is a measure of how the mean varies with
respect to variables such as time, temperature, and so on. The precision is a
measure of the deviation of the error from the mean.
Profile: According to IEEE1588 standard, profile is a set of all allowed PTP
features applicable to a device.
Timeout: According to IEEE1588 standard, timeout is the time in which a device
waits to receive synchronization messages. In case the message is not received
within this time interval, the clock that sends messages is considered out of
operation and the BMC algorithm is ran, and chooses a second master clock.
IEEE1588 defines a hierarchical topology composed of different types of clocks that
send and receive synchronization messages.
In hierarchical topology, a boundary clock is elected the grandmaster clock that
sends PTP messages for the entire network, which are also connected ordinary and
boundary clocks. The boundary clocks connected to the network are used as
intermediate time source for ordinary clock. The selection of the source clock is
performed by each receiver device, using the BMC algorithm.
The first revision of the IEEE1588 standard specifies only multicast network where a
PTP message sent by a network port can be received by all other ports connected to
the same network. The great advantage of the multicast network is that the master
clock sends only one packet of time Sync to the network, and it is received by all
slave devices connected to that network.
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