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11. UNDERSTANDING SYNCHRONISATION
PTP & Synchronisation
Precision Timing Protocol, specifically version 2, also known as PTPv2-IEEE-1588-2008, provides a method of synchronising clocks
across a network very accurately. For Media networks of a larger scale, it is important to understand your PTP configuration and to
ensure all switches and end-points are correctly configured for PTP. Getting this wrong will result in unstable audio.
All AoIP end-points contain their own clock, and PTP is used to synchronise those clocks. One device will act as the master, this could
be one of the AoIP end-points, or a PTP capable switch on the network, or a clock generator such as a Sonifex AVN-GMCS Grand
Master clock unit. A dedicated Grand-Master device is recommended for a broadcast media network.
The Master sends out time-of-day messages to all the slaved devices (devices that are referencing a master). It takes a finite amount
of time for a message to travel across a network, so by the time a slave receives a time-of-day message, it is already out-of-date as
time has moved on since it was originally time-stamped and sent out by the master. Slaves exchange "receipt request/response"
messages with the master, and by timing how long it takes to get a response, they are able to adjust the timestamps to factor in the
network latency, and therefore maintain their clock in relation to the master with a very high degree of accuracy.
On a small network with a few end-points connected to a single switch, cost effective non PTP aware switches like Cisco SG350
series, can be used as long as it is correctly configured for IGMP and QoS. As networks scale up though PTPv2 aware switches
become very important. PTPv2 aware switches adjust the time-stamps of PTP messages that they are forwarding on, to factor in the
delay caused by the switch - the amount of time it takes to receive and forward on the message.
Synchronising with PTPv2-IEEE1588
The following PTP Terms are used when deciding which device is Grandmaster.
Best Master Clock Algorithm:
This is used by all devices within the same
domain to determine which device is most
suitable to become the GrandMasterClock.
The following attributes are used to
determine this (in order of priority):-
Priority 1 – the user can assign a specific
static-designed priority to each clock,
preemptively defining a priority among
them.
A PTP CYCLE
By calculating the offset between the timestamps of these messages being sent and subsequently received.
A slave device can correctly adjust its clock to match that of the GMC to less than 1ms.
Class – each clock is a member of a
given class, each class getting its own
priority.
Accuracy – precision between clock
and UTC, in nanoseconds (ns).
Variance – variability of the clock.
Priority 2 – final-defined priority,
defining backup order in case the
other criteria were not sufficient.
Source Port ID - If the BMCA has
failed to choose a master from the above
criteria, it resorts to using the clock with
lowest port ID (usually its MAC address)
BMCA makes these checks in
order, and stops as soon as it has
identified a master at any stage (it
does not check the next step unless
there is no clear choice based on the
current step).
Once the PTP GMC has been
determined, all clocks will begin to
sync to the newly elected clock.
•
Master will send "sync_message"
to all slaves (packet timestamped
from GMC)
•
Slave will timestamp the packet
on reception
•
Slave replies with "delay_request"
to the GMC
•
GMC will timestamp the packet
on reception
•
GMC sends "delay_response" to
the slave.
•
This packet will include the GMC
receipt timestamp of the "delay_
request"
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