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DXD/PTP/GPS User Manual (rev 2.10)
16. Guidelines for configuring PTP (Precision Time Protocol)
Only a few years ago, setting up a network was not a task most audio or video engineers were concerned with. This was the
responsibility of the IT department. Nowadays, it has become an important part of more and more installations and, as we
move towards A/V over IP, careful advance planning of the network and proper configuration is necessary, demanding a new
set of skills.
Although configuring an A/V network includes many settings that do not concern the DXD-8, PTP is a complex standard and a
good understanding of network technology is required. The best source of information for PTP is the IEEE 1588 standard itself
available here: https://standards.ieee.org/standard/1588-2019.html. There are also many publications available from
different organizations such as SMPTE and AES with best practices recommendations.
PTP’s role is to synchronize the real-time clocks of the different nodes on a network but it does not make the network itself
synchronous. The traditional master/slave architecture is also very different when dealing with PTP thanks to the BMCA.
The list below only includes some very basic PTP requirements and recommendations and is not meant to be a comprehensive
check list.
16.1 REQUIREMENTS
•
PTP Domain
: Devices that need to be synchronized together via PTP must be part of the same domain. Be sure the PTP
domain is set properly on your DXD as messages with any other domain number will be ignored.
(menu 5.2).
•
Ports Timing
: per IEEE 1588, all devices on a PTP domain must have the same value set for these ports timing:
-
Announce Interval
-
Announce Receipt Timeout
(menu 5.4).
•
Delay Mechanism
: Two different delay mechanisms are available: E2E and P2P. End-to End mechanism is more
versatile because it can handle ordinary switches and routers but in large networks with lots of slaves, the master clock
and the network could be heavily burdened with large amount of delay request and response messages. With Peer–to-
Peer mechanism, delay is established between neighbors only.
-
P2P requires all of the devices to be PTP-aware as ordinary switches will not respond to peer-delay messages.
-
All the 1588 nodes on the same messaging path are required to use the same delay mechanism.
(menu 5.3).
16.2 RECOMMENDATIONS
•
PTP Profile
: It is recommended to use the same profile for all units in any particular domain. Failure to do so can lead
to unexpected results.
(menu 5.3).
•
Priority Fields
: If the DXD-8 is your preferred master clock, the simplest way to make it the Grandmaster is to set the Priority 1
field low enough to win the BMCA. However, it is also recommended that all candidate GMs would have the same Priority 1
value to avoid causing hierarchy issues as the Priority 1 field overrides any clock quality changes in the GM.
(menu 5.2).
Keep in mind that, with PTP, the BMCA always determines which device becomes Grandmaster. As conditions change
(loss of GPS for example), another Grandmaster could be selected, changing the state of the DXD-8 port. But this would
only be true if, as recommended above, all candidate GMs would have the same Priority 1 value.
