PTP/IEEE-1588 Frequently Asked Questions

Precision Time Protocol / IEEE-1588
What is PTP?
What is the accuracy of EndRun's PTP?
I want to put my Grandmaster on one subnet, with my servers and workstations on another subnet.  How will that work?
What is the difference between hardware and software timestamping?
Can PTP operate over a Wide Area Network (WAN)?
Can PTP be installed in the field?
Where can I get PTP Slave software?
How many PTP Slaves can your Grandmaster support?

What is PTP?
Precision Time Protocol (PTP) is a relatively new protocol that was developed to improve the time synchronization accuracy that is obtainable over a Local Area Network (LAN).  Specifications for PTP are defined in the IEEE-1588 standard.  In PTP terminology, the Grandmaster is the distributor of accurate time and the Slave is the receiver of this time.  The Slave synchronizes itself to the Grandmaster.

The most common network timekeeping protocol is the Network Time Protocol (NTP).  In NTP terminology, the Server is the distributor of accurate time and the Client is the receiver of this time.  The Client synchronizes itself to the Server.

With NTP you can get client synchronization accuracies in the millisecond range.  With PTP you can get slave synchronization accuracies in the nanosecond or microsecond range.  Synchronization accuracy depends not just on the PTP Grandmaster, but also on the network topology such as switch and slave hardware.  For an explanation of PTP read this white paper: Precision Time Protocol (PTP/IEEE-1588).
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What is the accuracy of EndRun's PTP?
Products listed below can be configured as a IEEE-1588/PTP Grandmaster Clock.  Here are the timestamp resolution and accuracy specifications:

Model Timestamp
Resolution
Timestamp
Accuracy to UTC (RMS)
Sonoma (GPS)
Network Time Server
8 nanoseconds 30 nanoseconds
Sonoma (CDMA)
Network Time Server
8 nanoseconds 10 microseconds (typical)
Meridian II
Precision TimeBase
8 nanoseconds 10 nanoseconds
Tycho II
Precision TimeBase
8 nanoseconds 25 nanoseconds
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I want to put my Grandmaster on one subnet, with my servers and workstations on another subnet.  How will that work?
The Grandmaster user interface allows you to modify the TTL Value (time-to-live value) in order to accomplish this.  You will also need to modify the TTL Value on your PTP Slave.
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What is the difference between hardware and software timestamping?
The main difference is in the synchronization accuracy that can be achieved.  With software timestamping as typically implemented (software-only approach), you can see slave synchronization accuracies between 10 and 100 microseconds.  You can achieve this level of accuracy with commonly used network hardware such as standard switches, and computers with software PTP slaves.

With hardware timestamping as implemented on a Sonoma it is possible to achieve time synchronization accuracies of 30 nanoseconds with an 8 nanosecond resolution.  However, in order to get this level of accuracy, both the Grandmaster and the Slave must be capable of hardware timestamping.  This means you will need to purchase specialized hardware to install in each Slave.  In addition, network switches must configured as transparent clocks or boundary clocks.  Read this white paper for network requirements to achieve sub-100 nanosecond synchronization: Precision Time Protocol (PTP/IEEE-1588).
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Can PTP operate over a wide area network (WAN)?
PTP Version 2 has been designed to span over a WAN.  However, performance is dependent on the network configuration.  For example, a network switch would need to be configured as a transparent clock or boundary clock in order to realize the superior synchronization capability.  Otherwise, synchronization of PTP becomes equivalent to NTP.
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Can PTP be installed in the field?
Yes.  All Sonoma, Meridian, Tycho II, Tempus LX, and Unison Time Servers are capable of operating PTP.  PTP is a relatively low-cost option that can be installed by you at any time.  All that is needed from you is the Ethernet address (MAC) and we can supply a software key and instructions for turning on PTP.  For older products, you may need to upgrade your software first.
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Where can I get PTP Slave software?
That question is answered here: PTP Slave Software.
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How many PTP Slaves can your Grandmaster support?
Over 2000.  But it depends on various settings and configurations that can increase or decrease the number of slaves that Sonoma can support.  Consider the following:

1.  If using a Boundary Clock, the Sonoma only interfaces with the network switch Boundary Clock.  In this case the capacity is limited by the Boundary Clock switch.

2.  When using a Transparent Clock, the capacity is limited by the frequency of the delay requests and the sync rate.  Sonoma will be able to provide all the slaves with the Sync Packets and Announce Packets.  But, there will be a limit for processing delay requests issued by the slaves.  Our implementation requires about 10 microseconds to handle a delay request / response.  The delay request is used to calculate the slave-to-master delay.  If your network is static the delay should not change and the Sonoma will announce to the slave to use 32-second delay request interval.

Even though the slaves randomize the delay request packets, the request can come in simultaneously.  What happens if delay requests show up simultaneously?  In this case, the Sonoma will not issue a delay response.  The slave will then randomize the delay request interval and issue the request again.  Some slaves will log a notification that a delay response was not received.
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