PTP FAQ
- What is PTP?
The Precision Time Protocol is a method by which a computer, communicating over a
Local Area Network can keep its local clock synchronized to a better time source,
commonly referred to as a Master Clock.
- My computer has a clock, why do I need PTP?
All commercially available computers do come with a built in clock. The built in clock
in an off the shelf desktop, server or laptop is only accurate to within a few seconds
per day. Depending on local conditions, such as high heat or vibration the clock built
into your computer can wander by minutes in a day. If you don't need your computer's
clock to be in line with the rest of the world's clocks by more than a few seconds
then you don't need PTP. Stop here, rest easy.
- I run an operating system that has Internet time synchronization built in, why would I use PTP?
Most operating systems (FreeBSD, MacOS X, Linux, Windows, etc.) have a built in network time
client based on the Network Time Protocol (NTP). NTP is meant to synchronize a clock over the Internet.
Many organizations, including companies and governments, set up very accurate clocks,
which are synchronized to a globally set standard, for use within their organization
or to the public. Because NTP works across the Internet it is not generally capable of
keeping a client computer's clock accurate to better than a second, though this is a rough
guess depending on how the computer is connected to the Internet.
If your application requires time to be accurate to within milliseconds or microseconds
then you'll want to use PTP and you'll want to synchronize your clocks to a nearby
master.
- Do I need to buy an expensive master clock to use PTP?
That depends on your application. The reason that your computer's clock is so inaccurate is
that the clock chip used is quite cheap, about $0.10, and possibly cheaper. You get what you
pay for, and most people aren't willing to pay for something better. The people who are
willing to pay for something better are willing to pay quite a bit more, and master clocks
can start at about $5000 and go up from there. The better materials are both rare
and hard to work with, which is part of what drives up the price. Oven baked crystals are
the next most expensive, followed by rubidium, and all the way up to cesium clocks, which are
also referred to as atomic clocks because they're using atomic decay to derive their time signal.
If you already have one of these you probably don't need this FAQ.
PTP does not require an expensive master clock but the protocol does depend on there being
a master and one or more slaves. If your application requires only that a group of machines
on a LAN be synchronized to each other, for instance in a factory automation or local audio
application, then an expensive master could be avoided by designating a normal server as
the master, and having it run both NTP and PTP. NTP would be used to get decent time from
the Internet, and PTP would be used to keep all of the slaves close to the time set by
the master. All the clients will only be as accurate as the system designated as the master
but this can work for systems that don't need to be synchronized within microseconds.
- Does the PTP Project endorse any master clocks for use with the PTP Daemon
The project does not endorse or verify any particular vendor's Grand Master Clock.
Several different models are available from a number of vendors including:
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