What's the difference between types of clocks?

What's the difference between types of clocks?

Posted on 10/06/2019 by Rob Smith
Many types of clocks are used in timing, especially Precision Time Protocol or PTP (IEEE 1588). Here’s a quick guide to the basics:

 

What is a master clock?

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The first type of clock you’ll encounter is a master clock. The master clock is the main source of time on a network. Usually you will set this up so that it is getting a time signal externally. The most common external source is GPS or another satellite network or combination. The master clock will then transmit the time signal across the network.

Used to: Provide time to a network

 

 

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What is a slave clock?

 

When using a master or grandmaster clock, other devices which sync to the master’s time are called slave clocks or slave devices. A slave clock is a device that syncs with the master or grandmaster clock but does not act as a source of timing. If your network is running PTP or NTP the slave clocks will take the time from the master clock, adjust to account for network lag, and sync themselves to this time.

Used when: receiving time at a device

If you want to learn more about how the slave clock deals with network lag check our article on PTP here ►

 

 

What is a transparent clock?

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Transparent clocks are used to route timing messages within a network. A transparent clock (often this is a PTP enabled switch) allows you to timestamp the packet before it reaches a slave device, allowing for the delay to be more accurately calculated. They operate in place of a normal switch, which can otherwise introduce inaccuracy when the network gets congested.

Transparent clocks also help manage the ordering of packets if they are coming from multiple sources. If the output port is busy it will queue packets, then stamp each packet with an arrival and departure time to account for the time it was waiting in the clock. 

Used when: Ethernet timing must pass through switches.

 

 

What is a boundary clock?

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A boundary clock in PTP is both a slave and a master clock. It will take the sync message in, adjust for delay, and then create a new master time signal to pass down the network to more devices. 

This allows you to have a clock that creates a new timing packet which is still correctly synced with the master, and can reduce the number of devices the master is directly sending time to. The packets are also able to resist some of the effects of degradation because they are reproduced from scratch. A transparent clock does not reproduce the signal in this way, but instead only adds delay information to the existing signal.

Note that a Boundary Clock can translate between signal types, and is also able to operate on more than one subnet. A transparent clock has neither of these abilities.

Used when: There are too many devices on a network for one master, long distances are involved, or signals need to be translated between protocols (such as PTP to IRIG-B).

 

 

What is a grandmaster clock?

When using a precision timing protocol (PTP) timing network, the grandmaster clock is the main distributor of time in a multi clock network, sending time downstream to other master clocks. In the above example regarding Boundary Clocks, the master clock could also be called a grandmaster.

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Sometimes more than one clock could feasibly be the grandmaster. To decide which clock becomes the master, the clocks automatically use the Best Master Clock Algorithm (BMCA). This determines which clock is the better or most accurate source of time for the network.

Used to: provide the best source of time when several are available, and provide time to other clocks on the network (PTP only)

 

Next up, find out what PTP is and why it's the future ►
What's the difference between types of clocks?