All PTP participants need to be PTP capable; this includes Ethernet switches but not the data sink (that is, the PC running DAQ software). The Clock in a DAQ module is named an Ordinary Clock. A Clock in an Ethernet Switch is named a Boundary Clock. The Master is selected automatically if no Grandmaster Clock delivers absolute time information. This mechanism is called Best Master Clock Algorithm (BMC).
Some DAQ topologies are line- or ring-structured with many switches, and thus Boundary Clocks use their own time control loops. As an alternative, the so called Transparent Clock (TC) allows an End-to-End (E2E) sync control and follow-up messages. The introduction of TCs allows for a far simpler solution to correct for variable switch latency. The basic idea of the TC is to measure the time a PTP event message has spent in the switch (the so-called residence time). The residence time is reported to the receiver by the message itself or by a subsequent Follow_up or Delay_Resp message. For this purpose a new message field has been added, the so-called Correction Field is a type of Time Interval that can be used to accumulate residence time along the path of the message, and may also be used for other kind of corrections. The key benefits are:
- No configuration required: Transparent clocks do not have to calculate and do not have to be considered in the calculation of the BMC algorithm, so they do not necessarily have to send or receive management messages.
- Quick reconfiguration of the network in case of faults.
- Faster setup times: in initialization phase and after change in topology, transparent clocks do not have to re-synchronize to a master clock before they can be considered part of a valid synchronized path.
- Perfect for small configurations.
Transparent Clocks using Peer-to-Peer (P2P) scale well with the number of devices attached to the subnet and can recover rapidly to changes in network topology. So this mechanism offers a high scalability and it’s best suited for deeply cascaded topologies (large scale systems using many switches in daisy chain).