Transparent Clock

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

Transparent clock

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

This means you have a new timing packet which is still correctly synced with the master, and can reduce the number of devices the master is directly talking to. The packets are also able to resist some of the effects of distance 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.

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.

Sometimes more than one clock could feasibly be the grandmaster. To decide which clock takes this role, 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.

The Transparent Clock feature is used by bridges or routers to assist clocks in measuring and adjusting for packet delay. The transparent clock computes the variable delay as the PTP packets pass through the switch or the router.

The original IEEE 1588-2002 standard for a precision clock synchronization protocol describes Ordinary Clocks, which are either Grandmaster Clocks or Slave Clocks. An Ordinary Clock (OC) always has a single port.

A BC generally has one port in the role of a slave clock and the remaining ports in the role of master clocks. In this case, the BC recovers the time of day within the slave clock function and relays it as a reference to the master clock functions.

In some applications, a TC is significantly easier to implement because the latency measurements and residence time compensation can be done in hardware, and the node does not need to provide a full slave clock implementation. Also, in a TC there is almost nothing to configure or to tune.

This document explains the difference between transparent clock and boundary clock. It also explains that Boundary clock could scale easily compared to transparent clock. The first section is the definition of the precision time protocol (PTP) functionality, ST 2059-1 and ST 2059-2, transparent clock and boundary clock definition. The next section shows the IP broadcast center clocking scheme and which PTP clocking is better.

The method is simple. A high precision clock, the master, will send out PTP sync messages using the User Datagram Protocol (UDP). Slaves will then receive the sync messages with the master time (t1). If hardware timestamping is not provided by the master, a Follow_Up message will be sent out to provide the time at which the initial sync message was sent out (t1). The slave stores the time at which it receives the Sync message (t2). After reception of the Sync message, the slave sends out a Delay_Request to the master clock (t3). The master finally answers with a Delay_Response message (t4).

The network gets more congested, and packet scheduling across the network can add delays, which in turn cause inaccuracy in time synchronization (PTP messages have different delays that are not compensated for). The transparent clock adjusts the PTP messages to remove the delays of its own packet processing, and thus compensates delays in PTP messaging.

In a spine-leaf architecture, as defined in our recent presentation: ( _Television_infrastructure_emMODULAR_emSFP_gateway ), the top of rack switches can adjust the delay inside the PTP packets (by adding a correction field in PTP message) to ensure it is viewed as transparently as possible.

The boundary clock ready switches possess a built-in PTPmaster clock. The switch will be the master for the endpoint devices attached to it. For stability, the switch will be a slave to anotherPTP master clock. In this scenario, the PTP master in the switch will communicate to a limited number of slaves. This way the boundary clock method ensures that PTP masters are not over solicited, and this will greatly improve the accuracy of the PTP time and the system scalability. The following picture shows a boundary clock system.

From the PTP ticks count, slaves accurately find the vertical and the horizontal pulses and align its video to the video reference. It is also possible to extract the audio clocks and finally to generate the Digital Audio Reference (DARS) if required. The following image shows the process of re-creating the clock for synchronization of signals.

Finally, the endpoints such as the emSFP gateways, emQUAD, emVIEW and emFUSION will work in both systems, transparent clock and boundary clock. But for scalability and accuracy, the boundary clock system should be implemented over the transparent clock.

Some engineers may be confused about key differences between a Precision Time Protocol (PTP) l transparent clock and a PTP boundary clock. This article will help clear up any confusion and suggest which of the two may be more suited to an IP broadcast center.

The method is simple. A high precision clock, the master, transmits PTP sync messages using User Datagram Protocol (UDP). Slaves then receive the sync messages with the master time (t1). If hardware timestamping is not provided by the master clock, a Follow_Up message will be sent out to provide the time at which the initial sync message was transmitted (t1). The slave stores the time at which it receives the Sync message (t2). After reception of the Sync message, the slave sends out a Delay_Request to the master clock (t3). The master finally answers with a Delay_Response message (t4).

As a network gets more congested, packet scheduling across the network can add more delays. The result can cause an inaccuracy in time synchronization because PTP messages have different delays that are uncompensated. The transparent clock adjusts the PTP messages to remove the delays of its own packet processing, and thus compensates for any delays in PTP messaging.

The boundary clock switches already possess a built-in PTP master clock. The switch acts as the master clock for any endpoint devices attached to it. To provide extra stability, the switch will be a slave to another PTP master clock.

In this scenario, the PTP master clock in the switch will communicate to a limited number of slaves. This way the boundary clock method ensures that PTP masters are not over solicited, which greatly improves the accuracy of the PTP time and the system scalability. The following figure shows a boundary clock system.

From the PTP ticks count, slaves can accurately find the vertical and the horizontal pulses and align its video to the video reference. It is also possible to extract the audio clocks and then generate the Digital Audio Reference (DARS) signal, if required. The following image shows the process of re-creating the clock for synchronization of signals.

Finally, Embrionix endpoints such as the emSFP gateways, emQUAD, emVIEW and emFUSION will work in both types of PTP designs, transparent clock and boundary clock. But for scalability and accuracy, the boundary clock system should be implemented whenever possible over the transparent clock.

By exchanging announce messages containing the priorities, time class, and time accuracy of GMs, clock nodes in a PTP domain elect a GM. The master nodes, member nodes, master ports, and subordinate ports are specified during the process. Then a loop-free, interconnected spanning tree with the GM as the root is generated for the PTP domain.

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