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WHITE PAPER TIME TRAVELS: A CLOSER LOOK AT PTP TIME TRAVELS: A CLOSER LOOK AT PTP Precision Time Protocol is a PTP – Why it’s necessary values gives the slave enough data to system requirement for the correct its internal clock to exactly latest IP media standards, The IEEE 1588 Precision Timing Protocol match the clock of the master. This including AES67 audio and (PTP) specifies a methodology for process is repeated periodically to SMPTE ST2110 video (as well as synchronizing devices to a single, ensure that each slave device maintains other popular formats such as shared clock across packet-based synchronization with the master. Figure Dante audio). This white paper networks, including Ethernet switches 1, on the next page, shows an overview describes key factors that must and IP routers. This standard, which was of this process, which is described in the be considered for successful PTP originally developed for laboratory and following paragraphs. deployments, including system industrial installations, is now architectures, PTP device fundamental for IP video/audio signal 1) The master, which knows the correct configuration, and multicast transport using SMPTE ST 2110 and IP time, begins the process by periodically network considerations. A few audio signals using AES67, among other generating a Sync message. The sync troubleshooting tips are also media applications. One of the main message is typically multicast to all of provided. uses of 1588 is for creating a common the devices on a particular subnet. It timebase for multiple video cameras contains header information that (GENLOCK), and also to synchronize identifies the master and includes a video and audio devices to a uniform precise time stamp of when that clock (time code). message left the master’s network interface. Depending on the hardware PTP Message Flow and software configuration of the master, the Sync information can be The PTP synchronization process works generated either as one message by exchanging a series of messages (one-step) or the master may issue two between a master device and a slave messages in rapid succession (two- device. During this process, the slave step). device can determine the exact amount of network delay from the master as a) Two-step operation is required well as determining if its clock is offset when the master is unable to include from the master. Knowing these two a hardware timestamp directly inside Artel • White Paper • 1 June 2018 WHITE PAPER IT’STIME ABOUT TRAVELS: (PRECISION) A CLOSER TIME LOOK AT PTP an outbound Sync message and therefore must send a Sync Follow-up message that contains the precise hardware timestamp. b) One-step messages are used when the master has the ability to include a precise timestamp derived from the hardware within the Sync message. Note that the selection of one-step or two-step operation is made as a configuration selection for both the master and the slave devices. In either case, once the slave receives a Sync (and optionally the Sync Follow- up) message, the slave will be able to determine precisely when the Sync message arrived at its hardware interface and to read the message(s) to determine the precise timestamp when the Sync message left the master’s hardware interface. The difference parameter of the slave device (which so that it is perfectly aligned to the between those two timestamps could make changing a master device master’s clock. Very tight accuracy represents the one way delay from the more difficult). The option of using levels are frequently achieved in this master to the slave, provided that there multicast or unicast technology to send process; slave clocks can often be is no offset between the slave and Delay Request messages is typically a aligned within a few nanoseconds of the master clocks. configuration parameter in the setup of master clock. slave and master devices. 2) The next step in the process is for the 3) Each time a master receives a Delay slave to send a Delay Request message Request message it must send out a Distributing the Clocks back to the master. Essentially, this is a Delay Response message to that device. message is very similar to the Sync Included in this message is the precise For a PTP network to operate properly, a message, except that the header data timestamp of when the Delay Request few general network requirements need fields are changed to show the slave message was received at the master’s to be in place. First and foremost, the device as the originator of the message. input as well as the identity of the PTP synchronization process described device that sent the Delay Request above requires a bidirectional network A Delay Request message can be sent message. Note that in systems with capable of transmitting IP packets from back using the same multicast group higher quantities of devices, the the master to the slave and back. This address as the original Sync message, or processing of a large number of delay enables all devices to receive Sync, it can be unicast directly to the Master’s request messages can become a burden Follow-Up, and Delay Response IP address. In the former case, all of the for the master. messages and to transmit Delay other devices that are members of the Requests. multicast group will receive the Once the slave device has received the message and have to parse it to Delay Response message, it has all the Each master device, or device that could determine that the message did not information necessary to calculate the potentially become a master, must have originate from the master. In the latter delay from the master and to determine the ability to receive Announce case, the slave device has to know the whether the slave’s internal clock has messages from every other master or master’s correct (non-multicast) IP any offset (positive or negative) from potential master device. Announce address; in some cases this address the master. The slave then uses these messages are sent on a regular basis needs to be set as a configuration two values to correct its internal clock from the active master and all other Artel • White Paper • 2 WHITE PAPER IT’STIME ABOUT TRAVELS: (PRECISION) A CLOSER TIME LOOK AT PTP devices that receive those messages should not attempt to become masters unless they determine that they would be a better master than the current master. All devices that could potentially become the master must execute a common Best Master Clock algorithm which is defined in the IEEE 1588 specification. Once a best master clock has been selected all other potential master devices must stop sending announce messages and should receive their timing from the common best master. This process is illustrated in Figure 2. Comparing two potential grandmaster clocks requires an A vs. B comparison based on multiple criteria. Table 1 shows an ordered list of the criteria that are used each time two clocks are compared. The process works by going matched pair should get preference in a through each individual criteria starting Within the list of criteria shown in working system. Also note that criterion from the top of the list and comparing Table 1, two priorities can be configured 6 is the ultimate tiebreaker: as long as clock A vs. clock B. if the two clocks are by the network administrator. Priority 1 the two clocks do not have identical matched identically for a given criteria is an eight bit field that can be used to MAC addresses (which they shouldn’t) then the comparison continues by going adjust specific devices to be high or low there will always be one clock selected to the next item on the list. As soon as priority; note that this criteria (since it is over the other. one clock is found to be preferred to the at the top of the list) is more important other clock, the process stops and that than all of the others. Priority 2 (which Criteria 2, 3 and 4 are related to the clock becomes the grandmaster. The is shown in the fifth row of Figure 3) is a quality of a particular clock and the other clock automatically stops sending secondary 8-bit priority field that can be different levels are defined in the IEEE announce messages and remains silent used to break ties between clocks that 1588 specification. Without going into as long as it continues to receive match in all of categories one through excessive amounts of detail, a clock that announce messages periodically from four. This criterion would be a good is connected to a better quality source, the selected grandmaster. place to define which clock of a one that has more accurate internal Artel • White Paper • 3 WHITE PAPER IT’STIME ABOUT TRAVELS: (PRECISION) A CLOSER TIME LOOK AT PTP electronics, and one that is more stable Transparent clocks sit in the path that that devices that are connected to these will be selected over other clocks. Note PTP (and all other) network traffic ports can synchronize directly to this that some of the values used in these passes through and do as much as they switch. The primary benefit of BCs in a fields made the specific to a particular can to make themselves invisible to the network is to reduce the amount of profile of clock usage. In particular, traffic Figure( 3). This is accomplished traffic coming from slave devices to a SMPTE has defined several additional by first calculating the amount of time master. Remember that master devices clock classes (criterion 2) that go in nanoseconds that a PTP message will regularly receive Delay Request beyond what is in the basic IEEE spends in transit going through a TC messages from every one of their slave standard.

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