AN1202: ITU-T Precision Time Protocol Grandmaster, Boundary Clock, and Slave Clock Mode ITU-T Precision Time Protocol (PTP) is an extension of IEEE1588 Precision Time Pro- tocol that tailors the use of PTP to telecommunication applications. PTP is a key solution KEY FEATURES in transmitting both frequency and time synchronization to achieve 5G network goals. • There are three critical clock types in the Telecommunication technology has been shifting to asynchronous protocols such as IEEE 1588 standard. Ethernet to save on cost. However, this has compromised the network’s ability to • Grandmaster is the top of the maintain timing accuracy. synchronization chain, the requirement for its accuracy is the most stringent. An obvious solution is to add a Global Poisitioning System (GPS) to every site to allow local clocks to synchronize to an accurate clock source. But this is both very expensive, • Boundary Clock receives its synchronization from the grandmaster contradicting the industry’s original intent of being cost effective and less reliable than clock or other Boundary clocks and sends 99.999% availabilty. the synchronization information to downstream boundary clocks and slave Another solution is Network Time Protocol (NTP); however, its accuracy is insufficient clocks. for many applications. The need for a higher accuracy protocol is rising as mobile mar- • Slave Clock receives its synchronization kets approach the challenge of a 5G network. from boundary clocks and it is the last clock in the synchronization chain. PTP is perfectly positioned for these applications because it provides the high accuracy time transfer without requiring a hardware change to the entire network. The protocol can also work in conjunction with SyncE to compensate for nodes that cannot process PTP data. This Application Note will discuss the generic IEEE1588 PTP features and how ITU-T expands on these features for telecommunication use, as well as implementation considerations. silabs.com | Building a more connected world. Rev. 1.1 AN1202: ITU-T Precision Time Protocol Grandmaster, Boundary Clock, and Slave Clock Mode IEEE 1588 Overview 1. IEEE 1588 Overview Although the focus of this paper is the ITU-T application of the IEEE 1588 Precision Timing Protocol, this section will first describe the foundational elements in terms of IEEE1588 before discussing the extensions in ITU-T. The IEEE1588 discussed here is IEEE1588-2008[1]. silabs.com | Building a more connected world. Rev. 1.1 | 2 AN1202: ITU-T Precision Time Protocol Grandmaster, Boundary Clock, and Slave Clock Mode IEEE 1588 Overview 1.1 Grandmaster, Boundary Clock, Slave Clock, and Transparent Clock There are four different types of clocks in the IEEE1588 architecture – grandmaster, boundary clock, slave clock, and transparent clock. In the following figure, the grandmaster is referred to as T-GM, boundary clock as T-BC, slave clock as T-TSC, and transparent clock as T-TC. These are the designated shorthand for PTP clocks in ITU-T, and they will be used throughout this Application Note. As shown in the figure below, the grandmaster receives Time of Day (ToD) and 1 pulse per second (1PPS) information from the GPS and converts it to 1588 timestamp messages. The 1588 messages are passed down to either a telecom time slave clock (T-TSC) or the slave side of a telecom boundary clock (T-BC). If the receiving clock is a T-BC, the master side of the T-BC will then transmit to the next boundary clock, transparent clock, or slave clock. The T-TSC receives the PTP timing information but is not responsible for transmitting it to downstream clocks. ToD 1588 1588 1588 ToD Clocks 1PPS T-GM T-BC T-TC T-TSC GNSS/GPS 1PPS ToD 1PPS Clocks Figure 1.1. IEEE1588 Architecture Block Diagram T-BC and T-TSC can also convert the 1588 messages into clock information in the form of ToD, 1PPS, and other relevant clock fre- quencies depending on the network needs. The telecom transparent clock (T-TC) is another plausible clock type in IEEE1588, it is less common in the ITU-T applications. The role of the transparent clock is to account for the inherent delay that occurred when packets passed through. The T-TC will record the en- trance and exit time of a packet and add the delay (called “residence time”) to the correction field of the packet. A T-TC does not syn- chronize its clock with the master’s clock. Table 18 from IEEEStd 1588-2008 represents the basic packet format [1]. Table 1.1. IEEE1588 Common Elements of a Packet IEEEStd 1588-2008 - Table 18 - Common Message Header Bits Octets Offset 7 6 5 4 3 2 1 0 transportSpecific messageType 1 0 reserved versionPTP 1 1 messageLength 2 2 domainNumber 1 4 reserved 1 5 flagField 2 6 correctionField 8 8 reserved 4 16 sourcePortIdentity 10 20 sequenceId 2 30 controlField 1 32 logMessageInterval 1 33 silabs.com | Building a more connected world. Rev. 1.1 | 3 AN1202: ITU-T Precision Time Protocol Grandmaster, Boundary Clock, and Slave Clock Mode IEEE 1588 Overview 1.2 Definition and Derivation of Two-Way Delay and Offset The most fundamental purpose of PTP is to synchronize the time between clocks, so the foundation of PTP is the Delay Calculation. To synchronize between two clocks, the master clock sends timestamps to the slave clock using one of two methods, One-Way Delay or Two-Way Delay. One-Way Delay can only achieve frequency synchronization while Two-Way Delay can achieve both frequency and time synchroniza- tion. Frequency synchronization allows each clock to have the same time base, i.e., a second lasts the same amount of time for all clocks. Time Synchronization is more stringent because the clocks also must be at the same clock time. One-Way Delay is used when the application only requires frequency synchronization. Two-Way Delay is used in applications with smaller error margins, such as the sub-µs market, where the time synchronization is also critical. In One-Way Delay, only the master transmits timing messages to the slave, and the transmission delay between them is not consid- ered. In Two-Way Delay, the slave responds to the master with a series of timing messages to calculate the transmission delay be- tween the master and the slave. For the math derivations in this section, the variable Offset is defined as the time deviance of the slave clock from the master clock; and the variable Delay is the time spent in transmission. In One-Way Delay mode depicted in the figure below, the master clock sends its clock information to the slave clock using Sync and Follow-up messages. The slave clock corrects its time offset according to the master’s clock time in the time stamps. There is no con- sideration of the transmission delay in One-Way Delay mode. Master Slave Master Slave Offset Sync ta Sync t1 Delay Follow-up t2 Follow-up t3 Delay Request Delay tb Sync t4 Offset Follow-up Delay Response One-Way Two-Way Figure 1.2. One-Way & Two-Way Delay Synchronization The Delay can be calculated using Two-Way Delay mode. Two-Way Delay mode requires additional communication between the slave and the master. A “Delay Request” is sent from the slave and the master responds with a Delay Response. These additional packets provide the slave with additional timestamps to subtract Delay from Offset. Four timestamps are obtained through the Two-Way exchange, t1, t2, t3, and t4. t1 is the time the master transmitted the Sync mes- sage, t2 is the time the slave received it, t3 is the time the slave transmitted the Delay Request message, and t4 is the time the master received it (which is transmitted to the slave via Delay Response). Note: t2 and t3 are measured with the slave’s local clock. Assuming the transmission is symmetrical, the following equations are true: Delay + Offset = t2 - t1 Delay - Offset = t4 - t3 When the two equations are added, the two Offset values cancel each other out, leaving the value of twice the Delay: (Delay + Offset) + (Delay - Offset) = 2Delay + Offset = 2Delay = t2 - t1 + t4 - t3 silabs.com | Building a more connected world. Rev. 1.1 | 4 AN1202: ITU-T Precision Time Protocol Grandmaster, Boundary Clock, and Slave Clock Mode IEEE 1588 Overview Again, assuming symmetry, the value of the Delay can be calculated: t2 - t1 + t4 - t3 Delay = 2 Similarly, for the offset: (Delay + Offset) - (Delay - Offset) = Delay - Delay + 2Offset = t2 - t1 - (t4 - t3) t2 - t1 - (t4 - t3) t2 + t3 - t1 - t4 Offset = = 2 2 The result is an Offset value the slave clock can adjust itself to without the error from the delay. Therefore, a more accurate clock. silabs.com | Building a more connected world. Rev. 1.1 | 5 AN1202: ITU-T Precision Time Protocol Grandmaster, Boundary Clock, and Slave Clock Mode IEEE 1588 Overview 1.3 Two-Step Versus One-Step In the previous section, the One-Way Delay and Two-Way Delay transmissions both start with the transmission of the Sync message. The Sync message transmits the timing information about the master clock. However, there are two methods in IEEE1588 to transmit master timing: One-Step and Two-Step. The difference between One-Step and Two-Step, is that Two-Step needs a Follow-up message to communicate when the previous Sync message was transmitted; while One-Step includes the transmission time of the Sync message in the message itself, as seen in the figure below. Master Slave Master Slave ta Sync ta Sync Follow-up tb Sync tb Sync Follow-up One-Step Two-Step Figure 1.3. One-Step & Two-Step Timestamp It is the master clock designer’s choice to produce either One-Step or Two-Step messages.