## 4 Characteristics of Different Time Distribution Systems
### White Rabbit
#### 4.3.1 Time Distribution Mechanism  
WR is built upon the Ethernet (IEEE 802.3) [88], Synchronous Ethernet (SyncE) [89], and PTP standards. It also incorporates a technique called Precise Phase Measurement. The integration of these technologies allows WR to achieve sub-nanosecond accuracy. A typical White Rabbit (WR) network, as illustrated in Figure 9, consists of WR nodes and WR switches connected by fiber links. From a data perspective, it functions as a standard Ethernet switched network with no hierarchy, allowing any node to communicate with any other node. However, in terms of time synchronization, a hierarchical structure exists, beginning with the WR master at the top, cascading down to other WR switches and subsequently to the nodes [90]. The top-level switch in the hierarchy, referred to as the grandmaster, receives absolute time along with the PPS signal and a 10 MHz reference from an external source, typically a Cesium or Rubidium oscillator disciplined by GPS [92]. During startup, the WR switch uses the timecode and PPS to establish the absolute UTC time and then relies solely on the 10 MHz signal for time calculations. Once the reboot process is complete, usually within a few minutes of powering on, the timecode service and PPS signal are no longer required, and the grandmaster switch can potentially disconnect from these sources. The accuracy of the round-trip time measurement is primarily determined by the precision of the 10 MHz  
Figure 9: The WR network architecture [91]. reference. The grandmaster switch distributes the time information to additional WR nodes through intermediate WR switches [90]. Typical PTP implementations use free-running oscillators at each node, leading to increasing time drifts between the master and slaves. This issue is addressed by the SyncE protocol, which transfers frequency over the Ethernet physical layer to ensure that all network nodes operate at precisely the same rate. Each WR switch utilizes the clock recovered from the data link to sample incoming data. For transmission, it employs an embedded PLL-based oscillator locked to the recovered clock, ensuring synchronized frequency across the network. As shown in Figure 10, on a link directly connecting two WR nodes, A and B, the L1 tx clock signal transmitted by node A at its port, which is in the master state, is synchronized to its local PTP clock. WR node B, with its port in the slave state, synchronizes its local PTP clock to the L1 rx clock signal received from node A. Consequently, the local PTP clocks of both WR nodes are frequency-traceable to the same source, the grandmaster, without relying on the PTP protocol. The PTP protocol is used solely to measure the link delay and offset from the master, enabling WR node B to adjust the time counter and phase of its local PTP clock [93]. The detailed path delay, frequency detector, and phase align sequence is shown in Figure 11.  
Figure 11: WR time synchronization scheme [91]. Figure 10: WR synchronization and syntonization scheme [93]. Synchronization in WR is achieved through phase adjustment (phase-steering) rather than directly altering the time counter value. The WR PLL not only synchronizes the local PTP clock to the recovered L1 rx clock signal but also maintains a specified phase offset between these two clock signals [93]. Each WR switch is equipped with a phase measurement module that uses phase/frequency detectors to periodically measure the phase difference between the recovered clock and the master clock [94]. This procedure ensures a high level of jitter elimination. Since it operates at the physical layer, its accuracy is unaffected by data transmission issues, such as packet delays or traffic load. The technology has demonstrated the capability to transfer highly accurate timing over long distances [91]. Same as PTP, WR calculates the path delay, but the difference is that WR considers the asymmetry in propagation time. In a WR network, PTP messages are handled not only by the grandmaster clock but also by the WR switches.