se the current value of the slave's clock (used for t3 and t4) could literally be years wrong compared to actual time (used for tl and t2). However, this does not matter since it is only time difference that matter, (t4 - t3), and not absolute time at this stage. Therefore the calculation stated before is always valid. However, what is important is that the slave's clock is ticking at the correct rate; otherwise, the (t4 - t3) calcu- lation would not be accurate. This is why the previously discussed syntonization step is important. Once the one-way delay from master to slave is known, the slave can use this to offset the time received from the master in the Sync messages (t1) and set its clock accordingly as shown in Fig. 8-18. At this point, both syntonization and synchronization have been achieved. Master Slave Delay Request Realtime that sync message arrives is t1+Af. If slave clock was already aligned then Af = t2-t1 but it isn't. Delay Response So slave must use delay measurement t4 sent back mechanism to determine Af + Ar (=A). to slave -A = om-os Assume Af = Dr Af=A/2 Network Delay Af = Master-to-Slave Ar = Slave-to-Master FIGURE 8-18 PTP synchronization process. PTP, Precision Time Protocol. Chapter 8 Synchronization in open radio access networks 285 This synchronization process is then repeated continuously, with several such message exchanges every second normally, to maintain synchronization of the slave clock to read time from the master. This ongoing activity is necessary since the syntonization from the ini- tial step can never be perfect, meaning that the slave clock will be ticking with a fractional error. Such an error can quickly accumulate if the clock is not regularly resynchronized. Of course, nothing is ever this simple in reality, and in real networks there a number of factors that can impact the ability of a two-way time transfer to transport time: Timestamping accuracy. The previous example assumes that we can timestamp messages as they are sent and received with arbitrary accuracy and pr