 and the Ethernet PHY transceiver to precisely mark the ar- rival or departure time of PTP packets. The accuracy (or inaccuracy) of the TSU, however small it is, can be directly observed on the eventual phase accuracy. The closer the timestamp for each indi- vidual PTP message is to the real ToD, the more accurate the PTP implementation will be. The resolution (or granularity) of the TSU also affects how accurately the packets can be timestamped. If the minimum value that the TSU can represent is limited to (say) 1 microsecond, then the timestamp cannot represent the precision of the ToD, even if it has a com- pletely accurate ToD available. Any jitter in the ToD transfer or the timestamps will affect the solu- tion when the slave clock uses the PTP formula to calculate the offset from master. Recall that the formula for a slave clock implementation is offset from master=(tz-t1+t5-ts)/2 = So, for example, if the resolution of a timestamp from the TSU is 1 microsecond, this means that the TSU can add jitter of up to 1 microsecond to the timestamps. With such a low resolution, even a perfect PTP implementation and ideal transport (with zero packet delay variation [PDV] and no link asymmetry), the resultant accuracy cannot be better than 500 ns. According to Institute of Electrical and Electronics Engineers (IEEE) standard 1588-2019 (clause 7.3.4.2), the timestamp shall be the time at which the event message timestamp point passes the reference plane marking the boundary between the PTP node and the network. Interestingly, this clause from 1588-2019 implicitly indicates that the timestamping boundary for any NE must be at precisely the same point for both arrival and departure of the messages. As shown in Figure 12-5, the points tingress and tegress would be the correct and symmetric locations on the reference plane to record the timestamps. So, even for the case of timestamping with hardware assistance, if the timestamp points of the re- ceived and transmitted PTP packets are not aligned, it results in asymmetry th