message was in transit. It is a similar approach for the reverse path; the error on the slave would be drev (the sign is reversed). This means that increasing the forward delay in- creases the negative TE in the slave, whereas increasing the reverse delay increases the positive TE. Combining these two measures results in a two-way time error. However, the slave clock might need to select a good subset of packets before using the packets to characterize TE. There might also be optional filtering of the time data with the appropriate bandwidth and some mechanism to stabilize the recovered clock. The output of this process is called the packet-selected two-way time error, or pktSelected2wayTE. There are other versions of the two-way TE that represent the value of the metric after it has been filtered. This sequence of measurements and its variations are used to directly characterize time error. For example, the specifications for noise tolerance use a form of pktSelected2wayTE (specifying the exact packet selection method) to define the input noise that a clock must tolerate. The specification uses the unfiltered version because it is a test for the tolerance to an input signal. This metric also turns up in the end-to-end network limits in G.8271.2. Figure 9-23 illustrates an example of this measure over time and shows several of the values de- rived from it; namely, minimum, maximum, and peak-to-peak. The other interesting value is the ab- solute value of the maximum pktSelected2wayTE, or max|pktSelected2wayTE, which caters for the fact that the important factor is the magnitude of the time error, and not the direction (values can be negative). Packet- selected two-way pktSelected2wayTE error pktSelected2wayTE Peak-to-peak pktSelected2wayTE |pktSelected2wayTE| Figure 9-23 Packet-Selected Two-Way Time Error So, for measuring an end-to-end network time error, the max|pktSelected2wayTE and the filtered version of the same measure measures the accuracy of the recovered phase/time from the packet stream. You will find m