ution and the circuit board layout within the WR network element (assumed to be mostly a fixed value and SO can be compensated for). White Rabbit extension to PTP (WRPTP), a PTP profile that includes a WR link setup process with extra messages and TLVs. Measurement and correction of asymmetry errors caused by the physical medium that are fixed and can be measured and corrected for. An example would be the difference in propagation ve- locity for two different As in a single BiDi fiber. Calibration and correction of asymmetry in the physical link between nodes, which is normally considered to be dynamic (such as cyclical temperature changes in the fiber over the day and seasons causing changes in the propagation speed). The process for measuring the two-way delay relies upon the E2E delay mechanism from PTP 1588-2008 (Annex J.3) with its associated accuracy to give a first-order estimate. Then, another process, using a technique called Digital Dual Mixer Time Difference (DDMTD), is used to measure/control phase difference between each end of the link at a much higher level of precision. The details of this method are beyond the scope of this book, but there are references at the end of the chapter (see reference with the term "DDMTD"). When building a WR-enabled network, it uses the Ethernet 802.1 bridge (switch) functionality with some WR extensions to ensure more deterministic performance with lower latency (much like TSN; see the earlier section dedicated to IEEE 802.1AS-2020). For highest precision, WR uses a single BiDi fiber for two-way communication with a different a in each direction. In this case, most of the asymmetry is caused by the difference in propagation velo- city in each direction, as the velocity in fiber is dependent on the frequency of the laser light. This can be accurately determined for each type of fiber (although that difference can change with tem- perature, which may also be modelled and corrected). The WRPTP profile specifies a modified BMCA (mBMCA), with the major difference being th