ck together. Using bidirectional optics (where upstream and downstream are carried over a single fiber on separate laser frequencies) is one technique that mitigates this problem, although it introduces another. The use of different As (frequencies) in the optical signal in a fiber cable causes asymmetry when the upstream and downstream signals use different As. These different frequencies travel at different speeds through the fiber and introduce a measurable asymmetry (multiple factors influence the size of this effect-it may be quite small, especially for short cable runs). This is an issue with bidirec- tional optics, although it is possible to model and correct it to a large extent, which is why bidirec- tional links are used to carry highly accurate time signals such as White Rabbit (discussed in "Chapter 7. Precision Time Protocol"). However, unlike dynamic asymmetry that is induced by changing traffic patterns, these forms of asymmetry are static and therefore can be measured and compensated for. See the sections "Asym- metry, "Correction and Calibration," and "Carrying Phase/Time over the Transport System" Chapter 9. 11.3.1 Multi-Profile with Profile Interworking The major approaches (using GNSS, FTS, and PTS) to delivering frequency, phase, and time are dealt with in the preceding sections, but many situations in living, breathing production networks are not SO uniform and do not always conform to a single approach. In some circumstances, there will be a difficult situation to overcome that requires a combination of techniques or a compromise outside the "one size fits all" solution. This is the case when the characteristics of the network or underlying transport require the engineer to combine the approaches from the FTS and PTS use cases. In those network locations where it is possible, G.8275.1 with SyncE is deployed using a PTP-aware network and transport, but G.8275.2 is used where the network no longer offers the level of PTP awareness that is needed. One common situation that arises is where the 