ed in an extended TLV. This extended QL TLV mechanism was developed for use with the eEEC values for QL. For further information on these changes, see the sections "Ethernet Synchronization Mes- saging Channel" and "Enhanced ESMC" in Chapter 6. Therefore, one important component to understand is the interworking between different SyncE generations. It is possible to have network nodes such as an eEEC supporting extended QL TLV; an EEC supporting standard QL TLV; and an EEC supporting extended QL TLV in the one network. These questions are addressed in the section "Interworking between different SyncE generations" in G.8264. Link to recommendation: https://handle.itu.int/11.1002/1000/13547 8.4 ITU-T Packet-Based Timing Recommendations The ITU-T develops its synchronization recommendations based on a solution-led approach. The process starts where other standards organizations define a problem to be solved. For example, 3GPP requires a solution to deliver accurate phase synchronization to 5G mobile radios. Taking that problem, the ITU-T puts together a range of recommendations to solve the problem, end to end. The ITU-T specifies all aspects of the problem, such as solution architecture, clock perform- ance, network topology, time error budgets, and, finally, the profiles to implement it all. See the sec- tion "Types of Recommendations" earlier in the chapter for more detail on these different categories. The end-to-end assumptions are then validated and confirmed by very thorough and sophisticated simulation. It is not just about PTP profiles; they arise as a result of the process, not the other way around. This approach is clearly demonstrated in this section on packet-based timing for both frequency and time and phase. It is for this reason that the authors strongly recommend going with the standards- based approach for implementation of timing. Following the path already laid out delivers predict- able outcomes, whereas doing it in your own way may not produce the results desired. There are many very large netw