o move is a risky undertaking, but some changes are apparent. Perhaps the most likely change to the Q13/15 recommendations will come because of the publica- tion of the new edition of IEEE 1588-2019 with v2.1 of the PTP protocol. According to clause 19 of IEEE 1588-2019, PTP v2.1 is designed to be interoperable with the current PTP v2.0. Of course, it will not be long before operators start to deploy PTP-capable nodes that support elements and fea- tures from PTP v2.1. If that network clock does not use any functionality introduced in v2.1, then it should be compatible with the existing deployments using v2.0. However, at some point, the ITU-T recommendations will have to align with the new version and start adopting some of the increased functionality available in the new 2019 edition of the IEEE 1588 standard. An example of this might be elements of the security features introduced with the new PTP edition. There will also be ongoing work on the details and results of the mathematical simulations that sup- port G.8271.1, G.8273.2, and G.8273.3. There is also work already underway to define the relative time error (rTE) between the ports of a boundary clock (in updates to G.8273.2). This limits the amount of time error that arises between different interface ports on a network element or between the PTP and 1PPS outputs. The only other change that looks likely will be the further adoption of new network topologies and limits (in G.8271.1) to specifically support the case of front-haul in the RAN using a network of highly accurate Class C T-BC boundary clocks. There is also an ongoing effort to support the concept of cluster-based synchronization in 5G RAN networks (see Appendix XII of G.8271.1 for an idea of what that looks like). Much of this adoption for front-haul is coming from 5G evolution in the RAN as well as industry initiatives around standardization in the radio segment of the mobile network (for example, the O- RAN Alliance). See Chapter 4 for more information on the front-haul standardization efforts