 "Chapter 8. ITU-T Timing Recommendations", for more details of how the ITU-T recommendations are structured. G.8265.1-Telecom Profile for Frequency Synchronization This recommendation defines a profile intended to be used to carry frequency across a packet net- work and where it is not an option to do that with SyncE. As shown in Figure 7-13, this profile ap- plies where the network elements have no support for PTP in any intermediate node between the PTP master and the PTP slave. Of course, given that limitation, using this profile over such a net- work does not guarantee that the performance requirements of a given application will be met. The main features of this profile are as follows: Domain number should be within a range of 4-23 with a default value of 4. Unicast UDP over IPv4 (Annex D of IEEE 1588-2008), and optionally PTP over IPv6 (Annex E of 1588-2008), can be used to transport the PTP messages. Message rates for Sync, Delay_Req, and Delay_Resp are from a minimum of 1 packet per 16 seconds to 128 per second. The message rate for Announce is from a minimum of 1 packet per 16 seconds to 8 per second with a default of 1 every 2 seconds. Unicast message negotiation is used-a mechanism allowing slaves to request timing service from a master in a unicast environment (see the earlier section "Negotiation" in this chapter and clause 16.1 of IEEE 1588-2008). The default interval to request service is 300 seconds, with a range of 60-1000 seconds. Defines an alternate BMCA (aBMCA), with the main feature being that the clock state is al- ways static, SO a master is always a master and a slave is always a slave-there is no dynamic changing of the port state between master and slave. One-step and two-step clocks are both allowed, with the master port free to use whatever method it prefers, and the slave required to accept either. One-way operation to recover frequency is allowed, although some implementations continue to use a two-way system (see the section "One-Way Versus Two-Way PTP" earlier in this chapter). P