s: gPTP defines performance characteristics (in Annex B), while 1588-2008 does not. There are requirements for the local (free-running) clocks, the time-aware systems (clocks), and the E2E performance. gPTP requires PTP transport with layer 2 (1588 Annex F), whereas 1588-2008 allows higher- layer transport (including IPv4/IPv6 in Annexes D and E). The 802.1AS document is addressed to an audience comfortable with 802 standards and covers PTP from that aspect (meaning it is not easy reading otherwise). gPTP defines PTP clocks as time-aware systems; ordinary clocks as time-aware "end stations"; and boundary clocks as time-aware "bridges." These time-aware nodes need to support the P2P delay mechanism and act much like the P2P TC from 1588-2008 (but participate in the BMCA). Therefore, with this P2P requirement, there is no possibility to mix unaware nodes into a timing distribution network. gPTP requires the use of two-step processing (with Follow_Up and the Follow_Up from the Pdelay_Resp), while 1588-2008 allows both one-step and two-step processing. The BMCA is the default BMCA from 1588-2008 with some small extensions. Ports, at the direction of the BMCA, can adopt one of four roles: Master: The port closest to the source of time from any downstream time-aware system Slave: The port closest to the root time-aware system Passive: Any port that is not master, slave, or disabled Disabled: A port that is disabled or not capable of 802.1AS processing Figure 7-15 illustrates the hierarchy of a timing network built using the BMCA from 802.1AS-2020 in combination with the Announce messages (a good way to think of it is that it is much like the Rapid Spanning-Tree Protocol). Grandmaster Time-aware bridge Time-aware Time-aware Time-aware bridge bridge bridge Time-aware Time-aware bridge bridge Master Slave Passive Disabled Time-aware Time-aware end station end station Figure 7-15 Example Hierarchy of Time-Aware Systems with Port States TSN becomes a topic again when discussing the timing requirements for the 5G radio acces