nce the switches are unaware of what PTP is. The second network, on the other hand, has every switch also func- tion as a boundary clock. A boundary clock can be thought of as a PTP slave and one or more masters all rolled into one. Rather than just transferring PTP packets as a switch does, it actually uses PTP itself to maintain a local clock. Whichever port the Grandmaster is attached to, either directly or through other boundary clocks or switches, acts as a PTP slave and recovers time in just the same way as a slave would. However, rather than using that time for some local function (such as synchronizing a cellular radio), it uses the time to become a local master on all its other ports-providing that time to slaves further down- stream (potentially through more boundary clocks or switches). This network is called a FTS network since every switch in it is also a PTP boundary clock-in other words, every switch understands and supports PTP. The final network is a cross between these two-a PTS net- work, in which some but not all of the switches are boundary clocks. So, what benefit does adding boundary clocks have? Well, remember that the biggest source of PDV is head of a line blocking happening to the PTP event messages as they pass through the switches in the network. However, if the switch is replaced by a boundary clock, no event messages actually pass through the switching function itself to get blocked. In other words, replacing a switch with a boundary clock eliminates the portion of overall PDV that would have come from that switch. Obviously, the more switches that are replaced by boundary clocks, the lower the overall PDV between the master and slave. It should now be clear why we said earlier that FTS is designed to provide the best time accuracy but over specifically architected networks, whereas PTS gives reduced accuracy, but over more general networks. The improved accuracy comes from virtually eliminating PDV altogether in an FTS network, but at the expense of needing to build the network wit