  We solve the Firing Squad Synchronization Problem (FSSP), for P systems based
on digraphs with simplex channels, where communication is restricted by the
direction of structural arcs. Previous work on FSSP for P systems focused
exclusively on P systems with duplex channels, where communication between
parents and children is bidirectional. Our P solution, the first for simplex
channels, requires cell IDs, strongly connected digraphs and some awareness of
the local topology (such as each cell's outdegree)---we argue that these
requirements are necessary. Compared to the known solutions for cellular
automata, our solution is substantially simpler and faster.
