b&w 3


syncronous vs. asyncronous operation



sync.

	   +---+     +---+     +---+
	   |   |     |   |     |
	---+   +-----+   +-----+



e.g traffic signals


10 sec on, 20 sec off

planned cycles








async.



	   +-+     +-+         +-+  +-+
	   | |     | |         | |  | |
	---+ +-----+ +---------+ +--+ +----

	   ^       ^           ^    ^
           |       |           |    |

	  water tank
	  empty indicator
   	  triggered



e.g refilling a water tank
	automatically



variable time periods



self-adjusting, subject to oscillation, loops etc. however.





in some cases either system can be used (e.g. traffic signals), in other cases

	one only may be practical


	examples

	1. launching a large rocket successfully (w. logic)

	2. navigating through a long complex journey to a destination (b. logic)


		example

		e.g. mthd 2. start with an approximate route and some essential supplies, 
			
			and adjust along the way


		     mthd 1. plan a route with 27 stages, arrive at stage 2, something unexpected

				happens and plan is no longer valid








