DDR3 Fly-by-topology and Write-leveling

The key is that the DQ/DQS signals are directly connected to each rank (or set of chips). So where the controls (clock, command, address, etc...) are connected in fly-by topology, the data and strobes are connected directly. This is why write-leveling is important. It indicates the skew between when the clock (and other control signals) arrive(s) and when the data (and strobes) arrive(s).

Also this is a nice explanation of rank and other stuff:
http://www.edaboard.com/ftopic342841.html