The SAS disk connection architecture is a point-to-point architecture. This means that the controller can communicate with more than one device at once.
Disk shelves are connected to the controller on a daisy chain called a stack. Stacks have a head (the first disk shelf in the chain) and a tail (the last disk shelf in the chain). If connectivity is interrupted somewhere on a stack, devices behind the break also lose connectivity. However, devices on other stacks are still available.