A memory system includes a controller, a first memory module, and a second memory module. The first memory module includes a first number of memory packages and a second number of memory packages. The second memory module includes a third number of memory packages and a fourth number of memory packages. The first and third numbers of memory packages are selected to correspond to a same rank based on control signals from the controller. The control signals are transmitted from the controller to the first and second memory modules through respective ones of a plurality of optical channels.