I'm trying to learn how communication works in a butterfly network.
The following is a butterfly network of order 3. I'm given the following description:
Each processor is connected to an output switch in rank 0 and to an input switch in rank k (in this case 3). In other words the switch in column j and rank 0 gets its input message from processor j and the switch in column j and rank k delivers its message to processor j. Messages only travel in one direction - towards switches of increasing rank.
I'm having trouble interpreting what it means by the input and output switches. At first I thought the messages were traveling from bottom to top, but the last statement makes it sound like it's traveling from top to bottom. Is the processor j mentioned found on top in rank 0? I'm trying to grasp how messages travel from one processor to another.