Compaq GS160 - Manual (Page 75)

GS80 Rack System Overview 4-5
The switch that interconnects the CPU modules, memory modules, and I/O riser
modules is built into the system drawer backplane.
In a two-drawer system, the system drawers are linked together through the
global ports and the distribution board. A directory module is required in each
system drawer in a two-drawer system.