Controllers (CTLs)
Upgrading from a two-controller (2N) VSP 5100 to a four-controller (4N) VSP 5500 requires adding two additional controller blades, doubling the total from two to four to deliver the higher I/O and cache capacity of the 5500.
Interconnect Switches (ISWs)
The VSP 5500 introduces external PCIe "node interconnect switches" - 1 U switch chassis that provide the high-speed fabric and management links between controller pairs. These ISW modules must be added to interconnect the expanded controller set in a 4-node configuration.
Channel Boards (CHBs)
Each new controller requires its own set of front-end channel boards for host connectivity. When you add two controllers, you also add their associated CHBs (FC, iSCSI, FICON, etc.) to furnish the additional host ports on the VSP 5500.