12.2 System components 12.2.1 Enclosure midplane This section describes each of the components that make up the IBM Storwize V5000 Gen2  system. The components are described in terms of location, function, and serviceability. The  enclosure midplane  connects the node or expansion canisters to the power supply units  and to the drives. The midplane is part of the enclosure midplane assembly, which consists of  the midplane and the front section of the enclosure. During the basic system configuration, vital product data (VPD) is written to the enclosure  midplane. On a control enclosure midplane, the VPD contains information, such as worldwide  node name (WWNN) 1, WWNN 2, machine type and model, machine part number, and serial  number. On an expansion enclosure midplane, the VPD contains information, such as  machine type and model, machine part number, and serial number. The enclosure midplane is initially generic and it is configured as a control enclosure  midplane or expansion enclosure midplane only when the VPD is written. After the VPD is  written, a control enclosure midplane is no longer interchangeable with an expansion  enclosure midplane and vice versa. Important:  The enclosure midplane must be replaced by a trained IBM service support  representative (SSR) only.  For more information about the midplane replacement process, see IBM Storwize V5000  Gen2 Knowledge Center. For a complete overview of maintenance tasks, see IBM Knowledge Center. Two node canister slots are on the top of the unit. The left slot is canister 1, and the right slot  is canister 2. Figure 12-1 shows the rear view of a fully equipped control enclosure. 12.2.2 Node canisters Figure 12-1 Rear view of a control enclosure with two node canisters (the Storwize V5020) Chapter 12. RAS, monitoring, and troubleshooting  625