Figure 4-3 z14 ZR1 I/O connectivity The PCIe slots in a drawer are organized into two I/O domains. Each I/O domain supports up  to eight features and is driven through a PCIe switch card. Two PCIe switch cards always  provide a backup path for each other through the passive connection in the PCIe+ I/O drawer  backplane. During a PCIe fanout card or cable failure, 16 I/O cards in two domains can be  driven through a single PCIe switch card.  The two switch cards are interconnected through the PCIe+ I/O drawer board (Redundant I/O  Interconnect, or RII). In addition, switch cards in same PCIe+ I/O drawer are connected to  PCIe fanouts across clusters in CPC drawer for higher availability.  The RII design provides a failover capability during a PCIe fanout card failure. Both domains  in one of these PCIe+ I/O drawers are activated with two fanouts. The flexible service  processors (FSPs) are used for system control. The domains and their related I/O slots are shown in Figure 4-2 on page 123. Each I/O domain supports up to eight features (FICON, OSA, Crypto, and so on.) All I/O  cards connect to the PCIe switch card through the backplane board. The I/O domains and  slots are listed in Table 4-1 on page 125. 124  IBM 14 Model ZR1 Technical Guide