z15 can be configured with of a maximum of five CPC drawers that are designed as a field  replaceable unit (FRU). Connections among the CPC drawers are established through  symmetric multiprocessing (SMP) cables. Each CPC drawer consists of two logical PU  (processor units) clusters and contains four central processor (CP) single-chip modules  (SCMs) and one storage controller (SC) SCM. In addition to SCMs, CPC drawers host  memory DIMMs, connectors for I/O, oscillator interface, and manifolds. z15 inherits I/O infrastructure reliability improvements from z14, including Forward Error  Correction (FEC) technology that enables better recovery of FICON channels.  FICON Express16SA and FICON Express16S+ features continue to provide a new standard  for transmitting data over 16 Gbps links by using 64b/66b encoding.  The z15 air-cooled configuration includes a newly designed front-to-rear radiator cooling  system. The radiator pumps, blowers, controls, and sensors are N+2 redundant. In normal  operation, one active pump supports the system. A second pump is turned on and the original  pump is turned off periodically, which improves reliability of the pumps. The replacement of  pumps or blowers is concurrent with no affect on performance.  A water-cooling system is also an option with the z15, with water-cooling unit (WCU)  technology. Two redundant WCUs run with two independent chilled water feeds. One WCU  and one water feed can support the entire system load. The water-cooled configuration is  backed up by the rear door heat exchangers in the rare event of a problem with the chilled  water facilities of the customer. RAS also includes the following enhancements:  The z15 continues to support concurrent addition of resources, such as processors or I/O  cards, to an LPAR to achieve better serviceability. If another SAP is required on a z15 (for  example, as a result of a disaster recovery situation), the SAPs can be concurrently added to  the CPC configuration. It is possible to concurrently add CP, zIIP, IFL, and ICF processors to an LPAR. This function  is supported by z/VM, and (with appropriate PTFs) by z/OS and z/VSE. It is possible to add  concurrently memory to an LPAR as well. This feature is supported by z/OS and z/VM. The z15 supports adding Crypto-Express features to an LPAR dynamically by changing the  cryptographic information in the image profiles. Users can also dynamically delete or move  Crypto-Express features. This enhancement is supported by z/OS, z/VM, and Linux on IBM  Z. GLYPH<SM590000> Integrated sparing  GLYPH<SM590000> Error detection and recovery improvements in caches and memory  GLYPH<SM590000> 25GbE RoCE Express2 GLYPH<SM590000> PCIe coupling links GLYPH<SM590000> Enhanced channel logging GLYPH<SM590000> A fixed HSA with its size increased to 256 GB on the z15 GLYPH<SM590000> OSA firmware changes to increase the capability of concurrent maintenance change level  (MCL) updates GLYPH<SM590000> System power cycle GLYPH<SM590000> CFCC level 24 GLYPH<SM590000> IBM RMF reporting Chapter 4. IBM z15 system design strengths  69