* Software ([[operating system]] services and, usually, [[middleware]] such as [[IBM DB2|DB2]]).
The Coupling Facility may be either a dedicated external system (a small mainframe, such as a [[System z9]] BC, specially configured with only coupling facility processors) or integral processors on the mainframes themselves configured as ICFs (Internal Coupling Facilities).<ref>{{cite web |url=httphttps://www.pcmag.com/encyclopedia_term/0,2542,t=Coupling+Facility&i=40413,00.asp |title=Coupling Facility Definition |publisher=PC Magazine.com |accessdate=April 13, 2009 }}</ref> It is recommended that at least one external CF be used in a parallel sysplex.<ref>{{cite web |url=http://www-ti.informatik.uni-tuebingen.de/os390/sysplex/sysplex/couplfac.pdf |title=Coupling Facility |accessdate=April 13, 2009 }}</ref> It is recommended that a Parallel Sysplex has at least two CFs and/or ICFs for redundancy, especially in a production data sharing environment. Server Time Protocol (STP) replaced the Sysplex Timers beginning in 2005 for System z mainframe models z990 and newer.<ref>{{cite web |title=Migrate from a Sysplex Timer to STP |url=http://publib.boulder.ibm.com/infocenter/zos/v1r9/index.jsp?topic=/com.ibm.zos.r9.e0zm100/sttostp.htm |publisher=IBM |accessdate=April 15, 2009 }}</ref> A Sysplex Timer is a physically separate piece of hardware from the mainframe,<ref>{{cite web |title=Sysplex Timer |url=http://www.symmetricom.com/resources/compliance-certifications/sysplex-timer/ |publisher=Symmetricom |accessdate=April 15, 2009 }}</ref> whereas STP is an integral facility within the mainframe's microcode.<ref>{{cite web |title=IBM Server Time Protocol (STP) |url=http://www-03.ibm.com/systems/z/advantages/pso/stp.html |publisher=IBM |accessdate=April 15, 2009 }}</ref>
With STP and ICFs it is possible to construct a complete Parallel Sysplex installation with two connected mainframes. Moreover, a single mainframe can contain the internal equivalent of a complete physical Parallel Sysplex, useful for application testing and development purposes.<ref>{{cite web |url=http://www.zjournal.com/index.cfm?section=article&aid=308 |title=MVS Boot Camp: IBM Health Checker |first=John E. |last=Johnson |publisher=z/Journal |accessdate=April 15, 2009 }}</ref>
