A Base Sysplex configuration is a sysplex with no Coupling Facilities. The Base Sysplex can be composed of one or more z/OS systems that have an XCF sysplex name and in which the authorized programs (members) use XCF services. XCF services are available in both single and multisystem environments. A multisystem environment is defined as two or more z/OS systems residing on one or more CPCs’ logical partitions connected through CTCs.
A Base Sysplex is the first step to implementing a Parallel Sysplex. A Parallel Sysplex is a Base Sysplex plus the use of the Coupling Facility. So, when you introduce the coupling facility, XCF exploits the coupling facility, using it as a link between z/OS systems.
The communication link between XCFs could be through channel-to-channel adapters (CTCs) that allow data movement between XCF buffers in the systems through an I/O operation. Another option for linking is a CF list structure.
CF – Coupling Facility
CPC – Central Processor Complex
CTC – Channel-To-Channel
XCF – Cross-system Coupling Facility