The next level of clustering uses basic shared DASD, but also has two channel-to-channel (CTC) connections between the systems. This clustering technique is known as a CTC ring.

The ring aspect is more obvious when more than two systems are involved. Figure 1 illustrates this clustering technique.

Figure 1. Basic sysplex

z/OS® can use the CTC ring to pass control information among all systems in the ring. The information that can be passed this way includes:

• Usage and locking information for data sets on disks. This allows the system to automatically prevent unwanted duplicate access to data sets. This locking is based on JCL specifications provided for jobs sent to the system.
• Job queue information such that all the systems in the ring can accept jobs from a single input queue. Likewise, all systems can send printed output to a single output queue.
• Security controls that allow uniform security decisions across all systems.
• Disk metadata controls so that RESERVE and RELEASE disk commands are not necessary.

To a large extent, batch jobs and interactive users can run on any system in this configuration because all disk data sets can be accessed from any z/OS image. Jobs (and interactive users) can be assigned to whichever system is most lightly loaded at the time.

When the CTC configurations were first used, the basic control information shared was locking information. The z/OS component doing this is called global resource serialization function; this configuration is called a GRS ring. The primary limitation of a GRS ring is the latency involved in sending messages around the ring.

A different CTC configuration was used before the ring technique was developed. This required two CTC connections from every system to every other system in the configuration. When more than two or three systems were involved, this became complex and required a considerable number of channels.

The earlier CTC configurations (every-system-to-every-system or a ring configuration) were later developed into a basic sysplex configuration. This includes control data sets on the shared DASD. These are used for consistent operational specifications for all systems and to retain information over system restarts.

Configurations with shared DASD, CTC connections, and shared job queues are known as loosely coupled systems. Multiprocessors, where several processors are used by the operating system, are sometimes contrasted as tightly coupled systems but this terminology is seldom used. These are also known as Symmetrical MultiProcessors (SMPs); the SMP terminology is common with RISC systems, but is not normally used for mainframes.