CSS (Channel SubSystem)

The Channel SubSystem is a subsystem that is used on all mainframe-based operating systems (including z/OS, Linux on IBM Z and z/VSE) to perform the I/O processing that cannot be offloaded from the mainframe nad needs to be performed on the system itself.
Offloading I/O processing to the channel subsystem frees up normal processors to do other work. This subsystem will perform the work of communicating with the external equipment for the purposes of I/O.

The Channel subsystem performs processing on the System Assistance Processors (SAP) and does not use the normal processors or memory.

While this subsystem is performing I/O operations for a task, other tasks can be performed on the normal processors. When the I/O operation is complete, the subsystem will interrupt the other process and resume the control of the process that requested the I/O operation.

The channel subsystem connects to external systems with “channel cables”.
Modern mainframes rely on FICOŃ and IBM zHyperLink Express to do this.

• FICON
  • Introduced in 1998, uses fiber-optic cable, supports 16000 devices per channel with a transfer rate 8GBps+ with a maximum distance of 100km (60 miles)
• IBM zHyperLink Express
  • Designed to reduce the I/O latency and improve I/O throughput. Introduced with the z14 mainframe, provides direct, short distance connection of up to 150 meters between the mainframe and DS8800 (or newer) disk storage. Link data rate of 8GBps.

in reality a channel is more than just a cable, it is also the subsystem reposonsible for communication, together with the protocols, technology and other software that supports this communication

Types of FICON channels

There are two types of FICON channels:

• CCW channel
  • Used to connect with external equipment that is not a mainframe, such as a disk and tape subsystems, network switches and printers
• Coupling Channels
  • Those are used to connect z/OS systems together or to connect z/OS with a coupling facility.

Central Processor Complex (CPC)

Central Processor Complex (CPC) can contain several z/OS systems and coupling facilities. All of these use coupling channels for communication.
Within a CPC these channels can be internal with no fiber cables required. These Internal Couplings (IC) channels are faster than channels using external fiber connections.

Those can be done with ICF!

HCD and defining the devices

Before an external device can be used, it must be defined to both the channel subsystem and z/OS
the z/OS programmer uses a tool called Hardware configuration Dialog (HCD) to perform this definition. It is an ISPF application.