Configuration Schematic

The principal difference between a mainframe and a PC or other server is the scale of the configuration, which may comprise hundreds of devices, all connected to the mainframe by fibre known as Channel Paths, each with their own identity known as a CHPID.

The Operating Systems (z/OS in this case) has an Input Output Supervisor (IOS) which only knows what devices are supported. It issues a Start Sub-Channel (SSCH) command on receipt of a READ/WRITE request from the application program.

The Channels Sub-system accepts the SSCH and determines the best route to the required device. There may be multiple routes both for performance and availability reasons.