Channel subsystem (CSS)

The heart of moving data into and out of a mainframe host is the channel subsystem, or CSS. The CSS is, from a central processor standpoint, independent of the processors of the mainframe host itself. This means that input/output (I/O) within a mainframe host can be done asynchronously.

When an I/O operation is required, the CSS is passed the request from the main processor. While awaiting completion of an I/O request, the main processor is able to continue processing other work. This is a critical requirement in a system designed to handle massive numbers of concurrent transactions.

Note: The processors that run the channel subsystem are called the system assist processors (SAP). There can be more than one SAP running the channel subsystem.

All LPARs within the central processor complex can make use of the channel subsystem.

Remember: A logical partition, or LPAR, is an independent subset of a CPC. Operating systems and applications running within an LPAR cannot distinguish the LPAR resources (memory, processors and devices) from those of a dedicated CPC. Effectively, an LPAR is a seamless division of a CPC into multiple simulated CPCs.

A simplified example of how the channel subsystem functionally resides within a central processor complex is shown in Figure 1. In this diagram, the large box represents an entire mainframe processor (CPC).

The asynchronous I/O is handled within the channel subsystem by a channel program. Each LPAR ultimately communicates using a subchannel. In addition, the channel subsystem can be used to communicate between LPARs.

Each CPC has a channel subsystem. Its role is to control communication of internal and external channels to control units and devices.

Figure 1. I/O and the mainframe

The channels permit transfer of data between main storage and I/O devices or other servers under the control of a channel program. Some of the other components in Figure 1 are described as follows:

Logical Partition

Within the central processor complex (CPC) are logical partitions that divide the CPC into independent machines that can run any mainframe architecture system control program (for example, z/OS, Linux, or z/VM). Partitions have access to CPC memory and subchannels.

Subchannel

The subchannel represents an I/O device. This is the mechanism by which an I/O request is passed (identified) to the channel subsystem itself.

Channel

The channel, represented by a channel path ID or CHPID, represents the actual communication path. A CHPID is the handle by which communication between the CPC and an external device is facilitated.

A CHPID must be unique, since it denotes a unique path of communication for the CPC. The maximum number of allowable CHPIDs within a channel subsystem is 256. Channels can be shared between LPARs.

Historically, a CHPID had a correspondence with a real physical channel connected to the CPC. However, for performance and enhanced capabilities, a CHPID now maps to a physical CHPID (PCHID) using a simple mapping table and a CHPID mapping tool, or CMT.

Control units

One of the main tasks of the channel subsystem is to communicate with storage devices such as tape and direct access storage devices (DASD). This is facilitated by a control unit (which is not shown in Figure 1). Although this is a significant aspect of the channel subsystem, this will not be discussed within this information since it is not a network device.

Logical channel subsystem (LCSS)

To facilitate the usage of more CHPIDs, the mainframe architecture supports a logical channel subsystem, or LCSS. The LCSS is functionally identical to the channel subsystem, but up to four LCSSs can be defined within a central processor complex. CHPIDs are unique within the LCSS only; consequently, the 256 CHPID limitation can be overcome.