## Channel paths 

A channel path  provides a connection between the channel subsystem and control unites that allows the channel subsystem to communicate with I/O devices. Depending on the type of connections, a channel path might be a physical connection to a control unit with I/O devices, such as FICON, or an internal logical control unit, such as HiperSockets. 

Each channel path in a channel subsystem features a unique 2-digit hexadecimal identifier that is known as a channel-path identifier (CHPID), which ranges 00 - FF. Therefore, a total of 256 CHPIDs are supported by a CSS, and a maximum of 1536 CHPIDs are available on a z14 server with six logical channel subsystems. 

By assigning a CHPID to a physical port of an I/O feature adapter, such as FICON Express16S+, or a fanout adapter (ICA SR) port, the channel subsystem connects to the I/O devices through these physical ports. 

A port on an I/O feature card features a unique physical channel identifier (PCHID) according to the physical location of this I/O feature adapter, and the sequence of this port on the adapter. 

In addition, a port on a fanout adapter has a unique adapter identifier (AID), according to the physical location of this fanout adapter, and the sequence of this port on the adapter. 

A CHPID is assigned to a physical port by defining the corresponding PCHID or AID in the I/O configuration definitions. 

## Control units 

A control unit  provides the logical capabilities that are necessary to operate and control an I/O device. It adapts the characteristics of each device so that it can respond to the standard form of control that is provided by the CSS. 

A control unit can be housed separately or can be physically and logically integrated with the I/O device, channel subsystem, or within the Z server. 

## I/O devices 

An  I/O device  provides external storage, a means of communication between data-processing systems, or a means of communication between a system and its environment. In the simplest case, an I/O device is attached to one control unit and is accessible through one or more channel paths that are connected to the control unit. 

## 5.1.2 Multiple subchannel sets 

A subchannel set is a collection of subchannels within a channel subsystem. The maximum number of subchannels of a subchannel set determines how many I/O devices that a channel subsystem can access. This number also determines the number of addressable devices to the program (for example, an operating system) that is running in the LPAR. 

Each subchannel has a unique four-digit hexadecimal number 0x0000 - 0xFFFF. Therefore, a single subchannel set can address and access up to 64 K I/O devices. 

MSS was introduced in z9 to extend the maximum number of addressable I/O devices for a channel subsystem. 

As with the z13 server, the z14 support four subchannel sets for each logical channel subsystem. It can access a maximum of 255.74 K devices for a logical channel subsystem and a logical partition and the programs that are running on it. 

