Channel paths Control units I/O devices 198  IBM z14 (3906) Technical Guide 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.  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. 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. 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. 5.1.2 Multiple subchannel sets