FICON Administrator's Guide v6.4.0 (53-1001771-01, June 2010)

4 FICON Administrator’s Guide

53-1001771-01

FICON concepts

Figure 1 shows how the traffic in a switched point-to-point configuration flows in a FICON

environment. The logical path of the traffic is defined as frames moving from the channel to the

switch to the control unit. FICON traffic moves from an LPAR (logical partition) and through the

channel, through a Fibre Channel link to the switch through the control unit, and ending at the

device. This is also called a channel path, which is a single interface between a central processor

and one or more control units along which signals and data can be sent to perform I/O requests.

The channel path uses the logical path to traverse the Fibre Channel fabric. The channel path is

defined using an ID, called the channel path ID (CHPID). This information is stored in the

Input/Output Configuration Program (IOCP) so that the same path is used. The IOCP is a z/OS

program that defines to a system the channels, I/O devices, paths to the I/O devices, and the

addresses of the I/O devices. The output is normally written to a z/OS or zSeries Input/Output

Configuration Data Set (IOCDS). The data set in the z/OS and zSeries processor (in the support

element) contains an I/O configuration definition built by the IOCP Work Load Manager.

FIGURE 1 FICON traffic

The traffic on the channel path communicates using channel command words (CCWs) that direct

the device to perform device specific actions, such as Seek, Read, or Rewind. In a FICON

environment, CCWs use the buffer credit process for information unit (IU) pacing. IU pacing is a

mechanism that limits the number of CCWs, and therefore the number of IUs, that can either

transmit (write) or solicit (read) without the need for additional control-unit-generated

acknowledgements called command responses. There are times when there are no more buffer

credits to pass back to the other end and a frame pacing delay occurs. This is the number of

intervals of 2.5 microsecond duration that a frame had to wait to be transmitted due to a lack of

available buffer credits.

FICON introduces the following concepts:

• FICON Control Unit Port (CUP)

The internal port in a switch that assumes an FC address such that it is the FC DID used to

direct FICON traffic to the FICON Management Server.

• FICON Manager

Host communication includes control functions such as blocking and unblocking ports, as well

as monitoring and error-reporting functions.