Brocade FICON Administrator’s Guide (Supporting Fabric OS v7.3.0) - Manual (Page 15)

FICON concepts
The following figure 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 a logical partition (LPAR) and through the channel, through
a Fibre Channel link to the switch through the control unit, and ends 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 Definition File (IODF) and may be
dynamically configured using the mainframe feature, zDAC (Discover and Auto Configuration). The
IODF is typically built using the hardware configuration definition (HCD).
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,
buffer credits are used at the Fibre Channel Protocol (FCP) level for flow control between optically
adjacent ports, while information unit (IU) pacing is the flow control mechanism used by the channel.
There are times when there are no more buffer credits to pass back to the other end of the link and a
frame pacing delay occurs. Frame pacing delay is the number of intervals of 2.5 microseconds that a
frame had to wait to be transmitted due to a lack of available buffer credits. Frame pacing delay
information is reported in the FICON Director Activity Report with the System z RMF feature.
FICON introduces the following concepts:
• FICON Control Unit Port (CUP)
The internal port in a switch that assumes an Fibre Channel (FC) address such that it is the FC
domain ID (DID) used to direct FICON traffic to the FICON Management Server (FMS).
• FICON Manager
Host communication includes control functions such as blocking and unblocking ports, as well as
monitoring and error-reporting functions.
• Hardware configuration definition (HCD)
HCD is an IBM interactive interface application that allows you to define the hardware configuration
for both the processor channel subsystem and the operating system running on the processor.
• Information unit
FICON concepts
FICON Administrator's Guide 13
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