Brocade FICON Administrator's Guide v7.1.0 (53-1002753-01, March 2013)

FICON Administrator’s Guide 3
53-1002753-01
FICON concepts
1
Latency guideline
The maximum supported distance for a FICON channel is 300 Km (1.5 msec of delay).
Synchronous mirroring applications are generally limited to 100 Km (0.5 msec of delay). Greater
distances require that the FICON Acceleration feature be used with FCIP. The FICON Acceleration
feature emulates control unit response to the channel to make the devices appear closer to the
channel than they actually are.
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 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
(Dynamic Auto-Discovery). 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 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
microsecond duration 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: