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

FICON Administrator’s Guide 3

53-1002753-01

FICON concepts

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: