Specifications

Chapter 1. Introduction 9
The communication path between a channel and a control unit is composed of two different
parts, the physical channel path and the logical path.
In a cascaded FICON Director topology, the physical paths are the FC links that provide the
physical transmission path between a FICON channel and a control unit.
A FICON (FC-SB-2) logical path is the relationship established between a FICON channel
image and a control unit image for communication during execution of an I/O operation and
presentation of status.
High-integrity data path
Another important value of FICON support of cascaded Directors is its ability to provide
high-integrity data paths. The high integrity function is an integral component of the FICON
architecture when configuring FICON channel paths through a cascaded fabric.
To support the introduction of FICON cascaded switching, IBM has worked with McDATA to
ensure robustness in the channel-to-control unit path is maintained to the same high standard
of error detection, recovery, and data integrity that has existed for many years with both
ESCON and the initial implementation of FICON.
End-to-end data integrity is designed to be maintained through the cascaded Director fabric.
Data integrity ensures any changes to the customer's data streams are always detected, and
the data frames (data streams) are delivered to the correct end point; an end point being a
FICON channel port or a FICON Control Unit port. For FICON channels, Cyclical
Redundancy Checking (CRC) and Longitudinal Redundancy Checking (LRC) are bit patterns
added to the customer data streams to allow for detection of any bit changes in the data
stream. With FICON support of cascaded switching, new integrity features are introduced
within the FICON channel and the FICON cascaded switch fabric to ensure detection and
reporting of any miscabling actions occurring within the fabric during operational use that may
cause a frame to be delivered to the wrong end point.
A FICON channel, when configured to operate with a cascaded switch fabric, requires the
switch fabric supports high integrity. During initialization, the FICON channel queries the
switch fabric to determine that it supports high integrity. If it does, the channel completes the
initialization process, allowing the channel to operate with the fabric.
Once a FICON switched fabric is customized to support FICON cascaded Directors and the
required World-Wide Node Name (WWNN) and Domain IDs have been added in the fabric
membership list, the Director checks that its Inter-Switch Links (ISLs) are attached to the
correct Director before they are made operational. If an accidental cable swap occurs, the
Director invokes logical path testing, reporting, isolation, and recovery. The high-integrity
fabric feature for cascaded FICON Directors protects against mis-cabling and mis-directing of
data streams.
High-integrity fabric architecture support includes:
򐂰 Fabric binding support
The ability of the fabric to prevent a switch from being added to the fabric that is not
configured to support the “high integrity fabric”; for example, all switches must be defined
to all other switches via a fabric membership list.
򐂰 Insistent Domain IDs support
Note: The cascaded FICON Director configuration is only supported by the zSeries (z800
and z900) servers.