Chapter 1. Introduction 7
channel when the channel is connected to an FC switch with the ability to use the domain and
port address portion of the 24-bit N_Port address (8 bits for the domain and 8 bits for the port)
to access multiple control units.
Figure 1-3 Switched point-to-point configuration
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 FICON switched point-to-point topology (with a single switch), the physical paths are the
FC links, or an interconnection of two FC links through an FC switch, that provide the physical
transmission path between a channel and a control unit.
A FICON (FC-SB-2) logical path is the relationship established between a channel image and
a control unit image for communication during execution of an I/O operation and presentation
of status.
1.3.2 Cascaded FICON Director configuration
In a cascaded FICON Director connection, at least three Fibre Channel (FC) links are needed
in the channel-control unit path. One is between the FICON channel card (N_Port) and the
FICON Director port (F_Port), then internally within the switch (through the backplane) to
another port (E_Port) that connects to the second FICON Director E_Port through the second
FC link, and then to a FICON adapter card in the control unit (N_Port) through the third FC
link. With this configuration, the connection between sites can consist of multiple FC links,
also known as Inter-Switch Links (ISLs).
An example of cascaded FICON Directors is shown in Figure 1-4 on page 8.
Multiple channel images and multiple control unit images can share resources of the Fibre
Channel link and Fibre Channel switches, such that multiplexed I/O operations can be
performed.
8 Getting Started with the IBM 2109 M12 FICON Director
Channels and control unit links can be attached to the Fibre Channel switches in any
combination, depending on configuration requirements and on available switch ports.
Sharing a control unit through a Fibre Channel switch means that communication from a
number of channels to the control unit can take place either over one switch-to-CU link (in the
case where a control unit has only one link to the Fibre Channel switch), or over multiple link
interfaces (in the case where a control unit has more than one link to the Fibre Channel
switch).
Just one Fibre Channel link is attached to a FICON channel in a cascaded FICON Director
configuration. However, from the FC switch (FICON Director), the FICON channel can
communicate with a number of FICON CUs on different ports of the second FC switch. Once
at the control unit, the same control unit and device addressing capability exists as for a
point-to-point configuration. However, the communication and addressing capability is greatly
increased for the channel when connected to an FC switch with the ability to use the domain
and port address portion of the 24-bit N_Port address (8 bits for the domain and 8 bits for the
port) to access multiple control units. Note that the domain address portion of the FC 24-bit
port address is different because there are two FC switches in the channel-to-control unit
path.
Figure 1-4 Cascaded FICON Director configuration
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, interconnected
by the FC switches, 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.

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