72 IBM eServer zSeries 900 Technical Guide
3.1 Connectivity overview
Input/output (I/O) channels are components of the z900 server Channel Subsystem (CSS).
They provide a pipeline through which data is exchanged between processors, or between a
processor and external devices. The most common type of device attached to a channel is a
control unit (CU). The CU controls I/O devices such as disk and tape drives.
Server-to-server communications are most commonly implemented using Inter-System
Channels (ISC), Integrated Cluster Bus (ICB) channels, and channel-to-channel (CTC)
As part of your system planning activity, you will make decisions about where to locate your
equipment, how it will be operated and managed, what the business continuity requirements
are for disaster recovery, tape vaulting, and so on. The type of software, operating systems,
and application programs you intend to use must support the features and devices on the
3.1.1 Configuration planning
A connectivity configuration plan needs to cover physical and logical aspects. The required
physical resources must be available and the logical definitions must follow some rules.
The following physical resources are required for connectivity:
Proper I/O cage (zSeries or compatibility) on a frame
I/O slot on an I/O cage
STI-M cards for zSeries I/O cage
STI-H, FIBB and CHA cards for compatibility I/O cage
Proper channel card on an I/O cage
Port on a channel card
The z900 Configurator includes, for a server configuration, all physical resources required for
a given I/O configuration, based on the supplied connectivity requirements.
Once the physical resources are installed, the definition phase starts. The channels must be
defined according to the architecture rules and the architecture’s implementation limits. As an
example, the maximum number of channels and CHPIDs is 256.
There are also other definition limits imposed by the server’s implementation, such as the
maximum number of subchannels in the Hardware System Area (HSA). The current limits are
63k subchannels in Basic mode or per logical partition, and 512k subchannels in total.
There are some addressing limits imposed by the architecture and by its implementation.
These limits are channel type dependent and must be taken into account when defining an
The most-used channel types for connectivity to control units, and to other servers, are
ESCON and FICON channels. Table 3-1 shows the addressing rules for ESCON and FICON
Chapter 3. Connectivity 73
Table 3-1 Addressing limits for ESCON and FICON channels
The maximum architected number of control unit images (CUADD parameter of IOCP) per
CU on FICON channels (256) is much higher than on ESCON channels (16), but in both
channel types the current implemented limit is the same (16 CU images).
One of the most important benefits of FICON channels is the maximum number of Unit
Addresses (UAs) per channel, which is currently implemented up to 16K addresses. ESCON
channels implement up to 1K of UAs per channel.
The number of UAs per physical CU that can be addressable by a channel is limited to 4K on
FICON channels and 1K on ESCON channels. The maximum number of UAs per logical
control unit (CUADD) is limited to 256 addresses on both channel types.
The Input/Output Configuration Program (IOCP) uses as input your I/O configuration
definitions, and checks the configuration rules before creating the I/O Configuration Data Set
(IOCDS), which is used during the server’s Power-On-Reset to build the I/O control blocks of
There are also some addressing limits that are control unit dependent, and that may impede a
logical partition activation if an addressing limit is exceeded. Table 3-2 shows, as an example,
the IBM Enterprise Storage Server (ESS) disk system addressing limits.
Table 3-2 Control Unit addressing limits example
The channel operational characteristics are also very important when deciding what type of
channel to use. Table 3-3 shows some differences between ESCON and FICON channels.
ESCON channels FICON channels
(FC or FCP modes)
CU images (CUADD) / CU:
Architected 16 256
UAs supported / channel:
Architected 1M 16M
Implemented 1K 16K
UAs / physical CU:
Architected 4K 64K
CU implemented 4K 4K
Addressable by a channel 1K 4K
UAs / logical CU (CUADD) 256 256
IBM Enterprise Storage Server ESCON channels FICON channels
(FC or FCP modes)
Logical paths / CU port 64 256
Logical paths / LSS 128 128