FICON attachment overview for a zSeries or S/390 host

This section provides an overview of FICON channel attachment on a S/390 or zSeries host.

You can use either longwave or shortwave adapters with the IBM^® S/390^® or zSeries^® host systems. The adapter for the IBM S/390 has one channel port. The adapter for the zSeries has two or four channel ports.

You can attach the FICON™ channels directly to a storage unit or you can attach the FICON channels to a fibre-channel switch. When you attach the FICON channels directly to a storage unit, the maximum number of FICON attachments is limited by the number of installed FICON I/O ports on the storage unit. When you use a storage unit host adapter to attach to FICON channels either directly or through a switch, the I/O port is dedicated to FICON attachment. It cannot be simultaneously attached to SCSI hosts.

When you attach a storage unit to FICON channels through one or more switches, the maximum number of FICON attachments is 128 per storage unit I/O port. The directors provide very high availability with redundant components and no single points of failure or repair.

You can use the IBM 2042 Model 001 (CNT 64-port director) or IBM 2042 Model 128 (CNT 128-port director). You can use either director to attach fibre-channel hosts and devices in addition to the FICON hosts and devices. For these configurations, the fibre-channel hosts should communicate only with the fibre-channel devices. The FICON hosts should communicate only with the FICON devices. Set up zones in the directors to guarantee that none of the fibre-channel hosts or devices can affect the FICON traffic.

When you attach FICON products to switches or directors, you cannot use cascaded switches on S/390 host systems and on zSeries 800 and 900 model systems without licensed internal code (LIC) patches. You cannot configure a fabric of multiple interconnected directors and have a FICON channel that is attached to one director communicate to a FICON control unit that is attached to another director. The FICON architecture prohibits this capability. The reason for the restriction is because the base S/390 and zSeries I/O architecture uses a single byte for addressing the I/O devices. This one-byte I/O address is not compatible with the fibre-channel, 3-byte port address. The FICON solution to this problem on these host systems is to disallow switch cascading.

However, you can use cascaded switches on zSeries 800 and 900 model host systems with LIC patches installed and on all other zSeries models. To support cascaded switches, the fabric must be what is generally called a high integrity fabric. A high integrity fabric ensures data integrity by detecting any miscabling within a FICON cascaded switch fabric that could lead to I/O data being delivered to the wrong destination. A high integrity fabric is one that both supports and is configured to use fabric-binding and insistent domain IDs. Fabric-binding is the ability of the fabric to prevent another switch from being added operationally to the fabric without the new switch being properly planned for in the fabric and for the switch to be configured with the high integrity attributes. An insistent domain ID is a function of the fabric that does not allow the switch address to be automatically changed when a duplicate switch address is added to the fabric. Instead, fabrics that use insistent domain IDs require an operator's overt action to change a switch address. The customization of fabric-binding and the setting of an insistent domain ID are normally done only at switch installation time or reinstallation time.

Note: Switches that do not support high integrity fabrics can only be used in a single-switch FICON fabric.

With fibre-channel and FICON-intermix mode, both Fibre-Channel Protocol (FCP) and FICON upper-level protocols can be supported within the same director when deployed independently by port. (Director ports operate in either fibre-channel or FICON mode). For a list of supported switches see: http://www-1.ibm.com/servers/storage/support/san/.