The Open Systems Adapter is actually a network controller that you can install in a mainframe I/O cage. The adapter integrates several hardware features and supports many networking transport protocols. The OSA card is the strategic communications device for the mainframe architecture. It has several key features that distinguish it from CCW-based communications.

Effectively, the OSA integrates the control unit and device into the same hardware. It does so by placing it on a single card that directly connects to the central processor complex I/O bus.

There are three main versions of the Open Systems Adapter:

• The OSA-2
• The OSA-Express
• The OSA-Express2

The OSA-2 card is no longer available, but a significant number of installations still utilize them. The OSA-2 card is of interest here because it could only run using CCW-based operations. OSA-Express and OSA-Express2 cards utilized a much faster method of direct access called Queued Direct I/O (QDIO).

In addition, OSA-Express provides significant enhancements over the OSA-2 in function, connectivity, bandwidth, data throughput, network availability, reliability, and recovery. Meanwhile, the OSA-Express2 card represents the latest and most capable card in the OSA lineup.

Figure 1 shows the OSA-Express2 and OSA-Express features that are available on mainframe servers.

Figure 1. OSA-Express and OSA-Express2 connectivity

Only the OSA-Express2 card supports the OSC and OSN channel types. The following information uses the term "OSA-Express" to denote a function that both OSA-Express and OSA-Express2 can support.