The OSN type is only available with OSA-Express2 and requires a z9 mainframe or later model. The primary intention of this type is to free organizations from the constraints of obsolete hardware: device types 3745 and 3746. The 374x device types, as they are called, are no longer manufactured or sold by IBM. A 374x host is required to run the Network Control Program (NCP). NCP is a significant functional component of subarea type SNA networks (more information about SNA networks is covered in the topic on Systems Network Architecture basics and implementation).

The OSN channel type allows an Open Systems Adapter to communicate with an NCP using Channel Data Link Control protocol (CDLC). CDLC cannot be used over an OSD or OSE channel type, and even with channel type OSN it can only communicate to other LPARs within the CPC. Historically, 374x devices were often connected to a parallel or ESCON channels, which support CDLC.

Where, then, will the NCP run? On a software program called Communications Controller for Linux (CCL). And, as mentioned, both LPARs must be within the same CPC, since the data flows do not enter the network.

In many cases, CCL provides the easiest way to migrate from older SNA-based network controllers to modern network devices. The CCL functional capabilities provide alternatives where either no viable solution existed before (XRF, for example) or where prior alternatives (such as SNI) posed significant implementation challenges.

Figure 1 shows a connectivity example. In this example, a channel type of OSE is used to communicate with CCL using LLC 802.2. With the OSN channel type, the communication would not flow out to the switch. Instead, CDLC would flow from LPAR to LPAR within the OSA-Express2 card itself.

Figure 1. CCL network connectivity