IBM SNA Interoperability Concepts
IBM SNA Interoperability Concepts |
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The creation of an SNA network requires the installation of specific networking software on SNA hardware components. Network software programs are available for each of the three primary hardware components: mainframes, FEPs, and cluster controllers. The combination of software and hardware installed in a device on an SNA network is referred to as a physical unit (PU), as shown in Figure A.3.
Figure A.3 A PU Is a Combination of Software and Hardware
In the preceding figure, note that the PU is the component that represents the hardware device (node) to the SNA network. Each PU type is identified by a number, with the highest number representing the top of the SNA hierarchy.
Note
The concept of a physical unit (PU) can be confusing. Please keep in mind that a PU is not an actual hardware device; it is a combination of software and hardware that represents a device to the SNA network.
Table A.3 identifies the PU types, the hardware component with which each PU type is associated, and typical IBM model numbers for each PU type.
Table A.3 PU Types
| PU Type | Hardware Component | Model Number |
|---|---|---|
| PU 5 | Mainframe (type 5 host node) | Type S/370, S/390 |
| PU 4 | FEP (type 4 node or communications controller) | IBM 3745, 3720 |
| PU 2 | Cluster controller (type 2 node) | IBM 3174, 3274 |
Note
PU type 3 has not been implemented. An additional PU type, PU 2.1 is identified in the section that describes peer-to-peer SNA networks, later in this appendix.
The following sections outline the software options for each hardware component, or node, and the resulting PU.
Virtual Telecommunications Access Method (VTAM) is the IBM mainframe program that controls communications between mainframe applications and the terminals and computers that connect to the mainframe.
VTAM contains the system services control point (SSCP), which is the networking focal point for a hierarchical network. The SSCP activates, controls, and deactivates network resources, such as FEPs, cluster controllers, terminals, and printers. The SSCP also monitors and logs the status of SNA components.
With the activation of VTAM and the SSCP, the mainframe becomes a PU 5.
Traditionally, VTAM and the SSCP communicate directly with the next lower level in the hierarchy, a PU 4 FEP. In an increasing number of cases however, the PU 5 communicates directly with a PU 2 that is either channel-attached or connected with an Open System Adapter.
The front-end processor (FEP), also called a communications controller or PU 4, runs a communications management program called Advanced Communication Function/Network Control Program (ACF/NCP). The FEP uses this software to manage the routing and communications on a hierarchical SNA network, offloading these tasks from the mainframe system. ACF/NCP is configured on the mainframe and then downloaded to the FEP. The FEP, as a PU 4, supports bi-directional communications with the next lower level in the hierarchy.
The cluster controller, or PU 2, contains administration software called the Configuration Support program, which controls the connections from the cluster controller to terminals and printers.
Note
A computer running SNA Server can be used in place of a traditional SNA cluster controller.
