Interoperability with IBM Host Systems

Downstream Connections

In hierarchical SNA environments, you configure 3270 communications between SNA nodes using SNA protocols. Usually those nodes are SNA Server–based computers and mainframes. A downstream system, however, is an SNA node that uses SNA Server as a PU gateway, as shown in Figure 10.12. To the downstream system, the SNA Server–based computer appears to be the actual mainframe providing the PUs and 3270 LUs.

Enlarge figure

Figure 10.12 Downstream Connections Using SNA Server

A downstream system in this type of environment must be a PU 2 device. For example, an IBM 3745 cluster controller can be a downstream device. A client running a terminal emulator that emulates a PU 2 and acquires LU sessions from the SNA Server–based computer can also be a downstream device.

On the computer running SNA Server, two connections are required to support downstream systems:

A host connection between the server and the mainframe. This can be any standard physical connection method supported by the mainframe.
A downstream connection between the server and the downstream system. This physical connection can be an 802.2/DLC, SDLC, or X.25 connection.

Once configured, SNA Server can manage the downstream LUs in a manner similar to how it manages other LUs, including assigning them to LU pools.

This type of configuration is useful in environments where the downstream system is unable to communicate directly with the mainframe because of hardware or network incompatibilities that the intermediate SNA Server–based computer can resolve.

Using SNA Server as a PU concentrator can also help reduce host configuration requirements. LUs from one or more PUs can be shared with multiple downstream devices, alleviating the need to configure each downstream device in VTAM on the mainframe system. The result is a more efficient use of host resources.

For more information about communications with hierarchical SNA networks, see the SNA Server version 4.0 documentation.