SNA Server provides 3270 connectivity through 3270 logical units (LUs). A 3270 LU is known as a dependent LU, because it requires a mainframe to function. Each 3270 LU defined within SNA Server is configured to use an existing connection to the mainframe system and corresponds to a matching LU resource allocated on the host computer, usually specified within VTAM. The 3270 LU definition in SNA Server is identified by a number that matches the number of the corresponding LU resource on the mainframe, and by a user-specified name.
The 3270 LU is further classified by the type of service provided over the connection. Like PUs, LU types are designated by numbers. For example, 3270 display data streams are known as LU 2. A 3270 LU can be configured as one of the following:
SNA Server client software is installed on each client PC. The client software manages communications between a 3270 application and the SNA Server computer. Applications designed to use the SNA Server client API use the logical units defined within SNA Server to establish a communications link from the client PC to the mainframe by means of SNA Server (see Figure 5.1). The link between the LU definition in SNA Server and the host LU resource is called a session. Sessions can be permanent and automatically started during initialization, or established on an as-needed basis. Concurrent sessions can share the same physical devices and communications links.
Figure 5.1 3270 Connectivity using SNA Server.