TN3270E description

A TN3270E client uses the TN3270E protocol to access the resources on a TN3270E server. However, the TN3270E client cannot complete the connection all the way to the target application because the TN3270E client communicates according to the TN3270E protocol, while the target application expects communication to be SNA protocol.

Figure 1 shows how the communication ultimately reaches the target application (in this example, TSO--but any SNA application can be targeted).

Figure 1. TN3270E protocol connection

In effect, the TN3270E server is nothing more than a protocol converter: on one side it maintains an RFC 2355-compliant TN3270E session; on the other side, it emulates a 3270 data stream terminal (including the 3174 control unit) to VTAM. The target application cannot tell the difference between a genuine 3270-attached terminal and a TN3270E server-emulated terminal.

Note: The TN3270E server is primarily a protocol converter (from a TN3270E to SNA and back again). However, the TN3270E server does intercept and alter some 3270 data stream flows. It can also alter the sequence of SNA flows. Consequently, it is more than just a converter.

Among the many capabilities of TN3270E, one is the ability of the TN3270E client specifically to request an LU to be used as the terminal LU. This level of control allows more control, from an SNA point of view.

TN3270E session setup

The characteristics of the TN3270E connection are negotiated during the start of the connection. For example, the client can request an LU name, a 3270 function can be negotiated, and, perhaps most significantly, the client must choose a 3270 device type.

Device types: There was more than one model of 3270 display terminal. IBM 3278 models 1 through 5 were the most common 3270 display terminals. TN3270E supports models beginning with 3278-2 through 3279-5.

TN3270E also supports the IBM-3287-1 printer device type.

The primary significance of device type selection is the number or rows and columns available in the TN3270E client window.

TN3270E printing

The TN3270E server has more than just 3270 terminal display capabilities. It can also support the SNA print data stream. Substituting "printer LU" for "terminal LU" through some SNA session-initiation differences, a print data stream can be emulated by the TN3270E server.

This means that an SNA application can direct a print data stream to an SNA printer LU as it always has. If that printer LU is a TN3270E-owned printer LU, then the TN3270E server accepts the print data stream from the application and forwards it to the TN3270E client running on the workstation. The workstation can then print the data using normal workstation printer facilities.

TN3270E allows the terminal session LU to identify implicitly the printer session LU that should be used. This is done using the ASSOCIATE command during the TN3270E printer connection setup. If an ASSOCIATE command is sent, then the TN3270E server selects a printer LU based upon the terminal LU to which the client is already connected. Obviously, the TN3270E server must be set up with a one-to-one mapping of terminal LUs-to-printer LUs.