Guide to TN3270 Emulation


TN3270 emulation is a communications standard that allows a remote terminal such as a Windows workstation to communicate with an IBM or IBM-compatible mainframe. Originally, 3270 display terminals, also known as “dumb terminals”, were used to connect to mainframes. TN3270 emulation replaces the use of dumb terminals for accessing mainframe applications.

3270 display terminals used a standard cipher set when interacting with a host system. This collection of codes or escape sequences, were used to perform basic screen display functions such as moving the cursor. Once PC’s became commonplace, it became apparent that users needed to be able to access the data and applications housed on mainframes from their desktop instead of using an additional display terminal. Software developers began developing TN3270 emulation software to fill this need. Using a TN3270 terminal emulator allowed PC’s to emulate display terminals, eliminating the need for an additional machine to access critical business applications and data.

PC’s are capable of processing data whereas display terminals were not. Consequently, TN3270 emulation provided extended capabilities that served to increase productivity. TN3270 emulation made advanced features such as host printing, file transfer and scripting possible.

RFC 1576 from the Internet Engineering Task Force (IETF), TN3270 Best Practices, outlines the sending, receiving and processing of the 3270 data stream and available 3270 telnet commands. TN3270E incorporates a set of enhancements to the TN3270 specification and is a superset of RFC 1576. TN3270E is referenced in the IETF RFC 2355 TN3270 Enhancements Specification. TN3270E addresses critical issues such as:

IBM® 3287 LU 1 and LU 3 host printing

Specific and associated LU’s

Standardized Attention and System Request keys

With TN3270E emulation, host printing is available through Data Stream Control (LU Type 3) printers and SNA Character String (LU Type 1 SCS) printers. The terminal emulator manages the host printer data streams using response requests and sequence numbering to control the rate and sequence of data transfer between the PC, mainframe and printer.

When a terminal emulator attempts to connect to the mainframe, the connection is assigned an LU or device name that is configured the host side. TN3270 emulation only supports the use of generic LU’s. With generic LU’s, the session established with the TN3270 server comes from an unnamed pool of LU’s. With generic LU pools, it is possible to run out of LUs. Also, it is likely that users will not be connected to the same LU on subsequent connection attempts.

With TN3270E, sessions can be configured to connect to a specific LU. This helps administrators ensure that their users will always be able to connect to LU’s of a particular type such as display LU’s or printer LU’s.

IETF RFC 1576 did not include a standards specification for the attention (ATTN) and system request keys. As a result, telnet handled these keys different than IBM. The ATTN key sends a signal command to the mainframe, and is used as a break key to interrupt the mainframe’s execution of the current program. Some session manager programs also use this key to switch between sessions. The system request key switches host connections from a LU-LU session to a SSCP-LU session. The use of two dissimilar methods for handling these keys resulted in functional inconsistencies. TN3270E addresses this issue by providing a standard method for implementing ATTN and system request keys.

Industry analysts estimate that up to 80% of the world’s data resides on mainframes. Using TN3270 emulation, users can access this data quickly and efficiently from their desktop PC’s.

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