A Beginner's Guide to TN5250 Emulation

TN5250 emulation uses the telnet protocol to create a TCP/IP connection to a remote host. Initially, IBM 5250 terminals were sold with IBM's System/34 minicomputer system. Today, TN5250, or telnet 5250, is typically used to access an IBM AS400 or iSeries server. TN5250 emulation software installed on a PC allows that PC to emulate the IBM 5250 terminal and communicate with the TN5250 server. The emulator receives the 5250 data stream from the TN5250 server and displays it in a terminal emulation window. This window was often referred to as a "green screen" because of its resemblance to the black background and green text display of a 5250 terminal. TN5250 emulation gives IBM AS400 and iSeries users the ability to access their host systems securely and offers features such as host printing, macros and file transfer that surpass the capabilities of the historic IBM 5250 terminal.

The screen display consists of protected and unprotected fields. As the name implies, protected fields are fields that are protected by the host. Data entry is not allowed within these fields. Unprotected fields are the fields used for entering data that will be transmitted back to the server and updated accordingly.

One common 5250 display feature is the column separator. The column separator is a feature that improves readability by separating columns of data on the display. Many TN5250 terminal emulators provide the ability to configure the column separator as a solid or dotted line, or to turn off this feature completely.

Many modern TN5250 emulators provide innovative features that extend the emulator's capability beyond that of a traditional 5250 terminal. Features such as file transfer and macro support transcend simply displaying the 5250 data stream.

Many TN5250 users perform various repetitive tasks on a routine basis. These tasks could range from something as simple as logging on to a particular host application to more complex tasks such as performing a specific set of commands for large batches of data. Using a terminal emulator, macro scripts can be used to automate these tasks.

Security

On the server side, the security model for the IBM iSeries is object-based, which simplifies deployment and enhances the system's ability to resist viruses. The iSeries can not only detect intrusion, but has built-in methods to prevent it and provides an audit log to track potential security risks. Both production and backup data are encrypted, and SSL VPN is used to ensure the network is secure.

For securing communications between the server and the client, many TN5250 emulators use secure sockets layer (SSL). With SSL, TN5250 terminal emulation sessions are protected from eavesdropping, tampering, or message forgery over TCP/IP. This feature can be critical for those organizations that use TN5250 or TN5250E for remote access across the Internet to access sensitive data on corporate mainframes. The SSL protocol was developed by Netscape Communications Corporation to provide security and privacy over the Internet.

Transport layer security (TLS) is another cryptographic protocol that can be used to secure transmissions between the TN5250 server and client. TLS is based on SSL version 3, and the two protocols are decidedly similar. According to the Internet Engineering Task Force (IETF) 2246:

"The differences between TLS and SSL 3.0 are not dramatic, but they are significant enough that TLS 1.0 and SSL 3.0 do not interoperate (although TLS 1.0 does incorporate a mechanism by which a TLS implementation can back down to SSL 3.0)."

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