During the last several decades, before the Internet Protocol's rise in popularity, large organizations established their own SNA networks. These SNA networks were used to communicate between remote end-users and the centralized mainframe. The display management protocol used to facilitate this communication within an SNA environment was called the 3270 data stream. At the end user's location in an SNA network was a device referred to as a 3270 terminal.

A 3270 terminal was a non-programmable (sometimes called "dumb") workstation. Stated more simply, it was a display screen with a keyboard attached; see Figure 1. The 3270 terminal had only rudimentary communications capabilities and was text-based. One of the earliest model 3270 terminal displays (3278 model 1) consisted of 12 rows and 80 columns of text characters, no more and no less. Eventually, a 24 x 80 screen size became the standard, with some alternate sizes available.

To give the old 3270 terminal credit, it did support a selector pen and even a magnetic strip reader. The selector pen was light-based (optical) and it was used to select options on the text screen, similar to how a mouse is used--but of course, the 3270 terminal did not support a mouse.

The 3270 terminal, containing a non-programmable display and keyboard (see Figure 1), was usually connected to a control unit using coaxial cable (although over time other connection choices became available). The device type for these control units, up until the time the Internet Protocol began displacing SNA networks, was called a 3174 control unit.

The 3174 had some programmability, which allowed an expansion of the capabilities and connectivity options of a 3270 workstation, but it still was a long way from today's GUI workstations. 3270 display terminals were attached to the 3174 using ports, with up to 64 terminals capable of connecting to a single 3174. The 3174 control unit had other capabilities as well, including printer support. At the time of the 3174's demise, it had even expanded to include support for Ethernet LAN.

During the most recent decade, corporate networks started implementing IP as the transport protocol on their backbone. Because so many SNA and 3270 applications existed, they looked at integrating the SNA protocol into their IP backbone. The technology used to move from SNA 3270 applications to TCP/IP is called TN3270, short for Telnet 3270.

Figure 1. IBM 3270 Display Terminal