Old vs New, TCP and SNA

We continue to look at standards, old and new, that have played a role in the evolution of terminal emulation. This articles discusses SNA and TCP and their roles in network communications.

TCP and SNA

TCP/IP is of course the name for a suite of communications protocols which forms the foundation of internet and network traffic. It's something which most computer savvy individuals are familiar with, or have at least heard of.

Before the emergence of TCP/IP as the dominant networking protocols alternatives were numerous and vendors like DEC (DECnet), Apple (AppleTalk) and Novell (IPX/SPX) offered competing and proprietary products. Amongst the many offerings of rival mainframe and hardware vendors IBM's propriety Systems Network Architecture (SNA) was a particularly prominent networking protocol stack. In a story familiar to the technologies of the era SNA was a proprietary protocol and compatible only with IBM hardware – however given that IBM dwarfed it's competitors in that particular market, it was arguably the most important. SNA was first made available to IBM's system System/370 mainframe systems in the mid 1970's. The technology was also deployed on IBM's subsequent midrange and server systems running OS/400 and AIX.

As a consequence the terminals used to connect to these systems and their early descendants did so via SNA. These were the 5250 and 3270 family of terminals, amongst the most successful and widely deployed terminals ever made. They continue to play an important role in the enterprise today through Windows terminal emulators like TTWin 4.

The information technology industry has, historically, gone through a period of standardization and consolidation in terms of standards and protocols, either through acceptance or attrition. However before TCP/IP emerged as the most important suite of networking protocols there was a period of overlap where there were several solutions offered to get SNA devices and TCP/IP devices to 'play nice' together within one network.

One such solution were SNA gateways, software and hardware which allowed TCP/IP based networks such as Windows desktops running terminal emulators and SNA devices such as IBM mainframes to operate on the same network. Microsoft SNA Server is and remains a popular implementation.

Other hardware solutions such as pc expansion cards (for example the IRMA board) and hybrid PCs produced by IBM were utilized to connect to SNA networks and provide 3270 terminal emulation.

In terms of terminal emulation it was the development of the TN3270 and TN5250 (telnet) based protocols which were crucial as it allowed both 3270 and 5250 sessions over TCP rather than SNA – contributing to the retirement of all kinds of hardware and software such as those mentioned above. These days you'll find that the majority of modern windows terminal emulation software emulating 3270 and 5250 terminals will use the telnet based protocol to make a host connections.

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