Systems Network Architecture

Applications, databases, and communication functions were mingled into the same protocol or product, which made it difficult to maintain and manage.

An expansion of interactive terminal-based systems would increase sales of terminals and more importantly of mainframe computers and peripherals - partly because of the simple increase in the volume of work done by the systems and partly because interactive processing requires more computing power per transaction than batch processing.

Hence SNA aimed to reduce the main non-computer costs and other difficulties in operating large networks using earlier communications protocols.

This had the following advantages and disadvantages: SNA at its core was designed with the ability to wrap different layers of connections with a blanket of security.

But eventually it serves a purpose to make a distinction between LU types, as the application must take the functionality of the terminal equipment into account (e.g. screen sizes and layout).

Within SNA there are three types of data stream to connect local display terminals and printers; there is SNA Character String (SCS), used for LU1 terminals and for logging on to an SNA network with Unformatted System Services (USS), there is the 3270 data stream mainly used by mainframes such as the System/370 and successors, including the zSeries family, and the 5250 data stream mainly used by minicomputers/servers such as the System/34, System/36, System/38, and AS/400 and its successors, including System i and IBM Power Systems running IBM i. SNA defines several kinds of devices, called Logical Unit types:[25] The primary ones in use are LU1, LU2, and LU6.2 (an advanced protocol for application to application conversations).

As mainframe-based entities looked for alternatives to their 37XX-based networks, IBM partnered with Cisco in the mid-1990s and together they developed Data Link Switching, or DLSw.

The actual encapsulation and decapsulation takes place in Cisco routers at each end of a DLSw peer connection.

Bull mainframes are fitted with Mainway for translating DSA to TCP/IP and VIP devices are replaced by TNVIP Terminal Emulations (GLink, Winsurf).

Both were largely obsolete by 2012. International Computers Limited (ICL) provided its Information Processing Architecture (IPA).

DECnet[28][29][30] is a suite of network protocols created by Digital Equipment Corporation, originally released in 1975 to connect two PDP-11 minicomputers.

SNA also competed with ISO's Open Systems Interconnection, which was an attempt to create a vendor-neutral network architecture that failed due to the problems of "design by committee".

[citation needed] The TCP/IP suite for many years was not considered a serious alternative by IBM, due in part to the lack of control over the intellectual property.

[citation needed] The 1988 publication of RFC 1041, authored by Yakov Rekhter, which defines an option to run IBM 3270 sessions over Telnet, explicitly recognizes the customer demand for interoperability in the data center.

[34] Third-party SNA software packages for VMS, such as the VAX Link products from Systems Strategies, Inc.,[34] were also available.