Systems Network Architecture

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Systems Network Architecture ( SNA ) is a network architecture that was developed by IBM in the 1970s and introduced in 1974 . SNA envisaged a hierarchical organization of the computer network, and the implementation required a mainframe and its peripherals intended for networks. This hierarchical network organization then controlled u. a. lots of "stupid" terminals . In contrast to the hierarchical organization of the SNA, there is, for example, the decentralized Internet protocol family .

The system software that implements this architecture is VTAM on the mainframe and Advanced Communication Function / Network Control Program (NCP) on the front end processors, as well as hard-wired screen and printer terminals.



The components in the SNA network are classified into network node types; In SNA terminology, these are called Physical Units (PUs) . These network nodes are linked to one another by a corresponding definition on the mainframe in such a way that connections (sessions) can be set up and cleared down between them.

The following physical unit types are defined:

  • PU Type 5, the central control point on the mainframe ( System Services Control Point )
  • PU Type 4, a front-end computer for line control
  • PU Type 3, which disappeared from the SNA documentation relatively early, was never implemented and was intended for the "intelligence" of the line.
  • PU Type 2, later expanded to PU Type 2.1, a control unit for the operation of screen and printer terminals
  • PU Type 1, a standalone screen or printer terminal. That guy is meaningless today.

The management area of ​​a PU Type 5 is called a "domain", the management area of ​​a PU Type 4 as a subset of the domain is called a "Subarea".

The physical units are only used to control connections. Communication with a user is not yet possible. Physical units of type 2 (.1) provide a further interface for this purpose; in SNA parlance this is called the Logical Unit (LU) . In the original approach of SNA (“classic SNA”, “Subarea-SNA”), a single logical unit could establish exactly one connection, an LU-LU session, to an application program on the mainframe at a specific point in time.


From a technical point of view, a logical unit is hardware (screen or printer) with a correspondingly implemented set of commands for controlling an SNA session through to formatted output on the end device, or an application program on a mainframe (PU_Type5). According to the implemented protocols there are different LU types. The most common are:

  • LU Type 2: a screen data terminal from the IBM 3270 product family
  • LU Type 1 or LU Type 3: a printer terminal
  • LU Type 0: freely defined format, preferably for communication between programs.

With the advent of intelligent hardware and the move away from “dumb terminals”, the functionality of logical units has increasingly been replaced by software solutions that emulated the original terminals . At the same time, the need arose to allow programs running peripherally to communicate directly with one another via the SNA network. The LU type 6.2, the interface for extended program-to-program communication ( APPC ) , which was previously only used between mainframe programs, was selected for this purpose . The main difference to LU Type 0 is that LU Type 6.2, in contrast to the freely definable format of LU Type 0, provides a standardized interface.

The LU type 6.2 also allows the establishment of parallel connections of this logical unit to one or more application programs, which can be located on a mainframe (PU type 5) or a peripheral node (PU type 2.1).


In contrast to the Internet protocol family commonly used in networks today, a number of services are already integrated as standard in SNA, such as the sending of documents and files (SNA Document Services (SNADS)) or the access of terminals and printers via other systems ( passthrough ).

Long before the OSI model for the organization of communication technology, SNA provided a self-contained architecture for a computer network.

Technical progress led to the introduction of the "new SNA" in 1985 : Advanced Peer-to-Peer Networking (APPN) removed the dependency on a central control point on the mainframe. Correspondingly powerful hardware could now establish and clear SNA connections directly. For this purpose, the physical unit of type 2.0 was expanded to include the necessary routing functionality, which the mainframe and front end computer had previously taken over, and was given the designation PU type 2.1.

In the meantime ( 2009 ) the front-end computers (PU 4) are history; the connection of VTAMs (PU 5) to one another and to PUs 2.1 is now made using the Internet protocol, more precisely through UDP tunnels (see tunnel (computer network) ). This technology has been practicable since around the year 2000, since OS / 390 Release 2.6 and is known under the name Enterprise Extender.


  • Werner Schäfer, Helmut an de Meulen: Systems Network Architecture. Addison-Wesley Verlag, Munich 2007, ISBN 978-0-201-56533-1 .
  • Hugo Schröer, Thomas Stalke: The SNA network architecture . Friedrich Vieweg & Sohn, Wiesbaden 1993, ISBN 978-3-322-91564-1 .
  • Carl A. Sunshine (Ed.): Computer Network Architectures and Protocols. Second Edition, Plenum Press, New York / London 1989, ISBN 978-1-4612-8093-4 .
  • Klaus Garbe: Management of computer networks. BG Teubner Verlag, Stuttgart 1991, ISBN 978-3-519-02418-7 .
  • Gurdeep S. Hura, Mukesh Singhal: Data and Computer Communications. Networking and Internetworking, CRC Press LLC, Boca Raton 2001, ISBN 0-8493-0928-X .

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