IBM Systems Network Architecture

Systems Network Architecture ( SNA) is a network architecture that was developed by IBM in the 1970s and introduced in 1974. SNA provided for a hierarchical organization of the computer network, and the implementation of set a mainframe along with the networks for certain peripherals ahead. This hierarchical network organization then headed among others many ' dumb ' terminals. In contrast to the hierarchical organization of the SNA there are, for example, decentralized Internet protocol family.

The system software that implements this architecture are VTAM on the mainframe and Advanced Communication Function / Network Control Program (NCP ) to the preprocessors ( 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 the SNA terminology, these are called Physical Units ( PUs ). These network nodes are linked together by such a definition on the mainframe that can be assembled and disassembled between them connections (sessions ).

Physical Unit following types are defined:

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

The administrative area of ​​a PU Type 5 is referred to as ' domain ', the administrative area of ​​a PU Type 4 as a subset of the domain is called ' subarea '.

The Physical Units are solely the control of connections. A communication with a user is hereby not given. For this purpose, provide Physical Units of type 2 ( .1 ) another interface available in the SNA parlance this is called Logical Unit ( LU). In the original approach of SNA ( ' classical SNA ', ' subarea SNA ') was able to build a single Logical Unit at any given time exactly one connection, an LU -LU session to an application program on the mainframe.


From a technical point of view, at a Logical Unit to hardware ( screen or printer) accordingly implemented supply of commands for controlling an SNA session through to the formatted output on the terminal. According to the implemented protocols, different LU types exist. The most common are:

  • LU Type 2 is a screen - data station from our family of IBM 3270
  • LU Type 1 or LU Type 3: a printer terminal
  • LU Type 0: free defined format, preferably for communication between programs.

With the advent of intelligent hardware and the renunciation of ' dumb terminals ' the functionality of Logical Units has been increasingly replaced by software solutions that emulated the original terminals. Similarly, the need arose to let programs communicate directly with each other via the SNA network. To this end, the new LU type 6.2 was introduced, the interface for Advanced Program -to- Program Communication (APPC ). The main difference for LU Type 0 was that APPC as opposed to free -definable format of the LU Type 0 provided a standardized interface.

The LU Type 6.2 also allowed for the first time the parallel structure of connections of a single logical unit to several application programs.


In contrast to today in networks commonly used Internet protocol suite a range of services are integrated as standard in SNA already, such as the transmission of documents and files ( SNA Document Services ( SNADS ) ) or the penetration of terminals and printers on other systems ( pass-through ).

Long before the OSI model for organizing the communication technology presented SNA already a self-contained architecture for a computer network is available.

The technical progress led in 1985 to the idea of ' new SNA ': Advanced Peer - to-Peer Networking sparked the function of a central control point on the mainframe to. According powerful hardware could now directly SNA connections to assemble and disassemble. For this purpose, the Physical Unit type 2.0 was to provide the required routing functionality that had been taken over mainframe and front end, extended and received the designation PU type 2.1.

In the meantime (2009) are the front-end (PU 4) history, the connection of VTAMs (PU 5) with each other, and with PUs 2.1, takes place today through the Internet protocol, accurate through the UDP tunnel (see Tunnel ( computer network ) ). This technology is known since about 2000, practicable for OS/390 Release 2.6 under the name Enterprise Extender.