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Systems Network Architecture (SNA) is IBM's proprietary networking architecture created in 1974. It is a complete protocol stack for interconnecting computers and their resources. SNA describes the protocol and is, in itself, not actually a program. The implementation of SNA takes the form of various communications packages, most notably VTAM which is the mainframe package for SNA communcations. SNA is still used extensively in banks and other financial transaction networks, as well as in many government agencies. While IBM is still providing support for SNA, one of the primary pieces of hardware, the 3745/3746 communications controller has been removed from market and will be dropped from support sometime after 2010. As a result most sites are working to remove the use of SNA from their networks and move to TCP/IP.
Advantages and Disadvantages
SNA removed link control from the application program and placed it in the Network Control Program. This had the following advantages and disadvantages:
Advantages
- Localization of problems in the telecommunications network was easier because a relatively small amount of software actually dealt with communication links. There was a single error reporting system.
- Adding communication capability to an application program was much easier because the formidable area of link control software which typically requires interrupt processors and software timers was relegated to system software and NCP.
Disadvantages
- Connection to non-SNA networks was difficult. An application which needed access to some communication scheme which was not supported in the current version of SNA faced obstacles. Before IBM included X.25 support (NPSI) in SNA, connecting to an X.25 network would have been awkward. Conversion between X.25 and SNA protocols could have been provided either by NCP software modifications or by an external protocol converter.
- At first glance, SNA networks appear to be very expensive in comparison to TCP/IP networks. For small networks, this may be true, but as the complexity of a large routed network grows, the SNA structure provides a cheaper path.
Logical Unit Types
SNA defines several kinds of devices, identifying each group with a Logical Unit grouping. LU0 provides for undefined devices, or build your own protocol. LU1 devices are printers. LU2 devices are dumb terminals. LU3 devices are printers using 3270 protocols. LU4 devices are batch terminals. LU5 has never been defined. LU6 provides for protocols between two applications. LU7 provides for sessions with 5250 terminals. The primary ones in use are LU1, LU2, and LU6.2 (an advanced protocol for application to application conversations).
Within SNA there are two types of datastream to connect local terminals and printers; there is the 3270 datastream mainly used by mainframes (zSeries family) and the 5250 datastream mainly used by minicomputers/servers such as the S/36, S/38, and AS/400 (nowadays called the iSeries).
Starting from version 5.2 of OS/400, SNA for client-access is no longer supported.
External links
- Cisco article on SNA
- APPN Implementers Workshop Architecture Document repository
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"Systems Network Architecture"