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ADVANTAGES AND DISADVANTAGES

SNA removed link control from the application program and placed it in the NCP. 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 that 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 data stream to connect local terminals and printers; there is the 3270 data stream mainly used by mainframes ( ZSeries family) and the 5250 data stream mainly used by minicomputers/servers such as the S/36, S/38, and AS/400 (now the ISeries ).

Starting from version 5.2 of OS/400 , SNA for Client-access is no longer supported.

The term 37xx refers to IBM's family of SNA communications controllers. The 3745 supports up to eight high-speed T1 circuits, the 3725 is a large-scale node and front-end Processor for a host, and the 3720 is a remote node that functions as a Concentrator and Router .


COMPETITORS

The proprietary networking architecture for Honeywell Bull mainframes is Distributed Systems Architecture (DSA). Communications package for DSA is TNVIP . Like SNA, DSA is also no more supported for client access. Bull mainframes are fitted with Mainway for translating DSA to TCP/IP and TNVIP devices are replaced by Terminal Emulations ( GLink , Winsurf ). GCOS 8 supports TNVIP SE over TCP/IP .


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