Editing Systems Network Architecture (section)

==Principal components and technologies==
Improvements in computer component technology made it feasible to build terminals that included more powerful communications cards which could operate a single standard [[communications protocol]] rather than a very stripped-down protocol which suited only a specific type of terminal. As a result, several [[list of network protocols|multi-layer communications protocols]] were proposed in the 1970s, of which IBM's SNA and [[ITU-T]]'s [[X.25]] became dominant later.

The most important elements of SNA include:
*[[IBM Network Control Program]] (NCP)<ref>{{cite web |publisher=Microsoft |title=SNA Functional Layers |website=[[Microsoft Docs]] |date=September 11, 2008 |url=https://learn.microsoft.com/en-us/previous-versions/windows/it-pro/windows-2000-server/cc977037(v=technet.10) |access-date=September 16, 2022}}</ref><ref>{{cite journal |author=W. S. Hobgood |title=The role of the Network Control Program in Systems Network Architecture |journal=[[IBM Systems Journal]] |year=1976 |volume=15 |issue=1 |pages=39–52 |doi=10.1147/sj.151.0039 |url=http://www.research.ibm.com/journal/sj/151/ibmsj1501E.pdf |access-date=August 26, 2006 |url-status=dead |archive-url=https://web.archive.org/web/20070316233256/http://www.research.ibm.com/journal/sj/151/ibmsj1501E.pdf |archive-date=March 16, 2007}}</ref> is a communications program running on the [[IBM 3705|3705]] and subsequent [[IBM 37xx|37xx]] communications processors that, among other things, implements the packet switching protocol defined by SNA. The protocol performed two main functions:
**It is a packet forwarding protocol, acting like modern [[switch]] - forwarding data packages to the next node, which might be a mainframe, a terminal or another 3705. The communications processors supported only hierarchical networks with a mainframe at the center, unlike modern routers which support [[peer-to-peer]] networks in which a machine at the end of the line can be both a [[client (computing)|client]] and a [[server (computing)|server]] at the same time.
**It is a multiplexer that connected multiple terminals into one communication line to the CPU, thus relieved the constraints on the maximum number of communication lines per CPU. A 3705 could support a larger number of lines (352 initially) but only counted as one peripheral by the CPUs and channels. Since the launch of SNA IBM has introduced improved communications processors, of which the latest is the [[IBM 3745|3745]].
*[[Synchronous Data Link Control]]<ref>{{cite manual |title=Synchronous Data Link Control Concepts |edition=Fifth |id=GA27-3093-4 |publisher=IBM |date=May 1992 |url=https://bitsavers.org/pdf/ibm/datacomm/GA27-3093-4_SDLC_Concepts_May1992.pdf}}</ref> (SDLC), a protocol which greatly improved the efficiency of data transfer over a single link:<ref>{{harv|Pooch|Greene|Moss|1983|p=310}}.</ref>
**It is a [[sliding window protocol]], which enables terminals and 3705 communications processors to send frames of data one after the other without waiting for an acknowledgement of the previous frame – the communications cards had sufficient memory and processing capacity to remember the last 7 frames sent or received, request re-transmission of only those frames which contained errors, and slot the re-transmitted frames into the right place in the sequence before forwarding them to the next stage.
**These frames all had the same type of envelope (frame header and trailer)<ref>{{harv|Pooch|Greene|Moss|1983|p=313}}.</ref> which contained enough information for data packages from different types of terminal to be sent along the same communications line, leaving the mainframe to deal with any differences in the formatting of the content or in the rules governing dialogs with different types of terminal.
:Remote terminals (e.g., those connected to the mainframe by telephone lines) and 3705 communications processors would have SDLC-capable communications cards.
:This is the precursor of the packet communication that eventually evolved into today's TCP/IP technology{{citation needed|date=November 2011}}. SDLC itself evolved into [[HDLC]],<ref>{{harv|Friend|Fike|Baker|Bellamy|1988|p=191}}.</ref> one of the base technologies for dedicated telecommunication circuits.
*[[VTAM]],<ref>{{cite news |last=Frank |first=Ronald A |title=IBM Delays Second Virtual TP Release; SD:C Impact Expected |newspaper=[[Computerworld]] |date=October 17, 1973 |url=https://books.google.com/books?id=pWBoOXVjuZ0C&dq=vtam+announced+intitle%3Acomputerworld&pg=PP1 |access-date=June 30, 2020}}</ref><ref>{{cite manual |publisher=IBM |title=Introduction to VTAM |id=GC27-6987-5 |date=April 1976 |url=https://bitsavers.org/pdf/ibm/sna/vtam/GC27-6987-5_Introduction_to_VTAM_Apr76.pdf}}</ref> a software package to provide log-in, session keeping, and routing services within the mainframe. A terminal user would log-in via VTAM to a specific application or application environment (e.g. [[CICS]], [[IBM Information Management System|IMS]], [[IBM DB2|DB2]], or [[Time Sharing Option|TSO]]/[[ISPF]]). A VTAM device would then route data from that terminal to the appropriate application or application environment until the user logged out and possibly logged into another application. The original versions of IBM hardware could only keep one session per terminal. In the 1980s further software (mainly from third-party vendors) made it possible for a terminal to have simultaneous sessions with different applications or application environments.