Systems Network Architecture
Systems Network Architecture[1] (SNA) is IBM's proprietary networking architecture, created in 1974.[2] It is a complete protocol stack for interconnecting computers and their resources. SNA describes formats and protocols but, in itself, is not a piece of software. The implementation of SNA takes the form of various communications packages, most notably Virtual Telecommunications Access Method (VTAM), the mainframe software package for SNA communications.
History
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SNA was made public as part of IBM's "Advanced Function for Communications" announcement in September, 1974,[3] which included the implementation of the SNA/SDLC (Synchronous Data Link Control) protocols on new communications products:
- IBM 3767 communication terminal (printer)
- IBM 3770data communication system
They were supported by
SNA was designed in the era when the computer industry had not fully adopted the concept of layered communication. Applications, databases, and communication functions were mingled into the same protocol or product, which made it difficult to maintain and manage.[5][6] SNA was mainly designed by the IBM Systems Development Division laboratory in Research Triangle Park, North Carolina, USA,[7] helped by other laboratories that implemented SNA/SDLC. IBM later made the details public in its System Reference Library manuals and IBM Systems Journal.
It is still used extensively in banks and other financial transaction networks, as well as in many government agencies. In 1999 there were an estimated 3,500 companies "with 11,000 SNA mainframes."[8] One of the primary pieces of hardware, the 3745/3746 communications controller, has been withdrawn[a] from the market by IBM. IBM continues to provide hardware maintenance service and microcode features to support users. A robust market of smaller companies continues to provide the 3745/3746, features, parts, and service. VTAM is also supported by IBM, as is the NCP required by the 3745/3746 controllers.
In 2008 an IBM publication said:
with the popularity and growth of TCP/IP, SNA is changing from being a true network architecture to being what could be termed an "application and application access architecture." In other words, there are many applications that still need to communicate in SNA, but the required SNA protocols are carried over the network by IP.[9]
Objectives of SNA
 | This section includes a improve this section by introducing more precise citations. (September 2022) ) |
IBM in the mid-1970s saw itself mainly as a hardware vendor and hence all its innovations in that period aimed to increase hardware sales. SNA's objective was to reduce the costs of operating large numbers of terminals and thus induce customers to develop or expand
Hence SNA aimed to reduce the main non-computer costs and other difficulties in operating large networks using earlier communications protocols. The difficulties included:
- Often a communications line could not be shared by terminals of different types, as they used different "dialects" of the existing communications protocols. Up to the early 1970s, computer components were so expensive and bulky that it was not feasible to include all-purpose communications interface cards in terminals. Every type of terminal had a hard-wiredcommunications card which supported only the operation of one type of terminal without compatibility with other types of terminals on the same line.
- The protocols which the primitive communications cards could handle were not efficient. Each communications line used more time transmitting data than modern lines do.
- Telecommunications lines at the time were of much lower quality. For example, it was almost impossible to run a dial-up line at more than 19,200 bits per second because of the overwhelming error rate, as compared with 56,000 bits per second today on dial-up lines; and in the early 1970s few leased lines were run at more than 2400 bits per second (these low speeds are a consequence of Shannon's Law in a relatively low-technology environment).
As a result, running a large number of terminals required a lot more communications lines than the number required today, especially if different types of terminals needed to be supported, or the users wanted to use different types of applications (.e.g. under CICS or TSO) from the same location. In purely financial terms SNA's objectives were to increase customers' spending on terminal-based systems and at the same time to increase IBM's share of that spending, mainly at the expense of the telecommunications companies.
SNA also aimed to overcome a limitation of the architecture which IBM's
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
The most important elements of SNA include:
- 3705 and subsequent 37xxcommunications 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 and a 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 3745.
- Synchronous Data Link Control[13] (SDLC), a protocol which greatly improved the efficiency of data transfer over a single link:[14]
- 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)[15] 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[HDLC,[16]one of the base technologies for dedicated telecommunication circuits.
- ). 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.
Advantages and disadvantages
 | This article contains a pro and con list. (November 2012) |
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.
- With the advent of Advanced Program-to-Program Communication (APPC) routing tables that explicitly defined endpoint to endpoint connectivity. APPN sessions would route to endpoints through other allowed node types until it found the destination. This is similar to the way that routers for the Internet Protocol and the Netware Internetwork Packet Exchange protocol function. (APPN is also sometimes referred to PU2.1 or Physical Unit 2.1. APPC, also sometime referred to LU6.2 or Logical Unit 6.2, was the only protocol defined to APPN networks, but was originally one of many protocols supported by VTAM/NCP, along with LU0, LU1, LU2 (3270 Terminal), and LU3. APPC was primarily used between CICS environments, as well as database services, because it contact protocols for 2-phase commit processing). Physical Units were PU5 (VTAM), PU4 (37xx), PU2 (Cluster Controller). A PU5 was the most capable and considered the primary on all communication. Other PU devices requested a connection from the PU5 and the PU5 could establish the connection or not. The other PU types could only be secondary to the PU5. A PU2.1 added the ability to a PU2.1 to connect to another PU2.1 in a peer-to-peer environment.[19])
Disadvantages
- Connection to non-SNA networks was difficult. An application that needed access to some communication scheme not supported in the current version of SNA would have 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.
- A sheaf of alternate pathways between every pair of nodes in a network had to be predesigned and stored centrally. Choice among these pathways by SNA was rigid and did not take advantage of current link loads for optimum speed.
- SNA network installation and maintenance are complicated and SNA network products are (or were) expensive. Attempts to reduce SNA network complexity by adding TCP/IPwas still seen as unfit for commercial applications e.g. in the finance industry until the late 1980s, but rapidly took over in the 1990s due to its peer-to-peer networking and packet communication technology.
- SNA's connection based architecture invoked huge state machine logic to keep track of everything. APPN added a new dimension to state logic with its concept of differing node types. While it was solid when everything was running correctly, there was still a need for manual intervention. Simple things like watching the Control Point sessions had to be done manually. APPN wasn't without issues; in the early days many shops abandoned it due to issues found in APPN support. Over time, however, many of the issues were worked out but not before TCP/IP became increasingly popular in the early 1990s, which marked the beginning of the end for SNA.
Security
SNA at its core was designed with the ability to wrap different layers of connections with a blanket of security. To communicate within an SNA environment you would first have to connect to a node and establish and maintain a link connection into the network. You then have to negotiate a proper session and then handle the flows within the session itself. At each level there are different security controls that can govern the connections and protect the session information.[20]
Network Addressable Units
Network Addressable Units in a SNA network are any components that can be assigned an address and can send and receive information. They are distinguished further as follows:[21]
- a System Services Control Point (SSCP) provides resource management and other session services (such as directory services) for users in a subarea network;[22]
- a Physical Unit is a combination of hardware and software components that control the links to other nodes.[23]
- a Logical Unit acts as the intermediary between the user and the network.[24]
Logical Unit (LU)
SNA essentially offers transparent communication: equipment specifics that do not impose any constraints onto LU-LU communication. But eventually it serves a purpose to make a distinction between LU types, as the application must take the functionality of the terminal equipment into account (e.g. screen sizes and layout).
Within SNA there are three types of data stream to connect local display terminals and printers; there is SNA Character String (SCS), used for LU1 terminals and for logging on to an SNA network with Unformatted System Services (USS), there is the
SNA defines several kinds of devices, called Logical Unit types:[25]
- LU0 provides for undefined devices, or build your own protocol. This is also used for non-SNA 3270 devices supported by TCAM or VTAM.
- LU1 devices are printers or combinations of keyboards and printers.
- LU2 devices are IBM 3270 display 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 IBM 5250 terminals.
The primary ones in use are LU1, LU2, and
Physical Unit (PU)
- PU1 nodes are terminal controllers such as IBM 6670 or IBM 3767
- PU2 nodes are cluster controllers running configuration support programs such as IBM 3174, IBM 3274, or the IBM 4701 or IBM 4702 Branch Controller
- PU2.1 nodes are peer-to-peer (APPN) nodes
- PU3 was never defined
- PU4 nodes are front-end processors running the Network Control Program (NCP) such as the IBM 37xx series
- PU5 nodes are host computer systems[26]
The term
SNA over Token-Ring
VTAM/NCP PU4 nodes attached to IBM Token Ring networks can share the same Local Area Network infrastructure with workstations and servers. NCP encapsulates SNA packets into Token-Ring frames, allowing sessions to flow over a Token-Ring network. The actual encapsulation and decapsulation takes place in the 3745.
SNA over IP
As mainframe-based entities looked for alternatives to their 37XX-based networks, IBM partnered with
Competitors
The proprietary networking architecture for
The networking architecture for
SNA also competed with
The TCP/IP suite for many years was not considered a serious alternative by IBM, due in part to the lack of control over the intellectual property.[
Non-IBM SNA implementations
Non-IBM SNA software allowed systems other than IBM's to communicate with IBM's mainframes and
Some Unix system vendors, such as Sun Microsystems with its SunLink SNA product line, including PU2.1 Server,[31] and Hewlett-Packard/Hewlett Packard Enterprise, with their SNAplus2 product,[32] provided SNA software.
Digital Equipment Corporation had VMS/SNA for VMS.[34] Third-party SNA software packages for VMS, such as the VAX Link products from Systems Strategies, Inc.,[34] were also available.
Hewlett-Packard offered SNA Server and SNA Access for its HP 3000 systems.[35]
Brixton Systems developed several SNA software packages, sold under the name "Brixton",[36][37][38] such as Brixton BrxPU21, BrxPU5, BrxLU62, and BrxAPPC, for systems such as workstations from Hewlett-Packard,[39] and Sun Microsystems.[40]
IBM supported using several non-IBM software implementations of APPC/PU2.1/LU6.2 to communicate with z/OS, including SNAplus2 for systems from HP,[41] Brixton 4.1 SNA for Sun Solaris,[42] and SunLink SNA 9.1 Support for Sun Solaris.[43]
See also
- Network Data Mover
- Protocol Wars
- TN3270
- TN5250
Explanatory notes
- ^ However, the 3745 simulator Communications Controller for Linux (CCL) is still available.
Notes
- ^ Peter H. Lewis (May 14, 1989). "A Link for All Operating Systems". The New York Times. Retrieved September 15, 2022.
- ^ (Schatt 1991, p. 227).
- ^ IBM Corporation. "IBM Highlights, 1970-1984" (PDF). IBM. Retrieved April 19, 2019.
- ^ IBM 3770 Family Batch Communications Terminal (PDF) (Report). Datapro.
and the 3790/3760 data entry/data communications ...
- ^ "Bridge your legacy systems to the Web". Datamation.
- ^ "Fujitsu Net Architecture". Computerworld. November 15, 1976. p. 99.
- ISBN 3-540-17707-8
- Informationweek. May 12, 1999. Retrieved September 16, 2022.
- ^ Networking on z/OS (PDF). IBM Corporation. 2010. p. 31.
"Networking on z/OS (web document)". IBM Corporation. - ^ devices that acted as DMA controllers for control units, which in turn attached peripherals such as tape and disk drives, printers, card-readers
- ^ "SNA Functional Layers". Microsoft Docs. Microsoft. September 11, 2008. Retrieved September 16, 2022.
- doi:10.1147/sj.151.0039. Archived from the original(PDF) on March 16, 2007. Retrieved August 26, 2006.
- ^ Synchronous Data Link Control Concepts (PDF) (Fifth ed.). IBM. May 1992. GA27-3093-4.
- ^ (Pooch, Greene & Moss 1983, p. 310).
- ^ (Pooch, Greene & Moss 1983, p. 313).
- ^ (Friend et al. 1988, p. 191).
- ^ Frank, Ronald A (October 17, 1973). "IBM Delays Second Virtual TP Release; SD:C Impact Expected". Computerworld. Retrieved June 30, 2020.
- ^ Introduction to VTAM (PDF). IBM. April 1976. GC27-6987-5.
- ^ IBM Systems Network Architecture and APPN PU2.1 References Guides
- ISBN 978-0738440941. Retrieved April 23, 2019.
- ^ Basic SNA terms and concepts
- ^ "z/OS Communications Server: SNA Network Implementation Guide (6)". IBM Knowledge Center. IBM Corporation. Retrieved October 3, 2015.
- ^ "z/OS Communications Server: SNA Network Implementation Guide (11)". IBM Knowledge Center. IBM Corporation. September 11, 2014. Retrieved October 3, 2015.
- ^ "z/OS Communications Server: SNA Network Implementation Guide (12)". IBM Knowledge Center. IBM Corporation. September 11, 2014. Retrieved October 3, 2015.
- ^ (Schatt 1991, p. 229).
- ^ Microsoft. "Physical Unit (PU)". Retrieved September 7, 2012.
- ^ "Distributed Systems Architecture".
- Hardcopy. pp. 62–65.
- ^ "DECnet for Linux". SourceForge. Archived from the original on October 4, 2009. Retrieved June 26, 2018.
- ^ "Networking Products Introduced by Digital". The New York Times. August 24, 1988.
- ^ SunLink SNA 9.1 PU2.1 Server Configuration Guide (PDF). Sun Microsystems. 1997.
- ^ "HP-UX SNAplus2 Software - Overview". HPE support.
- ^ Willett, Shawn; Wilson, Jayne (November 22, 1993). "Microsoft, Novell, IBM target host-to-LAN links". InfoWorld. Vol. 15, no. 47. p. 39.
- ^ Network World. p. 28.
VMS/SNA, software that runs under VMS in conjunction with a synchronous board, in a VAX configured with a BIbus, makes a single VAX appear as a PU 2 node.
- ^ "Software offerings accompany Spectrum announcement". Computerworld. Vol. 20, no. 9. March 3, 1986. p. 10.
HP also unveiled IBM connection capabilities with Systems Network Architecture (SNA) Server and Server Access software.
- ^ Brixton SNA Server - Red Hat Certified Software
- Network World. July 31, 1995.
- ^ Brixton PU2.1 SNA Server, retrieved September 14, 2022
- Network World. Vol. 10, no. 48. p. 15.
- ^ Orrange, Kate (March 9, 1992). "Brixton expands IBM/Sun linkup". InfoWorld. Vol. 14, no. 10. p. 41.
- ^ "HP SNAplus2 Configuration Requirements". IBM.
- ^ "Brixton 4.1 SNA for Sun Solaris Requirements". IBM.
- ^ "Configuring SunLink SNA 9.1 Support for Sun Solaris". IBM.
References
- Friend, George E.; Fike, John L.; Baker, H. Charles; Bellamy, John C. (1988). Understanding Data Communications (2nd ed.). Indianapolis: Howard W. Sams & Company. ISBN 0-672-27270-9.
- Pooch, Udo W.; Greene, William H.; Moss, Gary G. (1983). Telecommunications and Networking. Boston: Little, Brown and Company. ISBN 0-316-71498-4.
- Schatt, Stan (1991). Linking LANs: A Micro Manager's Guide. McGraw-Hill. ISBN 0-8306-3755-9.
- Systems Network Architecture General Information (PDF). First Edition. IBM. January 1975. GA27-3102-0.
- Systems Network Architecture Concepts and Products (PDF). Second Edition. IBM. February 1984. GC30-3072-1.
- Systems Network Architecture Technical Overview. Fifth Edition. IBM. January 1994. GC30-3073-04.
- Systems Network Architecture Guide to SNA Publications. Third Edition. IBM. July 1994. GC30-3438-02.
External links
- Cisco article on SNA
- APPN Implementers Workshop Architecture Document repository
- SNA protocols quite technical
- Related whitepapers sdsusa.com
- Advanced Function for Communications System Summary (PDF). Second Edition. IBM. July 1975. GA27-3099-1. Retrieved May 22, 2014.
- Systems Network Architecture Formats. Twenty-first Edition. IBM. March 2004. GA27-3136-20.
- Systems Network Architecture - Sessions Between Logical Units (PDF). Third Edition. IBM. April 1981. GC20-1868-2.
- Systems Network Architecture - Introduction to Sessions between Logical Units (PDF). Third Edition. IBM. December 1979. GC20-1869-2.
- Systems Network Architecture Format and Protocol Reference Manual: Architectural Logic (PDF). Third Edition. IBM. November 1980. SY20-3112-2.
- Systems Network Architecture: Transaction Programmer's Reference Manual for LU Type 6.2. Sixth Edition. IBM. June 1993. GC30-3084-05.
- Systems Network Architecture Type 2.1 Node Reference. Fifth Edition. IBM. December 1996. SC30-3422-04.
- Systems Network Architecture LU 6.2 Reference: Peer Protocols. Third Edition. IBM. October 1996. SC31-6808-02.
