Systems Network Architecture
Encyclopedia
Systems Network Architecture (SNA
SNA
SNA, Sna, or sna may refer to:In economics:* System of National Accounts, a conceptual framework that sets the international statistical standard for the measurement of the market economy* United Nations System of National Accounts...

) is IBM
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...

's proprietary networking
Computer network
A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....

 architecture created in 1974. It is a complete protocol stack
Protocol stack
The protocol stack is an implementation of a computer networking protocol suite. The terms are often used interchangeably. Strictly speaking, the suite is the definition of the protocols, and the stack is the software implementation of them....

 for interconnecting computer
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...

s 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 Virtual telecommunications access method (VTAM
VTAM
Virtual Telecommunications Access Method is IBM's software package that provides communications via telecommunication devices for mainframe environments. It is the implementation of Systems Network Architecture for mainframes...

) which is the mainframe
Mainframe computer
Mainframes are powerful computers used primarily by corporate and governmental organizations for critical applications, bulk data processing such as census, industry and consumer statistics, enterprise resource planning, and financial transaction processing.The term originally referred to the...

 package for SNA communications. 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
IBM 3745
The IBM 3745 is the latest and last of a 37xx family of communications controllers for the IBM mainframe environment. As of mid-2009 there were an estimated 7,000+ of the larger 3745 models still in active production status, down from 20,000 or more in 2007...

/3746 communications controller has been withdrawn from marketing by the IBM Corporation. However, there are an estimated 20,000 of these controllers installed and IBM continues to provide hardware maintenance service and micro code 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 IBM Network Control Program
IBM Network Control Program
The original IBM Network Control Program ran on the 3705 and supported access to older devices by application programs using Virtual Telecommunications Access Method ...

 (NCP) required by the 3745/3746 controllers.

Objectives of SNA

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 interactive terminal based-systems as opposed to batch
Batch processing
Batch processing is execution of a series of programs on a computer without manual intervention.Batch jobs are set up so they can be run to completion without manual intervention, so all input data is preselected through scripts or command-line parameters...

 systems. An expansion of interactive terminal based-systems would increase sales of terminals and more importantly of mainframe computers and peripherals - partly because of the simple increase in the volume of work done by the systems and partly because interactive processing requires more computing power per transaction than batch processing.

Hence SNA aimed to reduce the main non-computer costs and other difficulties in operating large networks using earlier communications protocols. The difficulties included:
  • A communications line could not be shared by terminals whose users wished to use different types of application, for example one which ran under the control of CICS
    CICS
    Customer Information Control System is a transaction server that runs primarily on IBM mainframe systems under z/OS and z/VSE.CICS is a transaction manager designed for rapid, high-volume online processing. This processing is mostly interactive , but background transactions are possible...

     and another which ran under TSO
    Time Sharing Option
    In computing, Time Sharing Option is an interactive time-sharing environment for IBM mainframe operating systems, including OS/360 MVT, OS/VS2 , MVS, OS/390, and z/OS.- Overview :TSO fulfills a similar purpose to Unix login sessions...

    .
  • 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-wired communications 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 300 bits per second because of the overwhelming error rate, as comparing 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
    Shannon–Hartley theorem
    In information theory, the Shannon–Hartley theorem tells the maximum rate at which information can be transmitted over a communications channel of a specified bandwidth in the presence of noise. It is an application of the noisy channel coding theorem to the archetypal case of a continuous-time...

     in a relatively low-technology environment). Telecommunications companies had little incentive to improve line quality or reduce costs, because at the time they were mostly monopolies and sometimes state-owned.

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 System/370
System/370
The IBM System/370 was a model range of IBM mainframes announced on June 30, 1970 as the successors to the System/360 family. The series maintained backward compatibility with the S/360, allowing an easy migration path for customers; this, plus improved performance, were the dominant themes of the...

 mainframes inherited from System/360
System/360
The IBM System/360 was a mainframe computer system family first announced by IBM on April 7, 1964, and sold between 1964 and 1978. It was the first family of computers designed to cover the complete range of applications, from small to large, both commercial and scientific...

. Each CPU could connect to at most 16 I/O channels
Channel I/O
In computer science, channel I/O is a generic term that refers to a high-performance input/output architecture that is implemented in various forms on a number of computer architectures, especially on mainframe computers...

 and each channel could handle up to 256 peripherals - i.e. there was a maximum of 4096 peripherals per CPU. At the time when SNA was designed, each communications line counted as a peripheral. Thus the number of terminals with which powerful mainframes could otherwise communicate was limited.

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
Communications protocol
A communications protocol is a system of digital message formats and rules for exchanging those messages in or between computing systems and in telecommunications...

 rather than a very stripped-down protocol which suited only a specific type of terminal. As a result several multi-layer communications protocols were proposed in the 1970s, of which IBM's SNA and ITU-T
ITU-T
The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....

's X.25
X.25
X.25 is an ITU-T standard protocol suite for packet switched wide area network communication. An X.25 WAN consists of packet-switching exchange nodes as the networking hardware, and leased lines, Plain old telephone service connections or ISDN connections as physical links...

 became dominant later.

The most important elements of SNA include:
  • IBM Network Control Program
    IBM Network Control Program
    The original IBM Network Control Program ran on the 3705 and supported access to older devices by application programs using Virtual Telecommunications Access Method ...

     (NCP) is a communications program running on the 3705 and subsequent 37xx communications processors that, among other things, implements the packet switching switching protocol defined by SNA. The protocol performed two main functions:
    • It is a packet forwarding protocol, acting like modern switch
      Switch
      In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another....

       - 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
      Peer-to-peer
      Peer-to-peer computing or networking is a distributed application architecture that partitions tasks or workloads among peers. Peers are equally privileged, equipotent participants in the application...

       networks in which a machine at the end of the line can be both a client
      Client (computing)
      A client is an application or system that accesses a service made available by a server. The server is often on another computer system, in which case the client accesses the service by way of a network....

       and a server
      Server (computing)
      In the context of client-server architecture, a server is a computer program running to serve the requests of other programs, the "clients". Thus, the "server" performs some computational task on behalf of "clients"...

       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
      IBM 3745
      The IBM 3745 is the latest and last of a 37xx family of communications controllers for the IBM mainframe environment. As of mid-2009 there were an estimated 7,000+ of the larger 3745 models still in active production status, down from 20,000 or more in 2007...

      .
  • Synchronous Data Link Control
    Synchronous Data Link Control
    Synchronous Data Link Control is a computer communications protocol. It is the layer 2 protocol for IBM's Systems Network Architecture . SDLC supports multipoint links as well as error correction. It also runs under the assumption that an SNA header is present after the SDLC header...

     (SDLC), a protocol which greatly improved the efficiency of data transfer over a single link:
    • SDLC included much more powerful error detection and correction
      Error detection and correction
      In information theory and coding theory with applications in computer science and telecommunication, error detection and correction or error control are techniques that enable reliable delivery of digital data over unreliable communication channels...

       codes than earlier protocols. These codes often enabled the communications cards to correct minor transmission errors without requesting re-transmission, and therefore made it possible to pump data down a line much faster.
    • It enabled 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 that the error detection and correction codes could not repair, 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) which contained enough information for data packages from different types of terminal to be send 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 (i.e. those connected to the mainframe by telephone lines) and 3705 communications processors would have SDLC-capable communications cards.
This is the precursor of the so called "packet communication" that eventually evolved into today's IP technology, and SDLC itself evolved into HDLC that is one of the base technology for dedicated telecommunication circuit.
  • VTAM
    VTAM
    Virtual Telecommunications Access Method is IBM's software package that provides communications via telecommunication devices for mainframe environments. It is the implementation of Systems Network Architecture for mainframes...

    , 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
    CICS
    Customer Information Control System is a transaction server that runs primarily on IBM mainframe systems under z/OS and z/VSE.CICS is a transaction manager designed for rapid, high-volume online processing. This processing is mostly interactive , but background transactions are possible...

     or TSO
    Time Sharing Option
    In computing, Time Sharing Option is an interactive time-sharing environment for IBM mainframe operating systems, including OS/360 MVT, OS/VS2 , MVS, OS/390, and z/OS.- Overview :TSO fulfills a similar purpose to Unix login sessions...

    ). 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 in to 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

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 APPN, routing functionality was the responsibility of the computer as opposed to the router (as with TCP/IP networks). Each computer maintained a list of Nodes that defined the forwarding mechanisms. A centralized node type known as a Network Node maintained Global tables of all other node types. APPN stopped the need to maintain 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 was similar to the way that TCP/IP routers function today.

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
    X.25
    X.25 is an ITU-T standard protocol suite for packet switched wide area network communication. An X.25 WAN consists of packet-switching exchange nodes as the networking hardware, and leased lines, Plain old telephone service connections or ISDN connections as physical links...

     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 IBM Advanced Peer-to-Peer Networking functionality were not really successful, if only because the migration from traditional SNA to SNA/APPN was very complex, without providing much additional value, at least initially. SNA software licences (VTAM) cost as much as $10000 a month for high-end systems. And SNA IBM 3745
    IBM 3745
    The IBM 3745 is the latest and last of a 37xx family of communications controllers for the IBM mainframe environment. As of mid-2009 there were an estimated 7,000+ of the larger 3745 models still in active production status, down from 20,000 or more in 2007...

     Communications Controllers typically cost over $100K. TCP/IP was 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 it deployed.

  • The design of SNA was in the era when the concept of layered communication was not fully adopted by the computer industry. Applications, database
    Database
    A database is an organized collection of data for one or more purposes, usually in digital form. The data are typically organized to model relevant aspects of reality , in a way that supports processes requiring this information...

    s and communication functions were mingled into the same protocol or product, to make it difficult to maintain or manage. That was very common for the products created in that time. Even after TCP/IP was fully developed, X window system was designed with the same model where communication protocols were embedded into graphic display application.

  • 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 the advent of the Web Browser which was the beginning of the end for SNA.

Logical unit types

Network Addressable Units in an SNA network are any components that can be assigned an address and can send and receive information. They are distinguished further as follows:
  • System Service Control Points, provide services to manage a network or subnetwork (typically in the mainframe),
  • Physical Units, a physical device or communications link (relating to boxes),
  • Logical Units, an access point to the network (relating to applications or subsystems such as CICS
    CICS
    Customer Information Control System is a transaction server that runs primarily on IBM mainframe systems under z/OS and z/VSE.CICS is a transaction manager designed for rapid, high-volume online processing. This processing is mostly interactive , but background transactions are possible...

     and TSO
    Time Sharing Option
    In computing, Time Sharing Option is an interactive time-sharing environment for IBM mainframe operating systems, including OS/360 MVT, OS/VS2 , MVS, OS/390, and z/OS.- Overview :TSO fulfills a similar purpose to Unix login sessions...

    ) or terminals.


SNA essentially offers transparent communication: equipment specifics don't 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).

SNA defines several kinds of devices, called Logical Unit types:
  • LU0 provides for undefined devices, or build your own protocol.
  • LU1 devices are printers.
  • LU2 devices are dumb IBM 3270
    IBM 3270
    The IBM 3270 is a class of block oriented terminals made by IBM since 1972 normally used to communicate with IBM mainframes. As such, it was the successor to the IBM 2260 display terminal. Due to the text colour on the original models, these terminals are informally known as green screen terminals...

     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
    IBM 5250
    IBM 5250 was originally a family of terminal devices sold with the IBM System/34 minicomputer systems. One model was the IBM 5251-11. It also connected to the later System/36, System/38 and AS/400 systems.- Historical origins :...

     terminals.

The primary ones in use are LU1, LU2, and LU6.2
LU6.2
Logical Unit 6.2 is an IBM-originated communications protocol specification dating from 1974, and is part of IBM's Systems Network Architecture....

 (an advanced protocol for application to application conversations).

Within SNA there are two types of data stream to connect local display terminals and printers; there is the 3270 data stream mainly used by mainframes (zSeries
ZSeries
IBM System z, or earlier IBM eServer zSeries, is a brand name designated by IBM to all its mainframe computers.In 2000, IBM rebranded the existing System/390 to IBM eServer zSeries with the e depicted in IBM's red trademarked symbol, but because no specific machine names were changed for...

 family) and the 5250 data stream mainly used by minicomputers/servers such as the S/36, S/38, and AS/400 (now System i).

Starting from version 5.2 of OS/400
OS/400
IBM i is an EBCDIC based operating system that runs on IBM Power Systems. It is the current evolution of the operating system named i5/OS which was originally named OS/400 when it was introduced with the AS/400 computer system in 1988....

, SNA for client-access is no longer supported.

The term 37xx
37xx
IBM 37xx is IBM's family of SNA communications controllers used mainly in mainframe environments.The IBM 3705 was the oldest of the family. The IBM 3725 was a large-scale node and front end processor, and the IBM 3720 was a remote node that functioned as a concentrator and router...

 refers to IBM's family of SNA
Systems Network Architecture
Systems Network Architecture 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...

 communications controllers. The 3745 supports up to eight high-speed T1
Digital Signal 1
Digital signal 1 is a T-carrier signaling scheme devised by Bell Labs. DS1 is a widely used standard in telecommunications in North America and Japan to transmit voice and data between devices. E1 is used in place of T1 outside North America, Japan, and South Korea...

 circuits, the 3725 is a large-scale node and front-end processor
Front end processor
A front end processor , or a communications processor, is a small-sized computer which interfaces to the host computer a number of networks, such as SNA, or a number of peripheral devices, such as terminals, disk units, printers and tape units. Data is transferred between the host computer and the...

 for a host, and the 3720
IBM 3720
The IBM 3720 was a communications controller made by IBM, suitable for use in an IBM System/390. Official service support was withdrawn in 1999 in favour of the IBM 3745. The IBM 3720 is unrelated to the similarly-numbered IBM 3270 display terminal system.-External links:*...

 is a remote node that functions as a concentrator
Concentrator
In telecommunication, the term concentrator has the following meanings:# In data transmission, a functional unit that permits a common path to handle more data sources than there are channels currently available within the path...

 and router.

Implementation and publication

SNA was made public as part of IBM's "Advanced Function for Communications" announcement in September, 1974, which included the implementation of the SNA/SDLC (Synchronous Data Link Control) protocols on new communications products:
  • IBM 3767
    IBM 3767
    IBM 3767 Communication Terminal is a serial printer terminal that employed wire matrix print-head technology and, for the first time, the Synchronous Data Link Control communications protocol set under IBM's Systems Network Architecture...

     communication terminal (printer)
  • IBM 3770 data communication system

They were supported by IBM 3704/3705 communication controllers and their Network Control Program, and by System/370 and their VTAM and other software such as CICS and IMS. This announcement was followed by another announcement in July, 1975, which introduced the IBM 3760 data entry station, the IBM 3790
IBM 3790
IBM 3790 was a computer system announced in 1975, one of the first distributed computing platforms. It preceded the IBM 8100, announced in 1979....

 communication system, and the new models of the IBM 3270
IBM 3270
The IBM 3270 is a class of block oriented terminals made by IBM since 1972 normally used to communicate with IBM mainframes. As such, it was the successor to the IBM 2260 display terminal. Due to the text colour on the original models, these terminals are informally known as green screen terminals...

 display system.

SNA was mainly designed by the IBM Systems Development Division laboratory in Research Triangle Park
Research Triangle Park
The Research Triangle Park is a research park in the United States. It is located near Durham, Raleigh, and Chapel Hill, in the Research Triangle region of North Carolina...

, North Carolina
North Carolina
North Carolina is a state located in the southeastern United States. The state borders South Carolina and Georgia to the south, Tennessee to the west and Virginia to the north. North Carolina contains 100 counties. Its capital is Raleigh, and its largest city is Charlotte...

, USA, helped by other laboratories that implemented SNA/SDLC. The details were later made public by IBM's System Reference Library manuals and IBM Systems Journal.

Competitors

The proprietary networking architecture for Honeywell Bull mainframes is Distributed Systems Architecture (DSA). Communications package for DSA is VIP. Like SNA, DSA is also no longer supported for client access. Bull mainframes are fitted with Mainway for translating DSA to TCP/IP and VIP devices are replaced by TNVIP Terminal Emulations (GLink, Winsurf). GCOS 8
General Comprehensive Operating System
General Comprehensive Operating System is a family of operating systems oriented toward mainframe computers.The original version of GCOS was developed by General Electric from 1962; originally called GECOS...

 supports TNVIP SE over TCP/IP.

See also

  • Network Data Mover


External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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