IBM жүйелік желі архитектурасы: тарихы мен мақсаты

Proprietary networking architecture created by IBM

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 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.

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 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:
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 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 System/370 mainframes inherited from System/360. Each CPU could connect to at most 16 I/O channels 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.

SNA removed link control from the application program and placed it in the NCP. This had the following advantages and disadvantages:

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 Peer to Peer Networking (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 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.)

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 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 $10,000 a month for high end systems. And SNA IBM 3745 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. 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.

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.

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.

As mainframe based entities looked for alternatives to their 37XX based networks, IBM partnered with Cisco in the mid 1990s and together they developed Data Link Switching, or DLSw. DLSw encapsulates SNA packets into IP datagrams, allowing sessions to flow over an IP network. The actual encapsulation and decapsulation takes place in Cisco routers at each end of a DLSw peer connection. At the local, or mainframe site, the router uses Token Ring topology to connect natively to VTAM. At the remote (user) end of the connection, a PU type 2 emulator (such as an SNA gateway server) connects to the peer router via the router's LAN interface. End user terminals are typically PCs with 3270 emulation software that is defined to the SNA gateway. The VTAM/NCP PU type 2 definition becomes a Switched Major Node that can be local to VTAM (without an NCP), and a "Line" connection can be defined using various possible solutions (such as a Token Ring interface on the 3745, a 3172 Lan Channel Station, or a Cisco ESCON compatible Channel Interface Processor).

The proprietary networking architecture for Honeywell Bull mainframes is Distributed Systems Architecture (DSA). The Communications package for DSA is VIP. 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 supports TNVIP SE over TCP/IP. The networking architecture for Univac mainframes was the Distributed Computing Architecture (DCA), and the networking architecture for Burroughs mainframes was the Burroughs Network Architecture (BNA); after they merged to form Unisys, both were provided by the merged company. Both were largely obsolete by 2012. International Computers Limited (ICL) provided its Information Processing Architecture (IPA). DECnet is a suite of network protocols created by Digital Equipment Corporation, originally released in 1975 to connect two PDP 11 minicomputers. It evolved into one of the first peer to peer network architectures, thus transforming DEC into a networking powerhouse in the 1980s. SNA also competed with ISO's Open Systems Interconnection, which was an attempt to create a vendor neutral network architecture that failed due to the problems of "design by committee". OSI systems are very complex, and the many parties involved required extensive flexibilities that hurt the interoperability of OSI systems, which was the prime objective to start 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. The 1988 publication of , authored by Yakov Rekhter, which defines an option to run IBM 3270 sessions over Telnet, explicitly recognizes the customer demand for interoperability in the data center. Subsequently, the IETF expanded on this work with multiple other RFCs. TN3270 (Telnet 3270), defined by those RFCs, supports direct client server connections to the mainframe using a TN3270 server on the mainframe, and a TN3270 emulation package on the computer at the end user site. This protocol allows existing VTAM applications (CICS, TSO) to run with little or no change from traditional SNA by supporting traditional 3270 terminal protocol over the TCP/IP session. This protocol is widely used to replace legacy SNA connectivity more than Data Link Switching (DLSw) and other SNA replacement technologies. A similar TN5250 (Telnet 5250) variant exists for the IBM 5250.

Non IBM SNA software allowed systems other than IBM's to communicate with IBM's mainframes and AS/400 midrange computers using the SNA protocols. Some Unix system vendors, such as Sun Microsystems with its SunLink SNA product line, including PU2.1 Server, and Hewlett Packard/Hewlett Packard Enterprise, with their SNAplus2 product, provided SNA software. Microsoft introduced SNA Server for Windows in 1993; it is now named Microsoft Host Integration Server. Digital Equipment Corporation had VMS/SNA for VMS. Third party SNA software packages for VMS, such as the VAX Link products from Systems Strategies, Inc.,

Brixton Systems developed several SNA software packages, sold under the name "Brixton", such as Brixton BrxPU21, BrxPU5, BrxLU62, and BrxAPPC, for systems such as workstations from Hewlett Packard, and Sun Microsystems. 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, Brixton 4.1 SNA for Sun Solaris, and SunLink SNA 9.1 Support for Sun Solaris.