Systems Network Architecture

History

• IBM 3767 communication terminal
• IBM 3770 data communication system

Objectives of SNA

• 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 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.

Principal components and technologies

• IBM Network Control Program is a communications program running on the 3705 and subsequent 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 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 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, a protocol which greatly improved the efficiency of data transfer over a single link:
• *SDLC included much more powerful error detection and correction 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" 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.
• VTAM, 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. 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 made it possible for a terminal to have simultaneous sessions with different applications or application environments.

  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 Peer-to-Peer Networking, routing functionality was the responsibility of the computer as opposed to the router. 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 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.. Physical Units were PU5, PU4, PU2

  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 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 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 cost as much as $10000 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.
• The design of SNA was in the era when the concept of layered communication was not fully adopted by the computer industry. Applications, databases and communication functions were mingled into the same protocol or product, which made it difficult to maintain and manage. That was very common for the products created in that time. Even after TCP/IP was fully developed, the 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 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.

Network Addressable Units

• a System Services Control Point provides resource management and other session services for users in a subarea network;
• a Physical Unit is a combination of hardware and software components that control the links to other nodes.
• a Logical Unit acts as the intermediary between the user and the network.

  Logical Unit (LU)

• 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.

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 nodes
• PU3 was never defined
• PU4 nodes are front-end processors running the Network Control Program such as the IBM 37xx series
• PU5 nodes are host computer systems

SNA over Token-Ring

SNA over IP