IBM Systems Network Architecture - Principal Components and Technologies

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 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) 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 - 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 (SDLC), 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" (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, 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 or TSO). 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.