In parallel with the development of the ARPANET, a number of standardised layered proto-col ‘stacks’ and protoproto-col suites for simplifying and standardising the communication between computer equipment were being developed independently by various different computer and telecommunications equipment manufacturers. Most of these protocols were ‘proprietary’. In other words, the protocols were based on the manufacturers’ own specifications and docu-mentation, which were kept out of the public domain. Many manufacturers believed at the time that ‘proprietary’ protocols gave both a ‘competitive advantage’ and ‘locked’ customers into using their own particular brand of computer hardware. But the principles of the various schemes were all similar, and the ideas generated by the various groups of developers helped in the development of the standardised protocols which came later.
All data communications protocols are based upon packet switching, a form of electronic inter-computer communication advanced by Leonard Kleinrock of MIT (Massachusetts Insti-tute of Technology — and later of UCLA — University of California in Los Angeles) in his paper ‘Information flow in large communication networks’ (July 1961). The term packet switching itself was coined by Donald Davies of the UK’s National Physical Laboratory (NPL) in 1966.
Packet switching is analogous to sending letters through the post — the data content, anal-ogous to a page of a letter is the user content (or payload) of a standard packet. The user content is packed in thepacket orframe (analogous to an ‘envelope’) and labelled with the destinationaddress. When the size of a single packet is too small for the message as a whole, then the message can be split up and sent as a sequence of numbered packets, sent one after another (see Figure 1.1). The networking nodes (which in different types of data networks have different names: routers, switches, bridges, terminal controllers, cluster controllers, front-end
Figure 1.1 Post Office analogy illustrating the principles of packet switching.
SNA (systems network architecture) 3 processors, etc.) all essentially work like a postal sorting office. They read the ‘address’ on each packet (without looking at the contents) and thenforward the packet to the appropriate next node nearer the destination.
The best-known, most successful and widely used of the 1970s generation of packet-switching protocols were:
• SNA (systems network architecture)— the networking protocols used for interconnecting IBM (International Business Machines) computers;
• DECnet— the networking protocols used for interconnecting computers of the Digital Equipment Corporation (DEC);
• X.25 (ITU-T recommendation X.25) and its partner protocol, X.75. This was the first
attempt, coordinated by the International Telecommunications Union standardisation sector (ITU-T), to create a ‘standard’ protocol — intended to enable computers made by different manufacturers to communicate with one another — so-called open systems interconnec-tion (OSI).
1.3 SNA (systems network architecture)
Thesystems network architecture (SNA) was announced by IBM in 1974 as a standardised communications architecture for interconnecting all the different types of IBM computer hard-ware. Before 1974, transferring data or computer programs from one computer to another could be a time-consuming job, sometimes requiring significant manual re-formatting, and often requiring the transport of large volumes of punched cards or tapes. Initially, relatively few IBM computers were capable of supporting SNA, but by 1977 the capabilities of the third generation of SNA (SNA-3) included:
• communication controllers (otherwise called FEPs or front end processors) — hardware which could be added to mainframe computers for taking over communication with remote devices;
• terminal controllers (otherwise calledcluster controllers) — by means of which, end-user terminals (teletypesor computerVDUs, video display units) could be connected to a remote host computer;
• the possibility to connect remote terminal controllers to the mainframe/communication controller site using either leaselines or dial-in lines;
• the possibility of multi-host networks (terminals connected to multiple mainframe computers — e.g., for bookkeeping, order-taking, personnel, etc. — by means of a single communications network).
Figure 1.2 illustrates the main elements of a typical SNA network, showing the typical star topology. Point-to-point lines across the wide area network (WAN) connect the front end processor (FEP or communications controller) at the enterprise computer centre to the terminals in headquarters and remote operations offices. The lines used could be eitherleaselines, point-to-pointX.25 (i.e.,packet-switched) connections,frame relayconnections or dial-up lines.
During the 1980s and 1990s, SNA-based networks were widely deployed by companies which used IBM mainframe computers. At the time, IBM mainframes were the workhorse of the computing industry. The mainframes of the IBM S/360, S/370 and S/390 architectures became well known, as did the components of the SNA networks used to support them:
Figure 1.2 A typical SNA network interconnecting IBM computer hardware.
• Front end processor (FEP or communication controller) hardware: IBM 3705, IBM 3725, IBM 3720, IBM 3745;
• Cluster controller hardware: IBM 3174, IBM 3274, IBM 4702, IBM 8100;
• VTAM (virtual telecommunication access method) software used as the mainframe com-munications software;
• CICS (communication information control system)mainframe management software;
• NCP (network control program)front end processor communications control software;
• NPSI (NCP-packet switching interface) mainframe/FEP software for use in conjunction with X.25-based packet-switched WAN data networks;
• TSO (time sharing option) software allowing mainframe resources to be shared by many users;
• NetView mainframe software for network monitoring and management;
• APPN (advanced peer-to-peer networking) used in IBM AS-400 networks;
• ESCON (enterprise system connection): a high-speed ‘channel’ connection interface between mainframe and front-end processor;
• Token ring local area network (LAN).
Due to the huge popularity of IBM mainframe computers, the success of SNA was assured. But the fact that SNA was not a public standard made it difficult to integrate other manufacturers’
network and computer hardware into an IBM computer network. IBM introduced products intended to allow the integration of public standard data networking protocols such as X.25 and Frame Relay, but it was not until the explosion in numbers ofPCs (personal computers) andLANs (local area networks)in the late 1980s and 1990s that IBM lost its leading role in
X.25 (ITU-T recommendation X.25) 5