Connectivity

owner(s):

Definition: Mainframe connectivity refers to the ability to interact with mainframe computers from remote locations via some electronic connection. This can be accomplished through an internet connection, through dial-up, through a leased line or private network. Since mainframes are physically large, and since they can support thousands of users simultaneously, it makes sense that users, even administrators and power users, don't need to be physically present to fully take advantage of the mainframe assets. Businesses which need the power of a mainframe but cannot justify owning their own can share a mainframe with other businesses without being concerned with where the mainframe is physically located.

Traditionally connectivity with a mainframe was over IBM's SNA (systems network architecture). This was the main protocol by which all devices connected with a mainframe, whether terminal, printer, plotter, etc. RJE (remote job entry) was a method of controlling jobs in the SNA scheme. While SNA was very effective in handling all sorts of connectivity issues, such as modem failures, SNA was large and complex, requiring dedicated switching minicomputers and a specialized professional staff to handle the maintenance. While this complexity can be justified in very large establishments, the trend has been to adding minicomputers and workstations to networks, making SNA more problematic. A variation on SNA was developed by IBM to better handle mixed devices, but the effect is to have two different SNA architectures.

A more recent alternative for mainframe connectivity is TCP/IP. This works very well with a hodge-podge of devices and computers. It was designed by the Department of Defense to withstand large sections of the network being destroyed. However problems with an IP network can go unreported or uncorrected. Also an IP network does require coniderable technical expertise to keep it going. However TCP/IP is well understood and widely accepted as the backbone of the internet.

It would appear that SNA works well when used in large commercial networks with centrally managed devices. But when users connect with mainframes with PCs and other ad-hoc equipment without central management, TCP/IP is probably more flexible. One hybrid scheme is for the mainframe itself and immediate devices to be controlled with an SNA architecture, but for remote devices like workstations to have a TCP/IP network with a network bridge connecting the two architecures. The internet itself can be the TCP/IP network, with a bridge to the SNA mainframe.