Academic journal article T H E Journal (Technological Horizons In Education)

Network-to-Mainframe Connectivity

Academic journal article T H E Journal (Technological Horizons In Education)

Network-to-Mainframe Connectivity

Article excerpt

With the proliferation of PCs and networks in the past decade, many universities have faced the problem of connecting PCs to the mainframes where most information resides. McGill University in Montreal is no exception. In the mid- 1980s, McGill had an extensive network of asynchronous, synchronous and coax-based facilities. In 1986, in conjunction with a new library automation system, a fiber-optic link was installed to provide access from our many IBM 3090 mainframe terminals (3270s) to several remote buildings. We also intended to use the fiber for a pilot token-ring network using Proteon's ProNET-10. At the same time, we started to experiment with Novell Netware Version 1.0 to interconnect PCs. We realized the importance of connectivity and began to interconnect several PC networks using the Pronet backbone.

Our network expanded rapidly, and so did the number of PCs on campus. In 1987, we extended our initial fiber-optic network with copper wiring to several other buildings and departments. Meanwhile, growing numbers of campus users were replacing their 3270 terminals with PCs and using IRMA cards for mainframe access. Before long, some UNIX systems were added as well, and bridges were installed to segment the growing "pilot" network.

The Need for Speed

Although the new high-bandwidth PC networks provided excellent connectivity among themselves, commercial vendors offered no high-speed paths to a System/370 host. In 1986, IBM had announced the Token-ring Interface Coupler (TIC), allowing System/ 370 hosts to be connected to an IBM Token-ring network. However, in order to take advantage of this connectivity, networks had to conform to IBM'S Systems Network Architecture (SNA) protocol, and IBM'S VTAM software had to be installed on all hosts. We were not prepared to convert our campuswide network to SNA, nor did we want to replace our by now extensive ProNET-10 network with IBM Token-ring.

Therefore, McGill decided to write its own 3270 emulator that would not rely on any one specialized network architecture and which could use the existing campus networks. The major problem we faced in designing our link between PC networks and the IBM mainframe was obtaining the high speeds we wanted. Most 3270 network solutions that exist today require the TIC as the host connection. Since IBM would not disclose how the TIC microcode in the 3174 could be used by any access method other than VTAM, we were forced to find an alternative solution. Emulating a remote 3270 control unit through a bi-synchronous line, as do some third-party vendors, also seemed undesirable; forcing PCs interconnected at 10 Mbps through a 56-Kbps gateway seemed to be a step backward in time. The answer was the most common interface of all: the 3270 coax.

The 3270 coax transfers data at more than 1Mbps between a PC and a control unit. Using IBM'S DFT (Distributed Function Terminal) protocol, the 3270 coax is capable of supporting transfer rates of up to 800 Kbps. With this protocol, the 3174 and 3274 cluster control units permit transparent transfer of data.

Given McGill's diversified network nature, we developed a product called NET3270. It is a compact yet extremely fast and complete emulator for DOS and OS/2 that runs on SPX Novell-compatible networks as well as on networks compatible with IBM'S NETBIOS. Work on NET3270 began in 1988, and the product was put into full production on campus in 1989.

A Dedicated PC Gateway

The link between the mainframe and the network is provided through a dedicated PC gateway. The system utilizes our proprietary software protocol that can multiplex more than a hundred 3270 terminal sessions over a single 3270 coax. The host system runs a server program McGill also developed, called VMSERV, to create and manage sessions from the workstations on the network. NET3270 workstations can connect to the gateway using either Novell's SPX protocol or IBM'S NETBIOS protocol. …

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