Pierre Goyette from McGill University talked about his large network of PCs on campus and his method for giving each of them 3270 host access.
When all of your PCs are on a token-ring network, it shouldn't be necessary to install an Irma card and coax line in each one. Only one PC in the ring should have to be coax-attached, if you have the right software support.
IBM will supply this software and attach your token-ring to a 3174 controller but there are problems with this implementation. You have to be using VTAM (McGill uses VM), the memory requirements are huge (250-400k per PC), and it only works on IBM-brand token rings. No Novell, no 3-Com. There are also unspecified security and configuration problems with the IBM solution.
TCP-IP isn't much better. There are more unspecified configuration problems, large memory requirements, and it is expensive. The 3279 emulation is incomplete, and the implementation is user-antagonistic.
So... they wrote their own. Their design considerations were:
NET3270 is a 3270 emulator that runs in a PC. It uses standard NETBIOS calls to communicate with a server program that runs in a dedicated PC; this dedicated PC connects to a 3174 controller via a DFT port, which in turn talks to a special application on the mainframe side. The PC multiplexes all the logical sessions while the mainframe application demultiplexes them.
NET3270 supports up to five sessions per PC, and includes a file transfer facility. The keyboard is reconfigurable, and colors can be changed by the operator. It can be made resident or nonresident (hot-key), and requires only 128K bytes to maintain five terminal sessions.
They only implement a subset of 3279-2 processing; no graphics, no programmed symbols. They did extend the 3270 operator interface compatibly however. Their "enhanced null processing" feature allows you to push blanks off the end of a field when doing inserts - something IBM should have done years ago.
File transfer speeds range from 8.5 KB/sec on a stock IBM PC, to 20 KB/sec on a PS/2 model 70. If you have several file transfers going simultaneously, the aggregate transfer rate approaches that of the 3174 controller: 100 KB/sec. 3174 controllers are roughly twice as fast as 3274 controllers due to microcode improvements.
McGill University is working on a MVS version. They haven't quite decided on what to do with NET3270 at this time; it isn't for sale, and it isn't in the public domain.