Remote file access over low-speed lines

A link between microcomputer and mainframe can be useful in several ways, even when, as is usually the case, the link is only a normal terminal line. One interesting example is the ‘Integrated application’, which divides a task between microcomputer and mainframe and can offer several benefits; in particular, reducing load on the mainframe and permitting a more advanced user interface than possible on a conventional terminal.

Because integrated applications consist of two co-operating programs, they are much more difficult to construct than a single program. It would be much easier to implement integrated applications concerned with the display and/or modification of data in mainframe files if the microcomputer could confine its dealings with the mainframe to a suitable file server. However, file servers do not appear practical for use over slow (compared to disc access speed) terminal lines.

It was proposed to alleviate the problems caused by the slow link with extended file operations, which would allow time-consuming file operations such as searching or copying between files to be done in the file server. It was discovered after attempting such a system that extended file operations are not, by themselves, sufficient; but, allied to a record-based file model and asynchronous operations (i.e. file operations that do not suspend the user program until they complete), useful results could be obtained.

This thesis describes FLAP, a file server for use over terminal lines which incorporates these ideas, and MMMS, an inter-application transport protocol used by FLAP for communication between the microcomputer file interface and the mainframe server.

Two simple FLAP applications are presented, a customer records maintenance program and a screen editor. Details are given of their construction and response time in use at various line speeds.