The Application Layer instantiates the End to End protocols for
the Cyclades Network. Two examples of these End to End Protocols are the Virtual
Terminal Protocol and the File Transfer
Protocol which are described below.
The Virtual Terminal Protocol is a set of conventions which define
a logical model for each class of terminals (called a Virtual Terminal) and a
specification about how to drive this virtual terminal. A Virtual Terminal is a
logical model in the sense that it defines an abstract model of a terminal in
terms of logical functions that different real terminals can interpret.
The virtual terminal layer resides
on top of the transport box. It provides additional functions required for
terminal oriented communications, for
example between a human user at a terminal and an application program in the
host computer. Virtual terminals have several implementations: an intelligent
terminal which directly handles the virtual terminal protocol; a terminal with
the additional logic in a concentrator required to handle the VTP; or an access
method providing application programs in a host with some kind of virtual
terminal access method.
The virtual terminal protocol covers several classes of terminals with
different functional capabilities corresponding to different classes of
Scroll-mode virtual terminal class - simple sequential terminals such as
teleprinters, displays in scroll mode, card readers and punches, and line
Page-mode virtual terminal class - simple terminals which offer direct
addressing within the pages.
Data entry virtual terminal class - terminals with field and attribute
Overlap class - terminals with elementary compatibility between classes