rfc782.txt

RFC 相关的技术文档

        A Virtual Terminal Management Model





        RFC 782





        prepared for 

        Defense Communications Agency
        WWMCCS ADP Directorate
        Command and Control Technical Center
        11440 Isaac Newton Square
        Reston, Virginia 22090








        by
        Jose Nabielsky
        Anita P. Skelton






        The MITRE Corporation
        MITRE C(3) Division
        Washington C(3) Operations
        1820 Dolley Madison Boulevard







                          TABLE OF CONTENTS



                                                                Page


LIST OF ILLUSTRATIONS                                             vi

1.0  INTRODUCTION                                                  1
1.1  The Workstation Environment                                   1
1.2  Virtual Terminal Management                                   2
1.3  The Scope                                                     3
1.4  Related Work                                                  4

2.0  THE VTM MODEL                                                 5
2.1  The VTM Model Components                                      7
2.2  The Virtual Terminal Model                                   10
     2.2.1  Virtual Terminal Connectivity                         11
     2.2.2  Virtual Terminal Organization                         11
            2.2.2.1  The Virtual Keys                             12
            2.2.2.2  The Virtual Controller                       12
            2.2.2.3  The Virtual Display                          12
     2.2.3  Virtual Terminal Architecture                         13
            2.2.3.1  Communication Variables                      13
            2.2.3.2  Virtual Display with File Extension          13
            2.2.3.3  Virtual Display Windows                      14
2.3  The Workstation Model                                        17
     2.3.1  The Adaptation Unit                                   17
     2.3.2  The Executive                                         18

REFERENCES                                                        19

















                                 iii

                        LIST OF ILLUSTRATIONS



                                                                Page

Figure Number

     2.1       The Virtual Terminal Model                          7
     2.2       The Workstation Model                               8
     2.3       VT 0 (expanded from previous figure)                9
     2.4       The Domains                                        14





































                                  v







1.0  INTRODUCTION

     Recent advances in micro-electronics have brought us to the  age
of the inexpensive, yet powerful, microprocessor.  Closely resembling
the advances of the 1960's which brought about  the  transition  from
batch  processing  to time-sharing, this technological trend suggests
the birth of decentralized architectures where the  processing  power
is  shifted  closer  to  the user in the form of intelligent personal
workstations.  The virtual terminal model described in this  document
caters to this anticipated personal computing environment.

1.1  The Workstation Environment

     A personal workstation is a computing engine which  consists  of
hardware  and  software dedicated to serve a single user.  As part of
its architecture, the workstation can invoke the resources of  other,
physically  separate  components, effectively extending this personal
environment well beyond the bounds of the single workstation.

     In this personal environment,  processing  resources  previously
shared  among  multiple  users  now become dedicated to a single one,
with a large part of these resources summoned to provide an effective
human-machine  interface.   As a consequence, modalities of input and
output that were unfeasible under the time-shared regime now become a
part of a conversational language  between user and workstation.  Due
to the availability of processing cycles, and the  closeness  of  the
user devices to these cycles, the workstation can support interactive
devices, and dialogue modes using these devices, which could  not  be
afforded before.

     The workstation can provide the  user  with  the  mechanisms  to
conduct  several  concurrent  conversations  with user-agents located
elsewhere in the global architecture.   One  such  mechanism  is  the
partitioning  of  the  workstation  physical  display  into  multiple
logical  displays,  with  one  or  more  of  these  logical  displays
providing a dedicated workspace between user and agent.

     The nature of the conversations on these logical  displays  need
not  be  limited  to  conventional  alphanumeric  input  and  output.
Conversations using input tools  such  as  positioning  and  pointing
devices  (e.g.,  mouse,  tablet, and such), and using high-resolution
graphics objects for output (e.g., line drawings, raster  blocks  and
images,  possibly  intermixed with text) should be possible on one or
more of these screens.

     Moreover, as long as the technological trend  continues  in  its
predicted  path,  one can postulate a workstation which could support
by the mid 1980's multi-media conversations using  voice  and  video,

                                  1






synchronized   with  text  and  graphics.   At  present,  multi-media
information   management   (i.e.,   acquisition,   processing,    and
dissemination)  is  an  active  research area, but eventually it will
become an engineering problem which, when  solved,  will  add  a  new
dimension  to  already feasible modes of interaction between user and
workstation.

1.2  Virtual Terminal Management

     All virtual terminal protocols  (VTPs)  provide  a  vehicle  for
device-independent,     bi-directional,     8-bit    byte    oriented
communications between two VTP users.  Most Vo so by invoking  a
device abstraction of real terminals, called a virtual terminal.

     As with a real device, a virtual  terminal  has  a  well-defined
architecture  with  its  own character sets and functions. A VTP uses
the architectural features of  the  virtual  terminal  to  provide  a
common  language,  an  intermediate  representation,  between its two
communicating entities.  However a  VTP  user  does  not  communicate
directly  with  this  virtual  terminal.   A function of a VTP is the
local mapping between the site-specific order codes and  the  virtual
terminal  domain,  thus allowing this adaptation to be transparent to
the VTP users.

     The model of a personal workstation as a dedicated  device  with
considerable   resources    affects  the  way  we  conceptualize  the
architecture of virtual terminals,  both  in  breadth  and  depth  of
function.   It also affects the way we view the virtual terminal vis-
a-vis its local correspondents, the personal  workstations,  and  its
remote correspondents, the other virtual terminals.

     This document presents a radical view of  virtual  terminals  as
resource  sharing  devices.   The  classical  concept  of  a  virtual
terminal as a two-way device with a  limited  architecture  has  been
dismissed.   Instead,  we  view a virtual terminal as an n-way device
with multiple correspondents sharing access to its virtual "keyboard"
and  "display."  In  this  model, a virtual terminal has two kinds of
correspondents:  adaptation units, and other virtual terminals.   The
adaptation  units  serve  as  interface  agents  between  the virtual
terminal and its users, providing the step transformation between the
user-specific   order   codes  and  the  virtual  terminal  interface
language.  In turn,  the  other  virtual  terminals  are  cooperating
co-equals  of the  virtual  terminal, interacting with it to maintain
global control and data store synchrony. Resembling the administrator
of  a  local  copy  of  a distributed data base, the virtual terminal
interacts with the other virtual  terminals  (the  remote  data  base
managers)  and  with  the  local  adaptation  units  (the  data  base
transformers) to provide read, write, and modify access to its  local

                                  2









data  store  (the  local  copy  of  the distributed data base), while
providing concurrency control to maintain a "single user  view"  when
so desired.

     To communicate with its correspondents, a virtual terminal  uses
two virtual languages. In the case where the correspondent is another
virtual terminal, it  uses  the  language  of  the  virtual  terminal
protocol;  in the case where the correspondent is an adaptation unit,
it uses an interface language closer to the physical architecture  of
the end-user, but a virtual language nevertheless.

     In essence, the virtual terminal has become a device in its  own
right,  free  from  a  single physical realization and also dedicated
ownership. As a result, a single workstation not only may request any
number  of  virtual  terminals,  but  a  number  of  workstations may
share -- and interact with -- a particular virtual terminal.

     The functional breadth of virtual terminals has  been  augmented
by  the  concept  of  virtual  terminal  classes.   Each  class is an
abstraction of a particular device architecture.  There  are  stream,
line,  logical  page,  physical page, and graphics virtual terminals,
all made up of:  a class-constrained data structure and its attendant
operations  (the virtual display); a general controlling element (the
virtual controller); and an input selector (the virtual keys).

     Finally, the functional depth of the virtual terminal  has  been
extended  by  architectural  features  previously  unavailable.   The
virtual terminal becomes a  multi-user  device  with  a  non-volatile
virtual  display available for selective viewing.  These concepts are
discussed is some detail in the chapter that follows.

1.3  The Scope

     An overview of the virtual terminal model and the management  of
communicating  virtual  terminals  is  presented.   A detailed design
description  of  the  data  structures  and  accompanying  addressing
functions  has been completed.  The operations and control mechanisms
are less complete.  Before  the  design  is  solidified,  an  initial
mimimal implementation will be made to validate the model.

     This document represents work in progress; current international
interest  in  virtual  terminal  protocols has motivated us to submit
this as an example of  mechanisms  that  a  virtual  terminal  should
support.   The  model  provides a framework for supporting device and
processing  capabilities  not  yet  commonly  available.   A  virtual
terminal  protocol standardization effort may not want to include all
the mechanisms that are described here, but it is our contention that
one should not preclude these extensions for the future.

                                  3







1.4  Related Work

     The concepts presented in this document  are  the  offspring  of
previous  work  in  the  area  of  personal  computing,  and  of user
interfaces to (distributed) systems.  The bibliography at the end  of
the  document  collects  this  material.  In  particular,  we want to
acknowledge the work done at the University of Rochester  on  virtual
terminals,(6)   work  which  has  influenced to a large degree how we
view user interfaces through a display.








































                                  4







2.0  THE VTM MODEL

     This section describes a virtual terminal management (VTM) model
whose  architecture  not  only  derives  from  a  quest  for  device-
independent, terminal-oriented communications, but  more  importantly
from a desire to provide effective human-machine interfaces.

     The VTM architecture  is  a  multi-user  structure  which  spans
several  building blocks. The underlying foundation to this structure
is provided by the cooperating  virtual  terminals.   Under  the  VTM
model,  these  cooperating  virtual  terminals  are  viewed as device
abstractions, all with  a  common  architecture,  exchanging  virtual
terminal  protocol  items  to  update each other's view of the world.
Resting on this foundation lie the adaptation units.  Associated with
a   single   end-user,   an   adaptation   unit   provides  the  step
transformation between user and virtual  domains.   In  a  sense  the
adaptation  unit  is  also  a virtual terminal, although one which is
much closer to the architecture of the end-user.  Finally, on top  of
this  supporting  structure  are  the  end-users, the application and
human processes, all interacting towards a common goal.

     Before embarking on a description of the VTM  model  components,
we  present  the  set of capabilities the VTM model provides its end-
users, either human or application.  After all,  the  motivation  for
the  model  and  its  underlying  concepts  stems  from our desire to
provide productive user environments.

     HUMAN  <--->  WORKSTATION

     o   Multiplexing the workstation physical display both  in  time
         and space.

         The workstation assigns to each user conversation a  logical
         terminal  with  a well-distinguished logical display.  Under
         the  user  control,  the  workstation  maps  these   logical
         displays  on  non-overlapping areas of the physical display,
         providing   a   dedicated   workspace   between   user   and
         correspondents.   Limited  only  by the area of the display,
         many logical displays could be  mapped  at  one  time,  each
         providing  display updates when so required.  Since the area