rfc809.txt

RFC 相关的技术文档

     facsimile  machine on the end of a telephone line. Thus
     the local facsimile machine could transmit data to  the
     computer  quite happily, believing that it was actually
     talking to a remote facsimile machine on the other  end
     of  a  telephone  wire.  Because of the property of the
     DACOM 6450 used in the experiment [16],  the  interface
     could  be  identical to one developed for connecting to
     an X25 network. The binary synchronous mode of the chip
     used  (SMC  COM5025) was appropriate to drive the DACOM
     machine.

       At the other side of the computer network there was a
     similar  computer  with an identical facsimile machine.
     The problem of transmitting  a  facsimile  picture  now
     appeared  simple:  data  was  taken  from the facsimile
     machine into the computer, transmitted over the network
     as  if  it was normal computer data, and then sent from
     the computer to the facsimile  machine  at  the  remote
     end.  The  data  being  sent  over  the network appears




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UCL FACSIMILE SYSTEM                              INDRA Note 1185

     exactly as any other computer data;  there  is  nothing
     special  about  it  to  signify  that  it  came  from a
     facsimile machine.  The  schematic  of  such  facsimile
     transfer system is shown in Fig. 2.



     facsimile
     machine
      +---+  interface
      !   !    +--+    +-----+
      !   ! == !  ! == !     ! computer
      +---+    +--+    +-----+
                          |
                           - - - - - -    computer
                         /             \  network

                         \             /             facsimile
                           - - - - - -               machine
                                      |    interface  +---+
                                   +-----+    +--+    !   !
                          computer !     ! == !  ! == !   !
                                   +-----+    +--+    +---+

                Fig. 2  Facsimile transfer system


       The experimental system was used to perform  a  joint
     experiment  between  UCL  and  two groups in the United
     States. Pictures were exchanged via the  ARPANET/SATNET
     [21],  [22]  between UCL in London, ISI in Los Angeles,
     and  COMSAT  in  Washington   D.C.   (Fig.   3).   This
     environment  was chosen because no equivalent group was
     available in the UK.

       One  problem   concerned   with   such   image   data
     transmission  is  the  quantity of data. Even with data
     compression,  a  single  page  of  facsimile  data  can
     produce  as  much  computer  data  as would normally be
     sufficient   for   sending   over   20,000   alphabetic
     characters  -  or  over a dozen typed pages. Thus for a
     given number of pages put into the system,  an  immense
     amount  of  computer  data is produced. This means that
     the transmission will be slower than for sending  text,
     and  that far more storage will be required to hold the
     data.

       Another problem was encountered which became only too
     apparent  when we implemented this system.  The network
     we were using was often unable  to  keep  up  with  the
     speed of the facsimile machine.  When this happened the




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UCL FACSIMILE SYSTEM                              INDRA Note 1185



                      US               UK
                           satellite
     COMSAT                   __
     +---+    +--+           /  \
     !   ! -- !  !           /  \
     +---+    +--+          /    \
       |          \        /      \
     +---+         \      /        \           UCL
     !fax!          \+--+/          \+--+    +---+
     +---+  ARPANET  !  !   SATNET   !  ! -- !   !
                    /+--+            +--+    +---+
                   /                           |
     ISI         /                          +---+
     +---+    +--+                           !fax!
     !   ! -- !  !                           +---+
     +---+    +--+
       |
     +---+
     !fax!
     +---+

     Fig. 3. The three participants of the facsimile experiments

     computer tried to slow down the facsimile machine.  The
     facsimile  machine  would  detect  this 'slowness' as a
     communication problem (as a telephone line would  never
     act  in  this  manner),  and would abandon the transfer
     mid-way through the page.

       This is because the the  facsimile  machine  we  were
     using  was never intended for use on a computer; it was
     designed and built for use on telephone lines.  Indeed,
     being  unaware that it was connected to a computer, the
     facsimile machine transmitted data at a constant  rate,
     which exceeded the limit that the network could accept.
     In other words, the computer network we were using  was
     not  designed for the transfer rate that we were trying
     to use over it.

       Both  these  problems  are  surmountable.   Facsimile
     machines are coming on the market that are designed for
     direct communication with a computer. These machines do
     not  mind  the delays on the computer interface and are
     tolerant of the stops and re-starts. On the other hand,
     if  there were a serious use of facsimile machines on a
     computer network, the network could be designed for the
     high  data rate required. Our problem was aggravated by




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UCL FACSIMILE SYSTEM                              INDRA Note 1185

     using a network that was never designed  for  the  data
     rates required in our mode of usage.

       Despite the problems we encountered being a result of
     the  experimental  equipment  we  were working with, we
     still had to  improve  the  situation  to  permit  more
     extensive communications to take place. The easiest way
     to do this was to introduce a local storage area in our
     computer   where  the  data  could  be  held  prior  to
     transmission.  The transfer of a page is  now  done  in
     three  stages.   First, the facsimile data is read from
     the facsimile machine and stored on a local disk.  This
     takes  place  at  high  speed  as  this is just a local
     operation.  When this is complete,  the  data  is  sent
     over  the  network  to  a  disk on the remote computer.
     Finally, the data from  that  disk  is  output  to  the
     remote  facsimile  machine.   This  improved  system is
     shown in Fig. 4.



                     computer network
      fax    computer    - - - -     computer   fax
     +---+   +-----+   /         \   +-----+   +---+
     !   ! = !     ! =     ==>     = !     ! = !   !
     +---+   +-----+   \         /   +-----+   +---+
        - - - + |        - - - -        | + - - >
              | | + - - - - - - - - - + | |
              | | |                   | | |
              V | |                   V | |
              +---+                   +---+
              !   !                   !   !
              !   !                   !   !
              +---+                   +---+
              disk                    disk

         Fig. 4.  The improved facsimile transfer system


       The idea  behind  this  method  is  to  decouple  the
     facsimile  machine from the network communications. The
     data is read from the facsimile machine at full  speed,
     without  the  delays  caused  by  the computer network.
     This also has the effect of being  more  acceptable  to
     the human operators: each page is now read in less than
     a minute.  The transmission over the network then takes
     place  at  whatever speed the network can sustain. This
     does not affect the facsimile machines at all; they are
     not involved in the sending or receiving. Only when all
     the data has been received at the remote  disk  is  the
     remote  facsimile  machine told that the data is ready.




                              - 7 -

UCL FACSIMILE SYSTEM                              INDRA Note 1185

     The facsimile machine is then given the data as fast as
     it will accept it.

       The disadvantage of such a system is that the  person
     sending  the  pages  does  not know how long it will be
     before they are actually printed at the other side.  If
     several  pages  are  input  in  quick succession by the
     operator, they will be stored on disk; it may  then  be
     some time before the last page is actually delivered to
     the destination. This is  not  always  a  disadvantage;
     where  many  operators  are  sending  data  to the same
     destination, it is a definite advantage to be  able  to
     input  the  pages and have the system deliver them when
     the  destination  becomes  free.  Such  a   system   is
     preferable to use of the current telephone system where
     the  operator  has  to  keep  re-dialing   the   remote
     facsimile machine until the call is answered.


     2.2 Interworking with Other Equipment

     2.2.1 Facsimile machines

       As was mentioned earlier, facsimile machines  produce
     a large amount of data per page due to the way in which
     the pages are encoded.  To reduce the data that has  to
     be  transmitted,  various  compression  techniques  are
     employed.  The manufacturers of facsimile machines have
     developed   proprietary  ways  in  which  the  data  is
     compressed and encoded.  Unfortunately this  has  meant
     that  interworking  of different facsimile machines has
     been impossible.  In the system described in  the  last
     section, exchange of pictures was only possible between
     sites that had identical facsimile  machines.  The  new
     set  of CCITT recommendations will reduce the extent to
     which differences in equipment persist.

       Having  the  data  on  a  computer   gives   us   the
     opportunity  to manipulate data in any way we wish.  In
     particular we could convert the data from the form used
     in  one  facsimile machine to that required by another.
     This means that interworking between different types of
     facsimile machines can be achieved.

       The development of this  system  took  place  in  two
     stages:  the  decompression  of the facsimile data from
     the coded form used in our  machine  into  an  internal
     data  form  and  the  recompression  of the data in the
     internal form into the encoded form  required  for  the
     destination  machine.  Two  programs  were developed to
     perform these two operations.




                              - 8 -

UCL FACSIMILE SYSTEM                              INDRA Note 1185

       At the same time we were developing  compression  and
     decompression  programs  for  machines  that  use other
     techniques.  In particular, we  developed  programs  to
     handle  the  recently approved CCITT recommendation for
     facsimile compression [15]. The CCITT came up with  two
     varieties of compression, depending upon the resolution
     being used.

       Unfortunately there were no facsimile machines on the
     network  that  use  the  CCITT  compression  technique.
     However, the programming of the  new  methods  achieved
     two  goals:  it proved that the data could be converted
     inside a small computer, so that machines of  different
     types could be supported on the network, and it enabled
     us  to  compare  the  compression  results.  These  are
     described  in  more detail in [13].  Essentially, these
     show that the DACOM technique  used  by  our  facsimile
     machine  is  comparatively  poor, and that considerably
     less data need be transmitted if some other  method  is
     used.  This  brings  up  another  possibility: we could
     change the compression of the data to reduce the volume
     for transmission and then change the data back again at
     the   destination.   This   may    save    considerable
     transmission  time,  especially  if  fast  computers or
     special hardware was easily available.   This  has  not
     been  tried  yet  in  our  system, as none of the other
     users on the network have the  capability  of  changing
     the  data  format  back  into  that  required  by their
     machines.

       There  are  many  other  more  efficient  compression
     schemes,  e.g.   block  compression  [7] and predictive
     compression [8], but we have not yet incorporated  them
     into our system.


     2.2.2 Output Devices

       One area that we have explored is the use of  devices
     other  than facsimile machines for outputting the data.
     Facsimile  machines  are  both  expensive  to  buy  and
     relatively  slow  to  operate. We have investigated the
     use of a TV-like screen to display the  data,  just  as
     character VDUs are commonly used to display text.  This
     activity requires bit-map displays, with an address  in
     memory  for each postion on the screen. Full colour and
     multiple shades can be used  with  appropriately  large
     bit-map  storage.   Although  simple  in principle, the
     implementation  of   the   relevant   techniques   took
     considerable effort.





                              - 9 -

UCL FACSIMILE SYSTEM                              INDRA Note 1185

       The problems arise in  the  way  that  the  facsimile
     image  is encoded. Raw facsimile images consist of rows
     of small dots, each dot recorded as a  black  or  white
     space. When these dots are arranged together they build
     up a picture in a similar manner to the way in which  a
     newspaper  picture is made up. Unfortunately the number
     of dots used in a facsimile page is not the same as the
     number  used  on  most screens. For instance, the DACOM
     facsimile machine uses 1726 dots across each page,  but
     across  a  screen there are usually just 512 dots. Thus
     to show the picture on the screen the 1726 dots must be
     'squeezed' into just 512 dots; stated another way, 1214
     dots must be thrown away without losing the picture!

       It is in reducing the number of picture elements that
     the  problem  arises.  We could just every third dot or
     so from the facsimile  page  and  just  display  those.
     Alternatively,  we  could  take three or more at a time
     and try to convert the group  of  them  into  a  single
     black  or  white  dot.   Unfortunately,  in  both these
     cases, data can get  lost  that  is  necessary  to  the
     picture.   For  instance,  a  facsimile  encoding of an
     architect drawing could easily end up with  a  complete
     line  removed,  radically  changing the presentation of
     the image.

       After much experimentation, we developed a method  of
     reducing  the  number  of  dots  without destroying the
     picture. This is  a  thinning  technique,  whereby  key
     elements  of  the picture are thinned, but not removed.
     Occasionally, when  the  detail  gets  too  fine,  some
     elements  are merged, but under these circumstances the
     eye would not have been able to see the detail  anyway.