rfc803.txt

RFC 的详细文档！

  | | |       |                                 |       | | |
  | | |      1|                                 |0      | | |
  | | | .-----------.                     .-----------. | | |
  | | ->|           |                     |           |<- | |
  | |   |     1     |                     |     2     |   | |
  | |   |    B-W    |<-----.       .----->|    W-B    |   | |
  | |   \-----------'      |       |      \-----------'   | |
  | |      |     A         |       |         A     |      | |
  | |      |     |         |0     1|         |     |      | |
  | |      \-----'         |       |         \-----'      | |
  | |         0          .-----------.          0         | |
  | |                    |           |                    | |
  | |                    |     4     |                    | |
  | |        RUN         |           |         RUN        | |
  | |      .-----.       \-----------'       .-----.      | |
  | |      |     |         A       A         |     |      | |
  | |      |     V         |       |         V     |      | |
  | |   .-----------.   1  |       |  1   .-----------.   | |
  | \-->|           |------'   0   \------|           |<--' |
  |     |     3     |<--------------------|     0     |     |
  \---->|    B-B    |-------------------->|    W-W    |<----'
        \-----------'          0          \-----------'      
                         
                Figure 3.  DFSA Model of Encoding 

Dacom 450/500 Facsimile Data Transcoding                        PAGE   6



2.2.  Formatting Considerations

     Data are encoded for transmission  by  the  Dacom  450  in  585-bit
frames,  consisting  of  a  24-bit  synchronization code, 37-bit leader,
512-bit information area and l2-bit checksum.  There are  two  kinds  of
frames  distinguished  by leader format, one for setup or initialization
and the other for the data itself.  Serial binary image data are  placed
in the data area of succeeding data frames.

     The header of each frame is shown in Figure 4.  The various  fields
are defined in Table 1 following the Figure.

    
           

   +-----------+--------+-------------------+----------+
   | Sync Code | Leader |        Data       | CRC Code |
   +-----------+--------+-------------------+----------+
        24    /    37    \       512             12
     .-------'            \----------------------.
    /                                             \
   +-------+-------+-------+-------+-------+-------+
   | Flags | Count | X Pos | Black | White | State |
   +-------+-------+-------+-------+-------+-------+
   |   7    \ 10      12       3       3       2
   |         \--------------------------.
   |                                     \
   +-----+-----+------+-----+-------+-----+
   | Seq | RUN | COFB | RPT | Spare | SUB |
   +-----+-----+------+-----+-------+-----+
      2     1     1      1      1      1

                   Figure 4. Frame Format

Dacom 450/500 Facsimile Data Transcoding                        PAGE   7



             Table 1. Header Field Definitions 

       

Field   Width   Function                Setup   Data
       (bits)                           Block   Block
-----------------------------------------------------

Sync Code  24   Synchronization         30474730 (octal)

Seq         2   Sequence number         00      00,01,10,11

RUN         1   Initialize-start        0       1

COFB        1   Unknown                 0       0

RPT         1   Unknown                 1       0

Spare       1   Unknown                 0       0

SUB         1   Indicates setup frame   1       0

Count      10   Number of bits in data  All 1's
                field (0 - 512)

X Pos      12   Current position on     All 1's
                scan line (0 - 1725)

Black       3   Current black field     All 1's
                length (2 - 7)

White       3   Current white field     All 1's
                length (2 - 7)

State       2   Current state (0 - 3)   All 1's

Data      512   Data (0 - 512 bits)

CRC Code   12   CRC checksum. Uses polynomial
                x**12 + x**8 + x**7  + x**5 + x**3 + 1

Dacom 450/500 Facsimile Data Transcoding                        PAGE   8



     Setup frames have additional information in  the  data  field;  the
various fields and their functions are described in Table 2.


        Table 2. Field Definitions for Setup Frame.
     

Field       Width       Function
--------------------------------

Start bit       1       Always zero

Speed bit       1       Set if express mode

Detail bit      1       Set if detail mode (speed and detail
                        bits both zero for quality mode)

14 inch         1       Set if 14-inch paper 

5 inch          1       Set if 5-inch inch paper (14-inch
                        and 5-inch inch paper bits both zero
                        for 11-inch paper)

Paper present   1       Set if paper present in scanner

Spare           5       Can have any value

Multi-page      1       Set if multi-page mode

               20       All 0's

              480       Alternate 1's and 0's


     The tailing setup frames differ from the leading setup frames  only
in  bits  which  indicate  whether  the system is operating in single or
multiple page mode and whether paper is present in the scanner.

     All n-bit numeric fields (except Seq) are transmitted by the  Dacom
450  machine  least-significant-bit  (LSB)  first  (i.e.   Count, X Pos,
Black,  White,  State, CRC, and run length words  in  the  data  field).
All other fields are transmitted left-most bit first.

     There are a few important points to be considered in regard to  the
header  of  a  data frame.  The header contains enough information about
the state of the decoding algorithm to be able to  re-establish  correct
decoding  in  the  event  of  loss  or  mutilation of a data frame.  The
decoding algorithm resets its state variables to  those  in  the  header
each  time  it  begins  decoding  a  new  data  frame.   One of the most
difficult problems encountered while constructing the decoding algorithm
was  the  correct synchronization of the algorithm as it proceeds across
the frame boundary with respect to the header information.  In order for
synchronization  to  be  maintained, the operation of the algorithm must

Dacom 450/500 Facsimile Data Transcoding                        PAGE   9



follow exactly that described in the previous section.

     This requirement for every data  frame  to  be  self-synchronizing,
leads  to  a  few  subtleties in the encoding algorithm which seem quite
natural, but were not very obvious in the beginning.

1.  Transition bits(s) labeling the arcs on the state transition diagram
    in  Figure  2  are  not broken across frames.  Similarly, individual
    run-length words are not broken across frames.

2.  If a frame ends with a transition, the  header  of  the  next  frame
    contains the state to which the transition is made.

3.  If a frame ends with a transition out of state  0  or  3,  then  the
    transition  bit (0 or 1) is inserted at the end of the current frame
    (not at the beginning of the next frame).

4.  The field lengths for black and white runs  in  the  header  include
    changes that may have been caused at the end of the previous frame.

5.  If a frame begins with a white  or  black  run,  then  this  run  is
    treated  (for  purpose of decreasing its field length) as if it were
    the beginning of a new run, since there is  no  information  in  the
    header to indicate otherwise.

     The decoding algorithm is  initialized  at  the  first  data  frame
received  after  the  sequence  of  setup  frames  at  the  beginning of
transmission.  The first data frame has a count of zero,  indicating  no
data  bits  are  in  the frame.  The second data frame begins the actual
document; however, its X position appears to be irrelevant.  Instead, we
assume the initial X position at this time is one pel to the left of the
right margin  (-l  mod  l726).   With  these  assumptions  succeeding  X
positions of the algorithm and the frame headers agree.

2.3.  The Decoding Program

     The decoding algorithm described above has been implemented in  the
PDP-11  MACRO-11 assembly language for the RT-11 operating system.  This
program contains extensive features for selectively dumping  frames  and
tracing  the operation of the algorithm.  It is designed to operate on a
file containing the raw data generated  by  the  machine  and  does  not
depend  upon  any  prior  reformatting  of  the  data.  However, it will
operate also on files in the so-called UCL format [4],  which  has  been
adopted  as  the standard for use in the Internet Program.  The existing
DCNET supporting software for the Dacom 450  uses  the  UCL  format  and
operates  normally  to  copy  data  directly between the machine and the
file, with decoding operations done at a later time.  However, there  is
no  intrinsic factor, except processing-rate limitations, why input data
could not be decoded directly from the machine.

     In operation, the program scans the input data one bit  at  a  time
and  searches  for  the  synchronization  pattern.   Note  that all data
processed are inverted from the natural interface conventions.   When  a

Dacom 450/500 Facsimile Data Transcoding                        PAGE  10



synchronization  pattern  is  found,  the  header  and data portions are
extracted  and  the  various  state  variable  checked  and  reset,   if
necessary.    Checksum   verification  is  performed  according  to  the
polynomial 1 + x**3 + x**5 + x**7 + x**8 + x**12.  In the case of  setup
frames  the  format  (detail, quality, express), page length (14, 8-l/2,
5-l/4) and multiple-page indicators are extracted from  the  data  area.
Finally,  under  control  of  specified  options,  the  header  and data
portions of the frame are printed with appropriate headings.

     The decoding algorithm itself is called for each  data  frame.   It
produces  an  output  consisting of a sequence of run-length pairs which