rfc809.txt
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attach the facsimile machine to a computer.
The facsimile machine is not like a standard piece of
computer equipment. We required a special hardware
interface to enable communication between the facsimile
machine and a small computer. This interface was made
to appear exactly like the telephone system to the
facsimile machine. Furthermore, the computer was
programmed to act exactly as if it were another
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.
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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.
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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.
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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
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