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Claims  |
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What is claimed is:
1. A method for shifting down a data transmission speed at a transmitter in
response to a request from a receiver for retransmission of data frames
having data errors in a facsimile system, comprising the steps of:
transmitting a plurality of data frames of image information from said
transmitter to said receiver at a first data transmission speed;
examining at said receiver whether or not each of said data frames contains
one or more data errors;
transmitting a request from said receiver to said transmitter for
retransmission of one or more data frames for which one or more data
errors have occurred at the receiver; and
shifting down said data transmission speed from said first data
transmission speed to a second data transmission speed which is used for
retransmitting said one or more data frames for which one or more data
errors have occurred, whereby said second data transmission speed is
determined selectively in accordance with a predetermined manner based on
information regarding a total number of data frames which have been
transmitted from said transmitter to said receiver and also a number of
data frames to be retransmitted wherein a Plurality of data transmission
speeds are previously provided for selection and one of said plurality of
data transmission speeds is automatically selected by comparing a
plurality of predetermined numbers with a ratio which ratio is formed by
said total number of data frames divided b said number of data frames to
be retransmitted.
2. A method for shifting down a data transmission speed at a transmitter in
response to a request from a receiver for retransmission of data frames
having data errors in a facsimile system, comprising the steps of:
transmitting a plurality of data frames of image information from said
transmitter to said receiver at a first data transmission speed;
examining at said receiver whether or not each of said data frames contains
one or more data errors;
transmitting a request from said receiver to said transmitter for
retransmission of one or more data frames for which one or more data
errors have occurred at the receiver; and
shifting down said data transmission speed from said first data
transmission speed to a second data transmission speed which is used for
retransmitting said one or more data frames for which one or more data
errors have occurred, whereby said second data transmission speed is
determined selectively in accordance with a predetermined manner based on
information regarding a total number of data frames which have been
transmitted from said transmitter to said receiver and also a number of
data frames to be retransmitted wherein a plurality of data transmission
speeds are previously provided for selection and one of said plurality of
data transmission speeds is automatically selected by comparing a
plurality of predetermined numbers with a ratio which ratio is formed by
said number of data frames number of data frames to be retransmitted
divided by said total number of data frames.
3. A facsimile machine comprising:
reading means for reading an original to be transmitted;
processing means for processing image information supplied from said
reading means to produce a plurality of data frames from said image
information;
transmitting means for transmitting said plurality of data frames to a
receiver facsimile machine at a remote location through a transmission
path; and
control means for controlling said reading means, said processing means and
said transmitting means, said control means controlling a data
transmission speed of said transmitting means such that the data
transmission speed is shifted down from a first speed to a second speed
when retransmitting one or more of said plurality of data frames, whereby
said second speed is determined automatically in accordance with a
predetermined program based on a total number of said plurality of data
frames and a number of said data frames to be retransmitted further
including a plurality of previously provided selectable data transmission
speeds and a means for automatically selecting one of said plurality of
data transmission speeds by comparing a plurality of predetermined numbers
with a ratio which ratio is formed by said number of data frames to be
retransmitted divided by said total number of data frames.
4. The facsimile machine of claim 3, further comprising first storing means
for temporarily storing image information and data frames.
5. The facsimile machine of claim 4, further comprising second storing
means for storing said predetermined program.
6. The facsimile machine of claim 3, wherein said processing means first
converts said image information supplied from said reading means into
digital image data before converting said image information into said data
frames.
7. The facsimile machine of claim 6, wherein said transmission line is a
public telephone line, and said facsimile machine further comprises a
MODEM for converting said data frames into suitable data appropriate for
transmission through said public telephone line.
8. The facsimile machine of claim 6, wherein said processing means includes
a compressing means for compressing said digital image data by coding,
said digital image data being compressed before being formed into data
frames.
9. The facsimile machine of claim 8 wherein said compressing means is a
CODEC.
10. A facsimile machine system having both a first transmission mode, in
which a plurality of data frames formed from image information are first
transmitted to a receiver and then one or more of said plurality of data
frames are retransmitted to said receiver upon request from said receiver,
and a normal transmission mode, in which image information is transmitted
directly to a receiver without being formed into data frames, comprising:
reading means for reading an original to be transmitted;
transmitting means for transmitting said image information to a receiver at
a remote location through a transmission path, said image information
being transmitted to said receiver after having being formatted in the
form of predetermined data frames when transmitting in said first
transmission mode and without having been formatted in the form of
predetermined data frames when transmitting in said normal transmission
mode;
inputting means for inputting one or more operational commands and/or
information to said transmitting means and said receiver;
storing means for storing first information regarding destination areas in
which said first transmission mode is to be used and second information
regarding a data frame size to be used in each of said destination areas;
and
control means for controlling said reading means, said transmitting means,
said inputting means and said storing means, said controlling means
comparing destination information input through said inputting means with
the destination areas stored in said storing means to determine whether or
not said destination information input through said inputting means
matches with one of said destination areas, whereby if there is a match,
said image information is transmitted to said receiver in said first
transmission mode, and, otherwise, said image information is transmitted
to said receiver in said normal transmission mode.
11. The facsimile machine of claim 10, wherein said storing means includes
a non-volatile memory.
12. The facsimile machine of claim 10, wherein said first transmission mode
is an error correction mode, whereby said retransmission is carried out
for one or more of said plurality of data frames for which one or more
data errors have occurred at said receiver upon receipt.
13. The facsimile machine of claim 10, wherein said normal transmission
mode is a mode in which said image information is first converted into
digital image information and compressed by coding according to a
predetermined coding method.
14. The facsimile machine of claim 13, wherein said transmission path is a
public telephone line and said coded digital image information is
modulated so as to be allowed to be transmitted through said public
telephone line.
15. The facsimile machine of claim 10, wherein said inputting means
includes an operation and display unit which may be manually operated by
an operator.
16. The facsimile machine of claim 10, wherein said destination areas are
those areas whose network conditions between communicating facsimile
machine are between extremely bad and good levels.
17. The facsimile machine of claim 10, wherein each of said data frames has
a HDLC data frame format.
18. The facsimile machine of claim 10, wherein a predetermined number of
said data frames are collected as a block and said data frames are
transmitted to said receiver block by block.
19. A facsimile machine system comprising:
means for producing a plurality of data frames of image information which
is to be transmitted;
transmitting means for transmitting said plurality of data frames to a
receiver facsimile machine at a remote location through a transmission
path; and
control means for controlling said means for producing and said
transmitting means, said control means controlling a data transmission
speed of said transmitting means such that the data transmission speed is
shifted down from a first speed to a second speed when retransmitting one
or more of said plurality of data frames, whereby said second speed is
determined automatically in accordance with a predetermined program based
on a total number of said plurality of data frames and a number of said
data frames to be transmitted further including a plurality of previously
provided selectable data transmission speeds and a means for automatically
selecting one of said plurality of data transmission speeds by comparing a
plurality of predetermined numbers with a ratio which ratio is formed by
said number of data frames to be retransmitted divided by said total
number of data frames.
20. The machine system according to claim 19, wherein said means for
producing includes a reading means for reading and original to be
transmitted and a processing means for processing image information
supplied from said reading means to produce said plurality of data frame.
21. The system of claim 20 further comprising a first storing means for
temporarily storing image information and data frames.
22. The system according to claim 21 further comprising second storing
means for storing said predetermined program.
23. A facsimile machine having both a first transmission mode, in which a
plurality of data frames formed from image information are first
transmitted to a receiver and then one or more of said plurality of data
frames are retransmitted to said receiver upon request from said receiver,
and a normal transmission mode, in which image information is transmitted
directly to a receiver without being formed into data frames, comprising:
means for converting information into image information to be transmitted;
transmitting means for transmitting said image information to a receiver at
a remote location through a transmission path, said image information
being transmitted to said receiver after being formatted in the form of
predetermined data frames when transmitting in said first transmission
mode and without having been formatted in the form of predetermined data
frames when transmitting in said normal transmission mode;
inputting means for inputting one or more operational commands and/or
information to said transmitting means and said receiver;
storing means for storing first information regarding destination areas in
which said first transmission mode is to be used and second information
regarding a data frames size to be used in each of said destination areas;
and
control means for controlling said reading means, said transmitting means,
said inputting means and said storing means, said controlling means
comparing destination information input through said inputting means with
the destination areas stored in said storing means to determine whether or
not said destination information input through said inputting means
matches with one of said destination areas, whereby if there is a match,
said image information is transmitted to said receiver in said first
transmission mode, and, otherwise, said image information is transmitted
to said receiver in said normal transmission mode.
24. The facsimile system according to claim 23, wherein said means for
converting includes a reading means for reading an original to be
transmitted.
25. The facsimile machine system of claim 23, wherein said storing means
includes a non-volatile memory.
26. The facsimile machine system of claim 23, wherein said first
transmission mode is an error correction mode, whereby said retransmission
is carried out for one or more of said plurality of data frames for which
one or more data errors have occurred at said receiver upon receipt.
27. The facsimile machine system of claim 23, wherein said normal
transmission mode is a mode in which image information is first converted
into digital image information and compressed by coding according to a
predetermined coding according to a predetermined coding method. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention generally relates to a facsimile machine, and, in
particular, to a facsimile machine having a function of shifting down its
data transmission speed depending on network conditions.
2. Description of the Prior Art
Typically, use is made of a telephone network as a transmission line for
facsimile data transmission. In the case of a telephone network, the
network condition is liable to change by noises or the like, in particular
when a long-distance communication, such as oversea communication, is to
be carried out. When a facsimile communication is carried out under such
condition, there occurs many errors in transmission of image information
so that there may be a case in which an image cannot be reproduced
accurately at the receiver.
In a typical facsimile machine, for example, of G3 type, the network
condition is examined by MODEM training prior to the initiation of
transmission of image information, and the data transmission speed for
transmitting image information is selectively determined depending on the
status of the network condition.
CCITT has recently announced an additional recommendation for a
communication control procedure called an error correction mode or simply
referred to as ECM hereinafter in the form of Annex A to Recommendation
T.30 on which facsimile machines of the G3 type are based. In the case of
carrying out facsimile communication according to this ECM mode, image
information is transmitted with a block comprised of a plurality of data
frames as a unit, and when errors have occurred in the transmitted image
information, those data frames for which the errors have occurred are
retransmitted. And, if the number of retransmissions has exceeded a
predetermined value, the data transmission speed is shifted down to carry
out retransmission of the data frames at a lower data transmission speed.
Suppose that the network condition was excellent immediately after the
initiation of transmission and then the network condition became extremely
inferior after the initiation of transmission of image information. In
this case, the data transmission speed was initially set, for example, at
9,600 bps for transmission of image information. However, the network
condition deteriorates and many errors start to occur in transmitted data.
As a result, a retransmission process is carried out repetitively over a
predetermined number of times. If the data are not transmitted properly
during this retransmission process, the data transmission speed is shifted
down, for example, to 7,200 bps, and another retransmission process with
this shifted down speed is carried out over a predetermined number of
times. If the data are still not transmitted properly, the data
transmission speed is further shifted down, for example, to 4,800 bps.
And, in this manner, the data transmission speed may finally be shifted
down to 2,400 bps.
In this manner, according to the conventional ECM mode, since the data
transmission speed is shifted down one step at a time when the
deteriorated network condition has occurred, it tends to take time until
the retransmission of image information has been successfully carried out,
which tends to prolong the overall communication time period.
In the above-described ECM mode, the image information after compression by
coding is divided into frames of 256 or 64 bytes (octet; 1 byte=1 octet=8
bits) from the beginning thereof and the image information of one frame is
shaped into a frame FLM of the type of HDLC (High Level Data Link Control)
procedure as shown in FIG. 14a before transmission. The frame FLM includes
a (front) flag sequence F of a predetermined bit pattern, an address field
A of a predetermined bit pattern (global address), a control field C of a
predetermined bit pattern unique to a particular facsimile machine, an
information field I, a frame check sequence FCS for error detection, and a
(rear) flag F arranged in the order mentioned. The information field I
includes a facsimile control field FCF in which a facsimile transmission
procedure signal is arranged and a facsimile information field FIF in
which various information added to the facsimile transmission procedure
signal is arranged. In this case, a facsimile coding data FCD of the
facsimile transmission procedure signal is arranged in the facsimile
control field FCF, and a frame number FNo indicating the order of the
frame and a frame data FDc which is a coding of one frame size FSZ are
arranged in the facsimile information field FIF. Since the frame number
FNo is defined by eight bits of binary numbers, it can range only from 0
to 255 consecutively, so that a consecutive series of 256 frames is set as
a block, and it is so structured that the receiver requests retransmission
of such a block. If the image information of one page could not have been
transmitted by one block, the remaining portion of the image information
is inserted into the next block for transmission.
When the receiver makes a request of retransmission, it sends a frame of
facsimile transmission procedure signal PPR (Partial Page Request signal)
to the transmitter. It is to be noted that although the facsimile
transmission procedure signal is transmitted in a frame format similar to
that of this partial page request signal PPR with necessary parameters, it
will be simply referred to as a PPR signal in the following description
for the sake of convenience. This PPR signal contains a particular bit
pattern PPR, which indicates it to be a PPR signal, in the facsimile
control field FCF and also contains an error map data EMp of 256 bits in
the facsimile information field FIF. In this error map data EMp, among one
block of block data which has been transmitted, a data "0" is assigned to
each of the frames for which no transmission error has occurred and a data
"1" is assigned to each of the frames for which one or more errors have
occurred, and these assigned data are arranged in the order of the frames.
Upon receipt of this PPR signal, the transmitter retransmits only the data
of those frames for which data "1" has been assigned in the error map data
EMp to the receiver. By repetitively issuing this request for
retransmission until all of the data errors have been eliminated, a
received image without errors can be recorded at the receiver.
In transmitting image information as described above, the frame size may be
set at one of two kinds, i.e., 256 bytes and 64 bytes. When this frame
size has once been set during a pre-transmission procedure between the
transmitter and the receiver, it remains valid until the transmission of
one page of image information has been completed and it cannot be altered.
When this frame size is set in a facsimile machine, if it is set
semi-permanently, for example, by operating a switch or the like mounted
on an internal circuit board provided in the facsimile machine, the
following disadvantages could occur.
That is, if the frame size semi-permanently set at 256 bytes, since the
number of frames constituting image information may be fewer, the amount
of information to be added to the frame data may be less. Thus, as
compared with the case in which the frame size is 64 bytes, the
transmission time period may be shortened. However, for example, as shown
in FIGS. 15a and 15b, if burst errors BN1 and BN2 have occurred in a
transmission line, the amount of data to be retransmitted increases
significantly as compared with the case in which the frame size is 64
bytes. Accordingly, if the network condition is inferior, the
retransmission time period could be longer for frame size (FSZ1) of 256
bytes than for frame size (FSZ2) of 64 bytes, so that the overall image
information transmission time period could also be longer for the 256 byte
case. In this manner, in the case where transmission is to be carried out
in the ECM mode, the frame size for which transmission can be carried out
efficiently differs depending on the network condition. On the other hand,
if the network condition is extremely good, since there is produced
virtually no data errors in image transmission, there is no need to use
the ECM mode. Similarly, if the network condition is extremely bad, since
many data errors would occur also during retransmission, the use of the
ECM is meaningless because it only prolongs the transmission time period
without merit.
In a prior art facsimile machine having both of ECM and normal modes of
operation, since the selection between the ECM and normal modes was not
always made properly and the frame size for use in the ECM mode was not
always set properly, the ECM mode was not used effectively at all times.
It may be so structured that such setting be carried out by an operator;
however, in such a case, since the operator is required to determine
appropriate parameters before transmission, which is cumbersome to the
operator and also impractical.
In the conventional G3 type normal mode, when transmitting image
information in facsimile communication, the image information is first
coded and the thus coded image information is normally stored in a FIFO
buffer temporarily. And then the coded image information is read out of
the buffer and transmitted to the receiver at the speed corresponding to
the data transmission speed set in the MODEM. In this case, the FIFO
buffer serves as an interface so as to absorb the discrepancy in timing
between the coding of image information and transmission of image
information from the MODEM.
In the case of the ECM mode, when constructing a HDLC data frame, the
insertion of "0" data so as to avoid the occurrence of the same bit
pattern as that of the flag in frame data and the CRC calculation for
error checking operation are carried out. For this reason, in the case of
facsimile communication in the ECM mode, during transmission, a block of
the coded image information is temporarily stored in the buffer memory to
thereby facilitate the carrying out of such processes as insertion of data
"0" in the frame data and CRC calculation and to allow for the
retransmission of image information. Thus, both in the conventional G3
type normal mode communication and the ECM mode communication, the image
information to be transmitted is stored in a buffer. And, in the prior art
facsimile machines, two separate buffers for storing image information
were provided for the above-described two kinds of communication.
As described above, in accordance with the conventional ECM mode, when the
network condition deteriorates after the initiation of transmission of
image information, the communication time period tends to become
prolonged. Furthermore, in the prior art facsimile machines, the ECM mode
was neither properly set nor effectively used. Moreover, in the prior art
facsimile machines, two separate buffers for temporarily storing image
information were provided, which constituted one of the reasons for
increased cost.
Japanese Patent Laid-open Pub. No. 58-198964, published Nov. 19, 1983,
discloses a data transmission system using a shift-down/shift-up condition
determining circuit 7 which calculates the number of retransmission
operations selectively carries out either a two-step shift-down operation
or a one-step shift-down operation depending on the manner of occurrence
of retransmission operations, i.e., either continuous or continual.
Japanese Patent Post-examination Pub. No. 60-36145, published Aug. 19,
1985, discloses a signal transmission system in which retransmission is
carried out only when transmission errors have occurred in frame
information having a high degree of importance and retransmission is not
carried out for frame information having a low degree of importance even
if transmission errors have occurred so as to shorten the overall
transmission time period.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, there is provided a
novel scheme for determining a data transmission speed in a facsimile
machine, according to which the degree of shift-down of data transmission
speed is determined based on a ratio between the total number of frames of
image information which have been transmitted and the number of frames
which have been indicated to contain data errors by the receiver. With
this novel scheme, the communication time period may be maintained at
minimum at all times. In this manner, since the data transmission speed is
determined on the basis of the total number of transmitted frames and the
number of frames which contain data errors as a result of transmission in
this aspect of the present invention, the data transmission speed can be
set properly and optimally depending on the network condition.
Accordingly, retransmission is prevented from being carried out
unnecessarily and thus the overall communication time period is always
maintained at a minimum.
In accordance with another aspect of the present invention, there is
provided a facsimile machine which includes a memory for storing
information regarding areas to which "retransmission upon error ocurrence"
is applied. The information to be stored includes destination areas to
which the ECM mode is to be applied and the frame size for each of the
destination areas. Thus, when the destination information input by an
operator belongs to one of the destination areas stored in the error
retransmission application area memory, the ECM mode is activated and the
frame size of the corresponding destination area stored in the memory is
selected to be used for transmission of image information. In this manner,
there is provided a structure to use the ECM mode most effectively. With
this structure, it is not necessary for the operator to manually set
appropriate conditions for using the ECM mode.
In accordance with a further aspect of the present invention, there is
provided a facsimile machine including a common buffer memory which is
used for temporarily storing coded image information until the
retransmission processing has been completed to transmit all of the image
information to the receiver properly during the ECM mode and for
temporarily storing coded image information until transmission of image
information has been completed during the conventional G3 type normal
mode. With this structure, a common buffer memory is shared between the
ECM and normal modes, so that it is only necessary to provide only one
buffer memory, which contributes to reduce the number of parts in a
facsimile machine.
It is therefore a primary object of the present invention to obviate the
disadvantages of the prior art as described above and to provide an
improved facsimile machine.
Another object of the present invention is to provide an improved facsimile
machine having a novel data transmission speed shift-down function
A further object of the present invention is to provide an improved data
transmission method and system, such as a facsimile machine, capable of
maintaining the overall transmission time at minimum at all times.
A still further object of the present invention is to provide a facsimile
machine capable of using an ECM mode effectively.
A still further object of the present invention is to provide a facsimile
machine which has high data transmission, high efficiency and is low cost.
Other objects, advantages and novel features of the present invention will
become apparent from the following detained description of the invention
when considered in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing the overall structure of a facsimile
machine constructed in accordance with one embodiment of the present
invention;
FIG. 2a is an illustration showing how to combine FIGS. 2aI and 2aII;
FIGS. 2aI and 2aII, when combined as shown in FIG. 2a, and 2b show a flow
chart illustrating the sequence of steps of a transmission process to be
carried out by the facsimile machine shown in FIG. 1;
FIG. 3 is a timing chart showing a transmission control procedure in a
normal transmission process;
FIG. 4 is a timing chart showing a transmission control procedure in a
retransmission process;
FIG. 5 is a timing chart showing one example of transmission control
procedure useful for explaining an alternative method of calculating an
error rate;
FIG. 6 is an illustration showing one example of a method for setting a
data transmission speed by a shift-down function;
FIG. 7 is a block diagram showing the overall structure of a facsimile
machine constructed in accordance with another embodiment of the present
invention;
FIG. 8 is an illustration showing the contents and the format of data
stored in the parameter memory 11 provided in the structure shown in FIG.
7;
FIG. 9 is an illustration showing how to combine FIGS. 9I and 9II;
FIGS. 9I and 9II, when combined as shown in FIG. 9, is a flow chart showing
a sequence of steps for determining whether or not to set an error
correction mode (ECM) and also a sequence of steps for determining a frame
size in the case where the ECM mode is set;
FIG. 10 is a timing chart useful for explaining a transmission procedure to
be carried out in the structure shown in FIG. 7;
FIG. 11a is a flow chart showing a sequence of steps of an image reading
process to be carried out at a transmitter in a further embodiment of the
present invention;
FIG. 11b is a flow chart showing a sequence of steps of an image
information transmission process to be carried out at the transmitter in
the embodiment shown in FIG. 11a;
FIG. 12a is a flow chart showing a sequence of steps of an image
information reception process to be carried out at a receiver in
association with the embodiment shown in FIGS. 11a and 11b;
FIG. 12b is a flow chart showing a sequence of steps of an image recording
process to be carried out at the receiver in the embodiment shown in FIG.
12a;
FIG. 13 is a schematic illustration showing the flow of image information
in the embodiment shown in FIGS. 11a, 11b, 12a and 12b;
FIG. 14a is an illustration showing an example of the format of a frame to
be used for transmitting image information arranged in the form of frames
in accordance with the conventional error correction mode;
FIG. 14b is an illustration showing an example of a signal indicating a
request for retransmission; and
FIGS. 15a and 15b are illustrations which are useful for explaining a
relationship between burst noise and a frame size.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to FIG. 1, there is shown in block form the overall structure
of a facsimile machine constructed in accordance with one embodiment of
the present invention. As shown, the illustrated facsimile machine
includes a CPU 1 which is in charge of the overall control of the present
facsimile machine and which also implements a facsimile transmission
control procedure process and a control program which is stored in a ROM
2. Also provided is a RAM 3 which provides a work area and a transmission
buffer. It is to be noted that the transmission buffer is designed to have
a memory capacity of at least 64 K bytes (K=1,024) so as to allow to store
one block of frames of 256 bytes. The facsimile machine also includes a
scanner 4 for reading an original to be transmitted at a predetermined
resolution and a plotter 5 for recording a received image on a sheet of
recording medium at a predetermined resolution. An operation and display
unit 6 is also provided as a man-machine interface through which various
operational commands and conditions may be supplied by an operator
There are also provided a CODEC 7 which compresses image information to be
transmitted by coding and decompresses received image information by
decoding and a MODEM 8 which modulates and demodulates digital image data
so as to allow such digital image data to be transmitted through a public
telephone network or the like, which is an analog network. A net control
unit 9 is also provided and it serves to establish a connection between
the present facsimile machine and a public telephone network. It is to be
noted that the net control unit 9 is provided with an automatic calling
and call receiving function. Furthermore, CPU 1, ROM 2, RAM 3, scanner 4,
plotter 5, operation and display unit 6, codec 7, MODEM 8 and net control
unit 9 are all interconnected through a system bus 10 so that data may be
exchanged between any two desired elements.
Now, the transmission operation of the facsimile machine having the
above-described structure will be described with particular reference to
FIGS. 2a and 2b. In the first place, an original to be transmitted is set
in the scanner 4 and then a predetermined transmission operation, such as
inputting of information regarding a destination of transmission, is
carried out at the operation and display unit 6 (step 21). As a result,
the net control unit 9 is activated (step 22), and when a call has been
received by a receiver facsimile machine, a transmission control procedure
is initiated as shown in FIG. 3, so that signals CED and DIS are first
transmitted to a transmitter from the receiver (step 23). In response
thereto, the transmitter transmits a signal DCS, together with a MODEM
training signal and a signal TCF (step 24). In the above-mentioned DCS
signal, the data transmission speed is designated at the highest speed of
9,600 bps.
In order to determine the status of the network, the receiver examines a
predetermined data transmitted by the TCF signal and then transmits a CFR
signal if the data error rate is equal to or lower than a predetermined
value. At the transmitter, upon receipt of this CFR signal (Y of step 25),
the scanner 4 is activated to have the original to be transmitted
optically read (step 26) to thereby convert visual image information into
electrical image information. The image information thus read is then
coded by the codec 7, and the thus coded image information is arranged in
the FIF section of a HDLC data frame, for example, by 256 bytes. These
data frames are collected to define one block of up to 256 frames. Thus,
in the case of 256 bytes per one frame, one block has a maximum of 64 k
bytes. One block of image information is then temporarily stored in the
RAM 3 and the total number of frames N is also stored in the RAM 3 (step
27). Then, this one block of image information is transmitted as image
information PIX continuously at the above-designated data transmission
speed of 9,600 bps (step 28).
Suppose that the original to be transmitted includes only one page and its
image information is 64 k bytes or less. Upon transmission of all of the
image information in succession, signals PPS and EOP are transmitted (step
29). The receiver receives the image information transmitted from the
transmitter and examines the presence of any data error. If no data error
has been found, then the receiver sends a signal MCF to the transmitter.
If the transmitter receives this MCF signal (Y of step 30), then it
transmits a signal DCN to have the network restored (step 31).
Now, a description will be had as to the case in which the network
condition was initially excellent, but it deteriorates during transmission
of image information PIX. In this case, as shown in FIG. 4, the
transmission of image information was initiated at the data transmission
speed of 9,600 bps similarly as the previous case, and, thereafter,
signals PPS and EOP are transmitted. When the network condition
deteriorates and there is produced a data error in the transmitted image
information, the receiver detects this data error and stores information
regarding which of the frames in one block of image information up to 256
frames contains the error. And, then, after having received a
PPS.multidot.EOP signal, the receiver transmits a PPR signal to apprise
the transmitter of the location of the frame which has produced the data
error.
Upon receipt of this PPR signal (N of step 30 in FIG. 2a), the transmitter
examines the number of retransmissions which have so far been carried out
(step 32). In the present case, since this is the first event of
retransmission (N of step 32), the image information corresponding to the
frame position indicated by the PPR signal is read out of the RAM 3. And,
then, the image information thus read out is retransmitted in a
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