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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a data transmission system and particularly to a
facsimile transmission system in which a receiver can send a request for
retransmission of data in the form of a block upon detection of an error
to the transmitter.
2. Description of the Prior Art
As well known in the art, in a facsimile communication system, in a
protocol procedure prior to transmission of messages or image data, the
transmission speed or rate of MODEM is selected to carry out MODEM
training, and if successful, transmission of image data at the selected
transmission speed is carried out. However, even if the MODEM training has
been carried out successfully, it does not insure the transmission of
image data in an excellent condition at all times. If the condition of
transmission line becomes poor during a transmission operation, image data
may be partly lost during transmission thereby hindering to transmit image
data in a complete form to a receiver.
In order to cope with the above-described inconveniences, the ARQ method
has been applied to a facsimile transmission system. In this case, image
data is divided into blocks, each containing a predetermined number of
bits, prior to transmission to a receiver. And, to each block data is
added a start flag, stop flag, command ( including block number ) and CRC
code so that the image data is formulated into a predetermined frame.
Thus, the image data is transmitted from a transmitter to a receiver block
by block. At the receiver, each block data is examined upon receipt and if
it is found that there is abnormality in CRC code, the block number
differs from the one expected at the receiver, and/or the flag is
destroyed, then the receiver automatically sends a signal for requesting
retransmission of block data to the transmitter.
In such a scheme, the same image data is repetitively transmitted from the
transmitter to the receiver whenever malfunction occurs during
transmission. Thus, it is insured that the image data received at the
receiver is always normal without loss and alterations, thereby allowing
the receiver to receive an image which is faithful to an original image
transmitted from the transmitter at all times. In a transmission system
equipped with such an ARQ function, the transmitter must receive a
response signal from the receiver while the transmitter is transmitting
the next following block data. Therefore, the above-described ARQ function
becomes inoperative when data must be transmitted through a transmission
route having a relatively large delay in transmission. For example, this
happens when a submarine cable or communications satellite is involved.
When a submarine cable is used, there is a transmission delay of
approximately 50 milliseconds in one way; whereas, when a communications
satellite is used, there is similarly a transmission delay of
approximately 300 milliseconds. In such a case, since there is a
relatively large delay in transmission, there is a chance that the
transmitter fails to receive a response signal from the receiver as to the
last preceding block data which has been just transmitted while the
transmitter is transmitting the next following block data.
SUMMARY OF THE INVENTION
It is therefore a primary object of the present invention to provide an
improved data transmission system.
Another object of the present invention is to provide a data transmission
system provided with an ARQ function which may remain operative even if
there is a relatively large delay in transmission.
A further object of the present invention is to provide a facsimile
transmission system capable of transmitting image data without loss and/or
alterations.
A still further object of the present invention is to provide an ARQ
equipped facsimile communication system which allows to transmit image
data through a transmission route having a relatively large transmission
delay, such as a submarine cable and communications satellite.
Other objects, advantages and novel features of the present invention will
become apparent from the following detailed 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 an ARQ-equipped
facsimile communication system constructed in accordance with one
embodiment of the present invention; and
FIGS. 2(a) through 2(d) are timing charts showing several wave forms which
are useful for understanding the operation of the system shown in FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1, there is shown an ARQ-equipped facsimile
communication system constructed in accordance with one embodiment of the
present invention. As shown, the system includes a transmitter 1, a
receiver 2 and a transmission line L extending between the transmitter 1
and the receiver 2. The transmitter 1 includes a transmission controller
11 which carries out the overall transmission control of transmitter 1, a
low speed MODEM 12 for use with various control signals and a high speed
MODEM 13 for use with image data to be transmitted. Similarly, the
receiver 2 includes a reception controller 21, a low speed MODEM 22 and a
high speed MODEM 23.
The transmitter 1 further includes a block data generator 14 as connected
to the transmission controller 11 and high speed MODEM 13. The block data
generator 14 receives compressed image data line by line and stores the
data temporarily, and, then, divides the data into blocks each having a
predetermined number of bits set by the transmission controller 11. Then a
start flag, a stop flag, a control bit and a CRC code are added to each of
the block data thus formed and the block data added with additional
information are supplied to the high speed MODEM 13.
The transmitter 1 also includes a response signal detector 15 connected to
the transmission line L for detecting a reception response signal
indicating the safe receipt of a block of data by the receiver and a block
data generator controller 16 for controlling the operation of the block
data generator 14 such that the next following block of data is
transmitted if the received response signal indicates ACK (acknowledgment)
and, on the other hand, the same block of data is retransmitted if the
received response signal indicates NACK (non-acknowledgment). Moreover, in
response to a command supplied from the transmission controller 11, the
block data generator controller 16 also functions to stop input of image
data into the block data generator 14.
Also provided in the transmitter 1 is a delay measuring signal generator 17
connected to supply a delay measuring signal for measuring a time delay of
a transmission route to the transmission line L. The transmitter 1 also
includes a delay response signal detector 18 for detecting a delay
response signal transmitted from the receiver 2 in response to the delay
measuring signal and a delay measurement controller 19 which activates the
delay measuring signal generator 17 and receives a detection signal from
the delay response signal detector 18 to measure a time delay in
transmitting data between the transmitter 1 and the receiver 2 through the
transmission line L.
On the other hand, the receiver 2 includes a data input circuit 24 which
decomposes the frame of received data and to store the resulting image
data and an error detector 25 which calculates a CRC code on the basis of
the received data and to compare the calculated CRC code with the
transmitter-added CRC code of the received data, whereby the error
detector 25 supplies as its output an ACK (acknowledgment) detection
signal if agreement is found between the two CRC codes or a NACK
(non-acknowledgment) detection signal if no agreement is found. There is
also provided an input control circuit 26 as connected between the data
input circuit 24 and the error detector 25. The input control circuit 26
controls the operation of the data input circuit 24 such that the data
input circuit 24 supplies image data to the next following stage only when
the error detector 25 supplies an ACK detection signal and there is an
agreement between the block number in the control frame and the block
number expected at the receiver 2. Also provided is a response signal
generator 27 which supplies an appropriate response signal to the
transmission line L depending upon the contents of a detection signal
supplied from the error detector 25.
The receiver 2 also includes a delay measuring signal detector 28 for
detecting the delay measuring signal transmitted from the transmitter 1
through the transmission line L and a delay response signal detector 29
which supplies a delay response signal to the line L in response to a
detection output supplied from the delay measuring signal detector 28.
It is to be noted that the above-described response signals are signals in
a frequency band, e.g., 330 Hz or 3,300 Hz, which is different from the
frequency band in transmitting facsimile signals. Thus, even during
transmission of image data, image data and response signals may be
transmitted at the same time using the same transmission line L.
When a facsimile communication is to be carried out with the
above-described structure, prior to entering into a predetermined
communication control procedure, the transmission controller 11 of
transmitter 1 supplies a command signal to the delay measurement
controller 19 to activate the delay measuring signal generator 17. Thus, a
delay measuring signal shown in FIG. 2(a) is supplied to the transmission
line L and then after elapsing a time period t.sub.1 it is detected by the
delay measuring signal detector 28 of the receiver 2, as indicated by FIG.
2(b). At the falling end of the delay measuring signal, the delay
measuring signal detector 28 apprises the delay response signal detector
29 of the fact that the delay measuring signal has been detected. And,
thus, the delay response signal detector 29 supplies a delay response
signal to the transmission line L. This delay response signal is detected
by the delay response signal detector 18 of the transmitter 1, as
indicated by FIG. 2(d), and the delay response signal detector 18 apprises
the delay measurement controller 19 of the fact that the delay response
signal has been detected.
Now, time t from activation of the delay measurement controller 19 by the
delay measuring signal generator 17 to detection of the delay response
signal by the delay response signal detector 18 is a sum of transmission
delay time t.sub.1, t.sub.2, signal width t.sub.3 of delay measuring
signal and processing time t.sub.4 at the receiver 2. Among these various
time periods, signal width t.sub.3 is a previously determined value and
the processing time t.sub.4 is also a predetermined value, so that the
total time delay T.sub.d may be expressed as follows:
T.sub.d =t-(t.sub.3 +t.sub.4)
Accordingly, after measuring time t, the delay measurement controller 19
carries out the above-described calculation to obtain a total delay time
T.sub.d, which is then output to the transmission controller 11. In
response to the received total delay time T.sub.d, the transmission
controller 11 determines the number n of bits in a block of data, for
example, according to the following table, which is then supplied to the
block data generator 14.
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Total Delay Time No. of Bits
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T.sub.b < T.sub.s /2
N
T.sub.s /2 .ltoreq. T.sub.b < T.sub.s
2N
T.sub.s .ltoreq. T.sub.b < 3T.sub.s /2
3N
3T.sub.s /2 .ltoreq. T.sub.b < 2T.sub.s
4N
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In the above table, N is an integer indicating a reference length of a
block of data, e.g., 2,048, and T.sub.s is time required to transmit the
data having N number of bits.
After determining number N of bits of a block of data in this manner, the
transmitter 1 enters into a predetermined communication procedure to carry
out transmission of image data as well known for one skilled in the art.
It is to be noted that a relation between time T.sub.d and number N of bits
should not be limited only to the above-described table. What is critical
here is that the system allows the transmitter to receive a response
signal supplied from the receiver while the transmitter is transmitting
the next following block of data. It is also to be noted that the
measurement of delay time T.sub.3 may also be carried out during the
facsimile communication procedure. In this case, use may be made of
optional signals in the procedure, such as NSS as a delay measuring signal
and CFR as a delay response signal. It should also be noted that the
above-described embodiment is a case in which the present invention is
applied to a facsimile communication system; however, the present
invention may also be applied to other data communication systems
employing the ARQ function.
As described above, in accordance with the present invention, the
transmission time delay is first measured and the number of bits of a
block of data to be transmitted is determined depending upon the measured
time delay, so that the ARQ function may be maintained operative even if
there is a relatively large time delay in a selected communication route.
While the above provides a full and complete disclosure of the preferred
embodiments of the present invention, various modifications, alternate
constructions and equivalents may be employed without departing from the
true spirit and scope of the invention. Therefore, the above description
and illustration should not be construed as limiting the scope of the
invention, which is defined by the appended claims.
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