JPH1093659A - Data transmission method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the error correction system for data transmission with respect to data transmission in a transmission system where the transmission side and the reception side have the same speed and are synchronous. SOLUTION: The transmission side divides information to be transmitted by a fixed bit length unit to take divided information as an information part 3 and constitutes a frame of the information part 3, a forward control part including information indicating the transmission order and information indicating whether there is an effective information part in a packet or not, and a backward information part 2 which informs the terminal of the other party whether a reception packet was normally received or not and whether synchronism is settled or not, and the transmission side transmits this frame structure to the reception side, and the reception side acquires reception frame synchronism based on the frame structure above and detects a transmission error about information in the frame after synchronization.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a data transmission method having a transmission system in which the transmission side and reception side have the same and synchronized speeds, and more particularly to a method for using an error-prone communication path. The present invention relates to an improvement in error correction means for data transmission.

[0002]

2. Description of the Related Art In data transmission using an erroneous communication path, an AR is generally used to improve error characteristics.
A technique called Q (Automatic Repeat Request) is used.

In ARQ, information is divided on the transmitting side into blocks of an appropriate length, and a number for indicating the order of data transmission, a command for retransmission control,
Information for identifying data, add an error detection code to determine whether there was a data error,
Transmission is performed by adding a synchronization flag or the like for determining the beginning and end of the block. Using the error detection code, the receiving side checks whether there is no error in the block and checks whether there is any missing packet by checking the number indicating the transmission order. If there is a packet omission or an error, the receiving side transmits the number of the block requiring retransmission and a command requesting retransmission to the transmitting side, and requests the transmitting side for retransmission.

As a typical example of ARQ, HDLC (High L
The protocol that uses the frame structure of the evel Data Link Control Procedure is most often used. Figure 7 shows the HD
4 shows a configuration of a packet in an LC procedure. A flag (hereinafter, referred to as FS: Flag Sequence) is a unique word for synchronization, and indicates the start and end of a packet frame at the beginning and end of the packet frame. Since the HDLC frame is of variable length and the end of the frame is recognized by FS, the data in the frame surrounded by FS includes F
If the same pattern as S appears, the receiving end cannot correctly recognize the end of the frame. Therefore, means for preventing this is required.

In the HDLC, this is prevented by a method called "0" insertion. On the transmitting side, when five consecutive "1" bits appear, "0" is inserted into the sixth bit to prevent the same pattern as FS. On the receiving side, except for the "0" bit at the sixth bit after the "1" continues for five bits, the sixth bit is "1", and is left as it is. If "1" continues for 7 bits or more, it is regarded as abnormal, the abort procedure is started, and the frame becomes an error. The address field is used to specify a data transmission partner. Therefore, the omitted protocol is often used in the point-to-point communication. The control field contains commands and responses for transmission / reception control, transmission sequence numbers, and the like. The information field contains data to be transmitted.

FIG. 6A shows a typical operation sequence at the time of normal transmission in the HDLC procedure, and FIG. 6B shows a typical operation sequence at the time of an error in a packet. For a packet received normally, an RR (Receive Ready) response is returned to notify the transmitting side of normal reception.

In data transmission using a communication path in which an error is likely to occur, a method of correcting an error is used in addition to the ARQ. As an error correction means in this error correction method, FEC (Forward Error Correction)
n) There is a code correction. Typical F
The EC includes a BCH (Bose-Chaudhuli-Hocqengham) code, a Reed-Solomon code, and the like.

When an error correction code is used, the processing delay and transmission efficiency are constant and predictable, so that not only data accuracy but also real-time transmission is prioritized, such as voice and image code transmission. ARQ as in the case and also the recording system
It is often applied when it cannot be used. However, if there are more errors than expected at the time of design,
Since the error cannot be detected and erroneous correction is performed, it is necessary to design the transmission path with a margin for the error rate and error characteristics.

[0009] Further, since the error correction code needs to have a greater redundancy than that used for error detection, if the number of correctable bits (error rate improvement rate) is to be increased, the error correction code is transmitted. The ratio of the correction code to the data increases, and the transmission efficiency deteriorates. In order to cope with a burst error, a block length larger than the burst length is required. By increasing the block length or using the cross-interleaving method, the characteristics against long burst errors can be improved.However, since error correction processing requires all data in the block, real-time processing at low bit rates is required. When used for code transmission of voice or moving images that require sexual characteristics, the block transmission time becomes long and the processing delay increases, so that the block length cannot be made too long.

In addition, when an error exceeding the error correction capability is detected, the error cannot be detected and an erroneous correction is performed. In addition, it is not possible for the upper-level application to change the operation in response thereto.

In general, the selection and design of error correction means are largely affected by the properties of the host application, the properties of the media, and the like. For example, in the case of real-time transmission of a voice code such as a telephone, the coding rate is generally a fixed rate, and real-time properties are required. In a technique such as the retransmission procedure, the effective transmission rate and the transmission delay fluctuate, so that a transmission rate higher than the voice code rate is required to guarantee a fixed rate, which is inefficient. Further, since it is necessary to reproduce the FEC frame on the receiving side, a synchronizing means is required.

FIG. 9 shows an example of the synchronizing means used in the FEC. Normally, a specific synchronization word is inserted and transmitted for each frame, and frame synchronization is established by detecting that the synchronization word is inserted at frame intervals on the receiving side. When a synchronization word is inserted, as shown in FIG. 9, there are a centralized synchronization method in which the synchronization word is inserted for each frame, and a distributed synchronization method in which the synchronization word is dispersedly inserted in a plurality of frames.

As another example, in the case of real-time transmission of a moving image, by adjusting the image quality with respect to the transmission rate,
Generally, the coding rate is made to follow the transmission rate by feedback control. Therefore, it is possible to cope with a certain rate fluctuation, but it is necessary to make the fluctuation of the transmission delay less than a certain time.

In general, when a wireless transmission path is used, the following properties are common as compared with a wired transmission path.
Since the power available at the time of reception is smaller than that of the wired transmission path, there are many random errors due to noise. Also, many long burst errors occur due to radio-specific interference such as fading / multipath. Therefore, if the FEC responds to this, a very long frame length is required, so that the processing delay increases. In addition, since the ratio of correction codes increases, the transmission efficiency decreases.

In the case of only the retransmission procedure, the entire frame is retransmitted even in the case of a one-bit error in the frame. Therefore, if retransmission due to a random error occurs with a relatively high probability, the transmission efficiency decreases. Further, there is no mechanism for guaranteeing a transmission delay time including retransmission.

Further, as shown in the frame configuration of FIG.
It is conceivable to combine the C code and the ARQ procedure.
The FEC code is inserted between the error detection code (FCS) and the flag indicating the end of the frame for transmission.
The error detection is performed on the data after the error correction by C, and the retransmission procedure is executed. When this procedure is used, a "0" insertion is performed for a pattern in which five or more "1" s are continuous to maintain the uniqueness of the flag, and the length is thereby changed.

However, if there is a transmission error immediately before the place where the “0” insertion is made, the inserted “0” bit is not deleted because the receiving side cannot detect the “0” insertion, so that the data length is reduced. The length is different on the receiving side and the transmitting side. In this case, since it cannot be corrected by a simple FEC such as BCH, it is necessary to cope with the retransmission procedure, or to adopt an FEC code that can cope with a bit loss / insertion error.

Generally, an FEC code that can cope with a bit insertion / erasure error requires a longer code than in a case where it does not cope with it, so that the transmission efficiency is reduced. The flag is
Since the frame is not protected by error correction and an error occurs in the flag, the frame cannot be detected, so that error correction by FEC becomes impossible. As described above, in the configuration in which the FEC code is simply added to the HDLC frame configuration, there is a problem that the error protection by the FEC code for a specific bit is weakened.

[0019]

As described above, in data transmission using a communication path in which an error is likely to occur,
In order to improve the error characteristics, ARQ (Automa
A technique called tic Repeat Request (automatic retransmission request) is used, and an error correction scheme or a technique combining the ARQ and the error correction scheme is used. However, in the case of only the retransmission procedure, the entire frame is retransmitted even in the case of a one-bit error in the frame, so that the transmission efficiency decreases when retransmission due to a random error occurs with a relatively high probability.

In the case of using the error correction method, since the processing delay and the transmission efficiency are constant and predictable, not only the accuracy of the data but also the transmission of the code of the voice or the image in which the real-time property is prioritized. Or in the case of A
It is often applied when RQ cannot be used. However, if there is an error more than expected at the time of design, it cannot be detected and erroneous correction is performed. It is necessary to design with a margin for the rate and error characteristics.

Further, when the ARQ and the error correction method are combined, in a configuration in which the FEC code is simply added to the ARQ frame configuration, the FRQ is applied to a specific bit error.
There is a problem that there is no error protection by the EC code. The present invention has been made in view of the above problems, and provides a data transmission method that solves these problems.

[0022]

According to the data transmission method of the present invention, basically, on the transmitting side, information to be transmitted is divided into fixed bit length units, and the divided information is used as an information part.
The information section, a forward control section including information indicating the data transmission order and information indicating whether or not there is a valid information section in the packet; and establishing whether or not the received packet has been normally received, and establishing synchronization. Whether or not a frame is configured from a backward information part for notifying the partner terminal, and transmitting the frame structure to the receiving side, and at the receiving side, based on the frame structure, synchronize the received frame, A transmission error is detected for information in a frame after synchronization. The receiving side notifies the transmitting side of the results of the detection of the transmission error and the synchronization using the backward control unit.

In the above method, the transmitting side always monitors whether or not the receiving side has established synchronization, and if the receiving side is out of synchronization, the transmission of the information frame is stopped and the synchronization is stopped. The phase shifts to a take phase in which a frame is transmitted to ensure that the sync word is unique in that phase.

In the above method, as means for ensuring that the synchronization word is unique, a synchronization phase for synchronization and a position of a forward control unit including a number indicating a data transmission order are provided in a synchronization phase. Sometimes, a unique synchronization pattern is inserted, and all other information is fixed to 0 or 1 for the other parts, and the synchronization word is used by the forward control unit in the data transmission phase for the information unit. It has a packet format in which the same bit pattern as the code determined to have no significant data is used as a synchronization word.

In the above-mentioned basic method, the synchronization word is omitted at the time of data transmission after the synchronization is established, and whether the sequence number of the forward control unit is a number continuous with the sequence number of the immediately preceding frame, If a retransmission request is made in the previous frame and the sequence number is that the retransmission frame has not been received, it is determined that synchronization is maintained, and the backward control unit is used as means for specifying a frame to be retransmitted. The sequence number of the last normally received frame is always inserted and transmitted, and the transmitting side determines the order of the sequence number of the backward control unit,
It has a procedure to compare the sequence number of its own transmitted frame with the order of the sequence number, determine that the sequence does not match as an error frame, perform retransmission, and further specify a frame to retransmit. A sequence synchronization bit having a value uniquely determined for the received sequence number and indicating that the sequence pattern of the sequence synchronization bit for the continuous sequence number is unique is transmitted to the backward control unit and transmitted. Means for synchronizing the sequence of the frame transmitted based on the sequence synchronization bit with the sequence of the backward control unit, and transmitting the bit for notifying the transmitting side whether the received packet has been normally received to the backward control unit. With

[0026]

FIG. 1 shows a flag configuration of a transmission frame according to the present invention. In FIG. 1, 1 is a synchronous word / forward control unit, 2 is a backward control unit, 3 is an information unit, 4 is an error detection code, and 5 is an error correction code. In the synchronization phase, synchronization word 1 for frame synchronization
is stored, and in the data transfer phase, the forward control unit 1b is stored. Forward control unit 1
b is control information sent from the transmitting side to the receiving side.
FIG. 2 shows a configuration example of the synchronous word / forward control unit 1. In FIG. 2, the forward control unit 1b includes:
A sequence number for indicating the order of the transmission frame and a bit indicating whether or not valid data exists in the information section 3 in the frame are encoded and stored. In this example, an example is shown in which the length of the synchronization word 1a and the length of the forward control unit 1b are the same, but they need not necessarily be the same.

Next, an example of the configuration of the backward control unit 2 is shown in FIG. In FIG. 3, backward control unit 2
Stores information for retransmission control of the data of the information section 1 sent from the other party. In the example of FIG. 3A, an area for inserting a sequence number for notifying the transmitting side of which frame has been received without error, and notifying the transmitting side whether the received frame is normally synchronized. The synchronization monitoring flag (hereinafter referred to as a synchronization monitoring bit) for performing the operation. At this time, the sequence number of the frame that was received without error at the end (including after error correction by FEC) is entered in the sequence number. By comparing the order of the sequence number of the backward control unit 2 with the order of the sequence number of the transmitted frame, it is possible to determine a normally received frame and a frame requiring retransmission.

In the case of FIG. 3B, a value of "1" or "0" determined by the transmitted sequence number is inserted in the sequence synchronization bit. A sequence number,
FIG. 4 shows an example of the correspondence between the sequence synchronization bits. The backward control unit 2 searches for the sequence pattern of the sequence synchronization bit received by the backward control unit 2 from the sequence synchronization bit predicted from the sequence number sequence of the transmitted frame, and the backward control unit 2 corresponds to any sequence number. Or to detect.

The synchronization monitoring bit is a bit for notifying the transmitting side whether or not the transmitted frame is synchronized on the receiving side, and has a value of syn.
c or async is entered. Here, a binary code indicating that the synchronization is normal is expressed as sync, and a binary code indicating that the synchronization is lost is expressed as async. S for the synchronization monitoring bit
When "sync" is set, it means that synchronization is achieved. When "asyn" is set, it indicates that synchronization is lost. If async is received, the process proceeds to the synchronization establishment phase. The retransmission bit is a bit for indicating to the transmitting side whether retransmission of the received frame is necessary or unnecessary. Ack as a value indicating normality,
Nack is entered as a value when retransmission is necessary. When ack is included in this bit, the transmitting side releases the data of the corresponding frame from the buffer, and when ack is included, retransmits.

In FIG. 1, reference numeral 4 denotes an error detection code, as described above. This error detection code 4 obtains an error detection code for the code of the block to be detected and inserts the code into this part. ,Send. On the receiving side, error correction is performed including the detection code using the error correction code 5 and error detection is performed on the error-corrected code. When an error is detected, the error is notified to the transmission side using the backward control unit 2 of the next transmission frame. The transmitting side monitors the backward control unit 2 of the received frame, and when notified of the frame in which the error is detected, retransmits the frame.

Reference numeral 5 denotes an error correction code as described above. Necessary arithmetic processing is performed on the block to be error-corrected to generate and transmit the error-correcting code 5.
On the receiving side, error correction processing is performed using the error correction code 5.

The data transmission method of the present invention has the following phases and performs the following operations. (1) Idle phase <idle state> All bits are transmitted with the same value (either "1" or "0" is a predetermined value). When there is a request to start processing from the host or when the synchronization word 1a is received, the process proceeds to the synchronization establishment phase of (2).

(2) Synchronization Establishing Phase <Phase for Establishing Synchronization> The synchronization word / forward controller 1
Enters. The synchronization monitoring bits of the backward control unit 2 include:
The value of async (out of synchronization) is stored.

In the information section 3, all bits are set to predetermined values (the same values as in the idle state).
The receiving side searches for a bit pattern that matches the synchronization pattern, and sets the matching position as the frame synchronization position. When it is confirmed that the synchronization word 1a is received at intervals of a predetermined number of frame lengths, a value of sync (synchronization establishment) is entered in the synchronization monitoring bit of the backward control unit 2 to notify the other party that reception synchronization has been established. I do.

After the synchronization of the received frame is established, it is monitored whether the synchronization monitoring bit of the backward control unit 2 is sync or async. If the received frame is not an error frame and the synchronization monitoring bit includes sync, the synchronization is established. It is determined that the connection has been established, and the synchronization of the backward
When the sequence number is used as in the case of (a), the process shifts to the transmission phase. Further, when the means as shown in FIG. 3B is used for synchronization of the backward control unit 2,
The process proceeds to the sequence synchronization establishment phase of (2) ′.

(2) 'Sequence synchronization establishment phase A sequence number is put in the transmission sequence number, and the value of the information bit is turned off. Upon receiving a value other than the synchronization word 1a in the synchronization word / forward control unit 1, the receiving side changes the synchronization monitoring bit to async, and sets the sequence synchronization bit corresponding to the received transmission sequence number to the backward of the next transmission frame. It is transmitted in the sequence synchronization bit of the control unit 2. The sequence synchronization bit pattern obtained from the transmitted sequence number is compared with the sequence synchronization bit pattern received by the backward control unit 2 to establish sequence synchronization. After the sequence synchronization is established, the synchronization monitoring bit is changed to "sync" again to notify the other party and wait until the synchronization monitoring bit from the other party becomes "sync". When synchronization is established in both directions, the process proceeds to the transmission phase (3).

(3) Transmission Phase <Data Transmission Phase> FIGS. 5A and 5B show a frame sequence when the backward control unit 2 uses FIG. 3A. FIGS. 6A and 6B show a frame sequence when the backward control unit 2 shown in FIG. 3B is used. The first () shows the forward control unit 1 b, and the second () shows the backward control unit 2. S0, S1,... Of the forward control unit 1b indicate transmission sequence numbers, and on / off indicates on / off of information bits. The information flag is flag information indicating whether or not valid information is included in the information section 3.

R0, R of the backward control unit 2 in FIG.
1, R2,... Are reception sequence numbers.
In this example, the sequence number of the last normally received frame is entered. FIG. 5A is a sequence diagram at the time of normal reception, and FIGS. 5B and 6B are sequence diagrams when an uncorrectable error occurs in a frame. In this example, retransmission is performed when there is an error in the transmission sequence number 2. The transmission procedure will be described below.

The transmitting side normally adds sequence numbers to frames in order and transmits the frames. The receiving side checks whether the frame is normal, and if it is normal, stores the sequence number in the backward control unit 2 of the next transmission frame and notifies the transmitting side. If the frame contains an uncorrectable error, the sequence number of the last normally received frame is stored in the backward control unit 2 and notified to the transmitting side. On the transmitting side, the sequence number notified by the backward control unit 2 is compared with the transmission order history of the sequence number of the frame transmitted by itself, and if they do not match, it is determined that the frame has an uncorrectable error. Judge and resend.

In the case of FIG. 3B, the area for storing the sequence number can be omitted, and the length of the backward control unit 2 can be made shorter than in the case of FIG. However, additional means for obtaining sequence synchronization is required, and a recovery procedure when the sequence synchronization is lost becomes complicated.

Hereinafter, a synchronization procedure and a recovery procedure will be described. The transmitting side internally generates a sequence synchronization bit corresponding to the transmission sequence number. Further, the receiving side generates a sequence synchronization bit based on the received sequence number, and notifies the transmitting side of the sequence synchronization bit in the sequence synchronization bit of the backward control unit 2 of the next transmission frame. The transmitting side compares the sequence pattern generated by the transmission sequence number while sliding the sequence synchronization bit notified by the backward control unit 2. If the patterns match, by determining the slide amount, the correspondence between the received backward control unit 2 and the sequence number of the transmitted frame is determined.

The receiving side sets the retransmission bit of the backward control unit 2 to a if the received frame is normally received.
When the ck receives an error frame, the acknowledgment is inserted and notified to the transmitting side. The transmitting side determines that the retransmission bit is na
In the case of ck, the frame is retransmitted. The sequence synchronization once established is constantly monitored, and when a sequence synchronization bit inconsistent with the transmitted frame sequence is received, the sequence synchronization is searched again. However, if the received frame is an error frame, the operation is performed assuming that the sequence synchronization bit of the frame is normal.

On the receiving side, it is determined whether or not the data has been normally received.
It can be determined based on the following criteria.

(A) When an error is detected by the error detection code after error correction. (B) The sequence number of the forward control unit 1b is a number other than the number predicted from the sequence number of the frame received so far. When both (a) and (b) do not apply, it is determined that the reception is normal.

When a predetermined number N of error frames are continuously received, it is determined that synchronization has been lost, and the process proceeds to a synchronization establishment phase. After the synchronization is established in the synchronization phase, the process returns to the transmission phase. In the retransmission procedure, the case where the error correction code and the ARQ are used together has been described, but the same retransmission procedure can be applied even when the error correction code is not used.

[0046]

By performing the above-described frame configuration and procedure, the bits for synchronization are not consumed in the transmission phase, so that the effective transmission rate can be increased. In addition, since information necessary for retransmission is constantly transmitted, when retransmission is not required, an effective transmission rate is guaranteed. Also, the delay required for retransmission can be reduced. By using ARQ in combination with an error correction code capable of correcting a small bit number error or a burst error of a short burst, frame retransmission due to a small bit number error is suppressed, and fluctuations in rate and delay are suppressed as much as possible. Can be.

[Brief description of the drawings]

FIG. 1 is a diagram showing a frame structure of the present invention.

FIG. 2 is a diagram illustrating a configuration example of a synchronous word / forward control unit.

FIG. 3 is a diagram illustrating a configuration example of a backward control unit;
FIG. 7A is a diagram illustrating a configuration example of a backward control unit in a case where backward synchronization is performed by a sequence number, and FIG. 6B is a case in which backward synchronization is performed by a transmission pattern of sequence synchronization bits.

FIG. 4 is a diagram showing a correspondence between a sequence of sequence synchronization bits and a sequence number.

5A and 5B are diagrams showing a sequence when FIG. 3A is used for the backward control unit, wherein FIG. 5A shows a frame sequence when there is no error frame, and FIG. 4 shows a frame sequence in the case.

6A and 6B are diagrams showing a sequence when FIG. 3B is used for the backward control unit, where FIG. 6A shows a frame sequence without an error frame, and FIG. 6B shows an error frame. 4 shows a frame sequence in the case.

FIG. 7 is a diagram showing a frame configuration of HDLC.

FIG. 8 is a diagram illustrating an example of a frame configuration when a correction by FEC is added to HDLC.

FIG. 9 is a diagram illustrating a configuration example of a frame of a synchronization unit used in FEC.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Synchronization word / forward control part 1a Synchronization word 1b Forward control part 2 Backward control part 3 Information part 4 Error detection code part 5 Error correction code part

Claims (6)

[Claims] 1. A data transmission method in a transmission system in which the speeds of a transmitting side and a receiving side match and are synchronized, wherein the transmitting side divides information to be transmitted in fixed bit length units, and divides the divided information into information. The information section, a forward control section including information indicating the data transmission order and information indicating whether or not a valid information section is present in the packet, and synchronizing whether the received packet has been normally received. A frame is composed of a backward information part for notifying the partner terminal whether or not the frame has been established, and the frame is transmitted to the receiving side, and the receiving side synchronizes the received frame based on the frame. A data transmission method for detecting a transmission error in information in a frame after synchronization. 2. The transmitting side constantly monitors whether or not the receiving side has established synchronization. If the receiving side is out of synchronization, the transmission of block data is stopped.
2. The data transmission method according to claim 1, further comprising: shifting to a phase for establishing synchronization, and transmitting a frame for ensuring that the synchronization word is unique in that phase. 3. As means for ensuring that the synchronization word is unique, a synchronization word for synchronization and a position of a forward control unit including a number indicating a data transmission order include a unique word in a synchronization phase. A synchronization pattern is inserted, and all other information is fixed to 0 or 1 for the other parts, and the synchronization word has no valid data in the information section in the forward control unit during the data transmission phase. 3. The data transmission method according to claim 2, wherein the data transmission method has a packet format in which the same bit pattern as the code judged as a synchronization word is used. 4. When data is transmitted after the establishment of synchronization, the synchronization word is omitted, and the sequence number of the forward control unit is a number that is consecutive to the sequence number of the immediately preceding frame, or retransmission is performed in the previous frame. 2. The data transmission method according to claim 1, wherein, when a sequence number for which a request is made and a retransmission frame has not been received is determined, synchronization is maintained. 5. As means for designating a frame to be retransmitted, the sequence number of the last normally received frame is always inserted into the backward control unit and transmitted, and the transmitting side transmits the sequence number of the backward control unit. 2. The data transmission method according to claim 1, further comprising a procedure of comparing the order and the order of the sequence numbers of the frames transmitted by itself, determining a frame whose sequence does not match as an error frame, and performing retransmission. 6. A means for designating a frame to be retransmitted, which has a value uniquely determined for a received sequence number and indicates that a sequence pattern of sequence synchronization bits for consecutive sequence numbers is unique. Bits are transmitted in the backward control unit, and the receiving side has means for synchronizing the sequence of the frame transmitted based on the sequence synchronization bit with the sequence of the backward control unit, and whether the received packet has been normally received. 2. The data transmission method according to claim 1, further comprising a backward control unit for providing a bit for notifying the transmission side of whether or not the data is transmitted.