JPS6138895B2 - - Google Patents

Description

Detailed Description of the Invention The present invention is a circuit for controlling the sequence number of transmitted and received frame information in a transmission station of a loop transmission system in which transmission between arbitrary stations is possible using a single transmission circuit using a variable block multiplexing method. In particular, the present invention relates to a simple sequence number control method using only one set of transmitting and receiving buffers for storing sequence numbers.

First, an outline of the loop transmission system will be explained with reference to FIG. Recently, there has been a trend to form a single distributed processing system by connecting information devices IDi such as computers and terminals scattered throughout factories, universities, buildings, etc., using transmission lines TL. As a transmission subsystem in a computer network that does not have a very wide area, such as in a campus, a system in which the transmission path is looped as shown in the figure and is generally called a data highway is attracting attention. The information equipment IDi joins the transmission subsystem via the transmission station STi. The monitoring station SST has functions such as generating a system clock, initializing transmission control, and monitoring transmission status. Assume now that information device IDi requests IDj to transfer data information. IDi then transmits the data information and the fact to the transmission station STi. Transmission station STi is a line
A search is made for a timing when the TL can be used exclusively, and when this is obtained, data information is sent in the direction shown in the figure to the transmission station STj to which IDj is connected. At this time, all transmission stations other than STi are in a state where the information received by the receiver is passed as is to the transmitter and sent out, and the information sent from the STi transmitter goes around the loop and is sent to the STi receiver. It is configured to return. When the station STj designated as the receiving station finishes receiving all the data information from STi, it moves from the previous path state to the same transmission state as STi, and completes sending response information indicating whether or not it has correctly received the information from STi. Then it goes back to pass state. Then, STj passes the received data information to IDj. This results in
Information transfer from IDi to IDj is accomplished.

However, if a transmission error occurs in the data information or response information and the transmission station cannot receive the data correctly, the data information is retransmitted from IDi or STi. Therefore, if IDj does not determine whether the received information is the one that was previously sent, double information will occur. For this reason, sequence numbers are attached to the information in the order of transmission, and the receiving side must always check this. In conventional systems, this function was performed by the ID, that is, by the user.

By the way, the above-mentioned loop transmission system is required to have a larger processing capacity and faster processing speed, so high-speed processing is now required at both the transmission subsystem and the user level. However, since user-level processing is performed by software, it is difficult to increase the speed compared to the transmission subsystem, and performing the above-mentioned sequence number processing on the user side makes the processing burden even heavier. Therefore, when this process is performed on the transmission station side, HDLC, which is a general-purpose communication control procedure, is used.
It is common to introduce the sequence number control method specified in (High Level Data Link Control). However, if this control method is introduced as is, in an N:M loop transmission system, the transmission station must store sequence numbers regarding all transmission partners. For this reason, as the number of transmission partner stations increases, more storage areas for sequence numbers are required, and their management becomes complicated. In addition, if there are changes or additions to the system,
Either notify all the transmission partner stations of the expansion station and re-correct the table of transmission partner stations, or create a storage area for the maximum number of stations determined by the system (for example, 256 stations) in advance to save this effort. It had the disadvantage that it had to be

That is, if the sequence number check method specified in HDLC is applied as is, in a system in which n transmission stations are connected to a loop transmission path, sets of sequence numbers for the maximum number of transmission destinations are stored. For example, in a system that performs transmissions with all stations except itself, a set of (n-1) sequence numbers is stored.

Each sequence number indicates a series of frame numbers in which one receiving station performs a series of exchanges with another transmission partner station. Therefore, by looking at each sequence number, it is possible to know how many frames of data have been received with the partner transmitting station. For example, if 8 frame transmission is assumed,
One sequence number can be expressed with 3 bits of information.

Therefore, if each station receives from (n-1) station partners and transmits 8 frames, each transmitting station:
A data buffer with a capacity of 3(n-1) bits is required.

Furthermore, the sequence number is updated (+1) every time one frame is sent by the destination transmitting station. Since the update is performed by an adder circuit, each receiving station requires (n-1) adder circuits.

The present invention aims to eliminate the complexity of the configuration of each transmission station and provides a sequence number control method that can easily cope with changes and additions to the system.

In a loop transmission system employing a variable block multiplexing method, only one station occupies a line for a certain period of time to transmit and receive data. Further, the transmitting station relinquishes the line exclusive right after receiving a normal response from the receiving station, and if there is a station that requests line exclusive possession, that station acquires the line exclusive right.

Therefore, focusing on these two functions, in the present invention, each station is provided with one set of transmitting and receiving buffers for storing sequence numbers, and this is transmitted to multiple partner transmitting stations (for example, (n-1) in the above example). It was decided that the system would be used in common for all transmitting stations (transmitting stations).

Here, in order to use it for (n-1) partner transmitting stations, updating (+1) and initializing the buffer becomes a problem.

The buffer refers to two buffers: a transmitting buffer and a receiving buffer. The transmission buffer is updated by adding 1 to the transmission sequence number stored in the transmission buffer, and the timing for updating (+1) is when transmission of one frame is completed. To update a reception buffer, add 1 to the transmission sequence number stored in the buffer, and the timing for updating (+1) is based on the transmission sequence number from the transmitting station and the reception sequence number in the reception buffer. This is when they match.

In addition to the transmitting station, the receiving station also has a transmitting buffer, but the transmitting buffer on the transmitting station side is particularly meaningful. Therefore, the above update can be considered to be an update of the transmission buffer on the transmitting station side.

Furthermore, although receiving buffers exist at transmitting stations as well as receiving stations, it is the receiving buffer at the receiving station that is particularly significant. Therefore, the above update can be considered as an update of the receiving buffer on the receiving station side.

Next, the initialization of the buffer will be described. The reception buffer is a line occupancy permission signal from the upstream line.
Initialize when POL is detected. Initialization was expressed as a reset state. The line occupancy permission signal POL from upstream is issued by the transmitting station and is an expression of intention to abandon line occupancy.

Furthermore, when a transmission frame is received by another station (meaning a station different from the above-mentioned transmitting station) from an upstream line, the receiving buffer is also initialized. This corresponds to a case where when the transmitting station relinquishes line occupation and sends out the line occupation permission signal POL, a station upstream from the receiving station takes possession of the line.

The transmission buffer initializes when it receives the line occupancy permission signal POL from the upstream line.

By this initialization, both the receiving sequence number and the transmitting sequence number are reset, and the station becomes ready to become a receiving station or a transmitting station at any time. For example, even if there is a transmission from another transmitting station,
The receiving station that has the reset sequence number does not reject the request, but rather accepts it.

Next, the present invention will be explained in detail using a specific transmission format.

FIG. 2 shows a frame format of information such as data and responses sent by a transmission station to which the present invention is applied, and is based on the frame format specified by HDLC.

Each piece of information is sandwiched between bit patterns F called flags to form a frame, and measures are taken so that the same bit pattern as the flag F does not occur in the information. The 3 bytes following the first flag are each destination station address DA, control information C indicating the type of frame, and source station address.
Consists of SA in order. The two bytes immediately preceding the flag at the end are frame check information FCS, which is used to detect transmission errors that occur within the frame. Information 1 is data or a response.
As shown in the figure, control information C is further divided into four pieces of information, the first bit is fixed at “0”, the next three bits are the transmission sequence number N(S), and the following one bit is the poll/final bit. Bit P/F and the next three bits represent the response sequence number N(R).

Since transmission in this loop system is a half-duplex procedure, the reception sequence number of the data frame is N.
(R), response frame transmission sequence number N
(S) is not used.

The frame shown in FIG. 2 is common to both the frame transmitted by the transmitter and the response frame received by the receiver. That is, both the transmitting station and the receiving station use the same frame when transmitting.

However, the contents of the frames differ between the transmitting station and the receiving station. For example, if station #1 is a transmitting station and station #n is a receiving station, the frame structure and operation will be as follows.

(i) Transmission operation by transmitting station #1 (Part 1).

In the transmitted frame, DA=#n, SA=
#1, N(S)=i, N(R)=0. However, N(S)=i indicates the number of transmission frames.
Therefore, in the first frame transmission, N(S)
= 0, updates by +1 every time it is transmitted, and outputs this update result as i.

Note that N(R) is N for transmitting station #1.
There is no meaning in setting (R). Therefore,
At transmitting station #1, N(R) is a constant value, N(R)=
Set it to 0.

(ii) Receiving operation by receiving station #n (Part 1).

Receiving station #n receives the frame according to (i) from transmitting station #1. Receiving station #n looks at DA,
Check that it is addressed to #n and receive it (after checking the FCS).

Next, look at N(S) and check whether or not the reception sequence number stored in the buffer at local station #n matches the N(S) in the reception frame. If they match, it is assumed that the reception is valid from the partner transmitting station #1, and the information is taken in. Note that the reception sequence number is initially reset to 0. If it is the same partner transmitting station #1, it is updated (+1) every time subsequent frames are received. Therefore, for the i-th frame transmission, at receiving station #n, the receiving sequence number is i for N(S)=i, and a match of i=i is obtained.

If the reception sequence numbers match, the reception sequence number at own station #n is set to (i+1).
Update (+1). This update is based on the received N
It may be an operation of adding +1 to (S) or an operation of adding +1 to the contents of the previously stored buffer, and there is no substantial change.

(iii) Response operation by receiving station #n (Part 1).

Receiving station #n transmits a response frame after the receiving operation (or through the process). This response frame is DA=#1, SA=#n, N(S)
=0, N(R)=i+1. Other I,
FCS etc. are no different from HDLC. Note that in the response frame, a response command may be entered in I.

Here, N(S)=i is a transmission sequence number, but it has no meaning for receiving station #n because it is not a transmitting station. It simply shows the number of times it has been sent. Therefore, N(S)=0
It may be.

(iv) Receiving operation by transmitting station #1 (Part 1).

Transmitting station #1 receives the response frame from the receiving station, checks DA=#1 to confirm that it is itself, and then checks N(R)=i+1 and determines that the transmitted frame has been correctly received. If there is still data to send, the sending sequence number (i+
1) is sent. As a result, the transmitting station #1
It matches the transmission sequence number (i+1). This is a kind of synchronization state on the so-called sequence number.

If there is no further data transmission, the transmission sequence number i is reset and a transmission permission signal POL is generated instead.

(v) Transmission operation by transmitting station #1 (Part 2).

If transmitting station #1 continues to transmit data, DA=#n, SM=#1, N(S)=i+
1, and a frame consisting of N(R)=0 is transmitted. Of course, I is also transmitted at the same time.

If transmitting station #1 does not continue to transmit data, it transmits the POL created in (iv).

When this POL is received, receiving station #n determines that there is no subsequent data transfer from transmitting station #1, and the buffer content (i+
1) Forcibly reset. As a result of this reset, receiving station #n is disconnected from transmitting station #1 and becomes in a state where there is no source station.
Therefore, next time, for example, when a new transmitting station #2 becomes a transmitting station for receiving station #n, the same operation as (i) will be followed by the operation (ii), but at that time, transmitting station #2 will become a transmitting station for receiving station #n. 2
The transmission sequence number is “0”,
In addition, since the reception sequence number at receiving station #n is also "0", a match between the two is obtained,
You can start reception correctly.

In addition, to reset the receive sequence number,
Source address SA that sent the previous frame
and the source address that sent this frame.
Perform this even if the SA does not match. This is a case where the previous transmitting station gives up exclusive possession of the line, and a station upstream from the receiving station becomes the transmitting station instead.
At that time, information I is received.

(vi) Receiving operation by receiving station #n (Part 2).

If the data continues to be transmitted from transmitting station #1, check DA = #n and SA = #1 (confirm SA = #1 if the data is transmitted from the same transmitting station #1). ), and if these can be confirmed, then N(S)=
Look at i+1 to see if there is a match between its own reception sequence numbers. Since its own received sequence number has become (i+1) due to the update in (ii), the sequence numbers match.

Upon seeing this coincidence, receiving station #n captures the transmission data at that time.

Next, the reception sequence number is updated in the same way as in (ii). In this case, it becomes (i+2).

On the other hand, when transmitting station #1 receives POL, if it has a line exclusive request, it performs the operation (i) as a receiving station.

In the case of this POL reception, the sequence number is reset as described in the second half of (vi).

(vii) Response operation by receiving station #n (Part 2).

Sends a response frame. The operation at that time is
Same as (iii). However, N(R)=i+2.

(viii) Receiving operation by transmitting station #1 (Part 2).

Receive a response frame. The operation is the same as (iv). However, confirmation of reception can be determined by the fact that it becomes (i+2).

Similar to (iv), if data continues to be sent, N
Send data as (S)=i+2. If no data is sent, a POL is generated.

Note that in (i) to (iv), the retransmission request and subsequent retransmission processing are omitted. It is explained in the examples. Furthermore, the reception sequence number at the transmitting station and the transmission sequence number at the receiving station may be given meaning. For example, transmitting station → receiving station
In some cases, the transmission format is such that the receiving station is the transmitting station and the transmitting station is the receiving station.

Next, the state of transmission and reception at each station will be explained more clearly using an example of a transmission time sequence shown in FIG.

In this example, transmission station ST ₁ first obtains the right to occupy the line.
A case is shown in which data is sent to STn, and then ST ₂ obtains the line exclusive right and sends data to STn-1.

The line exclusive right is obtained by the transmission requesting station which receives the transmission permission signal POL sent by the previous occupying station and converts this signal into the abort signal ABT and then frames the data frame.
Move on to sending DATA. For this reason, the transmission permission signal
POL is not sent downstream from the main office. In the diagram, 1
Although only frame sending is shown, it is generally possible to send a plurality of data frames consecutively and receive a collective response. The numbers in brackets of the data frame indicate the transmission sequence number, and the numbers in the brackets of the response frame indicate the reception sequence number. When the transmission of the data frame DATA is completed, it waits for the arrival of the response frame RESP from the receiving station while transmitting the time fill signal TF. In this way, the time fill is a signal sent by the transmitting station when there is no information to send.
When a response frame indicating normal reception is received and a reception sequence number that is one more than the transmission sequence number is received, all data frames DATA sent up to that point are considered to have been correctly received at the reception station. In this example, the reception sequence number is 1, but for example, if three frames, that is, data frames with transmission sequence numbers of 0, 1, and 2, are transmitted, it is correct if the reception sequence number 3 comes. In addition, the response frame is
RESP sending timing is data frame DATA
Directed by the inner pole/final bit. After receiving a normal response frame RESP, if the line is to be relinquished, a transmission permission signal POL is sent. Unlike data and response frames, the transmission permission signal POL often does not take a frame format so that it can be easily detected by hardware. At this time, the bit pattern selected is a pattern that does not appear in the data frame, response frame, or time fill signal. Note that if the transmitting station cannot receive the response frame RESP, it retransmits the data frame by counting the timeout period and requests transmission of the response again.

As can be seen from the above explanation, N-to-M transmission is possible in a loop transmission system that uses block multiplexing, but during a certain period of time, the transmitting station and receiving station are a fixed pair, and normal transmission between them is possible. After this is completed, another set of transmissions begins. Therefore, if the normal completion of the above-mentioned transmission is detected at the station designated for reception, the sequence number can be reset, so that it can be used commonly by many stations. In the present invention, the completion of normal transmission is known by receiving the transmission permission signal POL or by receiving a data frame from a station different from the station that has been transmitting up to now. This is shown in Figures 4 and 5.
This will be explained using figures.

FIG. 4 shows a case where the transmission station STi has obtained the transmission right and is sending a data frame DATA to STk. Transmission sequence number N(S) is m+1
At this time, the transmission of a response frame is requested by the poll/final bit P/F=1. The transmission station STk that received this receives the reception sequence number N(R).
returns a response frame RESP of m+2, notifying that it has successfully received frames with sequence numbers up to m+1. Also, the poll/final bit P/F is set to 1, indicating that no more frames will be sent.

In addition, the numbers shown in the column below transmission station STk in the figure are
This is the reception sequence number N(R) at STk. The reception sequence number N(R) is m+2 from the time the response frame RESP is sent. Received response frame RESP from transmission station STk
STi learns that all transmissions from its own station have been successful, sends a transmission permission signal POL, and temporarily relinquishes transmission rights. The transmission station STk that detected the transmission permission signal POL
It detects that reception from the STi is complete and sets the reception sequence number N(R) to 0. This makes it possible to prepare for the frame with transmission sequence number N(S)=0 that will be sent next to the local station. In this figure, there is no other transmission request and the transmitting station
Although the case where STi obtains the transmission right and sends out a data frame is shown, the same applies to the case where a station downstream from the transmission station STk obtains the transmission right. When the receiving sequence number N(R) = 0, the transmitting station
Since a frame with transmission sequence number N(S)=0 is received from STi, a response frame is returned with this as a normal frame. However, in some cases, unlike the above example, a transmission station upstream of transmission station STk may obtain the transmission right. FIG. 5 shows an example of this. This figure shows a case where STj detects the transmission permission signal POL sent out by the transmission station STi, acquires the transmission station, and transmits to STk.
Since the transmission station STj obtains the right to transmit, the transmission permission signal POL sent by the transmission station STi cannot be detected by the transmission station STk, and the reception sequence number N(R) cannot be reset. However, if the data frame DATA is received from a different transmission station than before, it can be assumed that the previous transmission has been completed normally. Therefore, as shown in the lower column of the transmission station STk in the figure, the transmission station address is set to the reception sequence number N. If it is stored together with (R), it is possible to manage the sequence number even in such a case. In other words, if a data frame DATA with a transmission sequence number N(S) of 0 is received from a station different from the previous transmission station before detecting the transmission permission signal POL, the reception sequence number N stored up to that point is
Receive unconditionally regardless of the value of (R), set the reception sequence number N(R) to 1, and N(S)≠0
In this case, N(R) is simply set to 0. By the above method, each transmission station can use only one set of transmitting and receiving buffers for storing sequence numbers, which can be shared by all stations.

FIG. 6 is a schematic configuration diagram of a transmission station STi to which the present invention is applied, and the position of the sequence number control circuit and its connection with peripheral circuits will be explained using this diagram. The received signal from the transmission line is converted into digital information by receiver k and taken into the station. When in the pass state, the signal is sent as is to the transmitter T via the shift register SR and multiplexer MPX, where it is converted back into a signal and sent out to the transmission line. When a transmission request occurs, the transmission control circuit TC activates the transmission permission signal detection circuit PPDC and waits for the detection of a transmission permission signal. When the transmission permission signal detection circuit PPDC detects a transmission permission signal, a transmission permission signal detection signal is generated.
Send POLD to the transmission control circuit. However, these control lines are not shown in the figure because they are not directly related to the main topic. Upon receiving this, the transmission control circuit TC activates the aport signal generation circuit ADTG and controls the multiplexer MPX to start transmitting information from its own station. When the transmission of the abort signal is completed, the frame generation circuit FRMG is activated to start transmitting the data information in the transmission information buffer SBUF. At this time, the transmission sequence number of the data frame is received from the sequence number control circuit SNCC as the local transmission sequence number MN(S). 1
Every time frame transfer is completed, a frame transmission completion signal TRNE is sent to the sequence control circuit SNCC. When the sending of the data frame is completed, the frame generation circuit FRMG sends out a time fill signal. When a response frame sent by the other receiving station is received, the transmission control circuit is notified of its arrival by an AND condition between the detection signal from the frame detection circuit MFDC addressed to the own station and the normal signal from the frame check information check circuit. The transmission control circuit TC fetches and examines the contents of the response frame via the reception information buffer RBVF, and if retransmission is necessary, the response frame reception sequence number RN stored in the control field check circuit CFCC is read.
(R) is taken into the transmission sequence section of the sequence number control circuit SNCC and retransmission is performed from the necessary data frame. On the other hand, if a normal response frame is received, a signal is generated from the transmission permission signal generation circuit POLG, and after completion, the multiplexer MPX is controlled to shift to the pulse state again.

Next, the operation when designated as a receiving station will be explained. When the multiplexer MPX selects the output of the shift register SR side, the data frame detection circuit MFDC detects the data frame addressed to the own station, and the frame check information check circuit FCSC checks that there are no transmission errors in the frame. When detected, this is transmitted to the transmission control circuit by AND gate AG.
Notify TC. At the same time, the sequence number control circuit SNCC fetches the received frame transmission sequence number RN(S) from the control field information stored in the control field check circuit FCSC, performs a sequence check of the frame, and if the result is normal, the sequence is returned. Sends the number normality detection signal SNOK to the transmission control circuit TC. In response to notification of these two signals, the transmission control circuit TC takes in frame information via the reception information buffer RBUF and sends it to the information equipment connected to the transmission station STi. Therefore, in the case of a data frame with an unreasonable sequence number or a retransmitted data frame, the arrival of the frame will be processed without being notified to the information equipment. The transmission control circuit TC, which is notified via the AND gate AG of the arrival of a data frame addressed to its own station with a normal frame check, creates a response frame to notify the transmitting station of the receiving status, such as normal or various error factors. The destination of the response frame uses the source station address SA from the source station address register SAR,
The received sequence number is the received sequence number N from the sequence number control circuit SNCC.
Use (R). When preparation is completed, the transmission control circuit TC sends information to the transmission information buffer SBUF, controls the frame generation circuit FRMG and multiplexer MPX to send a response frame to the line, and after completion, returns to the path state. After this, the transmission permission signal detection circuit
When the PPDC detects the transmission permission signal, it notifies the sequence number control circuit SNCC by the transmission permission signal detection signal POLD.

Here, the circuit configuration of the sequence number control circuit SNCC will be explained with reference to FIG. Here, the counter SSC is a counter for forming a transmission sequence number, and the register RSNR is a register for storing a reception sequence number. This counter
SSC also serves as a register, and is a counter SCC.
Together with RSNR, it is called a sending/receiving buffer.

The transmitting side only has a transmitting sequence counter SSC, and its output is the own transmitting sequence number MN.
(S). The configuration is such that it is possible to preset the value of the response frame reception sequence number RN(R) (the value of the intermediate frame at the time of retransmission). The counter value is reset to zero by the transmission permission signal detection signal POLP. Furthermore, the contents of the counter are incremented by one in response to input of the frame transmission completion signal TRNE.

The receiving side has one receive sequence number register.
RSNR, and the comparator COMP1 confirms that this output, that is, the reception sequence number N(R) and the reception frame transmission sequence number RN(S) match. Comparator COMP1
The match output is the sequence number normal detection signal
It becomes SNOK. Receive sequence number register
RSNR is a comparator that uses the output of the adder INCR that adds 1 to the received frame transmission sequence number.
It is configured so that it can be input by the coincidence output SNOK of COMP1, and is reset to zero by the transmission permission signal detection signal POLD via an OR gate. Furthermore, it is equipped with a pre-transmission station address register PSAR that stores the transmission source station address SA.
When a data frame addressed to the local station is received with a normal frame check, comparator 2 compares the output of the previous transmitting station address register with the source station address.
If they do not match, the contents of the receive sequence number register RSNR are reset to zero via the OR gate. Then the source station address
SA is input to the previous transmitting station address register PSAR. The reception sequence number register RSNR is the reception frame transmission sequence number after reset.
If RN(S) is zero, a one is written immediately.

As described above, the present invention has the advantage that in an N to M loop transmission system, buffers for storing frame sequence numbers can be configured as one transmitting/receiving set. Furthermore, we were able to gain flexibility in expanding the system.

[Brief explanation of the drawing]

Fig. 1 is an overall configuration diagram of an N to M loop transmission system to which the present invention is applied, Fig. 2 is a transmission frame format, and Fig. 3 is an example of a time sequence of a loop transmission control system to which the present invention is applied. 4 and 5 are transmission sequences for explaining the main points of the present invention, FIG. 6 is a schematic configuration diagram of a transmission station to which the present invention is applied, and FIG. 7 is a configuration of a sequence number control circuit according to an embodiment of the present invention. It is a diagram. STi...station (transmission station), SNCC...sequence number control circuit.

Claims (1)

[Claims] 1. A plurality of transmission stations equipped with a set of transmitters and receivers are connected in a loop through a transmission line, and at a certain time, only the only transmission station that has detected a transmission permission signal occupies the line. The station that has the exclusive right designates the only transmitting station as the receiving station and transmits information in frame format, and after the transmission is completed, it receives a response frame from the receiving station, confirming that the transmitted information has been correctly received. In an N-to-M loop transmission system in which transmission is performed between arbitrary transmission stations by sending a transmission permission signal and determining the next line-occupying station when confirmed, each station has a counter for a transmission sequence number and a reception sequence number. A transmission station that has a number storage buffer and has obtained exclusive rights to the line initially transmits frames with the transmission sequence number set to 0.
If you want to continue transmitting, add 1 to the transmission sequence number each time you transmit a frame, and then
When relinquishing line occupation, a line permission signal is sent, and the transmission station designated as the receiving station transmits a signal if the transmitting sequence number in the received frame is equal to the memorized receiving sequence number, or if it is different from the previous one. When a frame addressed to the local station is detected from a station, the value obtained by adding 1 to the value of the received transmission sequence number is stored as the reception sequence number along with the transmission station address, and when sending a response frame, the reception sequence number is attached and the above transmission right is granted. A frame sequence number control method characterized in that when it is detected by reception of the line permission signal that the transmission station obtained has given up possession of the line, the reception sequence number is initialized.