JP2007129423A - Wireless access control method taking into account permissible delay time, access point, terminal, and program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wireless access control method or the like taking into account a permissible delay time in a wireless system wherein an access point and a terminal are connected on the basis of the DCF system of the IEEE802.11. <P>SOLUTION: When the terminal transmits a data packet needing the permissible delay time to the access point, the terminal transmits an RTS including the permissible delay time Ta to the access point. Then the access point transmits a first CTS permitting immediate transmission of the data packet or a second CTS for holding the transmission of the data packet to the terminal depending on the remaining time on the basis of the permissible delay time Ts of the RTS. In the case of receiving the first CTS, the terminal immediately transmits the data packet to the access point and in the case of receiving the second CTS, the terminal holds the transmission of the data packet to the access point. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a radio access control method, an access point, a terminal, and a program in consideration of an allowable delay time.

A cellular system is a typical wireless communication system in which an access point centrally controls a plurality of terminals. In the radio access control method applied to the cellular system, the following multiple access methods are used.
Frequency division multiple access (FDMA)
Time Division Multiple Access (TDMA)
Code Division Multiple Access (CDMA)

  The access point allocates radio resources (frequency slot, time slot, code, etc.) to each terminal. This control corresponds to a lower sublayer of the data link layer and is performed by a MAC (Media Access Control) that defines a frame (data transmission / reception unit) transmission / reception method and the like.

  The MAC protocol includes a centralized control type and an autonomous distributed type. In the centralized control type, an access point (or base station) performs media access control of a plurality of terminals, and the terminals follow the control results. In the autonomous decentralized type, terminals are controlled in an autonomous decentralized manner without centralized control in specific terminals. Both methods are aimed at maximizing the communication (throughput) of transmitted data and satisfying the required QoS (Quality of Service).

  A typical MAC is a Connection Oriented type and a Contention-Free type. This method allocates radio resources in a fixed manner to communicating terminals, and is mainly suitable for voice communication.

  On the other hand, many Packet Oriented type MACs have been proposed and studied, and there are also Contention type and Contention-Free type. A typical Contention type is IEEE 802.11 DCF (Distributed Coordinated Function) based on CSMA (Carrier Sense Multiple Access) adopted in wireless LAN systems.

  FIG. 1 is a sequence diagram of DCF in the prior art.

  DCF is an access method based on CSMA / CA (Carrier Sense Multiple Access with Collision Avoidance), in which a terminal having a data packet to be transmitted senses a carrier first, and data packets of other terminals are transmitted. After confirming that there is no data packet, the data packet is transmitted. Thereby, it is possible to avoid a collision with a data packet transmitted from another terminal.

(S101) The source terminal A waits for a backoff time for a data packet to be transmitted after a certain DIFS (DCF InterFrame Space) time has elapsed. The back-off time is an actual waiting time until a data packet is transmitted, and is determined by a contention window (CW: Contention Windows) generated by each terminal using a random number. Since the contention window is generated by random numbers, the transmission timing of data packets between terminals is very low.
Backoff time = random value in contention window x slot time

  According to S101, the transmission source terminal A wants to transmit a data packet to the destination terminal B, so that the back-off time elapses. After the back-off time has elapsed, an RTS (Request to Send) is transmitted to the destination terminal B. At this time, the other terminal D is also ready to transmit the data packet, and the back-off time has elapsed. When the other terminal D detects the RTS transmitted from the transmission source terminal A, it suspends (temporarily stops) the count-down of the back-off time. Then, the other terminals C and D that have detected the RTS shift to a NAV (Network Allocation Vector) period and postpone the use of the channel until the transmission of the DATA frame from the transmission source terminal A to the destination terminal B is completed. . The time when the transmission of the DATA frame is completed is when an ACK transmitted from the destination terminal B to the transmission source terminal A is detected.

(S102) The destination terminal B that has received the RTS transmits a CTS (Clear to Send: request acceptance) to the transmission source terminal A after the elapse of SIFS (Short InterFrame Space) time.

(S103) The source terminal A that has received the CTS transmits a DATA frame to the destination terminal B after the SIFS time has elapsed.

(S104) The destination terminal B that has received the DATA frame transmits ACK to the transmission source terminal A after the SIFS time has elapsed. The other terminals C and D that have confirmed that the ACK is transmitted from the destination terminal B to the transmission source terminal A stop the NAV period.

(S105) The other terminal D resumes the countdown of the carried back back-off time. Thereafter, the other terminal D can transmit the RTS when the back-off time expires. As in S101, all other terminals that have received the RTS shift to the NAV period.

  In the wireless LAN system, there is a hidden terminal problem in which collision occurs when terminals that cannot sense each other transmit at the same time. Therefore, as described above, the exchange of RTS-CTS (particularly, transmission of CTS) shifts to a NAV period during which other terminals cannot use the channel, thereby improving the hidden terminal problem.

  In EDCF (Enhanced DCF) standardized by IEEE802.11e, AIFS (Arbitration IFS) whose value can be set according to the QoS class is defined instead of DIFS, and priority control is possible. Also, a contention window for determining the transmission standby time can be set for each QoS class. Therefore, by reducing the contention window in the QoS class with higher priority, RTS transmission can be performed in a short standby time, and after receiving the CTS, it is possible to transmit the DATA frame.

  As a conventional technique, there is a technique in which a predetermined calculation process is performed in advance for the purpose of shortening a delay time from the generation of data to be transmitted to the actual transmission (see, for example, Patent Document 1). .

  In addition, for example, when a link layer frame transmitted on a wireless line in wireless ATM has an excessive delay due to deterioration of a transmission path state, the packet is discarded, and a transition to transmission of the next packet is made (for example, a patent) Reference 2).

JP-A-11-284629 JP 2000-216813 A

  Some applications that send and receive data packets are meaningless when the delay time limit is exceeded. For example, in the case of an online game or a telemedicine system, it is necessary to minimize the transmission delay of the data packet, and the transfer of the data packet exceeding the allowable delay time (allowable delay time) does not make sense. If the data packet is transferred beyond the allowable delay time, the application must be a low-quality service.

  In EDCF, priority control within the same QoS class cannot be performed, and radio access control that secures an allowable delay time cannot be performed. Further, the packet transmission permission of the low priority class cannot be obtained, and transmission request packets are frequently generated from the terminal, which causes a collision.

  In addition, according to the technique described in Patent Document 1 described above, radio resources that satisfy the allowable delay time are not allocated. Furthermore, according to the technique described in Patent Document 2 described above, the packet transmission order is not specified, and transmission processing is assumed to be performed in the order of packet arrival. In this case, when packets are transmitted on a first-come-first-served basis regardless of the allowable delay time, there are packets that do not complete transmission within the allowable delay time.

  Therefore, the present invention has an object to provide a wireless access control method and the like in consideration of a remaining time with respect to an allowable delay time for a wireless system in which an access point and a terminal are connected based on the IEEE 802.11 DCF method. To do.

  The present invention relates to a wireless communication system in which a low delay service that requires a low delay time as a QoS request as a service class and a normal service that does not exist, and in particular, the low delay service is given priority in the uplink from the terminal to the access point. The present invention relates to a centralized control type MAC protocol that allows communication. In the downlink, it is easy for the access point to schedule the transmission of all data packets, whereas in the uplink, the access point and the terminal that wants to transmit the packet do not know each other's information. Is difficult.

According to the present invention, in a wireless access control method in a wireless system in which an access point and a terminal are connected based on the IEEE 802.11 DCF method,
A first step of transmitting an RTS including the allowable delay time Ta to the access point when the terminal transmits a data packet requiring the allowable delay time Ta to the access point;
In response to the remaining time based on the allowable delay time Ta of the RTS, the access point is the first CTS (CTS-I) that immediately allows the data packet to be transmitted to the terminal, or the second that causes the data packet to be suspended. A second step of transmitting a CTS of (CTS-P);
When the terminal receives the first CTS, the terminal immediately transmits a data packet to the access point. When the terminal receives the second CTS, the terminal has a third step of deferring transmission of the data packet to the access point. It is characterized by.

According to another embodiment of the wireless access control method of the present invention,
When transmission of a data packet is suspended, the terminal transmits the next data packet when receiving the first CTS from the access point, or the remaining time for the allowable delay time Ta becomes 0 In this case, it is also preferable to discard the data packet.

According to another embodiment of the wireless access control method of the present invention,
For the second step, the access point has an allowable delay time Ta, a time Td required to complete transmission of the DATA frame, and a margin time α.
If Ta− (Td + α)> 0, send the second CTS,
When Ta− (Td + α) = 0, it is also preferable to schedule the transmission timing of the data packet from the terminal by transmitting the first CTS.

Furthermore, according to another embodiment of the wireless access control method of the present invention,
The access point monitors the radio resource usage rate, and it is also preferable to shorten the margin time α if the radio resource usage rate is high and to increase the margin time α if the radio resource usage rate is low.

Furthermore, according to another embodiment of the wireless access control method of the present invention,
When the terminal transmits RTS to the access point but cannot receive CTS from the access point, the RTS includes again the time obtained by subtracting the time already passed from the allowable delay time included in the RTS, and transmits to the access point. It is also preferable.

Furthermore, according to another embodiment of the wireless access control method of the present invention,
The terminal
When the first CTS transmitted from the access point to another terminal is detected, a transition is made to the NAV period in which the use of the channel is postponed,
It is also preferable to stop the NAV period when a data packet is transmitted from another terminal to the access point by the first CTS, and then an ACK transmitted from the access point to the other terminal is detected.

According to the present invention, in the access point to which the terminal is connected based on the IEEE 802.11 DCF method,
RTS receiving means for receiving an RTS including the allowable delay time Ta from the terminal when the terminal transmits a data packet requiring the allowable delay time Ta to the access point;
Depending on the remaining time based on the allowable delay time Ta of the RTS, the terminal transmits a first CTS (CTS-I) that immediately allows the transmission of the data packet or a second CTS that suspends the transmission of the data packet to the terminal. CTS determination means for determining whether to do;
CTS transmitting means for transmitting the determined first CTS or second CTS (CTS-P).

According to another embodiment of the access point of the present invention,
The CTS determination means has an allowable delay time Ta, a time Td necessary for completing the transmission of the DATA frame, and a margin time α.
If Ta− (Td + α)> 0, send the second CTS,
When Ta− (Td + α) = 0, it is also preferable to transmit the first CTS.

Also, according to another embodiment of the access point of the present invention,
A radio resource monitoring means for monitoring a radio resource usage rate;
Preferably, the radio resource monitoring unit notifies the CTS determination unit to shorten the margin time α if the radio resource usage rate is high and to increase the margin time α if the radio resource usage rate is low.

According to the present invention, in a terminal connected to an access point based on the IEEE 802.11 DCF scheme,
RTS transmitting means for transmitting an RTS including the allowable delay time Ta to the access point when transmitting a data packet requiring the allowable delay time Ta to the access point;
From the access point, depending on the remaining time based on the allowable delay time Ta of the RTS, the first CTS (CTS-I) that immediately allows the transmission of the data packet, or the second CTS that holds the transmission of the data packet ( CTS receiving means for receiving CTS-P);
A data packet transmitting means for immediately transmitting a data packet to the access point when receiving the first CTS, and suspending transmission of the data packet to the access point when receiving the second CTS; To do.

According to another embodiment of the terminal of the present invention,
When transmission of a data packet is suspended, the terminal transmits the next data packet when receiving the first CTS from the access point, or the remaining time for the allowable delay time Ta becomes 0 In this case, it is also preferable to discard the data packet.

Also, according to another embodiment of the terminal of the present invention,
When the RTS is transmitted to the access point but the CTS cannot be received from the access point, it is also preferable to include the time obtained by subtracting the time already passed from the allowable delay time included in the RTS, and transmit to the access point again. .

Furthermore, according to another embodiment of the terminal of the present invention,
When the first CTS transmitted from the access point to another terminal is detected, a transition is made to the NAV period in which the use of the channel is postponed,
It is also preferable to stop the NAV period when a data packet is transmitted from another terminal to the access point by the first CTS, and then an ACK transmitted from the access point to the other terminal is detected.

According to the present invention, in a program for causing a computer mounted on an access point connected to a terminal to function based on the IEEE 802.11 DCF method,
RTS receiving means for receiving an RTS including the allowable delay time Ta from the terminal when the terminal transmits a data packet requiring the allowable delay time Ta to the access point;
Depending on the remaining time based on the allowable delay time Ta of the RTS, a first CTS (CTS-I) that immediately permits transmission of the data packet to the terminal, or a second CTS (CTS that suspends transmission of the data packet) -P) CTS determination means for determining whether to transmit,
The computer is caused to function as a CTS transmission unit that transmits the determined first CTS or second CTS.

According to the present invention, in a program for causing a computer mounted on a terminal connected to an access point to function based on the IEEE 802.11 DCF method,
RTS transmitting means for transmitting an RTS including the allowable delay time Ta to the access point when transmitting a data packet requiring the allowable delay time Ta to the access point;
From the access point, depending on the remaining time based on the allowable delay time Ta of the RTS, the first CTS (CTS-I) that immediately allows the transmission of the data packet, or the second CTS that holds the transmission of the data packet ( CTS receiving means for receiving CTS-P);
When the first CTS is received, the data packet is immediately transmitted to the access point, and when the second CTS is received, the computer is caused to function as a data packet transmitting means for deferring transmission of the data packet to the access point. Features.

  According to the present invention, it is possible to realize a radio access control method and the like in consideration of a remaining time with respect to an allowable delay time for a radio system in which an access point and a terminal are connected based on the IEEE 802.11 DCF scheme.

  In particular, by transmitting a second CTS (CTS-P) indicating suspension of data packet transmission to the RTS transmitted from the terminal, the retransmission of the RTS from the terminal is suppressed, and a wasteful packet is generated. Can avoid packet collisions and improve throughput and delay time. Further, not only can an allowable delay time be ensured, but transmission is often performed at the limit of the allowable time, so that delay jitter is also reduced.

  Hereinafter, the best mode for carrying out the present invention will be described in detail with reference to the drawings.

  According to the present invention, the IEEE802.11 DCF scheme is characterized by the frame structure of RTS and CTS exchanged between an access point and a terminal. The RTS includes an allowable delay time. The CTS includes information on whether to immediately permit transmission of a data packet (CTS-I: Immediately) or hold (CTS-P: Pending).

Table 1 below is a configuration diagram of the RTS frame.

Table 2 below is a configuration diagram of the CTS-I / P frame.

  The frame control unit is provided with a field (6 bits) indicating a frame type, and there is a Reserve portion as a control type. Using this Reserve part, CTS-I and CTS-P are defined.

  FIG. 2 is a first sequence diagram in the present invention.

(S201) When the terminal B wants to transmit a DATA frame to the access point, the terminal B transmits an RTS to the access point after the back-off time has elapsed. At this time, the terminal A also desires to transmit a DATA frame to the access point, and counts down the back-off time. Terminal A suspends the countdown of the backoff time by detecting the RTS transmitted from terminal B to the access point.

  Here, it is assumed that there is a margin in the remaining time for the allowable delay time for the DATA frame to be transmitted by terminal B. Further, the DATA frame that the terminal A intends to transmit may have a low priority. In this case, the RTS of Table 1 is configured to include the priority of the DATA frame requested for transmission.

(S202) The access point that has received the RTS from the terminal B determines whether to transmit CTS-I or CTS-P to the terminal B based on the allowable delay time included in the RTS. For example, it can be determined by the following equation.
Ta: Allowable delay time Td: Time required to complete transmission of DATA frame α: Margin time Ta− (Td + α)> 0: Transmit CTS-P Ta− (Td + α) = 0: Transmit CTS-I

  It is also preferable to vary the margin time α according to the radio resource usage rate of the access point. If the radio resource usage rate is high, it is possible to secure a long time until the CTS-I is transmitted by shortening the margin time α. On the other hand, if the radio resource usage rate is low, the time until CTS-I is transmitted can be shortened by increasing the margin time α. This is because it is not preferable to secure a low delay because the transmission of the DATA frame is unnecessarily suspended and transmitted with a margin of the allowable delay time even though the radio resource usage rate is low. However, it is preferable from the viewpoint of reducing delay jitter that the transmission is performed at the limit of the allowable delay time.

  Here, when the allowable delay time included in the RTS is relatively long and the access point determines that the remaining time with respect to the allowable delay time is also relatively long, the CTS-P is transmitted to the terminal B. Thereby, the terminal B temporarily suspends transmission of the data packet. Here, the terminal A restarts the countdown of the back-off time by detecting the CTS-P transmitted from the access point to the terminal B.

(S203) Terminal A expires the back-off time and transmits an RTS to the access point. Here, it is assumed that the allowable delay time included in the RTS transmitted from the terminal A is shorter than the allowable delay time of the RTS previously transmitted from the terminal B.

(S204) When the access point receives the RTS from the terminal A, the access point determines that the data packet from the terminal A should be received immediately. At this time, the access point transmits CTS-I to terminal A.

  Terminal C has not transmitted RTS to the access point. The state of the terminal C is when there is no data packet to be transmitted or when the back-off time has elapsed even if there is a data packet to be transmitted. Here, the terminal C detects the CTS-I transmitted from the access point to the terminal A. At this time, the terminal C shifts to the NAV period, and postpones the use of the channel until the transmission of the DATA frame from the terminal A to the access point is completed. The time when the transmission of the DATA frame is completed is when the ACK transmitted from the access point to the terminal A is detected.

(S205) The terminal A that has received the CTS-I from the access point transmits a DATA frame to the access point. Also, other terminals receiving CTS-I set NAV and refrain from transmission.

(S206) When the access point completes the reception of the DATA frame from the terminal A, the access point transmits an ACK to the terminal A.

  Terminal C detects an ACK transmitted from the access point to terminal A. At this time, the terminal C cancels the NAV period and can use the channel. When terminal C suspends the count-down of the back-off time in S204, it restarts the count-down of the back-off time in S206.

(S207) Thereafter, the access point grasps that the remaining time for the terminal B is decreasing. At this time, the access point transmits CTS-I to terminal B.
(S208) The terminal B that has received the CTS-I from the access point transmits a DATA frame to the access point.

  The terminal B discards the DATA frame when the remaining time with respect to the allowable delay time Ta becomes 0 without receiving the CTS-I from the access point. As a result, the DATA frame that has become meaningless beyond the allowable delay time is discarded, and the next DATA frame is preferentially transmitted, so that the allowable delay delay time of the next DATA frame can be secured.

  As another embodiment, there is a case where the RTS is retransmitted when the CTS cannot be received even though the terminal transmits the RTS to the access point. In this case, the time obtained by subtracting the time that has already passed from the allowable delay time included in the previously transmitted RTS is included in the retransmitted RTS again and transmitted to the access point. This is because the DATA frame has already been put on hold.

  FIG. 3 is a functional configuration diagram of an access point and a terminal in the present invention.

  The basic configuration of both the access point and the terminal is the same as that of the conventional wireless LAN system, and FIG. 3 shows only functional units newly added or improved to the conventional technology.

  The terminal 1 includes a physical layer frame transmission / reception unit 101, a DATA / ACK transmission / reception unit 102, a carrier sense unit 103, a backoff time determination management unit 104, an RTS transmission unit 106, and a CTS-I / P reception unit 107. Have

  The physical layer frame transmission / reception unit 101 transmits / receives a physical layer frame to / from the access point 2 via a wireless link.

  The DATA / ACK transmission / reception unit 102 transmits a DATA frame to the access point 2 and receives an ACK via the physical layer frame transmission / reception unit 101. The processes of S205 and S208 described above are realized.

  The carrier sense unit 103 senses the channel usage status of other terminals via the physical layer frame transmission / reception unit 101. In particular, RTS transmitted by other terminals and CTS-I / P transmitted by the base station are detected.

  The backoff time determination management unit 104 starts counting down the backoff time derived from the contention window when a DATA frame transmission request is generated. When the RTS transmitted from another terminal is detected by the carrier sense unit 103, the countdown is suspended. However, the countdown is restarted when CTS-P from the access point is detected. The process of S203 described above is realized.

  The RTS transmission unit 106 transmits an RTS frame including information on allowable delay time.

  The CTS-I / P receiving unit 107 receives CTS-I or CTS-P transmitted from the access point 2. The information is notified to the DATA / ACK transmission / reception unit 102, and transmission or holding of the DATA frame is controlled.

  The access point 2 includes a physical layer frame transmission / reception unit 201, a DATA / ACK transmission / reception unit 202, an RTS reception unit 206, a CTS-I / P transmission unit 207, a CTS-I / P determination unit 209, and an allowable delay time. And a management unit 208.

  The CTS-I / P transmission unit 207 transmits CTS-I or CTS-P to the terminal 1 based on the determination result of the CTS-I / P determination unit 209.

  The allowable delay time 208 calculates the remaining time for the allowable delay time included in the RTS for the terminal that has received the RTS. Count down from the allowable delay time and manage until the remaining time becomes zero. The CTS-I / P determination unit is notified of the remaining time.

  The CTS-I / P determination unit 209 determines whether to transmit CTS-I or CTS-P. The above-described processes of S202, S204, and S207 are realized.

  Note that when the access point receives a plurality of RTSs of the same low-delay service, it processes them in the order of arrival. That is, the transmission of the DATA frame following the RTS that arrives later does not hinder the transmission of the DATA frame following the RTS that arrived earlier.

  As described above, according to the present invention, on the basis of the IEEE 802.11 DCF method, a wireless access control method and the like considering a remaining time with respect to an allowable delay time is realized for a wireless system in which an access point and a terminal are connected. it can.

  According to the above-described various embodiments of the present invention, those skilled in the art can easily make various changes, modifications, and omissions in the technical idea and scope of the present invention. The above description is merely an example, and is not intended to be restrictive. The invention is limited only as defined in the following claims and the equivalents thereto.

It is a sequence diagram of DCF in a prior art. It is a sequence diagram in the present invention. It is a functional block diagram of the access point and terminal in this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Terminal 101 Physical layer frame transmission / reception part 102 DATA / ACK transmission / reception part 103 Carrier sense part 104 Backoff time determination management part 106 RTS transmission part 107 CTS-I / P reception part 2 Access point 201 Physical layer frame transmission / reception part 202 DATA / ACK Transmission / reception unit 206 RTS reception unit 207 CTS-I / P transmission unit 208 Allowable delay time management unit 209 CTS-I / P determination unit

Claims (15)

In a wireless access control method in a wireless system in which an access point and a terminal are connected based on the IEEE 802.11 DCF method,
A first step of transmitting an RTS including the allowable delay time Ta to the access point when the terminal transmits a data packet requiring the allowable delay time Ta to the access point;
Depending on the remaining time based on the allowable delay time Ta of the RTS, the access point first transmits a data packet to the terminal (CTS-I) or transmits a data packet. A second step of sending a second CTS to be suspended (CTS-P);
When receiving the first CTS, the terminal immediately transmits the data packet to the access point, and when receiving the second CTS, the terminal suspends transmission of the data packet to the access point. A wireless access control method.   When the transmission of the data packet is suspended, the terminal transmits a next data packet when receiving the first CTS from the access point, or the remaining time for the allowable delay time Ta The radio access control method according to claim 1, wherein when the data reaches 0, the data packet is discarded. For the second step, the access point has an allowable delay time Ta, a time Td required for completing the transmission of the DATA frame, and a margin time α.
If Ta− (Td + α)> 0, send the second CTS,
3. The radio access control method according to claim 1, wherein when Ta− (Td + α) = 0, the transmission timing of the data packet from the terminal is scheduled by transmitting the first CTS. 4. .   The access point monitors a radio resource usage rate, and shortens the margin time α if the radio resource usage rate is high, and lengthens the margin time α if the radio resource usage rate is low. The radio access control method according to claim 3, wherein:   If the terminal transmits an RTS to the access point but cannot receive a CTS from the access point, the terminal again includes a time obtained by subtracting the time already elapsed from the allowable delay time included in the RTS, and The radio access control method according to claim 1, wherein transmission is performed to an access point. The terminal
When the first CTS transmitted from the access point to another terminal is detected, a transition is made to a NAV period in which the use of the channel is postponed,
When the first CTS transmits a data packet from the other terminal to the access point, and then detects an ACK transmitted from the access point to the other terminal, the NAV period is stopped. The radio access control method according to claim 1, wherein the radio access control method is one of the following. Based on the IEEE 802.11 DCF method, in an access point to which a terminal is connected,
RTS receiving means for receiving an RTS including the allowable delay time Ta from the terminal when the terminal transmits a data packet requiring the allowable delay time Ta to the access point;
In accordance with the remaining time based on the allowable delay time Ta of the RTS, the terminal is a first CTS (CTS-I) that immediately permits transmission of a data packet, or a second that suspends transmission of the data packet. CTS determination means for determining whether to transmit CTS;
CTS transmitting means for transmitting the determined first CTS or second CTS (CTS-P). The CTS determination means has an allowable delay time Ta, a time Td required for completing the transmission of the DATA frame, and a margin time α.
If Ta− (Td + α)> 0, send the second CTS,
The access point according to claim 7, wherein the first CTS is transmitted when Ta− (Td + α) = 0. A radio resource monitoring means for monitoring a radio resource usage rate;
The radio resource monitoring unit notifies the CTS determination unit to shorten the margin time α if the radio resource usage rate is high and to lengthen the margin time α if the radio resource usage rate is low. The access point according to claim 8. In a terminal connected to an access point based on the IEEE 802.11 DCF method,
RTS transmitting means for transmitting an RTS including the allowable delay time Ta to the access point when transmitting a data packet requiring the allowable delay time Ta to the access point;
From the access point, the first CTS (CTS-I) that immediately permits the transmission of the data packet according to the remaining time based on the allowable delay time Ta of the RTS, or the second that suspends the transmission of the data packet CTS receiving means for receiving CTS (CTS-P) of
A data packet transmitting means for immediately transmitting the data packet to the access point when receiving the first CTS, and suspending transmission of the data packet to the access point when receiving the second CTS; A terminal characterized by that.   When the transmission of the data packet is suspended, the terminal transmits a next data packet when receiving the first CTS from the access point, or the remaining time for the allowable delay time Ta The terminal according to claim 10, wherein when the data reaches 0, the data packet is discarded.   When the RTS is transmitted to the access point but the CTS cannot be received from the access point, the RTS includes the time obtained by subtracting the time already passed from the allowable delay time included in the RTS, and is transmitted to the access point. The terminal according to claim 10 or 11, characterized in that: The terminal
When the first CTS transmitted from the access point to another terminal is detected, a transition is made to a NAV period in which the use of the channel is postponed,
When the first CTS transmits a data packet from the other terminal to the access point, and then detects an ACK transmitted from the access point to the other terminal, the NAV period is stopped. The terminal according to any one of claims 10 to 12, characterized in that: In a program for causing a computer mounted on an access point connected to a terminal to function based on the IEEE 802.11 DCF method,
RTS receiving means for receiving an RTS including the allowable delay time Ta from the terminal when the terminal transmits a data packet requiring the allowable delay time Ta to the access point;
Depending on the remaining time based on the allowable delay time Ta of the RTS, a first CTS (CTS-I) that immediately permits transmission of a data packet to the terminal, or a second that suspends transmission of the data packet CTS determination means for determining whether to transmit CTS (CTS-P);
A program that causes a computer to function as a CTS transmission unit that transmits the determined first CTS or second CTS. In a program for causing a computer mounted on a terminal connected to an access point to function based on the IEEE 802.11 DCF method,
RTS transmitting means for transmitting an RTS including the allowable delay time Ta to the access point when transmitting a data packet requiring the allowable delay time Ta to the access point;
From the access point, the first CTS (CTS-I) that immediately permits the transmission of the data packet according to the remaining time based on the allowable delay time Ta of the RTS, or the second that suspends the transmission of the data packet CTS receiving means for receiving CTS (CTS-P) of
When the first CTS is received, the data packet is immediately transmitted to the access point, and when the second CTS is received, a computer is used as a data packet transmitting means for deferring transmission of the data packet to the access point. A program characterized by functioning.

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