KR20150083686A - Method of restricting slot access ateempts in wireless lans - Google Patents

Abstract

In the present invention, disclosed is a method for limiting an access trial based on a slot in a wireless LAN where an access point is allowed to determine a parameter to limit an access trial on a plurality of stations in a slot set wherein the station tries to access again in a different slot based on the parameter to limit the access trial when the station has a failure of a channel access in an allocated slot.

Description

[0001] METHOD OF RESTRICTING SLOT ACCESS ATEEMPTS IN WIRELESS LANS [0002]

The following embodiments relate to a method for controlling channel access in a wireless communication system.

The wireless LAN accesses a channel medium distributedly through DCF (Distributed Coordination Function) or EDCA (Enhanced distributed channel access) to preempt a channel, and then transmits a frame. However, as the number of stations existing in a basic service set (BSS) composed of an access point (AP) and a plurality of stations increases, the distributed channel occupation method has a limit. When many stations try to access a channel to occupy limited resources, collision probability and transmission latency increase dramatically.

IEEE 802.11ah must support access of up to 8,000 stations. It is not possible to allocate different time slots to each of up to 8,000 stations. In the standard specification, a set of several time slots is referred to as a restricted access window (RAW), and this RAW is assigned to a specific station group. The stations in the group are allocated a limited number of time slots included in the RAW, and connect only to the slot to which they are assigned.

Since multiple stations can be assigned to one slot in this way, there may be stations that can not use the slot. In addition, if the stations allocated in the front slot in the RAW can not occupy a medium in the slot, the channel can be attempted again through the DCF scheme in the next consecutive slot. Here, a problem arises in the prior art.

The previously proposed channel access method in IEEE 802.11ah does not adequately perform contention resolution when multiple stations are allocated to RAW. This is because, as described above, stations that failed to occupy the channel in the previous slot continuously appear as a new contender in the next slot.

The backoff counter of the stations that failed to occupy the channel in the previous slot is likely to maintain a substantially minimum value, so that collisions due to transmission of the stations in the next slot may occur. Such a collision is highly likely to generate a garbage packet that the access point can not decode. A technique for solving such a problem is required.

Embodiments are based on the assumption that, in a wireless LAN system, an access point transmits information about a slot and a set of slots (for example, RAW in 802.11ah) to each station, and a station recognizes A method is provided in which the access point provides additional information so that the station can efficiently determine the slot usage.

Embodiments provide techniques for reducing the probability of collisions in a slot. In addition, since the station that fails to preempt the channel does not have to keep waking up continuously until the channel is preempted, the embodiments provide a technique for reducing power consumption. In addition, embodiments also provide a technique for prioritizing uplink / downlink traffic, since the access point specifies uplink access restriction and downlink access restriction for each group of stations. Embodiments can provide a technique for reducing the performance and power consumption of the entire system based on these points.

Embodiments can improve the performance of the entire WLAN due to the reduction of the collision probability and reduce the power consumption in the WLAN system with the power limitation when compared with the IEEE 802.11ah proposed in the prior art.

A method of operating an access point along one side comprising: assigning a set of slots comprising a plurality of slots to a group comprising a plurality of stations; Determining parameters limiting access attempts of the plurality of stations within the set of slots; And transmitting a beacon signal including the parameters to the plurality of stations.

At this time, the parameters may limit the number of access attempts of the plurality of stations for slot occupancy in the slot set.

In addition, in the step of determining the parameters, a first parameter for limiting an access attempt of a first station included in the plurality of stations and a second parameter for limiting access attempts of a second station included in the plurality of stations, The parameters can be determined differently.

In addition, each of the parameters may limit the number of uplink access attempts of the corresponding station and the number of downlink access attempts of the corresponding station.

Further, in the step of determining the parameters, an uplink access attempt of any one of the stations included in the plurality of stations may be restricted differently from a downlink access attempt of the one station.

Determining the number of uplink access attempts of the station corresponding to each of the parameters based on the priority of the uplink traffic and the priority of the downlink traffic, The number of times may be limited.

In addition, each of the plurality of slots may include time resources between beacon intervals.

In addition, the method of operation of the access point includes dividing a time resource between beacon intervals into slots of uniform length; And grouping the slots into a plurality of sets of slots.

In addition, the method of operating the access point may further include grouping a plurality of stations included in the service set of the access point into a plurality of groups.

The method of operation of the access point may further include assigning a plurality of slots included in the slot set to a plurality of stations included in the group, Lt; RTI ID = 0.0 > information < / RTI >

The beacon signal may further include information on the set of slots assigned to the group.

The method of operation of the access point may further include: assigning a second set of slots differentiated from the set of slots to a second group distinct from the group; And determining second parameters that limit access attempts of a plurality of second stations in the second group within the second set of slots, wherein in the transmitting, A beacon signal including two parameters may be transmitted to the plurality of stations and the plurality of second stations.

A method of operating a station included in a service set of an access point according to another party includes receiving a beacon signal from the access point; Extracting a slot allocated to the beacon signal and a parameter limiting an access attempt; Attempting channel access in the slot; And if the channel access in the slot fails, retrying channel access in another slot based on the parameter.

At this time, the step of retrying the channel access may include retrying the channel access in the next slot of the slot that fails to access the channel within the limit number of access attempts indicated by the parameter.

In addition, the method of operating the station may further include operating in a low power mode until the next beacon signal is received if the channel access fails for a limited number of access attempts indicated by the parameter.

In addition, the parameter limits each of the number of uplink access attempts and the number of downlink access attempts, and the step of retrying the channel access comprises: determining a number of uplink access attempts and a downlink access attempt And determining whether to retry the channel access based on any of the number of times.

The method further includes extracting information related to a slot set allocated to the beacon signal in the beacon signal, wherein in the step of retrying the channel access, the plurality of slots included in the slot set The channel access may be retried.

Further, in the step of attempting to access the channel, a slot access may be attempted by the DCF method. Further, in the step of retrying the channel access, the slot access can be retried by the DCF scheme.

1 illustrates a basic service set of an access point in accordance with an embodiment;
2 illustrates a set of slots and slots in accordance with one embodiment;
3 is a diagram illustrating an RPS IE according to an embodiment;
4 illustrates a RAW assignment field according to one embodiment;
5 is an operational flow diagram illustrating a method of operation of an access point in accordance with an embodiment.
6 is an operational flow diagram illustrating a method of operation of a station in accordance with one embodiment.
7 is a block diagram illustrating an access point and a station in accordance with one embodiment.

Hereinafter, embodiments will be described in detail with reference to the accompanying drawings.

1 is a diagram illustrating a basic service set (BSS) of an access point according to one embodiment. Referring to FIG. 1, a basic service set 100 according to an embodiment includes an access point 110 and a plurality of stations. A plurality of stations may be grouped into a plurality of groups (120, 130, 140, 150).

Each of the plurality of groups 120, 130, 140, and 150 includes a plurality of stations, and the group may be referred to as a station group. Each of the plurality of stations may be serviced by the access point 110. Each of the plurality of stations may include a sensing terminal, a metering terminal, and the like.

2 is a diagram illustrating a slot and a slot set according to an embodiment. Referring to FIG. 2, an access point according to an exemplary embodiment may divide the time domain into a plurality of slots having a predetermined length, taking air occupation as a time domain. The access point can allocate the thus configured slot to a station connected to itself or a station group composed of several stations. Hereinafter, a slot may be referred to as a time slot.

More specifically, the access point may transmit allocation information to the station or group of stations via a beacon signal. The access point periodically transmits a beacon signal. The access point may divide the beacon interval 230 between the first beacon signal 210 and the second beacon signal 220 into a plurality of slots. For example, the access point may divide the beacon interval 230 into slots of uniform length.

The access point may group a plurality of slots into a plurality of slot sets 241, 242, 243, 244. As will be described in detail below, the access point may allocate a plurality of sets of slots 241, 242, 243, 244 to a plurality of station groups.

This slot allocation allows channel occupation only to a limited station or group of stations authorized by the access point, thereby preventing an unauthorized station from occupying the channel, thereby reducing the probability of collision. At the same time, energy can be saved by transitioning to a doze state at a time when the stations are not allocated to the station.

Embodiments can be applied to IEEE 802.11ah. IEEE 802.11ah is a standard that operates in the sub 1 GHz band and supports a number of stations (e.g., sensors, meters, etc.).

IEEE 802.11ah must support access of up to 8,000 stations. It is not possible to allocate different time slots to each of up to 8,000 stations. In the standard specification, a set of several time slots is referred to as a restricted access window (RAW), and this RAW is assigned to a specific station group. The stations in the group are allocated a limited number of time slots included in the RAW, and connect only to the slot to which they are assigned.

Each of the stations includes information included in a RAW parameter set information element (RPS IE) included in a beacon signal periodically transmitted by an access point, a traffic indication map information element (traffic indication map IE) It is possible to know the time slot allocated to itself. In this case, for fairness between stations, time slots are assigned to stations in a random manner, and it is also possible that several stations are assigned to the same slot. This is because the number of stations belonging to the group may be larger than the number of the limited slots.

Since multiple stations can be assigned to one slot in this manner, the stations access the channel in a DCF manner when accessing the slot. In this case, a station can not use the slot. In addition, if the stations allocated in the front slot in the RAW can not occupy a medium in the slot, the channel can be attempted again through the DCF scheme in the next consecutive slot.

Here, a problem arises in the prior art. The previously proposed channel access method in IEEE 802.11ah does not adequately perform contention resolution when multiple stations are allocated to RAW. This is because, as described above, stations that failed to occupy the channel in the previous slot continuously appear as a new contender in the next slot.

The backoff counter of the stations that failed to occupy the channel in the previous slot is likely to maintain a substantially minimum value, so that collision due to transmission of several stations in the next slot may occur. Such a collision is highly likely to generate a garbage packet that the access point can not decode.

Embodiments can provide a technology for improving the performance of the entire WLAN due to the reduction of the collision probability and reducing power consumption in the WLAN system with power limitation when compared with the IEEE 802.11ah proposed in the prior art.

Embodiments are applicable to both wireless LAN systems and wireless communication systems using slot access in addition to IEEE 802.11ah. However, for convenience of explanation, an embodiment of IEEE 802.11ah will be described below. Hereinafter, in a wireless LAN system according to an exemplary embodiment, an access point notifies each station of information on a slot and a set of slots (for example, RAW in 802.11ah), and the station transmits a slot It is assumed that it operates under a known protocol.

Embodiments provide a method and protocol for providing additional information so that the station can make a slightly more evolved decision when the access point informs each station about the slot and the set of slots (RAW) as described above. In IEEE 802.11ah, it is called an RPS IE to provide stations belonging to a specific RAW with information that can calculate the characteristics of the RAW and accessible slots.

Referring to FIG. 3, an RPS IE 300 according to an embodiment includes a plurality of RAW allocation information 330, 340, and 350. The RPS IE 300 may further include an element ID 310 and a length 320.

Each of the plurality of RAW allocation information 330, 340, and 350 may include information on characteristics of a plurality of RAWs, such as RAW allocation information. Here, since the RPS IE is carried in a beacon every beacon interval, the RAW information may have a valid value only for a time between beacons or beacons containing RPS IEs.

According to embodiments, the individual stations may obtain information about the slot to which they will access based on the RAW information to which the RPS IE belongs. For example, IEEE 802.11ah allows one RAW to be assigned to one slot per station. If the channel fails to occupy the allocated slot, the station may attempt to occupy the channel in the next slot.

The embodiments thus assume that the station may attempt to occupy the channel across multiple slots. In this situation, a phenomenon may occur in which a plurality of stations are driven to occupy a channel in one slot.

More specifically, there may be a situation in which a very large number of stations are gathered to access from a slot existing behind the RAW. In this situation, if the backoff counters of two or more stations are the same, the probability of a collision occurring later increases greatly.

Once such a collision occurs, the slot will not function and will be discarded. At this time, the stations allocated to the next slot and the stations that failed to transmit previously operate simultaneously on the DCF basis to perform contending.

In this case, there is a high probability that a collision occurs in the corresponding slot. Thousands of stations are dense in the embodiment, and since the access time of the station is very limited, the probability of collision within the slot is very large. Such a collision will degrade the performance of the entire system, and the stations will also remain in the wake up state until channel occupancy is available, thereby attempting to occupy the channel, thereby damaging the power consumption.

In order to solve the problem of degradation of overall system performance and power consumption due to such collision and channel busy, embodiments propose RPS IE field as shown in FIG.

Referring to FIG. 4, the RAW Assignment field (RAW Assignment field) 400 among the fields of the RPS IE includes an access restriction field 470. The RAW allocation field 400 includes a RAW control field 410, a RAW slot definition field 420, a RAW start time field 430, a RAW group field 440, a channel indication field 450, 460).

More specifically, the RAW control field 410 includes information for reducing the RAW type and the frame size. For example, the RAW type may include regular RAW, sounding RAW, non-TIM RAW, and the like. The RAW slot definition field 420 includes slot information such as the duration of one slot, the number of slots in the RAW, and the like. The RAW start time field 430 specifies the time at which the corresponding RAW starts, and the time may be based on a transmission time of a packet containing RPS information. The RAW group field 440 includes station group information belonging to the RAW. The channel indication field 450 includes available channel information, and the periodic operation parameter field 460 may include periodic information for a non-TIM station.

The access restriction field 470 includes information that the station restricts an access attempt. For example, the access restriction field 470 may include information about how many times the station is allowed to access the slot for occupancy within the RAW allocated to it.

The access restriction field 470 may be divided into an uplink restriction field 471 and a downlink restriction field 472. The uplink restriction field 471 includes information for limiting an uplink access attempt of the station, and the downlink restriction field 472 includes information for limiting a downlink access attempt of the station. For example, the uplink limit field 471 may include information on how many times the station is allowed to access the slot for occupancy within the RAW allocated to it. The downlink limit field 472 may include information about how many times the station is allowed to access the slot for occupancy within the RAW allocated to it.

From the beacon signal transmitted by the access point, the individual stations can receive the RPS IE and know the RAW information to which they belong. The RAW information to which the user belongs can be updated by looking at the RAW allocation field. The stations can know which slot they can access through the information in the RAW allocation field. Also, through the access restriction field value, it is possible to grasp how many access attempts are possible on the uplink and the downlink.

The access point can spread the policy to station group priorities, group emergency situations, or group power situations by providing different access limit values to each station group. In addition, since the station tries to transmit within the RAW as much as the access limitation, it can solve the problem of the IEEE 802.11ah standard which can continuously try to occupy the channel.

In this way, it is possible to prevent the collision between the stations described above by making an attempt to occupy the channel in the slot to which it belongs and, if this fails, to restrict the attempt in the next slot. Also, The phenomenon of drooling can be prevented.

Further, since the effect is accompanied by the effect, the user tries only as much as the access restriction without waiting until the channel occupation, and abandons the channel occupation after the limitation, so that it does not act as a provisional contender to other stations, It increases the possibility of reducing consumption.

The reason for further distinguishing between uplink and downlink access restriction will be described. Uplink traffic and downlink traffic do not always have the same priority. For example, in the case of gas meter information, uplink traffic may be more important than downlink traffic. (E.g., individual meters), rather than downlink traffic sent by an access point (e.g., a central terminal) in the role of collecting gas meter information, Traffic can be more important.

Since the importance of uplink and downlink traffic is different in this way, it is effective to set the access limit to a different value. To receive uplink traffic from a particular group of stations at the access point as quickly as possible, as in the above example, the uplink access limit may be set to a value higher than the downlink access limit. Of course, these values may vary depending on the number of slots in RAW and the number of stations belonging to RAW and having uplink traffic.

Embodiments can reduce the probability of collisions in a RAW slot by individually setting access limit values for each RAW. As a result, the embodiments can prevent the stations failing to occupy the channel from continuing to try to acquire a channel while going over to the next slot.

Embodiments may also reduce the number of content stations in a RAW slot. Embodiments can thereby reduce the probability of collisions within slots. A station with a backoff counter at a lower value toward the rear slot of the RAW may be crowded, and these stations may cause a collision when making an access attempt. Embodiments can reduce the probability of collision because they block this transmission attempt after a certain limited number of times.

Also, in the conventional IEEE 802.11ah standard, the station may continue to wake up until the channel is preempted. In the worst case, stations that can not occupy the channel may be awake from the full RAW. In this case, power can be wasted. Embodiments can reduce the power consumption of the stations by restricting the number of channel occupancy attempts by placing access restrictions.

In addition, the access point according to the embodiments can differently determine the number of access restrictions according to the traffic characteristics of the station group assigned to one RAW. Through this, the access point can positively adjust the station's channel occupancy attempt pattern. For example, the access point may provide a channel control technique for sending / receiving an urgent packet.

5 is a flowchart illustrating an operation method of an access point according to an exemplary embodiment of the present invention. Referring to FIG. 5, in step 510, an access point assigns a set of slots including a plurality of slots to a group including a plurality of stations. In step 520, the access point determines parameters that limit access attempts of a plurality of stations within the set of slots. In step 530, the access point transmits a beacon signal including parameters to a plurality of stations. The steps described in FIG. 1 through FIG. 4 may be applied to each step shown in FIG. 5, so that a detailed description will be omitted.

6 is a flowchart illustrating an operation method of a station according to an exemplary embodiment of the present invention. Referring to FIG. 6, in step 610, the station receives a beacon signal from an access point. In step 620, the station extracts the slot allocated to itself and the parameter limiting the access attempt in the beacon signal. In step 630, the station attempts channel access in the slot assigned to it. In step 640, the station determines whether channel access is successful. If the channel access fails, the station determines in step 650 whether to restrict access based on the parameters. For example, if the number of cumulative access attempts is less than the number of access attempt limits indicated by the parameter, the station may determine that access is not yet restricted. If the access is not yet restricted, in step 660 the station retries the channel access in the other slot. The steps described in FIG. 1 through FIG. 4 may be applied to each step shown in FIG. 6, so that a detailed description will be omitted.

7 is a block diagram illustrating an access point and a station in accordance with one embodiment. Referring to FIG. 7, an access point 710 according to an embodiment includes an allocation unit 711, a determination unit 712, and a transmission unit 713. The assigning unit 711 may assign a set of slots including a plurality of slots to a group including a plurality of stations. The determination unit 712 can determine parameters that limit access attempts of a plurality of stations in the set of slots. The transmitting unit 713 may transmit a beacon signal including parameters to a plurality of stations.

The station 720 according to one embodiment includes a receiving unit 721, an extracting unit 722, an access unit 723, and a determining unit 724. The receiving unit 721 can receive a beacon signal from the access point. The extracting unit 722 may extract a slot allocated to itself in the beacon signal and a parameter limiting the access attempt. The access unit 723 may attempt to access the channel in the slot allocated to it. The determination unit 724 may determine whether or not the channel access is successful. In addition, when the channel access fails, the determination unit 724 can determine whether to restrict access based on the parameter. If access is not yet restricted, the access unit 723 can retry channel access in another slot.

The steps described in FIG. 1 through FIG. 6 may be applied to each step shown in FIG. 7, so that a detailed description will be omitted.

The embodiments described above may be implemented in hardware components, software components, and / or a combination of hardware components and software components. For example, the apparatuses, methods, and components described in the embodiments may be implemented within a computer system including, for example, a processor, a controller, an arithmetic logic unit (ALU), a digital signal processor, such as an array, a programmable logic unit (PLU), a microprocessor, or any other device capable of executing and responding to instructions. The processing device may execute an operating system (OS) and one or more software applications running on the operating system. The processing device may also access, store, manipulate, process, and generate data in response to execution of the software. For ease of understanding, the processing apparatus may be described as being used singly, but those skilled in the art will recognize that the processing apparatus may have a plurality of processing elements and / As shown in FIG. For example, the processing unit may comprise a plurality of processors or one processor and one controller. Other processing configurations are also possible, such as a parallel processor.

The software may include a computer program, code, instructions, or a combination of one or more of the foregoing, and may be configured to configure the processing device to operate as desired or to process it collectively or collectively Device can be commanded. The software and / or data may be in the form of any type of machine, component, physical device, virtual equipment, computer storage media, or device , Or may be permanently or temporarily embodied in a transmitted signal wave. The software may be distributed over a networked computer system and stored or executed in a distributed manner. The software and data may be stored on one or more computer readable recording media.

The method according to an embodiment may be implemented in the form of a program command that can be executed through various computer means and recorded in a computer-readable medium. The computer-readable medium may include program instructions, data files, data structures, and the like, alone or in combination. The program instructions to be recorded on the medium may be those specially designed and configured for the embodiments or may be available to those skilled in the art of computer software. Examples of computer-readable media include magnetic media such as hard disks, floppy disks and magnetic tape; optical media such as CD-ROMs and DVDs; magnetic media such as floppy disks; Magneto-optical media, and hardware devices specifically configured to store and execute program instructions such as ROM, RAM, flash memory, and the like. Examples of program instructions include machine language code such as those produced by a compiler, as well as high-level language code that can be executed by a computer using an interpreter or the like. The hardware devices described above may be configured to operate as one or more software modules to perform the operations of the embodiments, and vice versa.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. For example, it is to be understood that the techniques described may be performed in a different order than the described methods, and / or that components of the described systems, structures, devices, circuits, Lt; / RTI > or equivalents, even if it is replaced or replaced. Therefore, other implementations, other embodiments, and equivalents to the claims are also within the scope of the following claims.

Claims (20)

In an operating method of an access point,
Allocating a set of slots including a plurality of slots to a group including a plurality of stations;
Determining parameters limiting access attempts of the plurality of stations within the set of slots; And
Transmitting a beacon signal including the parameters to the plurality of stations
/ RTI >
The method according to claim 1,
Wherein the parameters limit the number of access attempts of the plurality of stations for slot occupancy in the set of slots.
The method according to claim 1,
In determining the parameters,
Wherein a first parameter limiting an access attempt of a first station included in the plurality of stations and a second parameter limiting an access attempt of a second station included in the plurality of stations are determined differently, How it works.
The method according to claim 1,
Each of the parameters limiting the number of uplink access attempts of the corresponding station and the number of downlink access attempts of the corresponding station.
The method according to claim 1,
In determining the parameters,
Wherein an uplink access attempt of any one of the plurality of stations and a downlink access attempt of any one of the plurality of stations are restricted differently from each other.
The method according to claim 1,
In determining the parameters,
The number of uplink access attempts of the station corresponding to each of the parameters and the number of downlink access attempts of the corresponding station are limited based on the priority of the uplink traffic and the priority of the downlink traffic. .
The method according to claim 1,
Wherein each of the plurality of slots comprises a time resource between beacon intervals.
The method according to claim 1,
Dividing a time resource between beacon intervals into slots of uniform length; And
Grouping the slots into a plurality of sets of slots
Further comprising the steps of:
The method according to claim 1,
Grouping a plurality of stations included in a service set of an access point into a plurality of groups
Further comprising the steps of:
The method according to claim 1,
Allocating a plurality of slots included in the slot set to a plurality of stations included in the group
Further comprising:
Wherein the beacon signal further comprises allocation information that the plurality of slots are assigned to the plurality of stations.
The method according to claim 1,
Wherein the beacon signal further comprises information regarding the set of slots assigned to the group.
The method according to claim 1,
Assigning a second set of slots differentiated from the set of slots to a second group distinct from the group; And
Determining second parameters that limit access attempts of a plurality of second stations included in the second group within the second set of slots
Further comprising:
In the transmitting step,
The beacon signal including the parameters and the second parameters is transmitted to the plurality of stations and the plurality of second stations.
A method of operating a station included in a service set of an access point,
Receiving a beacon signal from the access point;
Extracting a slot allocated to the beacon signal and a parameter limiting an access attempt;
Attempting channel access in the slot; And
If the channel access in the slot fails, retrying channel access in another slot based on the parameter
Gt; station. ≪ / RTI >
14. The method of claim 13,
The step of retrying the channel access
Retrying channel access in the next slot of the slot that failed to access the channel within the limit number of access attempts indicated by the parameter
/ RTI >
14. The method of claim 13,
If the channel access fails for a limited number of access attempts indicated by the parameter, operating in a low power mode until receiving the next beacon signal
≪ / RTI >
14. The method of claim 13,
The parameter limits the number of uplink access attempts and the number of downlink access attempts, respectively,
Wherein retrying the channel access comprises:
Determining whether to re-attempt channel access based on either the number of uplink access attempts and the number of downlink access attempts depending on the type of channel access
/ RTI >
14. The method of claim 13,
Extracting information related to a slot set allocated to the beacon signal in the beacon signal
Further comprising:
And in the step of retrying the channel access, the channel access is retried within a plurality of slots included in the set of slots.
14. The method of claim 13,
In attempting to access the channel,
A method of operating a station in which slot access is attempted in a DCF manner.
14. The method of claim 13,
In the step of retrying the channel access,
And the slot access is retried in the DCF manner.
A computer-readable recording medium having recorded thereon a program for executing the method according to any one of claims 13 to 19.

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