WO2010085093A2

WO2010085093A2 - Method and apparatus for accessing channel in contention based communication system

Info

Publication number: WO2010085093A2
Application number: PCT/KR2010/000379
Authority: WO
Inventors: Yongho Seok
Original assignee: Lg Electronics Inc.
Priority date: 2009-01-21
Filing date: 2010-01-21
Publication date: 2010-07-29
Also published as: WO2010085093A3

Abstract

A method and apparatus of determining a slot time in a contention based communication network is provided. An access point(AP)receives a transmission mode field indicating whether each of a plurality of stations supports directed transmission from each of the plurality of stations constituting a basic service set (BSS), determines the slot time for attempting contention-based access on the basis of the transmission mode field obtained from each of the plurality of stations constituting the BSS, and transmits a mode field comprising the slot time to the plurality of stations constituting the BSS.

Description

METHOD AND APPARATUS FOR ACCESSING CHANNEL IN CONTENTION BASED COMMUNICATION SYSTEM

The present invention relates to wireless communications, and more particularly, to a method and apparatus for accessing a channel.

An institute of electrical and electronics engineers (IEEE) 802.11 very high throughput (VHT) is one of wireless local access network (WLAN) systems newly proposed in recent years to support a throughput of 1Gbps or higher. Researches on the VHT are ongoing independently in two different aspects, that is, IEEE 802.11ac using a band of 6GHz or lower and IEEE 802.11ad using a band of 60GHz.

A channel access mechanism can be classified into contention-based access and non-contention-based access. The contention-based access and the non-contention-based access are differentiated according to whether a backoff process is performed. When using the non-contention-based access, a station attempts channel access without the backoff. When using the contention-based access, if a channel is busy, the station re-attempts channel access after a backoff time elapses. Carrier sense multiple access with collision avoidance (CSMA/CA) which is one type of the contention-based access is used in IEEE 802.11.

The WLAN system employs a directional antenna to obtain a wider coverage and a higher throughput. This is because the use of the directional antenna allows transmission of the same amount of data at a shorter time by using the same frequency resource in comparison with the case of using an omni-directional antenna. However, utilization of the directional antenna may result in coverage extension, but may disadvantageously cause a more severe hidden node problem.

In a contention-based system, the shorter the backoff time, the higher the channel access efficiency. The backoff time can be decreased by the use of the directional antenna. However, in general, a station using the directional antenna coexists with a station using the omni-directional antenna in the WLAN system.

Accordingly, there is a need for a channel access method considering the coexistence with the station using the omni-directional antenna and the hidden node problem caused by the directional antenna.

The present invention provides a method and apparatus for determining a backoff time for channel access in a contention-based wireless local access network (WLAN) system.

The present invention also provides a channel access mechanism in a contention-based WLAN system and a method of determining a frame transmission mode.

In an aspect, a method of determining a slot time in a contention based communication network is provided. The method includes receiving a transmission mode field indicating whether each of a plurality of stations supports directed transmission from each of the plurality of stations constituting a basic service set(BSS), determining the slot time for attempting contention-based access on the basis of the transmission mode field obtained from each of the plurality of stations constituting the BSS, and transmitting a mode field comprising the slot time to the plurality of stations constituting the BSS.

The transmission mode field may be received by using an association request frame or a probe request frame.

The mode field may be transmitted by being included in any one of a beacon frame, an association response frame, and a probe response frame.

If it is determined that the plurality of stations in the BSS consist of only steerable stations on the basis of the transmission mode field, the slot time may include a short request to send (RTS) frame transmission time which is a time consumed in directed RTS frame transmission.

The slot time may further include a short clear channel assessment (CCA) time which is a time consumed in CCA for directed transmission.

If it is determined that steerable stations and non-steerable stations coexist in the BSS, the slot time may include a long RTS frame transmission time which is a time consumed in omni-directed RTS frame transmission and a CCA time which is a time consumed in CCA for omni-directed transmission.

If it is determined that the BSS consists of only steerable stations on the basis of the transmission mode field, the slot time may include a CCA time which is a time consumed in CCA for omni-directed transmission.

The mode field may further include a frame transmission mode field indicating whether each station transmits a frame either omni-directionally or directionally after channel access.

The frame transmission mode field may include a control frame transmission mode subfield for setting a transmission mode of a request to send (RTS) frame and a clear to send (CTS) frame.

The frame transmission mode field may include a data frame transmission mode field for setting a transmission mode of a data frame and an acknowledgement (ACK)/negative-ACK (NACK) frame.

In another aspect, A channel access method of a station in a contention based communication network is provided. The method includes transmitting a transmission mode field which is information indicating whether directed transmission using a directional antenna is supported, receiving a mode field comprising a slot time determined by considering the transmission mode field, and performing a channel access procedure through a backoff process based on the slot time, and receiving or transmitting a frame.

The mode field may further include a frame transmission mode field indicating whether the frame is transmitted either omni-directionally or directionally.

According to the present invention, an overhead generated in a process of contention-based channel access can be reduced in a wireless local access network (WLAN) system in which a station using a directional antenna and a station using an omni-directional antenna coexist.

In addition, a backoff time can be determined according to a type of stations constituting a basic service set (BSS).

FIG. 1 is a schematic view showing an exemplary structure of a wireless local access network (WLAN) system to implement an embodiment of the present invention.

FIG. 2 is a flowchart showing an example of a carrier sense multiple access with collision avoidance (CSMA/CA) operation.

FIG. 3 shows an example of a type of inter frame space (IFS) used in an institute of electrical and electronics engineers (IEEE) 802.11 medium access control (MAC) and a relation among IFSs.

FIG. 4 shows an example of a collision avoidance mechanism using random backoff.

FIG. 5 shows a channel access mechanism according to an embodiment of the present invention.

FIG. 6 is a flowchart showing an example of a method of determining a slot time of an access point (AP) according to an embodiment of the present invention.

FIG. 7 shows an example of a mode field according to an embodiment of the present invention.

FIG. 8 is a block diagram showing a wireless apparatus according to an embodiment of the present invention.

FIG. 1 is a schematic view showing an exemplary structure of a wireless local access network (WLAN) system to implement an embodiment of the present invention. The WLAN system includes one or more basic service sets (BSSs). The BSS is a set of stations (STAs) which are successfully synchronized to communicate with one another. The BSS can be classified into an infrastructure BSS and an independent BSS (IBSS). The infrastructure BSSs (BSS1 and BSS2) shown in FIG. 1 include

STAs

10, 30 and 40, access points (APs) 20 and 50. The AP is a STA providing a distribution service. The

APs

20 and 50 are connected by means of a distribution system (DS). The IBSS operates as Ad-hoc mode and does not include any AP. The IBSS constitutes a self-contained network since connection to the DS is not allowed. A plurality of infrastructure BSSs can be interconnected by the use of the DS. An extended service set (ESS) is a plurality of BSSs connected by the use of the DS. In the same ESS, a non-AP STA can move from one BSS to another BSS while performing seamless communication.

The STA is an arbitrary functional medium including a medium access control (MAC) and wireless-medium physical layer (PHY) interface conforming to the institute of electrical and electronics engineers (IEEE) 802.11 standard. The STA may be a AP or a non-AP STA. A non-AP STA may be a portable terminal operated by a user. The non-AP STA may be simply referred to as an STA. The non-AP STA may be referred to as a wireless transmit/receive unit (WTRU), a user equipment (UE), a mobile station (MS), a mobile terminal, a mobile subscriber unit, etc. The AP is a functional entity for providing connection to the DS through a wireless medium for an associated STA. Although communication between non-AP STAs in an infrastructure BSS including the AP is performed via the AP in principle, the non-AP STAs can perform direct communication when a direct link is set up. The AP may be referred to as a centralized controller, a base station (BS), a node-B, a base transceiver system (BTS), a site controller, etc.

An IEEE 802.11 protocol includes a medium access control (MAC) layer and a physical layer. The basic structure of the MAC includes a distributed coordination function (DCF) based on a carrier sense multiple access with collision avoidance (CSMA/CA).

FIG. 2 is a flowchart showing an example of a CSMA/CA operation. A station attempting channel access waits until a network allocation vector (NAV) value becomes 0 (step S200). An NAV is a timer indicating time information when a wireless medium (or channel) is reserved. The station sets the NAV to a time expected to use a wireless medium including all frames required to complete a current operation. Other stations wait until the NAV is counted down to 0. If the NAV is not 0, a virtual carrier sensing function indicates that the medium is busy. If the NAV is 0, the virtual carrier sensing function indicates that the medium is idle.

When the NAV value of the station becomes 0, a wireless medium is sensed (step S210). It is determined whether the wireless medium is idle (step S220). If the wireless medium is not idle, transmission is delayed for a random time (step S230). If the wireless medium is idle, frame transmission is attempted (step S240). This frame may be a request to send (RTS) frame. If collision occurs as a result of transmission of the RTS frame, transmission is delayed again for a random time, and then returning to step S200, channel access is attempted again (step S250).

IEEE 802.11 MAC uses an inter frame space (IFS) to define a minimum time required for a station intending to perform wireless communication to wait until a next operation is performed after sensing an idle state of a wireless medium. Various types of priorities are provided by the IFS. The lower the IFS value, the higher the priority.

A shorter IFS (SIFS) is a time interval from a time of completely transmitting one data frame to a time of transmitting an acknowledgement (ACK) frame, a clear to send (CTS) frame, etc. The SIFS provides a highest priority level among IFSs. The SIFS has a fixed value according to a physical layer by considering a time required for a station that has transmitted a data frame to enter a state capable of receiving another data frame.

A point IFS (PIFS) is used by an access point (AP) to obtain a medium access right earlier than other stations when operating based on a point coordination function (PCF).

A distributed IFS (DIFS) is used when all stations operating based on a DCF transmit data and a management frame. Transmission in this case has a lower priority than PCF-based transmission.

An extended IFS (EIFS) is used by a DCF-based station to provide a sufficient time capable of transmitting an ACK frame to a receiving station when data frame transmission has an error.

FIG. 3 shows an example of a type of IFS used in the IEEE 802.11 MAC and a relation among IFSs. When a wireless medium is currently used by another station, frame transmission is delayed until the wireless medium is no longer used. When the wireless medium is cleared, a process is suspended during a corresponding IFS. After the IFS, backoff is started. A backoff time for the backoff is defined by the following equation.

MathFigure 1

Random() is a pseudo-random integer drawn from a uniform distribution over the interval [0,CW], where CW is an integer within the range of values of the PHY characteristics aCWmin and aCWmax, aCWmin ≤ CW ≤ aCWmax. Herein, a slot time ‘aSlotTime’ is a value given according to the PHY characteristics of a station.

FIG. 4 shows an example of a collision avoidance mechanism using random backoff. During a time when a station 1 transmits a frame 400, a wireless medium is busy, and thus a station 2 and a station 3 set an NAV. When the wireless medium is in an idle state, the station 2 and the station 3 set a backoff time. For example, assume that the station 2 sets a backoff time of 2, and the station 3 sets a backoff time of 4. The station 2 and the station 3 decrease the backoff time after a DIFS. The station 2 of which the backoff time becomes 0 earlier than the station 3 transmits a frame 420. The station 3 decreases the backoff time after the transmission of the frame 420 and the DIFS, and transmits a frame 430 when the backoff time becomes 0.

The stations can support at least one type of antennas among three types of antennas, that is, a single antenna, a sector antenna, and a phased array antenna. The single antenna is a non-steerable antenna which cannot regulate a pattern of propagation, whereas the sector antenna and the phased array antenna are steerable antennas which can regulate a pattern such as a propagation direction, phase, etc.

Hereinafter, a station including a non-steerable antenna is referred to as a non-steerable STA, and a station including a steerable antenna is referred to as a steerable STA. The non-steerable STA supports only omni-directed transmission, whereas the steerable STA supports directed transmission.

FIG. 5 shows a channel access mechanism according to an embodiment of the present invention. Stations (STAs) 1 and 2 attempt channel access by performing a backoff process in a contention period. When a backoff timer becomes 0, the STA 1 transmits a directed RTS frame 510 to an AP through beamforming. The directed RTS frame 510 is an RTS frame which is directionally transmitted. In comparison thereto, an omni-directed RTS frame is an RTS frame which is omni-directionally transmitted. According to a characteristic of directed transmission, a transmission period required for transmission of the directed RTS frame 510 may be shorter than a transmission period required for transmission of the omni-directed RTS frame. The AP transmits an omni-directed clear-to-send (CTS) frame 520 to the STAs so that all STAs belonging to a BSS can listen. The STA 2 receiving the CTS frame 520 sets an NAV 530 and thus stops channel access during a transmission opportunity (TXOP) of the STA 2 which is currently using a wireless medium. The STA 1 transmits a data frame 540, and the AP transmits an ACK 550 for the data frame 540. Upon completion of the TXOP of the STA 1, backoff is resumed after a DIFS elapses.

There is a need to regulate a backoff time of the steerable STA. In particular, since a physical characteristic changes as directed transmission is introduced, the present invention proposes to decrease a probability of slot time collision.

The steerable STA can define a slot time ‘aSlotTime’ used in the backoff process as follows.

MathFigure 2

Herein, 'aPropDriftMargin' denotes a transmission delay time considering a distance between an STA and an AP, 'aShortRTSDur' denotes a transmission time of a directed RTS frame, 'aSIFSTime' denotes an SIFS time, 'aCCATime' denotes a time required for clear channel assessment (CCA), and ‘aRxTxSwitchTime’ denotes a time required when a physical media dependent (PMD) sub-layer switches from a reception mode to a transmission mode, wherein the PMD sub-layer is a lower portion of a MAC that serves to transmit a radio frequency (RF) signal to another 802.11 STA,. The same will equally apply hereinafter.

However, it is difficult to apply the slot time of Equation 2 above to the non-steerable STA. This is because a transmission time of an omni-directed RTS frame is longer than a transmission time of a directed RTS frame, and thus the slot time of Equation 2 may not be enough to ensure a time required for transmitting the omni-directed RTS frame.

Therefore, when the non-steerable STA and the steerable STA coexist in the BSS, the slot time can be obtained by the following equation.

MathFigure 3

Herein, 'aLongRTSDur' denotes a transmission time of an omni-directed RTS frame. This is a unit of backoff when the backoff is performed in a contention period for channel access in a situation where the non-steerable STA and the steerable STA coexist.

When comparing the slot time of Equation 2 with the slot time of Equation 3, the slot time of Equation 3 is longer than the slot time of Equation 2 due to a transmission time of the omni-directed RTS frame. In a system where the non-steerable STA and the steerable STA coexist, the slot time is greater than a sum of an omni-directed RTS frame transmission time (i.e., aLongRTSDur), a time required in CCA for an omni-directed CTS frame (i.e., aCCATime), and an SIFS time (i.e., aSIFSTime). When the slot time is increased as described herein, it may result in the increase of a convention overhead in the backoff process.

The present invention proposes a channel access mechanism in which an overhead is decreased by performing a backoff process on the basis of a regulated slot time after determining a slot time adaptively according to a type of stations constituting the BSS, that is, according to situations, for example, where the BSS consists of only non-steerable STAs, where the BSS consists of only steerable STAs, and where the BSS consists of both the steerable STAs and the non-steerable STAs.

FIG. 6 is a flowchart showing an example of a method of determining a slot time of an AP according to an embodiment of the present invention.

The AP obtains a transmission mode field from each of stations constituting a BSS (step S610). The transmission mode field is an information element indicating whether each station is a steerable STA supporting directed transmission or a non-steerable STA supporting omni-directed transmission.

The transmission mode field may be received by using an association request frame or a probe request frame which is transmitted by the station to the AP in a BSS participation process.

Before the transmission mode field is received, the AP may transmit a message for requesting the transmission mode field transmission from each station.

The AP determines a backoff time on the basis of the transmission mode field received from each of stations constituting the BSS (step S620). By using the transmission mode field, the AP can know whether the BSS includes only the steerable STA, or only the non-steerable STA, or both the steerable STA and the non-steerable STA, and thus determines a slot time included in the backoff time.

More specifically, the AP may determine the slot time as follows.

When all stations constituting the BSS are steerable STAs, the slot time (i.e., aSlotTime) can be determined by Equation 4.

MathFigure 4

Herein, ‘aShortCCATime’ denotes a clear channel assessment time (CCATime) for directed transmission. In this case, both of the AP and the station use directed transmission and reception, and the AP does not consider an RTS transmission time (i.e., aRTSDur) when determining the slot time. In other words, an RTS frame, a CTS frame, a data frame, and an ACK frame are directionally transmitted. Accordingly, a hidden node problem may not be solved by the CCA. Such a problem can be solved by a capture effect.

Alternatively, when all stations constituting the BSS are steerable STAs, the slot time (i.e., aSlotTime) can be determined by Equation 5.

MathFigure 5

When the RTS frame is directionally transmitted to the AP and the CTS frame is omni-directionally transmitted, the slot time of Equation 5 can be used. When determining the slot time, a clear channel assessment time (i.e., aCCATime) and an RTS frame transmission time are considered. Since directed transmission is performed in this situation, a relatively short clear channel assessment time (i.e., aCCATime) and an RTS frame transmission time (i.e., aShortRTSDur) are considered. Accordingly, the slot time is decreased and thus waste of radio resources is reduced, thereby increasing efficiency.

When steerable STAs and non-steerable STAs coexist in the BSS, the slot time (i.e., aSlotTime) can be determined by Equation 6.

MathFigure 6

Both of the clear channel assessment time and the RTS frame transmission time are considered. In this case, since a non-steerable STA not supporting directed transmission exists in the stations constituting the BSS, aLongCCATime and aLongRTSDur are respectively regarded as the clear channel assessment time and the RTS frame transmission time by considering the non-steerable STAs performing omni-directed transmission.

Alternatively, when all stations constituting the BSS are non-steerable STAs, the slot time (i.e., aSlotTime) can be determined by Equation 7.

MathFigure 7

In this case, the RTS frame transmission time is not considered in the slot time. Since all stations support omni-directed transmission and omni-directed reception, the hidden node problem can be solved by the CCA performed on an RTS frame, a CTS frame, a data frame, and an ACK frame which are omni-directionally transmitted/received.

As described above, the AP obtains transmission mode information of the stations constituting the BSS, and adaptively determines the slot time on the basis of the obtained transmission mode information, thereby increasing utilization efficiency of radio resources.

When the slot time is determined, the AP transmits a mode field including the determined slot time to each station of the BSS (step S630). The mode field may be transmitted to the stations of the BSS in various manners. The mode field may be transmitted by using a management frame or a control frame. For example, the mode field may be included in a beacon frame, an association response frame, a probe response frame, or the like in an information element format when transmitted to each of stations constituting the BSS.

A station receiving the determined slot time may perform channel access by performing backoff based on the determined slot time at a later time, and may use the determined slot time when an IFS is set.

According to another embodiment of the present invention, when each of stations constituting the BSS transmits a frame after channel access, the AP may report to the station whether to perform omni-directed transmission or directed transmission. Specifically, frame transmission mode information indicating whether an RTS frame, a CRS frame, a data frame, and an ACK frame for the transmitted data frame will be transmitted either directionally or omni-directionally may be transmitted in an information element format together with the aforementioned determined slot time or may be transmitted by using an additional frame.

FIG. 7 shows an example of a mode field according to an embodiment of the present invention. A mode field 700 includes a slot time field 710 and a frame transmission mode field 720.

The slot time field 710 includes a slot time determined by the aforementioned slot time determining method. The slot time included in the slot time field 710 may be indicated in a time unit or may be expressed by an index.

The frame transmission mode field 720 is a field for indicating whether each station transmits a frame omni-directionally or directionally after channel access. The frame transmission mode field 720 may include a control frame transmission mode subfield 724 and/or a data frame transmission mode subfield 726.

The control frame transmission mode subfield 724 sets a transmission mode of an RTS frame and/or a CTS frame. The control frame transmission mode subfield 724 indicates whether the RTS frame and/or the CTS frame are transmitted directionally or omni-directionally.

The data frame transmission mode subfield 726 sets a transmission mode of a data frame and/or an ACK/NACK frame. The data frame transmission mode subfield 726 indicates whether the data frame and/or the ACK/NACK frame are transmitted directionally or omni-directionally.

The frame transmission mode field 720 may have a length of 4 bits, wherein 2 bits are used for the control frame transmission mode subfield 724 and the other 2 bits are used for the data frame transmission mode subfield 726.

Table 1 shows an example of determining a transmission mode of a frame according to a value of the control frame transmission mode subfield 724.

Table 1

In Table 1, ‘X’ indicates that a corresponding frame is not used, ‘D’ indicates that a corresponding frame is directionally transmitted, and ‘O’ indicates that a corresponding frame is omni-directionally transmitted. That is, when the control frame transmission mode subfield value is set to 0, a station does not use an RTS frame and a CTS frame, and when it is set to 1, the station directionally transmits the RTS frame, and the AP omni-directionally transmits the CTS frame.

Table 2 shows an example of determining a data frame and an ACK frame according to a value of the data frame transmission mode subfield 726.

Table 2

Similarly to Table 1, ‘D’ indicates that a corresponding frame is directionally transmitted, and ‘O’ indicates that a corresponding frame is omni-directionally transmitted. According to the data frame transmission mode subfield value, a station determines a reception mode depending on a data frame transmission mode, and transmits an ACK frame.

As described above, the AP determines a slot time and a transmission mode of a control frame and a data frame according to whether stations constituting the BSS consist of only steerable STAs, whether the stations consist of only non-steerable STAs, or whether the stations consist of both the steerable STAs and the non-steerable STAs, and then reports the determination result to the stations. Therefore, an optimal slot time and an optimal frame transmission mechanism can be configured adaptively according to a type of the stations constituting the BSS, and thus channel access and data frame transmission can be performed with a decreased overhead and radio resources can be effectively utilized.

FIG. 8 is a block diagram showing a wireless apparatus according to an embodiment of the present invention. A wireless apparatus 800 may be an AP or a non-AP station.

The wireless apparatus 800 includes a processor 810, a memory 820, a transceiver 830, and an antenna 850. The transceiver 830 transmits/receives a radio signal, and implements an IEEE 802.11 physical layer. The transceiver 830 supports directed transmission through the antenna 850. The processor 810 is coupled to the transceiver 830, and implements an IEEE 802.11 MAC layer. When the processor 810 processes an operation of the AP among the aforementioned methods, the wireless apparatus 800 is the AP. When the processor 810 processes an operation of the non-AP station among the aforementioned methods, the wireless apparatus 800 is the non-AP station.

The processor 810 and/or the transceiver 830 may include an application-specific integrated circuit (ASIC), a separate chipset, a logic circuit, and/or a data processing unit. The memory 820 may include a read-only memory (ROM), a random access memory (RAM), a flash memory, a memory card, a storage medium, and/or other equivalent storage devices. When the embodiment of the present invention is implemented in software, the aforementioned methods can be implemented with a module (i.e., process, function, etc.) for performing the aforementioned functions. The module may be stored in the memory 820 and may be performed by the processor 810. The memory 820 may be located inside or outside the processor 810, and may be coupled to the processor 810 by using various well-known means.

The aforementioned embodiments include various exemplary aspects. Although all possible combinations for representing the various aspects cannot be described, it will be understood by those skilled in the art that other combinations are also possible. Therefore, all replacements, modifications and changes should fall within the spirit and scope of the claims of the present invention.

Claims

A method of determining a slot time in a contention based communication network, the method comprising:

receiving a transmission mode field indicating whether each of a plurality of stations supports directed transmission from each of the plurality of stations constituting a basic service set (BSS);

determining the slot time for attempting contention-based access on the basis of the transmission mode field obtained from each of the plurality of stations constituting the BSS; and

transmitting a mode field comprising the slot time to the plurality of stations constituting the BSS.
The method of claim 1, wherein the transmission mode field is received by using an association request frame or a probe request frame.
The method of claim 1, wherein the mode field is transmitted by being included in any one of a beacon frame, an association response frame, and a probe response frame.
The method of claim 1, wherein, if it is determined that the plurality of stations in the BSS consist of only steerable stations on the basis of the transmission mode field, the slot time comprises a short request to send (RTS) frame transmission time which is a time consumed in directed RTS frame transmission.
The method of claim 4, wherein the slot time further comprises a short clear channel assessment (CCA) time which is a time consumed in CCA for directed transmission.
The method of claim 1, wherein, if it is determined that steerable stations and non-steerable stations coexist in the BSS, the slot time comprises a long RTS frame transmission time which is a time consumed in omni-directed RTS frame transmission and a CCA time which is a time consumed in CCA for omni-directed transmission.
The method of claim 1, wherein, if it is determined that the BSS consists of only steerable stations on the basis of the transmission mode field, the slot time comprises a CCA time which is a time consumed in CCA for omni-directed transmission.
The method of claim 3, wherein the mode field further comprises a frame transmission mode field indicating whether each station transmits a frame either omni-directionally or directionally after channel access.
The method of claim 8, wherein the frame transmission mode field comprises a control frame transmission mode subfield for setting a transmission mode of a request to send (RTS) frame and a clear to send (CTS) frame.
The method of claim 9, wherein the frame transmission mode field comprises a data frame transmission mode field for setting a transmission mode of a data frame and an acknowledgement (ACK)/negative-ACK (NACK) frame.
A channel access method of a station in a contention based communication network, the method comprising:

transmitting a transmission mode field which is information indicating whether directed transmission using a directional antenna is supported;

receiving a mode field comprising a slot time determined by considering the transmission mode field; and

performing a channel access procedure through a backoff process based on the slot time, and receiving or transmitting a frame.
The method of claim 11, wherein the mode field further comprises a frame transmission mode field indicating whether the frame is transmitted either omni-directionally or directionally.
An access point (AP) operating by accessing a channel in a wireless local access network (WLAN) system, comprising:

a radio frequency (RF) unit; and

a processor functionally coupled to the RF unit,

wherein the processor is configured to:

obtain transmission mode information indicating whether each station supports directed transmission from all stations constituting a basic service set (BSS);

determine the slot time on the basis of the transmission mode information obtained from the all stations constituting the BSS; and

transmit the slot time to the all stations constituting the BSS.
A station operating by accessing a channel in a wireless local access network (WLAN) system, comprising:

a radio frequency (RF) unit; and

a processor functionally coupled to the RF unit,

wherein the processor is configured to:

transmit transmission mode information which is information indicating whether directed transmission using a directional antenna is supported;

receive a slot time determined by considering the transmission mode information; and

perform a channel access procedure through a backoff process based on the slot time, and receive or transmit a frame.