KR20100072959A - System and method for communication in wireless local area network - Google Patents

Abstract

Disclosed are a communication system and method for performing a backoff for performance improvement of a WLAN system. In order to provide a communication system and method in a wireless local area network that can prevent a collision by a hidden node, the channel monitor unit for determining whether or not to start a new back off counting; A representative backoff slot determination unit for determining a backoff in units of virtual slots when it is determined by the channel monitor unit to start a new backoff counting unit; A backoff counter unit for counting a backoff determined by the representative backoff slot determination unit; And a data transmitter for accessing the channel and transmitting data when the channel is idle at the time when the backoff counter is counted by the backoff counter, wherein the virtual slot constitutes a backoff window section. A communication system is provided in which timeslots are grouped in a predetermined number. According to the present invention, in an environment where hidden nodes are present so that signals cannot be detected from each other, each node is distributed and selects a backoff, that is, a delay time, thereby preventing collision by the hidden node.

Description

Communication system and method in wireless local area network {SYSTEM AND METHOD FOR COMMUNICATION IN WIRELESS LOCAL AREA NETWORK}

The present invention relates to wireless communications, and more particularly, to a communication system and a method for performing a backoff for improving the performance of a wireless local area network (WLAN) system.

Recently, with the development of wireless communication technology and the proliferation of wireless devices, the demand for high-speed and reliable data transmission through a wireless link is increasing. The WLAN developed in response to such a request basically consists of terminals [Station, STA], which are mobile data communication equipment, and base stations [Access Point, AP] that can exchange data with the terminals. Base stations and terminals located in the same wireless service area are known as a base service set (BSS). In this specification, a terminal, a station, or a base station that performs communication will be mixed with a node.

Terminals located in one radio service area transmit or receive data using radio resources allocated from a base station. At this time, the base station allocates the radio resource in the form of a phase resource. The phase resource means a time interval in which the terminal or the base station can transmit data.

1 is a view showing the structure of a WLAN system to which the present invention is applied.

As shown in FIG. 1, each base station [AP, 20, 21] is connected to a wired network [10], and a plurality of terminals [STA, 32, 34, 36, 42, 44] is the IEEE 802.11 series of physical [Physical, PHY] is connected to each base station [20, 21] through a radio link according to the [Media Access Control (MAC)] layer protocol and transmits and receives data through a plurality of radio channels. The terminals [32, 34, 36] and the base station [20] located in the same wireless service area [30] form one basic service set [BSS], and the terminals located in the wireless service area [40] [ 42 and 44 and the base station 21 form different BSSs. Terminals located in each BSS may exchange data with each other through a corresponding base station. The main functions of the base stations [20, 21] include the delivery of data traffic, access to other networks [e.g. wired network [10], support of roaming, synchronization within one BSS, support of power management and one BSS Control of media access to support time-bound services within the network.

In particular, the MAC layer of a base station [AP, 20, 21] or a plurality of terminals [STA, 32, 34, 36, 42, 44] in a WLAN system is a layer that controls the transmission of data, WLAN Act as a key element in the system. The MAC layer defines a distributed coordination function (DCF), and the DCF transmits one first-in, first-out (FIFO) to all terminals requiring wireless medium access in an asynchronous transmission method. Control to have a transmission queue.

Accordingly, in the DCF mode, the MAC layer checks whether a predetermined medium is busy, and after a certain terminal uses a channel to avoid collision of the wireless medium, a random terminal is terminated at the end of each frame. Consider backoff, or delay time. Also, a positive acknowledgment of a frame transmitted through the wireless medium is used to confirm a request for retransmission on the receiving side of the transmitted frame.

The MAC layer then uses IFS [Inter Frame Space] to define the minimum start to wait for the next operation after detecting that the wireless medium is idle. The IFS provides various kinds of priorities. The smaller the IFS value, the higher the priority.

First, DIFS (Distributed Inter Frame Space) is a time interval used when transmitting user data or a management frame of a terminal operating by DCF. In addition, a SIFS (Shorter Inter Frame Space) means a time interval before one frame is transmitted and an ACK frame is transmitted, and is the highest priority time interval. In addition, the SIFS has a fixed value according to the physical layer in consideration of the time until the terminal transmitting the frame can receive another frame.

In accordance with the IEEE 802.11 standard, DCF, adopted as a contention-based channel access function in the Media Access Control [MAC] layer, is used for carrier sensing multiple access / collision avoidance using binary exponential backoff. Carrier Sense Multiple Access with Collision Avoidance, CSMA / CA].

CSMA / CA detects a carrier on a transmission path in a WLAN and avoids collision so that collision does not occur. Unlike carrier-detection multi-access / collision detection [CSMA / CD], which transmits data after checking network status, the acknowledgment signal does not collide without a collision. When you confirm that it is sent, you are sending data.

According to the DCF, the terminal determines the data transmission time. The terminal operates as a BSS or IBSS [Independent BSS]. For reference, there are two types of IEEE 802.11 networks, one is an infrastructure network, that is, a network in which an AP is used in all communication processes, and the other is an ad hoc network, that is, a network that communicates directly between terminals without an AP. . Time is divided into repetitive cycles called super frames. Each super frame starts with a beacon frame. The remaining time is divided into optional contention free period (CFP) and contention period (CP). DCF operates during CP.

2A and 2B are conceptual views illustrating the concept of random backoff defined in the DCF mode.

In order to solve the hidden node problem described later, DCF optionally defines a RTS / CTS [Request to send / Clear to send] mechanism.

A transmitting terminal that wants to transmit a data frame first determines whether a channel state is idle by using a CSMA / CA mechanism. In DCF mode, if the channel is idle, the node waits for DIFS and sends data. When the data frame is transmitted after DIFS has elapsed, a predetermined random backoff, or delay time, is provided to prevent collision between terminals. When the channel is in an idle state, a random backoff operation for data transmission may be performed, such as when a data frame transmission by another terminal is finished and DIFS passes. According to the random backoff mechanism when the channel is in an idle state, the transmitting terminal randomly selects a backoff, that is, a delay time within a contention window (CW), that is, a backoff window. When the channel is in the idle state and the channel is in the idle state, the data frame is transmitted after waiting for a backoff, or a delay time. However, when there is another terminal using the channel while waiting, the apparatus stops waiting for the backoff, that is, delay time, and waits for the remaining delay time when the channel is idle, and then transmits data. When the devices that have the same backoff or delay time accidentally try to transmit data at the same time, the data transmission fails and retransmits. In case of retransmission, the size of CW, the range in which the delay time is selected, is exponentially Increases.

In the DCF mode, the terminal that wants to transmit the data frame monitors the state of the channel. This process is performed until the idle period is equal to DIFS (Distributed inter frame space). The terminal detecting the idle DIFS waits for a random backoff interval before transmitting. The backoff counter is decremented in slots while it is detected that the channel is in an idle state. If the channel is detected to be in use while the backoff counter is in operation, the counter stops and resumes operation when an idle state greater than DIFS is detected. When the backoff becomes 0, the terminal transmits a frame. The backoff time in each transmission is chosen uniformly in the range [1, CW] in timeslots. Where CW is the current backoff window size. On initial transmission attempt, CW is equal to the initial backoff window size, CWmin. CW is increased by twice until the maximum backoff window size, CWmax.

After the receiving terminal successfully receives the frame, the receiving terminal transmits an ACK packet after SIFS. If the transmitting terminal does not receive the ACK packet within a designated time or detects another frame transmission, the transmitting terminal reschedules the frame transmission according to the backoff policy.

According to the RTS / CTS mechanism, a transmitting terminal that wants to transmit a data frame transmits an RTS frame before transmitting the data frame. The RTS frame is transmitted according to the backoff policy. If the RTS frame is successfully delivered to the receiving terminal, the receiving terminal responds with a CTS frame. The data frame is then transmitted from the transmitting terminal, followed by the ACK packet from the receiving terminal. These four frames [RTS, CTS, data, ACK] are distinguished by SIFS. That is, the RTS / CTS frame performs a channel reservation function for data frame transmission.

As described above, according to the conventional IEEE 802.11 standard, DCF is defined without adjustment of AP, and the core of DCF is to select a random integer between 0 and CW variable values to avoid collision between terminals, and then select the selected integer. It waits for the slot corresponding to and then transmits (Random Backoff).

As described above, according to the conventional technology of transmitting an RTS packet by determining a random backoff, that is, a delay time according to an RTS / CTS and a random backoff mechanism, a hidden node exists so that signals between the hidden nodes are hidden from each other. In an undetectable environment, collisions between RTS packets transmitted by hidden nodes are not avoided.

3A, 3B, and 3C are conceptual views for explaining a hidden node problem.

In FIG. 3A, node A and node C are hidden nodes. In other words, node A wants to transmit data to node B, but node A, which does not belong to node C's signaling coverage, cannot detect node C's signal, so node B is node RTS packet currently transmitted from node C. You can not confirm that you are receiving. Accordingly, as shown in FIG. 3B, the node T transmits a length T [slot unit, in which the RTS packet is 18 slots in the case of IEEE 802.11b, and 1 slot is 20 microseconds] based on the transmission time of the RTS packet. If the random backoff determined by the node A is included during the period [T1, T2-1] before and after, the node A transmits the RTS packet to the next time slot of the random backoff, and as a result, the node A transmits the random backoff. The RTS packet and the RTS packet transmitted by the node C collide with each other. In FIG. 3B, the transmission time point of the RTS packet transmitted by the node C is indicated by x, and the RTS packet transmission interval of the node A, in which the RTS packet transmitted by the node A and the RTS packet transmitted by the node C, may collide with each other, It is marked as a vulnerable period.

Due to such a hidden node problem, there is a problem that the performance of the entire network is reduced. 3C is an experimental graph showing that the probability of packet collision due to a hidden node problem increases as the number of nodes increases. The single hop shown in FIG. 3C means an experimental result in a situation in which all nodes can sense each other's signals. As shown in Fig. 3c, according to the prior art, it can be seen that the collision probability by the hidden node is considerably high compared to the case of the single hop.

The present invention has been proposed to solve the above problems, and in the environment where a hidden node is present and cannot detect each other's signal, each node is distributed and selects a backoff, that is, a delay time, to the hidden node. It is an object of the present invention to provide a communication system and method in a wireless local area network that can prevent collisions caused by a collision.

Other objects and advantages of the present invention can be understood by the following description, and will be more clearly understood by the embodiments of the present invention. Also, it will be readily appreciated that the objects and advantages of the present invention may be realized by the means and combinations thereof indicated in the claims.

In order to achieve the above object, the present invention provides a channel monitor for determining whether it is time to start a new back-off counting; A representative backoff slot determination unit for determining a backoff in units of virtual slots when it is determined by the channel monitor unit to start a new backoff counting unit; A backoff counter unit for counting a backoff determined by the representative backoff slot determination unit; And a data transmitter for accessing the channel and transmitting data when the channel is idle at the time when the backoff counter is counted by the backoff counter, wherein the virtual slot constitutes a backoff window section. A communication system is provided in which timeslots are grouped in a predetermined number.

In addition, the present invention to achieve the above object the first step of receiving data; Determining whether it is time to newly start backoff counting based on the received data; Determining a backoff in virtual slot units if it is determined that it is time to newly start backoff counting; Counting the determined backoff; And a fifth step of accessing the channel and transmitting data when the channel is in the dormant state when all of the backoffs are counted, wherein the virtual slot has a predetermined number of time slots constituting a backoff window interval. It provides a communication method that is grouped.

According to the present invention, in an environment where hidden nodes are present so that signals cannot be detected from each other, each node is distributed and selects a backoff, that is, a delay time, thereby preventing collision by the hidden node.

The following merely illustrates the principles of the invention. Therefore, those skilled in the art, although not explicitly described or illustrated herein, can embody the principles of the present invention and invent various devices that fall within the spirit and scope of the present invention.

In addition, all conditional terms and embodiments listed herein are in principle clearly intended to be understood solely for the purpose of understanding the concept of the invention and are not to be limited to the specifically listed embodiments and states. do.

It is also to be understood that the detailed description, as well as the principles, aspects and embodiments of the invention, as well as specific embodiments thereof, are intended to cover structural and functional equivalents thereof.

In addition, these equivalents should be understood to include not only equivalents now known, but also equivalents to be developed in the future, that is, all devices invented to perform the same function regardless of structure.

Thus, for example, it should be understood that the block diagrams herein represent a conceptual view of example circuitry embodying the principles of the invention. Similarly, all flowcharts, state transitions, pseudocodes, and the like are understood to represent various processes performed by a computer or processor, whether or not the computer or processor is substantially illustrated on a computer readable medium and whether the computer or processor is clearly shown. Should be.

The functionality of the various elements shown in the figures, including functional blocks represented by a processor or similar concept, can be provided by the use of dedicated hardware as well as hardware capable of executing software in association with appropriate software. When provided by a processor, the functionality may be provided by a single dedicated processor, by a single shared processor or by a plurality of individual processors, some of which may be shared.

In addition, the explicit use of the terms processor, control or similar terminology should not be interpreted exclusively as a citation of hardware capable of executing software, and is not intended to be used to store digital signal processor [DSP] hardware or software without limitation. It should be understood that it implicitly includes ROM, RAM, and nonvolatile memory.

Other hardware for the governor may also be included.

Similarly, the switches shown in the figures may be presented conceptually only. It is to be understood that the action of such a switch can be performed manually or through the interaction of program control and dedicated logic via program logic or dedicated logic. Certain techniques may be selected by a designer with a more detailed understanding of the disclosure.

In the claims of this specification, components expressed as means for performing the functions described in the detailed description include all types of software including, for example, a combination of circuit elements or firmware / microcode, etc. that perform the functions. It is intended to include all methods of performing a function which are combined with appropriate circuitry for executing the software to perform the function.

The invention, as defined by these claims, is equivalent to what is understood from this specification, as any means capable of providing such functionality, as the functionality provided by the various enumerated means are combined, and in any manner required by the claims. It should be understood that.

The above objects, features and advantages will become more apparent from the following detailed description taken in conjunction with the accompanying drawings. First of all, in adding reference numerals to the components of each drawing, it should be noted that the same components have the same number as much as possible even if displayed on different drawings. In addition, in describing the present invention, when it is determined that the detailed description of the related known technology may unnecessarily obscure the subject matter of the present invention, the detailed description thereof will be omitted. Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

4A and 4B are conceptual views illustrating a packet transmission method according to the present invention.

As shown in Figs. 4A and 4B, according to the present invention, instead of transmitting a packet to a random time slot determined according to a random backoff, each node can transmit a packet only in a specific time slot, that is, to access a channel. Group time slots in the backoff window interval. In the present specification, the grouped time slot group [VIII] is called a virtual slot [VRT], and the last time slot among the time slot groups constituting the virtual slot [VS] is called a representative backoff slot. 4A and 4B show the representative backoff slots as T1 and T2. According to the present invention, the node is determined to be backoff in units of virtual slots [VS], and thus channel access is performed at the next time slot of the representative backoff slots represented by T1 and T2. Thus, according to the prior art shown in FIG. 3B, each time slot that can be determined as a random backoff slot has a collision probability of (1 / [2T-1]), while the virtual slot according to the invention shown in FIG. 4A. Representative backoff slots T1 and T2 of [VS] have a collision probability of (1/2). Therefore, according to the present invention, it is possible to improve the performance of the entire network by the reduced probability of collision compared to the prior art.

According to an embodiment of the present invention, if a node does not receive an ACK packet transmitted from another node during a virtual slot [VS] period, i.e., until the time point of the representative backoff slot, the node enters a channel in a slot after the representative backoff slot. Access. According to the present invention, a backoff counter based on the virtual slot [VS] section is determined by Equation 1 below.

backoff counter = i * VS

i = Random [0, B]

B = ceiling (CW / tVS)

Here, VS is a virtual slot, i is an index of the virtual slot, the number of virtual slots that the node should currently count, tVS means the length of the virtual slot. CW is the current backoff window size.

On initial transmission attempt, CW is equal to the initial backoff window size, CWmin. CW is, for example, increased by 2 times until the maximum backoff window size, CWmax. ceiling (number) is a function that returns a variable number rounded up to the nearest integer. For example, ceiling (2.5) rounds 2.5 to the nearest integer and returns 3. ceiling (CW / tVS) rounds the result of (CW / tVS) to the nearest integer and returns the rounded result to B.

I is an arbitrary integer selected in the range of [0, B] in the initial transmission attempt, while i is decremented in virtual slots while the backoff counter unit counts in time slots. Meanwhile, as described above, when there is another terminal using the channel during the backoff period, the backoff, that is, waiting for the delay time is stopped, and when the channel is idle again, the backoff counting is resumed. The channel is accessed after waiting for the remaining delay time in time slots, where i is the number of virtual slots that the node should currently count, while i is counted in time slots while the backoff counter counts in time slots. Is reduced. That is, i is the index of the virtual slot, which serves as a variable representing the total backoff length in the case of the first transmission attempt, and stops backoff, that is, waiting for delay, and backoff when the channel is idle. When a channel is accessed after resuming counting and waiting for a residual delay in units of time slots, the function serves as a variable representing the length of the residual delay.

For example, when an ACK packet is received according to an embodiment of the present invention, the operation of the backoff counter unit according to the present invention may be defined by Equation 2 below.

Taccess = {[now + DIFS], [now + DIFS + i * tVS]} (in seconds)

Here, Taccess means a backoff period, that is, a delay period, and is defined as a period of {[now + DIFS] and [now + DIFS + i * tVS]}. now means a time when the channel is in an idle state, for example, when a node receives an ACK packet. That is, according to the present invention, the backoff window is defined as a time from the end point of DIFS in which counting is started by receiving an ACK packet from the time point corresponding to [i * tVS]. I is an index of the virtual slot, and in the case of the first transmission attempt, is an integer selected from the range of [0, B] as defined in Equation 1 above, and resumes backoff counting. ] In case of transmitting data after waiting for the remaining delay time, i is decremented in virtual slots while the backoff counter unit counts in time slots as the number of virtual slots that the node should currently count.

Here, in order to explain an embodiment of the present invention, the figure illustrates a case in which an ACK packet is received according to the RTS / CTS mechanism, but the present invention is not limited to this embodiment and the present invention belongs to the present invention. It is obvious to those skilled in the art. Therefore, it is to be understood that the present invention is not limited to the embodiment shown in the drawings. Therefore, according to the present invention, the variables now and DIFS defined in Equation 2 may be replaced with other possible variables. In this case, Equation 2 means "when the counting of the backoff counter unit is stopped, the backoff counter unit is not operated until the next backoff count start time [wait without deceasing backoff counter until next Taccess time]" It can be understood as.

Figure 4b is an embodiment of the present invention, the node determines the backoff, i.e., i = 2 for the length of the maximum virtual slot according to Equation 1 [backoff counter = 2VS], [Equation 2] According to, the ACK packet is received to access a channel in a time slot after the virtual slot corresponding to 2tVS [i = 2] has elapsed from the end of DIFS in which counting has started, that is, after the representative backoff slot. For example, an operation of performing RTS packet transmission is shown. Rbc shown in FIG. 4B is an index of a virtual slot.

5 is a conceptual diagram illustrating a process of accessing a channel by nodes A, B, and C having a hidden node relationship according to an embodiment of the present invention.

In FIG. 5, node A exchanges an RTS packet and a CTS packet with a receiving node, receives an ACK packet from the receiving node after transmitting a data packet [DATA], and again determines a backoff [i = 2] to attempt channel access. do.

Node B resumes backoff counting after node A completes the transmission of data packet [DATA] and receives an ACK packet from the receiving node, accessing the channel after a predetermined backoff period in accordance with the present invention. After exchanging the RTS packet and the CTS packet and transmitting the data packet [DATA], an ACK packet is received from the receiving node, and the channel is determined again [i = 4].

Node C resumes backoff counting according to the channel occupancy of nodes A and B during the backoff period, looking for an opportunity to access the channel after a predetermined backoff period in accordance with the present invention.

Since backoff counting is performed in units of time slots, the timing at which backoff counting is stopped due to channel occupied by another node may not coincide with a virtual slot unit. For example, as shown in Fig. 5, when a channel is occupied by node A and node A transmits an RTS packet, node B's backoff counting stop time may correspond to a time slot other than the representative backoff slot. Can be. FIG. 5 shows that the backoff counting interruption times of nodes A, B, and C correspond to time slots other than the representative backoff slots [501 to 507]. However, the time point for resuming backoff counting is not the stopping point of the previous backoff counting but the starting point of the virtual slot including the time slot corresponding to the stopping time.

The ACK packet transmitted by the receiving node can be synchronized based on the ACK packet since all nodes that are affiliated with the receiving node, for example, the AP, can be received regardless of whether they are hidden nodes. Do.

According to the present invention, by synchronizing the virtual slots based on the ACK packet transmitted by the receiving node, the time corresponding to the end time of the backoff counting, not the time of the last backoff counting, is not the time of resuming backoff counting. You can synchronize to the starting point of the virtual slot that contains the slot.

For example, in FIG. 5, node B shows a process of resuming backoff counting after node A completes the transmission of data packet [DATA] and receives an ACK packet from the receiving node. Here, when Node A exchanges the RTS packet and the CTS packet with the receiving node, Node B stops backoff counting in the virtual slot [Rbc = 1] section [501] corresponding to a time slot other than the representative backoff slot. do. However, after the node A completes the transmission of the data packet [DATA] and receives the ACK packet from the receiving node, the node B backs off by synchronizing the virtual slot [Rbc = 1] based on the received ACK packet. The time point at which the counting resumes is to be the starting point of the virtual slot [Rbc = 1] including the time slot corresponding to the time point at which the backoff counting is stopped [501].

Similarly, Node A and Node C synchronize the virtual slots (Rbc = 1 for Node A, Rbc = 4 and Rbc = 3 for Node C) based on the received ACK packets, thereby allowing Node A and Node C to back. The time point for resuming off counting is a virtual slot [Rbc = 1 for node A, Rbc = 4 for node A] containing a time slot corresponding to the time at which backoff counting is stopped [503, 505, 507]. And Rbc = 3].

According to the synchronization, the backoff period [Taccess] according to [Equation 2] is defined.

6 is a block diagram showing a communication system according to an embodiment of the present invention, Figure 7 is a flow chart showing a communication method according to an embodiment of the present invention.

As shown in Fig. 6, a communication system according to an embodiment of the present invention includes a packet receiver [601], a channel monitor [603], a representative backoff slot determiner [605], a backoff counter [607] and It includes a packet transmission unit [not shown].

The packet receiver 601 receives a packet. The channel monitor 603 checks whether the packet received by the packet receiver 601 is an ACK packet and checks whether the channel is in an idle state, that is, when it is time to start backoff counting. In the case, it is determined whether the backoff counting should be restarted or resumed.

The ACK packet is data for determining a point of time for the backoff counting according to an embodiment of the present invention (see the variable now in Equation 2). As described above, to illustrate an embodiment of the present invention. Figure 2 illustrates the case where an ACK packet is received according to the RTS / CTS mechanism, but it is obvious to those skilled in the art that the present invention is not limited to this embodiment. Therefore, it is to be understood that the present invention is not limited to the embodiment shown in the drawings. Therefore, according to the present invention, the variables now and DIFS defined in Equation 2 may be replaced with other possible variables.

On the other hand, the determination of whether the back off counting should be newly started or resumed by the channel monitor 603 may be performed by simply checking whether the time slot has passed. For example, this may be done by checking whether the last backoff counting has been performed until the last virtual slot.

As another embodiment, the determination of whether the backoff counting should be newly started or resumed by the channel monitor unit 603 is determined by whether the packet has been transmitted in the previous channel access period. Can be. For example, node A of FIG. 5 starts backoff counting anew because it received the ACK packet after transmitting the data packet (Rbc = 2 of node A). On the other hand, since node B stopped backoff counting without any packet transmission because it was occupied by node A, the next backoff counting resumes (node Bbc Rbc = 1). In addition, since node C stopped backoff counting without any packet transmission because it was occupied by node A and node B, the next backoff counting resumes [node C's Rbc = 4 and Rbc = 3]. .

As a result of the determination performed by the channel monitor unit 603, when it is determined that the backoff counting should be newly started, the representative backoff slot determining unit 605 determines the backoff according to Equation 1 above. The representative backoff slot determination unit 605 determines a backoff counter based on the virtual slot [VS] section, which is required for the initial transmission attempt. For example, Node A and Node B in Fig. 5 determined backoffs of 2VS and 4VS for the initial transmission attempt, respectively (Rbc = 2 of Node A and Rbc = 4 of Node B).

On the other hand, when it is determined by the channel monitor unit 603 that the backoff counting should be resumed, or by the representative backoff slot determination unit 605, a virtual slot [VS] necessary for the initial transmission attempt. After the backoff based on the interval is determined, the backoff counter 607 synchronizes the virtual slots based on the ACK packet received by the packet receiver 601 and backoff defined by Equation 2 above. The backoff counting is performed in the section, that is, the delay section [Taccess]. I is decremented in virtual slots while the backoff counter portion 607 counts in time slots. That is, step S707 is repeatedly performed until i = 0, and when i = 0, the packet transmitter [not shown] accesses a channel and transmits an RTS packet, for example. When the channel monitor unit 603 detects that the channel is occupied by another node while the backoff counter unit 607 counts in the time slot unit, the backoff counter unit 607 stops counting. The starting point of the virtual slot including the time slot corresponding to the point at which the backoff counting is stopped is the point at which the next backoff counting resumes.

As shown in Fig. 7, according to an embodiment of the present invention, the packet receiver [601] receives a packet [S701]. The channel monitor 603 checks whether the packet received by the packet receiver 601 is an ACK packet and checks whether the channel is in an idle state, that is, when it is time to start backoff counting. In this case, it is determined whether backoff counting should be newly started or resumed [S703].

The ACK packet is data for determining a point of time for the backoff counting according to an embodiment of the present invention (see the variable now in Equation 2). As described above, to illustrate an embodiment of the present invention. Figure 2 illustrates the case where an ACK packet is received according to the RTS / CTS mechanism, but it is obvious to those skilled in the art that the present invention is not limited to this embodiment. Therefore, it is to be understood that the present invention is not limited to the embodiment shown in the drawings. Therefore, according to the present invention, the variables now and DIFS defined in Equation 2 may be replaced with other possible variables.

On the other hand, the determination of step S703 may be performed by simply checking whether the time slot has passed. For example, this may be done by checking whether the last backoff counting has been performed until the last virtual slot.

As another embodiment, the determination of step S703 may be confirmed by whether the packet has been transmitted before. For example, node A of FIG. 5 starts backoff counting anew because it received an ACK packet after transmitting the data packet (Rbc = 2 of node A). On the other hand, since node B stopped backoff counting without any packet transmission because it was occupied by node A, the next backoff counting resumes (node Bbc Rbc = 1). In addition, since node C stopped backoff counting without any packet transmission because it was occupied by node A and node B, the next backoff counting resumes [node C's Rbc = 4 and Rbc = 3]. .

As a result of the determination in step S703, when it is determined that backoff counting should be newly started, the representative backoff slot determining unit 605 determines the backoff according to Equation 1 above (S705). In step S705, a backoff counter based on a virtual slot [VS] interval, which is required for the initial transmission attempt, is determined. For example, Node A and Node B in Fig. 5 determined backoffs of 2VS and 4VS for the initial transmission attempt, respectively (Rbc = 2 of Node A and Rbc = 4 of Node B).

On the other hand, if it is determined that the backoff counting should be resumed as a result of the determination in step S703, or the backoff counter based on the virtual slot [VS] section required for the initial transmission attempt in step S705, After the determination, the backoff counter unit 607 synchronizes the virtual slots based on the ACK packet received by the packet receiving unit 601 and backs up in the backoff period, that is, the delay period [Taccess] defined by Equation 2 above. Off counting is performed [S707]. In step S707, i is decremented in units of virtual slots while the backoff counter unit 607 counts in units of time slots. That is, step S707 is repeatedly performed until i = 0 [S709]. If i = 0, the packet transmitter [not shown] accesses the channel and transmits, for example, an RTS packet [S711]. Although not shown in the figure, in step S707, if the channel monitor unit 603 detects that the channel is occupied by another node while the backoff counter unit 607 counts in units of time slots, the backoff counter Division 607 stops counting. The starting point of the virtual slot including the time slot corresponding to the point at which the backoff counting is stopped is the point at which the next backoff counting resumes.

8A, 8B, and 8C are graphs comparing experimental results according to the communication method of the present invention and the prior art communication method. FIG. 8A is a collision probability graph, and FIG. 8B is an aggregate network throughput [bps]. ]], FIG. 8C is a graph showing a packet drop rate. In the experimental environment, all nodes are located at the cell edge at uniform intervals from the receiving node [AP], and are 250m as a transmission range and a carrier sensing range. As a UDP packet, the transceiver node exchanges RTS / CTS packets. The virtual slot [VS] unit is 19. As shown in Figs. 8a, b and c, the communication method of the present invention shows superior performance compared to the prior art indicated by CWmin = 20,31.

9A, 9B, and 9C are graphs comparing another experimental result according to a communication method of the present invention and a communication method of the prior art, FIG. 9A is a collision probability graph, and FIG. 9B is an aggregate network throughput. [bps]] and FIG. 9C is a graph showing a packet drop rate. As an experimental environment, the transmission packet is a UDP packet having a packet size of 200, and the experimental environment is the same as that of Figs. 8A, B and C except that no RTS / CTS packet exchange is performed between the transmitting and receiving nodes. Similarly, the communication method of the present invention, as shown in Figures 9a, b, c, shows superior performance compared to the prior art indicated by CWmin = 20, 31.

10A and 10B are tables comparing the results of another experiment according to the communication method of the prior art [IEEE 802.11 standard] and the communication method of the present invention, respectively, and show the change of the fairness index in seconds when the TCP packet is used. For reference, the closer the fairness index is to 1, the more fair it is. As shown in Figs. 10A and 10B, according to the communication method of the present invention, it is more fair and superior in performance compared to the prior art.

The method of the present invention as described above may be implemented as a program and stored in a computer-readable recording medium (CD-ROM, RAM, ROM, floppy disk, hard disk, magneto-optical disk, etc.).

The present invention described above is not limited to the above-described embodiments and the accompanying drawings, and various substitutions, modifications, and changes are possible in the technical field of the present invention without departing from the technical spirit of the present invention. It will be clear to those of ordinary knowledge.

1 is a view showing the structure of a WLAN system to which the present invention is applied;

2A and 2B are conceptual views illustrating the concept of random backoff defined in the DCF mode;

3A, 3B, and 3C are conceptual views for explaining a hidden node problem;

4A and 4B are conceptual views illustrating a packet transmission method according to the present invention;

5 is a conceptual diagram illustrating a process of accessing a channel by nodes A, B, and C having a hidden node relationship with each other according to an embodiment of the present invention;

6 is a block diagram showing a communication system according to an embodiment of the present invention;

7 is a flowchart illustrating a communication method according to an embodiment of the present invention;

8A, 8B, and 8C are graphs comparing experimental results according to a communication method of the present invention and a communication method of the prior art;

9A, 9B, and 9C are graphs comparing another experimental result according to a communication method of the present invention and a communication method of the prior art;

10A and 10B are tables comparing the results of still further experiments according to the communication method of the prior art [IEEE 802.11 standard] and the communication method of the present invention, respectively.

Claims (24)

In a communication system, A channel monitor unit determining whether it is time to newly start backoff counting; A representative backoff slot determination unit for determining a backoff in units of virtual slots when it is determined by the channel monitor unit to start a new backoff counting unit; A backoff counter unit for counting a backoff determined by the representative backoff slot determination unit; And When the channel is idle at the time when the backoff is counted by the backoff counter, the data transmission unit for accessing the channel and transmitting data Including, The virtual slot The time slots constituting the backoff window interval are grouped in a predetermined number. Communication system. The method of claim 1, The representative backoff slot determination unit The backoff counter is determined based on Equation 1 below. Communication system. [Equation 1] backoff counter = i * VS i = Random [0, B] B = ceiling (CW / tVS) Where VS is the virtual slot, i is the index of the virtual slot, tVS is the length of the virtual slot, and CW is the current backoff window size. The method of claim 2, The channel monitor unit Determining whether to start backoff counting based on an ACK packet transmitted to the communication system; Communication system. The method of claim 3, The back off counter unit Counting the backoff after the time corresponding to the Distributed Inter Frame Space (DIFS) defined by the IEEE 802.11 standard has elapsed from the time when the ACK packet is transmitted to the communication system. Communication system. The method of claim 4, wherein The back off counter unit Counting the backoff during the backoff period [Taccess] defined by the following equation (2) Communication system. &Quot; (2) " Taccess = {[now + DIFS], [now + DIFS + i * tVS]} However, now is a time point when the ACK packet is transmitted to the communication system. The method of claim 5, The back off counter unit Counting in time slots and reducing the index i of the virtual slots in the virtual slots Communication system. The method of claim 6, The back off counter unit If it is detected that the channel is occupied by another node while performing the backoff counting, the backoff counting is stopped. Communication system. The method of claim 7, wherein The channel monitor unit To determine if it's time to resume backoff counting Communication system. The method of claim 8, The channel monitor unit Determine whether it is time to resume backoff counting by checking whether the last backoff counting has been performed up to the last virtual slot. Communication system. The method of claim 8, The channel monitor unit It is determined whether it is time to resume backoff counting based on whether the communication system has transmitted data in the previous channel access period. Communication system. The method of claim 8, The back off counter unit If it is determined by the channel monitor that it is time to resume backoff counting, Performing a backoff counting by synchronizing the virtual slot at the time point at which the backoff counting resumes Communication system. The method of claim 11, The back off counter unit Backoff counting is performed by starting the virtual slot including the time slot corresponding to the point at which the previous backoff counting was interrupted as the start point of backoff counting. Communication system. In the communication method of the communication system, A first step of receiving data; Determining whether it is time to newly start backoff counting based on the received data; Determining a backoff in virtual slot units if it is determined that it is time to newly start backoff counting; Counting the determined backoff; And A fifth step of accessing the channel and transmitting data when the channel is in an idle state when all of the backoffs are counted Including, The virtual slot The time slots constituting the backoff window interval are grouped in a predetermined number. Communication method. The method of claim 13, The third step is The backoff counter is determined based on Equation 1 below. Communication method. [Equation 1] backoff counter = i * VS i = Random [0, B] B = ceiling (CW / tVS) Where VS is the virtual slot, i is the index of the virtual slot, tVS is the length of the virtual slot, and CW is the current backoff window size. The method of claim 14, The second step is Determining whether to start backoff counting based on an ACK packet transmitted to the communication system; Communication method. The method of claim 15, The fourth step is Counting the backoff after the time corresponding to the Distributed Inter Frame Space (DIFS) defined by the IEEE 802.11 standard has elapsed from the time when the ACK packet is transmitted to the communication system. Communication method. The method of claim 16, The fourth step is Counting the backoff during the backoff period [Taccess] defined by the following equation (2) Communication method. &Quot; (2) " Taccess = {[now + DIFS], [now + DIFS + i * tVS]} However, now is a time point when the ACK packet is transmitted to the communication system. The method of claim 17, The fourth step is Counting in time slots and reducing the index i of the virtual slots in the virtual slots Communication method. The method of claim 18, The fourth step is If it is detected that the channel is occupied by another node while performing the backoff counting, the backoff counting is stopped. Communication method. The method of claim 19, The second step is To determine if it's time to resume backoff counting Communication method. 21. The method of claim 20, The second step is Determine whether it is time to resume backoff counting by checking whether the last backoff counting has been performed up to the last virtual slot. Communication method. 21. The method of claim 20, The second step is It is determined whether it is time to resume backoff counting based on whether the communication system has transmitted data in the previous channel access period. Communication method. 21. The method of claim 20, The fourth step is If it is determined by the channel monitor that it is time to resume backoff counting, Performing a backoff counting by synchronizing the virtual slot at the time point at which the backoff counting resumes Communication method. 24. The method of claim 23, The fourth step is Backoff counting is performed by starting the virtual slot including the time slot corresponding to the point at which the previous backoff counting was interrupted as the start point of backoff counting. Communication method.

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