KR101220150B1 - Proactive transmission method for reliable multicast in wireless lan - Google Patents

Abstract

PURPOSE: An active transmission method for reliable multicast in a wireless LAN is provided to calculate the whole number of data packets required for restoring information packets of all receiving terminals by using packet error rate information of the receiving terminal. CONSTITUTION: An AP encodes an information block composed of k pieces of information packets to generate a transmission block composed of n pieces of packets. The packets of the transmission block are transmitted to a plurality of receiving terminals. The AP transmits a busy tone to the receiving terminals. The receiving terminals receiving the busy tone separately transmit AcK packets in order of numbers granted in a multicast membership process by the AP. The AP receives the Ack packets transmitted from the receiving terminals. The receiving terminals calculates the number of packets required for block restoration based on a value of an NIP field and a value of a PER filed included in the Ack packets.

Description

Proactive Transmission Method for Reliable Multicast in Wireless LAN

The present invention relates to an active transmission method for reliable multicast in a wireless LAN. In particular, the present invention provides reliability for multicast transmission in an IEEE 802.11 wireless LAN, and provides excessive control overhead due to the use of a large number of control packets during error recovery. The present invention relates to an active transmission method for reliable multicast in a wireless LAN capable of preventing occurrence.

In order to transmit data to multiple receiving terminals, the multicast method of one-to-many transmission is more effective than the unicast method of one-to-one transmission to each terminal. Multicast can save network resources and reduce data distribution time by simultaneously transmitting data to all receiving terminals. Many applications, such as IPTV, video conferencing, and televised lectures, require multicast transmissions to provide better quality of service.

IEEE 802.11 WLAN has been widely used for providing Internet service through wireless because of its easy installation and low cost. The IEEE 802.11 standard defines a MAC (Medium Access Control) protocol for channel sharing among multiple receiving terminals. In the standard, DCF (Distributed Coordination Function) is designed for contention-based channel access. DCF has two transmission modes. A basic method having a two-step exchange process (DATA-ACK) and a RTS / CTS (Request To Send / Clear To Send) method having a four-step exchange process (RTS-CTS-DATA-ACK). In DCF, access to radio channels is controlled by Interframe Space (IFS) between packet transmissions. Three IFSs are defined in the standard: SIFS (Short IFS), PIFS (Point coordination function IFS), and DIFS (DCF IFS).

Collisions in the media are solved using a binary exponential backoff algorithm. RTS / CTS / ACK packets and binary exponential backoff are only used for unicast transmissions. In multicast transmissions, on the other hand, data packets are transmitted without this method. Since there is no feedback such as an ACK from the receiving terminal, the transmitting terminal does not know whether the receiving terminal receives the transmitted data packet without error. Because of this lack of feedback, IEEE 802.11 DCF does not support error recovery for data packets in multicast transmissions and does not guarantee reliability.

Many protocols have been proposed to ensure the reliability of multicast transmissions. These protocols fall into two broad categories: reader-based and ACK-based. In the reader-based protocol, the transmitting terminal retransmits the data packet only when there is no ACK from the reader terminal. In the ACK-based method, the transmitting terminal continuously retransmits the data packet until receiving the ACK from all receiving terminals. These methods still suffer from reliability and efficiency because they use too many control packets during error recovery, causing excessive control overhead.

Accordingly, the present invention has been made in view of the above circumstances, and an object of the present invention is to provide an active transmission method for reliable multicast in a simple and efficient WLAN for reducing feedback overhead and for reliable multicast. .

여기서, N_i는 수신 단말 i를 위해 필요한 데이터 패킷 수,

는 수신 단말 i의 PER 값, X_i는 수신 단말 i의 블록 복구에 필요한 패킷 수, 즉 X_i는 (k-NIP_i), NIP는 수신 단말 i가 Ack 패킷을 통해 전송한 NIP, NIP(Number of Insufficient Packets)는 수신 단말이 블록을 디코딩하기에 부족한 패킷의 수이며; g) 단계 f)에 의해 연산된 N_i 값들 중 최대 수로 추가 패킷들을 상기 AP에서 상기 다수의 수신 단말들로 전송하는 단계; 및 h) 단계 g) 이후, 상기 AP에서 FR(Feedback Request) 패킷을 생성하고, 상기 FR 패킷을 상기 다수의 수신 단말들에 전송하는 단계;를 포함한다.
실시 예에 따라서는, 상기 패킷들 각각은 MA(Multicast Address), 상기 블록의 순서 번호, 블록 크기, 및 블록 내에서의 패킷 순서 번호를 나타내는 필드들을 포함한다.
실시 예에 따라서는, 상기 FR 패킷은 비트맵 필드 및 상기 비트맵 필드의 크기를 나타내는 비트맵 크기 필드를 포함한다.Active transmission method for reliable multicast in a wireless LAN to achieve the above object is a) in the AP, block encoding an information block consisting of k information packets to generate a transport block consisting of n packets Making; b) transmitting the n packets of the transport block from the AP to a plurality of receiving terminals; c) after performing step b), transmitting busy tone at the AP to the plurality of receiving terminals; d) transmitting, by the plurality of receiving terminals receiving the busy tone, Ack packets in the order of numbers assigned to the multicast subscription from the AP; e) receiving, at the AP, the Ack packets transmitted by each of the plurality of receiving terminals; f) calculating the number of packets N _i required for block recovery in each of the plurality of receiving terminals based on a value of a PER field and a value of a NIP field included in each of the received Ack packets;

Here, N _i is the number of data packets required for the receiving terminal i,

Is the PER value of the receiving terminal i, X _i is the number of packets required for block recovery of the receiving terminal i, that is, X _i is (k-NIP _i ), NIP is NIP, NIP (Number transmitted by the receiving terminal i through the Ack packet) of Insufficient Packets) is the number of packets that the receiving terminal lacks to decode the block; g) sending additional packets from the AP to the plurality of receiving terminals at the maximum number of N _i values computed by step f); And h) after step g), generating a Feedback Request (FR) packet at the AP and transmitting the FR packet to the plurality of receiving terminals.
According to an embodiment, each of the packets includes fields indicating a multicast address (MA), an order number of the block, a block size, and a packet order number within the block.
According to an embodiment, the FR packet includes a bitmap field and a bitmap size field indicating the size of the bitmap field.

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The present invention is a combination of both Forward Error Correction (FEC) and ARQ, using the packet error rate information of each receiving terminal to calculate the total number of data packets necessary for all receiving terminals to recover the information packet and the corresponding number By transmitting as many packets to receiving terminals and receiving one ACK, the feedback process can be minimized, thereby improving transmission efficiency.

1 to 3 are diagrams illustrating formats of data packets, ACK packets, and FR packets used in an active transmission method for reliable multicast in a wireless LAN according to an embodiment of the present invention.
4 is a diagram for describing block transmission between an AP and a receiving terminal according to an embodiment of the present invention.
5 and 6 are diagrams showing the performance according to the data packet error rate.
7 and 8 are diagrams illustrating performance according to the number of receiving terminals.

Hereinafter, with reference to the accompanying drawings will be described in detail with reference to the embodiment of the present invention an apparatus and an operation method. It will be appreciated that those skilled in the art will readily observe that certain changes in form and detail may be made therein without departing from the spirit and scope of the present invention as defined by the appended claims. To those of ordinary skill in the art. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.

1 to 3 are diagrams illustrating formats of data packets, ACK packets, and feedback request (FR) packets used in an active transmission method for reliable multicast in a wireless LAN according to an embodiment of the present invention.

As shown in Fig. 1, the data packet format according to the present invention uses a multicast address (MA) instead of an address for each receiving terminal. These packet formats cooperate with the format of IEEE 802.11. Also, fields including a block index, a block size, and a packet packet sequence number within a block are included in a block currently being transmitted. Without these fields, it is impossible to obtain block information on the transmitted data packet and thus it is impossible for the receiving terminal to perform block decoding.

As shown in FIG. 2, the ACK packet includes a NIP field including NIP (Number of Insufficient Packets) information. This information means the number of packets that each receiving terminal lacks to decode the block. In other words, this value is obtained by subtracting the number of data packets received without error from the block size k. In addition, the ACK packet includes a packet error rate (PER) field for transmitting packet error rate (PER) information to an AP (not shown). The value of this field is measured and set by each receiving terminal (not shown). The PER value is a real number in the range [0, 1] and cannot be transmitted in 1 byte. Accordingly, in the present invention, the PER value is quantized. In other words, the method cuts into a predetermined section and considers the same value within the section. For example, after multiplying the PER value measured by each receiving terminal to 255 and rounding to the first decimal place, only the integer part value is taken and set in the PER field of FIG. 2 and transmitted. Upon receiving this, the AP divides the PER field value by 255 to obtain a real PER value. Although some losses may occur during the quantization process, they are not significant because their values are very small, up to 1/510.

As shown in FIG. 3, a FR (Feedback Request) packet is used by the AP to request transmission of an ACK packet from a receiving terminal. m represents the size of the bitmap. Since the bitmap value m is in bytes, it is set to a value divided by 8 with respect to the number of reception terminals (R) joined to the current multicast group. That is, [R / 8]. [x] means round to x. The bitmap is set to 1 bit for each receiving terminal and set to 1 if feedback transmission is required and to 0 if not necessary.

In order to use a bitmap, a unique number for distinguishing each multicast receiving terminal is required. Although each receiving terminal has a MAC address, it can be distinguished. However, since this address does not guarantee continuity and thus it is very inefficient to use in a bitmap, there must be a separate identifier to guarantee continuity. In the present invention, when the receiving terminal joins the multicast group, the AP guarantees continuity by using a method of assigning a number starting from 1 to each receiving terminal. An unassigned number may be generated due to a receiving terminal that is withdrawn in the middle. To prevent this, a newly subscribed terminal always allocates the smallest number among the unassigned numbers. Allocate a bitmap using the new separator number. That is, the i th bit of the bitmap is used for the receiving terminal i.

Although the present invention describes only the data / ACK transmission process without defining the RTS / CTS exchange process, those skilled in the art will understand that various methods proposed in the related art can be selected and used when the RTS / CTS exchange process is required.

The AP divides the stream received through the Internet into several blocks consisting of k information packets. The block encoding method is applied to encode n packets. That is, (n-k) parity packets are generated and added to the block to allow packet loss on the wireless channel. In the present exemplary embodiment, the data packet is not distinguished from the information packet and the parity packet unless otherwise specified. After encoding, a data packet is configured as shown in FIG. 3, and the AP transmits the packet to the receiving terminal in block units.

The method according to the present invention uses an active transmission method for transmitting more data packets than the actual number of information packets according to the channel state of each receiving terminal in order to minimize feedback from the receiving terminal. If only the number of information packets is transmitted, an error occurs in the receiving terminal, and additional data packet transmission is required to recover the block. To this end, each receiving terminal should send a feedback requesting an additional transmission to the AP. In order to minimize this, the present invention calculates the required number of data packets in consideration of the data packet error rate of each receiving terminal and transmits the corresponding packet. The data packet error rate of each receiving terminal is obtained through the PER field of the ACK packet.

The AP calculates the number of data packets to be transmitted in consideration of the PER value transmitted by each receiving terminal to transmit the data packets. The number N _{i of} data packets required for the receiving terminal i is shown in Equation 1 below.

Where P ⁱ _err and X _i represent the PER value of the receiving terminal i and the number of packets needed to recover the block, respectively, X _i is (k-NIP _i ), and NIP _i is transmitted by the receiving terminal i through the ACK packet. One is NIP. In the first transmission of each block, NIP is k because the receiving terminal needs a data packet of block size (k). Since the actual number of packets to be transmitted by the AP is an integer, the AP is rounded to the value of N _i calculated by Equation 1.

After calculating the number of packets required for each receiving terminal, the AP selects the largest value and transmits a data packet, as shown in Equation 2 below. The highest value is used because all receiving terminals must be satisfied.

5/(1-0.1))개의 패킷 전송이 필요하다. 수신 단말 R2에게는 6(

5/(1-0.2))개가 필요하다. 그러므로, AP는 큰 값인 6을 선택하고 6개의 패킷을 전송한다.For example, when two receiving terminals R1 and R2 exist and each receiving terminal has PERs of 0.1 and 0.2, respectively, five packets should be transmitted, the AP can transmit 6 packets through Equations 1 and 2 And get 6 packets. That is, in order to transmit five packets without error to the receiving terminal R1, on average, 5 (

5 / (1-0.1)) packets are required. 6 to the receiving terminal R2

5 / (1-0.2)) is required. Therefore, the AP selects a large value of 6 and transmits 6 packets.

According to the present invention, the AP transmits a busy tone or a FR packet to request feedback from a receiving terminal. These two signals are transmitted one time slot time after transmitting the last packet in the active transmission method of the present invention. Busy tone is one time slot in length and means to send ACK packet to all receiving terminals. In contrast, an FR packet means that only a receiving terminal included in a bitmap transmits an ACK packet. The reason for using busy tones rather than regular feedback request packets is that on a wireless channel, even if a packet is transmitted at a lower transmission rate, there is a possibility of loss. If the request packet is lost, the receiving terminal cannot transmit the ACK packet. In order to prevent this, the busy tone is used in the present invention. In addition, the busy tone is shorter than using FR packets. The receiving terminal receiving the busy tone transmits an ACK packet in the order of the number assigned when joining the multicast group. The usage time of busy tone and FR packet is different as follows. Busy tones are used when feedback is required from all receiving terminals and FR packets are used when feedback is needed from some receiving terminals. In general, the busy tone is used since feedback is required from all receiving terminals after the first transmission of each block, and the FR packet is used since feedback is required only from some receiving terminals from the second transmission.

Hereinafter, the embodiment of the present invention described above will be described in more detail.

4 is a diagram for describing block transmission between an AP and a receiving terminal according to an embodiment of the present invention.

Referring to FIG. 4, the AP obtains the number n of packets required for the first transmission based on the values obtained through Equations 1 and 2, and transmits corresponding packets. After transmitting the nth packet, the AP transmits a busy tone. Since the receiving terminal R1 does not want additional packet transmission because, for example, k or more error-free packets have been received, the receiving terminal R1 transmits an ACK packet by setting the NIP field to zero. Since the receiving terminal R2 has received (k-1) error-free packets, one additional data packet is needed to recover the block, so the NIP field is set to 1 to transmit the ACK packet. The receiving terminal R3 lacks two data packets and sets the NIP to 2 and transmits it. After receiving the ACK packet from the receiving terminal, the AP further transmits the data packet by the number calculated in Equations (1) and (2) (assuming 3 in this embodiment). After transmitting three additional packets, the AP transmits the FR packets to all of the receiving terminals. The FR packet includes a bitmap having information on whether feedback is requested for each receiving terminal. Since the receiving terminal R1 recovered the block in the first transmission, since no further feedback is required, the receiving terminal R1 is set to 0 at the first position of the bitmap, and the receiving terminals R2 and R3 are set to 1 at the second and third positions because the feedback is needed. . Each receiving terminal transmits the ACK packet in order only after the FR packet is set to 1 in the bitmap. The transmission time of each receiving terminal is determined by multiplying the number (r) in which the bitmap is set to 1 among the receiving terminals having a number ahead of themselves with respect to the time T required for transmitting the ACK packet. That is, the receiving terminal R2 transmits the ACK packet at 0xT time because there is no preceding number receiving terminal set to 1 in the bitmap, and the receiving terminal R3 transmits the ACK packet at 1xT time because there is one preceding number receiving terminal set to 1. send. After receiving the FR packet from the AP, the receiving terminals R2 and R3 do not want additional packet transmission because they have received more than k error-free packets. Therefore, the NIP field is set to 0 to transmit the ACK packet. When the AP receives an ACK packet having NIP set to 0 from all receiving terminals, the AP ends transmission of the current block and starts transmission of the next block.

In order for the method according to the invention to work well, busy tone must be distinguishable from normal packets. The transmission time is used for this. Since every packet has a 20 ms physical layer preamble and a header, a time for packet transmission requires at least three time slot times. The value of 20 Hz is the value calculated in Table 1 to be described below. Busy tones, however, only require one time slot time. Therefore, transmission of busy tone and normal packet can be easily distinguished.

Hereinafter, the performance of an active transmission method for reliable multicast in a wireless LAN according to the present invention is compared and analyzed by a conventional broadcast ARQ (BARQ) method. The parameters used in the simulations are listed in Table 1 below.

Parameter Value Data Bit Rate 54 Mbps Control Bit Rate 6 Mbps Slot Time 9 us SIFS 16 us Propagation delay 1 us CRC 4 Octets PHY PLCP Preamble Length 16 us PHY PLCP Header Length 5 Octets Block size 20 Packets Packet Size 1500 Octets CWmin 31

In the IEEE 802.11 WLAN standard, multicast data packets are transmitted at the lowest transmission rate to provide multicast services to all receiving terminals. However, the simulation was performed in the IEEE 802.11a network environment with a data rate of 54 Mbps and a control packet of 6 Mbps. In the simulation, we considered the topology where there is no unicast transmission and only the AP performs multicast transmission. Therefore, no collision occurs in the simulation. In addition, it is assumed that the data packet error between receiving terminals is independent and that the error rate of the control packet is 20% of the data packet error rate.

5 and 6 are diagrams showing the performance according to the data packet error rate. The number of receiving terminals is ten. In both methods in Figure 5, the efficiency decreases as the data packet error rate increases. In the active transmission method (PTRM) for reliable multicast in the wireless LAN according to the present invention, as the data packet error rate increases, the receiving terminal will be lost for more packets and more packets are needed to recover the block. Decreases. In the conventional BARQ method, the performance is reduced because the error packet is retransmitted.

However, since the independent loss between receiving terminals cannot be solved, it can be seen that the efficiency is drastically lowered because more packets are transmitted than the proposed method. The proposed method solves the problem of independent loss between receiving terminals through FEC. In the figure, when the data packet error rate is 0, the performance difference according to the pure operation principle is shown regardless of the FEC operation. In the wireless LAN of the present invention, the active transmission method for reliable multicast requires one feedback after transmitting several data packets, but the BARQ method requires feedback for all transmitted packets, and thus performance is not good.

6 is a diagram illustrating a delay according to a data packet error rate. In both methods in Figure 6, the delay increases as the data packet error rate increases. In the wireless LAN of the present invention, the active transmission method (PTRM) for reliable multicast calculates the number of packets to be transmitted according to the channel state of each receiving terminal and transmits the packets accordingly. Then ask for feedback. Therefore, the increasing delay value is not large. However, the BARQ method not only requests feedback for every packet but also performs multiple retransmissions for one packet. That is, if any one of several receiving terminals receives an error, the backoff window value is doubled and the data packet is retransmitted after the contention process is performed. Therefore, as the error rate increases, the delay increases sharply.

7 and 8 are diagrams illustrating performance according to the number of receiving terminals. The data packet error rate is 0.1. In both figures, the performance decreases as the number of receiving terminals increases. In the BARQ method, as the number of receiving terminals increases, the size of the field for time unit allocation included in the data packet increases and the feedback time increases, thereby degrading performance. In the active transmission method (PTRM) for reliable multicast in the wireless LAN of the present invention, a feedback time for each receiving terminal to transmit an ACK packet increases, thereby reducing performance. However, regardless of the number of receiving terminals, the proposed present invention (PTRM) always has better performance than the BARQ method.

IEEE 802.11 WLANs do not support reliable multicast. To this end, existing methods still have problems in reliability and efficiency because excessive control overhead is generated due to the use of many control packets during error recovery, and retransmission is performed to satisfy all receiving terminals. The present invention provides a new method for actively determining the number of packets to be actively transmitted according to the data packet error rate of the receiving terminal using the FEC to solve this problem. In addition, the present invention requests one feedback after transmitting several packets without requesting feedback for each packet. Simulation results show that the method according to the present invention always maintains a good performance regardless of the data packet error rate and the number of receiving terminals.

The above description is merely illustrative of the technical idea of the present invention, and those skilled in the art to which the present invention pertains may make various modifications and changes without departing from the essential characteristics of the present invention. Accordingly, the embodiments disclosed in the present invention are not intended to limit the technical spirit of the present invention but to describe the present invention, and the scope of the technical idea of the present invention is not limited by these embodiments. The scope of protection of the present invention should be construed according to the following claims, and all technical ideas within the scope of equivalents should be construed as falling within the scope of the present invention.

Claims (5)

In the active transmission method for reliable multicast in a wireless LAN,
a) at the AP, block encoding an information block consisting of k information packets to generate a transport block consisting of n packets;
b) transmitting the n packets of the transport block from the AP to a plurality of receiving terminals;
c) after performing step b), transmitting busy tone at the AP to the plurality of receiving terminals;
d) transmitting, by the plurality of receiving terminals receiving the busy tone, Ack packets in the order of numbers assigned to the multicast subscription from the AP;
e) receiving, at the AP, the Ack packets transmitted by each of the plurality of receiving terminals;
f) calculating the number of packets N _i required for block recovery in each of the plurality of receiving terminals based on a value of a PER field and a value of a NIP field included in each of the received Ack packets;

Here, N _i is the number of data packets required for the receiving terminal i,

Is the PER value of the receiving terminal i, X _i is the number of packets required for block recovery of the receiving terminal i, that is, X _i is (k-NIP _i ), NIP is NIP, NIP (Number transmitted by the receiving terminal i through the Ack packet) of Insufficient Packets) is the number of packets that the receiving terminal lacks to decode the block;
g) sending additional packets from the AP to the plurality of receiving terminals at the maximum number of N _i values computed by step f); And
h) after step g), generating a Feedback Request (FR) packet in the AP and transmitting the FR packet to the plurality of receiving terminals; active transmission method for reliable multicast in a WLAN .
delete The method of claim 1, wherein each of the packets is
And a field indicating a multicast address (MA), an order number of the block, a block size, and a packet order number within the block. delete The active transmission method of claim 1, wherein the FR packet includes a bitmap field and a bitmap size field indicating a size of the bitmap field.

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