CN112153747A - Wireless mesh network-based segmented competition beacon sending method - Google Patents
Wireless mesh network-based segmented competition beacon sending method Download PDFInfo
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Abstract
The invention discloses a beacon sending method of a segmented competition type in a wireless mesh network, which comprises the following three aspects: the division of periods and intervals, interval selection rules and beacon frame transmission rules. The time is divided into equal-length periods, and each period is divided into a plurality of equal-length intervals. Nodes are allocated to a certain interval at the start of each period according to an interval selection rule according to the case where a beacon frame is transmitted in the previous period, so that nodes that have transmitted the beacon frame most recently are allocated to a later interval, and nodes that have not transmitted the beacon frame most recently are allocated to an earlier interval. The node plans to transmit a beacon frame at the start time (PBTT) of the allocated interval. In each period, if the node receives a beacon frame before the PBTT, or before the PBTT starts a beacon frame back-off timer, or before the beacon frame back-off timer is decreased to 0, the node cancels the transmission of the beacon frame of the period; otherwise, the beacon frame is transmitted when the back-off timer is decremented to 0.
Description
Technical Field
The present invention relates to the field of beacon frame transmission in infrastructure-less networks, and in particular, to a method for avoiding beacon frame collision in a wireless mesh network.
Background
The normal operation of the wireless network is not to leave the periodic broadcast of beacon (beacon) frames. The beacon frame of the wireless network has two roles. First, the beacon frame announces the existence of the network, facilitating the access of new nodes. Second, the beacon frame contains necessary parameters required for the network to operate, such as time information, network name, network function, etc.
The transmission mode of the beacon frame is related to the network structure. For an infrastructure network, such as a mobile cellular network, Wi-Fi (IEEE 802.11), IEEE 802.16 network, IEEE 802.20 network, etc., beacon frames are sent by a central node, such as a base station in a cellular network, an access point in a Wi-Fi network, etc., and other nodes cannot send beacon frames. The central node periodically transmits the beacon frame according to a certain mode without negotiation with other central nodes or non-central nodes. For infrastructure-less networks, such as independent networks (independents networks) in IEEE 802.11, mesh networks (mesh networks), and Zigbee networks based on IEEE802.15.4, all nodes participate in the transmission of beacon frames together. Since there is no central node coordinating the beacon frame transmissions of the nodes, different nodes may transmit beacon frames simultaneously, with the two beacon frames interfering with each other at their common neighbor nodes, a phenomenon known as "collision". The three beacon frame transmission modes for the infrastructure-less network specified in the standard are summarized as follows:
(1) an independent network. All nodes in the independent network contend for transmission of beacon frames at certain intervals. The Time at which the Beacon frame is transmitted in contention is referred to as a Target Beacon Transmission Time (TBTT). At each TBTT moment, all nodes suspend the back-off Timer of non-Beacon frames, randomly determine the initial value of a Beacon Backoff Timer (BBT), and contend to transmit Beacon frames in a carrier sense multiple access/collision avoidance manner. And if the beacon frame sent by other nodes is received before the BBT is decreased to 0, the sending of the beacon frame at this time is cancelled and the BBT is deleted. Otherwise, the beacon frame is sent when the BBT decrements to 0.
(2) A mesh network. And the beacon frame transmission of the mesh network adopts a negotiation mode. TBTT is distributed on time axis at equal intervals, mesh node inserts the sending of beacon frame into the head of sending queue at TBTT time. The transmission of the beacon frame follows the same channel contention rules as the transmission of the general frame. In order to avoid TBTT Collision of a plurality of adjacent Mesh nodes, a Mesh Beacon frame Collision Avoidance (Mesh Beacon Collision Avoidance, hereinafter referred to as MBCA) method is adopted as a standard. The method attaches the sending time and interval of the beacon frame of the neighbor node in the beacon frame, so that all mesh nodes can obtain the rule that other nodes in a two-hop range send the beacon frame. Based on the information, part of the mesh nodes adjust their own timers to make the TBTT of the mesh nodes staggered with the TBTT of other nodes in the two-hop range, thereby avoiding collision.
(3) IEEE802.15.4 networks. In an IEEE802.15.4 network, only a Full-Function Device (FFD) can transmit a beacon frame. When the FFD acts as a pan (personal Area network) coordinator, it transmits beacon frames at certain intervals. When the FFD does not act as the PAN coordinator, the FFD determines the time for sending the beacon frame according to the received time of the beacon frame sent by the PAN coordinator associated with the FFD and the StartTime parameter. The interval at which the FFD transmits the beacon frame is constant, which is the length of one superframe. When the macbeacon order parameter is equal to 15, the FFD does not transmit a beacon frame unless a beacon frame request is received.
In the mesh network, the standard MBCA method can completely avoid the conflict of beacon frames under the condition that nodes are sparse. However, this complete beacon frame collision avoidance comes at the cost of frequent exchange of beacon frame transmission information. In addition, when nodes are densely distributed, the MBCA method is likely to fail to negotiate a beacon transmission rule that can completely avoid collision, thereby causing beacon frames of some nodes to collide continuously. Aiming at the defects of the MBCA method, the invention provides a segmented competitive beacon sending method.
Disclosure of Invention
The present invention is directed to solving the problem of transmitting beacon frames for wireless mesh networks. The transmission method of the beacon frame established by IEEE 802.11s depends on negotiation of each node, and a large overhead is caused. The invention provides a segmented competitive beacon sending method, which does not need negotiation among nodes and does not cause continuous beacon frame collision.
The invention providesA wireless mesh network-based segmented competition beacon transmission method divides time into equal-length periods, each period is divided into N equal-length intervals, wherein N is an integer greater than or equal to 2, and the N is marked as P in sequence from front to back according to the time1、P2、……、PN(ii) a At the beginning of each period, the nodes are distributed to a certain interval in the period according to the interval selection rule according to the condition that the beacon frames are transmitted in the previous period, so that the nodes which have transmitted the beacon frames recently are distributed to the interval at the later time, and the nodes which have not transmitted the beacon frames recently are distributed to the interval at the earlier time; the node plans to transmit a beacon frame at the start time of the allocated interval, which is called a planned beacon transmission time; at the beginning of each period, the node decides whether and when to transmit a beacon frame according to the following beacon frame transmission rules:
step 1: judging whether a beacon frame rollback timer is maintained at the starting moment of the period, if so, cancelling the sending of the beacon frame of the period, and if not, turning to the step 2;
step 2: judging whether a beacon frame is received before the scheduled beacon transmission time in the period, if so, canceling the transmission of the beacon frame in the period, and if not, turning to the step 3;
and step 3: before starting the beacon frame rollback timer, judging whether the beacon frame is received or not after the scheduled beacon transmission time of the period, if so, cancelling the transmission of the beacon frame of the period, and if not, starting the beacon frame rollback timer, and turning to the step 4;
and 4, step 4: and judging whether the beacon frame is received or not before the beacon frame rollback timer is decremented to 0, if so, cancelling the sending of the beacon frame in the period, deleting the beacon frame rollback timer, finishing the judgment of the period, and otherwise, sending the beacon frame when the beacon frame rollback timer is decremented to 0.
Further, the number N of intervals included in each period is determined by the following equation:
where R is the farthest distance that the receiving node can correctly receive the frame sent by the sending node, ρ is the average density of the nodes, ceil (x) represents the smallest integer greater than or equal to x.
Further, the interval selection rule of the node at the beginning of each period is as follows:
1) when N ═ 2: if the node last sent a beacon frame at or before the previous 2 nd cycle, it selects P1Has a probability of q1Wherein 0.5<q1<1, selecting P2Has a probability of 1-q1(ii) a If the node last sent the first 1 st cycle of the beacon frame, it selects P1Has a probability of 1-q1Selecting P2Has a probability of q1；
2) When N is present>And 2, time: if the node last sent a beacon frame at or before the first Nth period, it selects P1Has a probability of q1Selecting P2Has a probability of 1-q1The probability of selecting other intervals is 0; if the node last transmitted the beacon frame the first nth period, where 1<n<N, then it selects PN-n+1Has a probability of q2，0.5<q2<1, selecting PN-nHas a probability of
Selection of PN-n+2Has a probability of
The probability of selecting other intervals is 0; if the node last sent the first 1 st cycle of the beacon frame, it selects PN-1Has a probability of 1-q1Selecting PNHas a probability of q1The probability of selecting the other interval is 0.
The method can avoid the continuous collision of the beacon frames. Since the nodes are randomly allocated to the respective intervals according to the probability distribution, two nodes that collide in the present period are not necessarily allocated to the same interval in the subsequent period, and the probability that the beacon frames of the two nodes collide again decreases. In node-dense scenarios, the MBCA method in IEEE 802.11s may not be able to negotiate a beacon frame transmission scheme that completely avoids collisions. In this case, the beacon frames transmitted by two nodes continuously collide, and the common neighbor nodes of the two nodes cannot receive the beacon frames of the two nodes. The method reduces the possibility of continuous conflict of two node beacons in a dense network by using an interval division mode based on node density and an interval selection mode based on probability.
Drawings
Fig. 1 is a flowchart of a beacon frame transmission rule;
FIG. 2 is a schematic diagram of cycle and interval division;
fig. 3 is a diagram of a finite state machine for beacon frame transmission rules;
FIG. 4 is a schematic diagram of a cross-cycle beacon frame decision process, in which a node receives a beacon frame before BBT decrements to 0, and the node cancels the transmission of the beacon frame of the i-1 th cycle;
fig. 5 is a schematic diagram of a beacon frame decision process across cycles, in which a node does not receive a beacon frame until the BBT decrements to 0, and the node transmits the beacon frame when the BBT decrements to 0.
Detailed Description
The invention provides a wireless mesh network-based segmented competitive beacon sending method, which comprises three parts: the method comprises the following steps of dividing periods and intervals, selecting intervals and sending beacon frames according to the rules:
(1) division of periods and intervals
Each node in the wireless mesh network is equipped with a timer having a time synchronization function. The time is divided into equal-length periods of length T0. The 1 st cycle starts from time 0 of the timer. The time range of the ith cycle is defined as [ (i-1) T0,iT0). Each period is divided into N equal-length intervals, and the length of each interval is
Wherein N is an integer greater than or equal to 2. The N intervals are sequentially called P from front to back according to time1、P2、……、PN。PkThe range of the interval (k-1, 2, …, N) is defined as
The start Time of each interval is referred to as a Candidate Beacon Transmission Time (hereinafter referred to as CBTT).
The number of intervals N included in each cycle depends on the density of the nodes. Assuming that the distance threshold of the neighbor node is R, namely when the distance between two nodes is less than or equal to R, the two nodes can directly communicate; otherwise direct communication is not possible. Calculating according to the network coverage and the number of nodes to obtain the average density of the nodes as rho, wherein the unit is 'number/unit area', and then the average of the node signal coverage contains pi rho R2And (4) each node. The value of N is set as follows:
wherein ceil (x) represents the smallest integer greater than or equal to x.
(2) Interval selection rule
Each node selects different intervals according to a specific probability distribution at the starting Time of each period according to the condition that the node sends beacons in the previous periods, and the CBTT shows that the node is scheduled to send the beacons at the CBTT at the beginning of the corresponding interval, and the CBTT is called the scheduled Beacon Transmission Time (PBTT) of the node in the period. Interval selection rules are divided into two cases:
1. when N ═ 2:
a) if the node last sent a beacon frame at or before the previous 2 nd cycle, it selects P1Has a probability of q1(0.5<q1<1) Selecting P2Has a probability of 1-q1；
b) If the node last sent the beacon frame first1 cycle, then it selects P1Has a probability of 1-q1Selecting P2Has a probability of q1；
2. When N > 2:
a) if the node last sent a beacon frame at or before the first Nth period, it selects P1Has a probability of q1Selecting P2Has a probability of 1-q1The probability of selecting other intervals is 0;
b) if the node last sent the beacon frame in the first nth period (1)<n<N), then it selects PN-n+1Has a probability of q2(0.5<q2<1) Selecting PN-nHas a probability of
Selection of PN-n+2Has a probability of
The probability of selecting other intervals is 0;
c) if the node last sent the first 1 st cycle of the beacon frame, it selects PN-1Has a probability of 1-q1Selecting PNHas a probability of q1The probability of selecting other intervals is 0;
the "n-th cycle" means that the sequence number of the current cycle minus the sequence number of the cycle is n.
The above-mentioned "last beacon frame transmission is in the first nth period" means that the start time of the last beacon frame transmission is within the range of the first nth period. Since the transmission of the beacon frame needs to last for a certain period of time, it is possible that the transmission of a certain beacon frame spans two periods.
(3) Beacon frame transmission rules
Based on the interval selection rule, the beacon frame transmission rule of each node in each period (denoted as period S) is as follows:
step 1: at the beginning of the period S, it is determined whether the BBT is maintained. The BBT is a backoff counter set for contention to transmit beacon frames. If yes, canceling the sending of the beacon frame of the period S, and finishing the judgment of the period; otherwise, go to step 2.
Step 2: it is determined whether a beacon frame is received before the PBTT within the period S. If yes, canceling the sending of the beacon frame of the period S, and finishing the judgment of the period; otherwise, go to step 3.
And step 3: it is determined whether a beacon frame is received after the PBTT of period S before starting the BBT. If yes, canceling the sending of the beacon frame of the period S, and finishing the judgment of the period; otherwise, starting the BBT and turning to the step 4.
And 4, step 4: it is determined whether a beacon frame is received before the BBT decrements to 0. If yes, canceling the sending of the beacon frame of the period S, deleting the BBT, and finishing the judgment of the period; otherwise, the beacon frame is sent when the BBT decrements to 0.
The above beacon frame transmission rule can be described by a flowchart shown in fig. 1.
The "receiving the beacon frame" in a certain time range means that the time when the physical layer reports to the MAC layer that the beacon frame is correctly received is within the time range. Since the transmission of the beacon frame needs to last for a certain period of time, the time at which the beacon frame starts to be transmitted may not be within the time range. The time range indicated by "before" the event a does not include the time at which the event a occurs, and the time range indicated by "after" the event a includes the time at which the event a occurs.
The BBT determines whether or not its value is decremented in units of time slots from the initial value. The initial value is determined according to a specific method. When the decreasing condition is satisfied in a certain time slot, the value is decreased, otherwise, the BBT is suspended. The start and restart of the BBT needs to satisfy a specific precondition. When the BBT is not started, an event that satisfies the preconditions will trigger the BBT to start. In the case of time synchronization, neighboring nodes may select the same interval at a certain period, and they contend to transmit the beacon frame on the same PBTT. The node whose BBT is decreased to 0 first sends the beacon frame, and other nodes receiving the beacon frame cancel the sending of the beacon frame in the period.
The beacon frame transmission rule is a segmented contention-based beacon transmission method. In particular, all nodes are allocated to different intervals at the beginning of the cycle according to an interval selection rule. The node which has transmitted the beacon frame most recently is allocated to a later interval, and it is highly likely to receive the beacon frame transmitted by other nodes before its PBTT, thereby canceling the transmission of the beacon frame of the period. The node which has not transmitted the beacon frame recently is allocated to the interval with the earlier time, which is advantageous in competing for transmitting the beacon frame, and the probability of transmitting the beacon frame in the period is high. A node that has transmitted a beacon frame most recently is less likely to continue transmitting a beacon frame in this period, while a node that has not transmitted a beacon frame for a long time is more likely to transmit a beacon frame in this period. The segmented competition type beacon frame transmission method enables beacon frames transmitted by nodes to be distributed on a time axis uniformly, and enables adjacent nodes to transmit the beacon frames at different time intervals. In addition, the larger the node density is, the larger the number of intervals included in one period is, which means that each node is dispersed to more intervals in one period, and the probability of beacon frame collision is reduced.
The invention does not require the synchronization of the timers of all nodes in the network and does not depend on a specific time synchronization method. Each node implements a segmented contention based beacon transmission method in accordance with a local timer.
The invention is further illustrated with reference to the figures and examples.
The example considers a wireless mesh network with nodes equipped with omni-directional antennas, but the invention is equally applicable to directional antennas.
(1) Division of periods and intervals
The nodes divide the period and interval according to a local timer. The starting moment of each cycle being the cycle length T0Integer multiples of. T is0Is set according to the application scenario and physical layer characteristics, in this example T is taken0＝100ms。
The value of the number N of intervals comprised by a cycle is related to the distance threshold R. In this example, if the receiver is located within a circle having a center at the transmitter and a radius at R, the receiver can correctly receive the frame, pi ρ R, transmitted by the transmitter without interference2I.e. the average number of nodes in the area. Node pointThe average density ρ is derived from the actual network deployment. In this example, ρ is 2.5 × 10-4m-2According to the formula (1), N is calculated to be 4.
When T is0When N is 4 and 100ms, the period and interval are divided as shown in fig. 2.
(2) Interval selection rule
The interval selection rule involves q1And q is2And (4) determining. q. q.s1Is that the node is assigned to P1Or PNThe probability of (c). And q is2Is that the node is assigned to P2、……、PN-1The probability of (c). In general case q1And q is2Should satisfy 0.5<q2≤q1<1。
Setting q1＝0.9，q20.8. In the case where N is 4, the interval selection rule is as follows:
1. if the node last sent a beacon frame at or before the previous 4 th cycle, it selects P1Is 0.9, P is selected2The probability of (2) is 0.1, and the probability of selecting other intervals is 0;
2. if the node last sent the previous 3 rd cycle of the beacon frame, it selects P1Is 0.1, P is selected2The probability of P is 0.8, and P is selected3Is 0.1, P is selected4The probability of (a) is 0;
3. if the node last sent the 2 nd preceding cycle of the beacon frame, it selects P1Is 0, P is selected2The probability of P is 0.1, and P is selected3Is 0.8, P is selected4The probability of (a) is 0.1;
4. if the node last sent the first 1 st cycle of the beacon frame, it selects P3Is 0.1, P is selected4The probability of (2) is 0.9, and the probability of selecting other intervals is 0;
the seeds for each node to generate the random number should be independent of each other.
(3) Beacon frame transmission rules
An equivalent description of the beacon frame transmission rules is a finite state machine as shown in fig. 3. The finite state machine represents a state transition situation of a node with respect to transmission of a beacon frame for a specific period. The various states, events and actions are defined as follows.
The state is as follows:
1) i: the node executes the step 1 in the initial state of the period starting moment;
2) w _ 1: the node waits for receiving a beacon frame before the PBTT and executes the step 2;
3) w _ 2: when the time reaches PBTT of the period, the node prepares to start BBT, and executes step 3;
4) w _ 3: BBT is started, and step 4 is executed;
5) f: the judgment of the period is finished;
event:
1) BBT _ ON: maintaining the BBT at the beginning of the cycle;
2) BBT _ OFF: BBT is not maintained at the beginning of the cycle;
3) BCN _ RCVD: receiving a beacon frame;
4) PBTT _ RCHD: the time reaches the PBTT of the period;
5) CDTN _ STSFD: the precondition for starting the BBT is satisfied;
6) BBT _ EXPRD: BBT decreases to 0;
and (4) action:
1) cncl (): canceling the transmission of the beacon frame of the period;
2) intl _ BBT (): determining an initial value of the BBT;
3) strt _ BBT (): starting the BBT;
4) dlt _ BBT (): deleting the BBT;
5) trsmt _ bcn (): transmitting a beacon frame;
and the node determines the PBTT of the period at the beginning of each period according to the interval selection rule. And then determines whether and when to transmit the beacon frame in the present period according to the beacon frame transmission rule. The transmission rule of the beacon frame is described below according to a finite state machine. A node experiences two states at each cycle: an initial state I and an end state F. The initial state I is the state of the node at the beginning of the period, and the end state F is the state after the beacon frame transmission decision of the period is completed.
The initial state of the node at the starting time of the period is the I state, and whether the BBT is maintained or not is judged at the current time. If the BBT is maintained (BBT _ ON event), it means that the decision process of the previous cycle has not been completed yet, and the transmission of the beacon frame of the present cycle is cancelled (cncl () action), and the decision process of the present cycle is completed (F state). If the BBT is not maintained at this time (BBT _ OFF event), the node enters W _1 state by listening to the beacon frame before the PBTT of this period arrives. Fig. 4 and 5 show two beacon frame decision processes across periods. In fig. 4, the node maintains the BBT at the start time of the ith period, and cancels the transmission of the beacon frame of the ith period according to step 1. Then, the node receives the beacon frame sent by other nodes, and the BBT is not decremented to 0 at this time. According to step 4, the node cancels the transmission of the beacon frame of the (i-1) th period and deletes the BBT. In fig. 5, the node maintains the BBT at the start time of the ith period, and cancels the transmission of the beacon frame of the ith period according to step 1. Then, the BBT is decremented to 0, the node transmits a beacon frame (the beacon frame corresponds to the i-1 th period), and the BBT is automatically deleted when the BBT is decremented to 0.
The node in the W _1 state waits for which of two events, BCN _ RCVD and PBTT _ RCHD, occurs first. If the BCN _ RCVD occurs first, that is, the beacon frame is received before the PBTT of the present period, the beacon frame transmission of the present period is cancelled (cncl () action), and the decision process of the present period is ended (F state). If PBTT _ RCHD happens first, i.e. no beacon frame is received before PBTT, then BBT is initialized (intl _ BBT () action), BBT is ready to be started to send beacon frame, and node moves to W _2 state.
The node in the W _2 state waits for which of the two events BCN _ RCVD and CDTN _ STSFD occurs first. If BCN _ RCVD occurs first, namely the beacon frame is received before BBT is started, the beacon frame transmission in the period is cancelled (cncl () action), and the judgment process in the period is ended (F state). If CDTN _ STSFD occurs first, i.e. no beacon frame is received before BBT starts, BBT is started (strt _ BBT () action), a beacon frame is sent when BBT decrements to 0, and the node transitions to W _3 state.
The node in the W _3 state waits for the first occurrence of two events, BCN _ RCVD and BBT _ EXPRD. If the BCN _ RCVD occurs first, that is, the beacon frame is received before the BBT decrements to 0, the sending of the beacon frame in the present period is cancelled (cncl () action), the BBT is deleted (dlt _ BBT () action), and the decision process in the present period is ended (F state). If BBT _ EXPRD occurs first, i.e., no beacon frame is received before BBT decrements to 0, a beacon frame is sent (trsmt _ bcn () action), and the decision process of this cycle ends (F state).
The initialization of the BBT is essentially to select a random number within a certain range, and reference may be made to an initial value setting method of the BBT of the stand-alone type network in IEEE 802.11-1999. Specifically, an integer is randomly selected from [0,2 × aCWmin ] as an initial value of BBT in a uniform distribution. aCwmin can be 7.
The start and restart of the BBT needs to satisfy a precondition. The precondition for BBT start-up and restart of an independent network defined by IEEE 802.11-2016 is that the duration of the channel being idle reaches SIFS + aSlotTime. SIFS is the minimum spacing between frames and aSlotTime is the unit of time to determine channel busy and idle, which are defined in IEEE 802.11-1999. When the BBT is started, if the channel is determined to be idle in aSlotTime, the BBT decrements by 1, otherwise, the BBT is suspended. When the BBT is suspended, the BBT restarts if the restart preconditions are met. The restarted BBT starts to decrement at the value of the last pause.
The precondition and decrementing for BBT initiation involves the definition of channel busy and idle. According to the definition of IEEE 802.11-1999, a channel is considered to be in a "busy" state when one of the following conditions is satisfied, otherwise, the channel is considered to be in an "idle" state:
1) the node is transmitting a frame;
2) the node monitors that other nodes send frames;
3) a Network Allocation Vector (NAV) of the node is not 0;
when the PBTT _ RCHD event occurs, the node shall suspend initiating the frame transmission sequence and suspend sending the broadcast frames except the beacon frame, and resume initiating the frame transmission sequence and sending the broadcast frames except the beacon frame after the decision of the beacon frame is completed (i.e. reaching the F state). If the node maintains a Non-Beacon Backoff Timer (NBBT) for contending to initiate a frame transmission sequence at the PBTT time, the node suspends the Timer and restarts the NBBT after the Beacon frame is determined to be complete (i.e., to reach the F state). The restarted NBBT starts decrementing at the value at the last pause.
Claims (3)
1. A wireless mesh network-based segmented contention-based beacon transmission method is characterized in that: dividing time into equal-length periods, wherein each period is divided into N equal-length intervals, N is an integer greater than or equal to 2, and the intervals are marked as P from front to back in sequence according to time1、P2、……、PN(ii) a At the beginning of each period, the nodes are distributed to a certain interval in the period according to the interval selection rule according to the condition that the beacon frames are transmitted in the previous period, so that the nodes which have transmitted the beacon frames recently are distributed to the interval at the later time, and the nodes which have not transmitted the beacon frames recently are distributed to the interval at the earlier time; the node plans to transmit a beacon frame at the start time of the allocated interval, which is called a planned beacon transmission time; at the beginning of each period, the node decides whether and when to transmit a beacon frame according to the following beacon frame transmission rules:
step 1: judging whether a beacon frame rollback timer is maintained at the starting moment of the period, if so, cancelling the sending of the beacon frame of the period, and if not, turning to the step 2;
step 2: judging whether a beacon frame is received before the scheduled beacon transmission time in the period, if so, canceling the transmission of the beacon frame in the period, and if not, turning to the step 3;
and step 3: before starting the beacon frame rollback timer, judging whether the beacon frame is received or not after the scheduled beacon transmission time of the period, if so, cancelling the transmission of the beacon frame of the period, and if not, starting the beacon frame rollback timer, and turning to the step 4;
and 4, step 4: and judging whether the beacon frame is received or not before the beacon frame rollback timer is decremented to 0, if so, cancelling the sending of the beacon frame in the period, deleting the beacon frame rollback timer, finishing the judgment of the period, and otherwise, sending the beacon frame when the beacon frame rollback timer is decremented to 0.
2. The method of claim 1, wherein the method comprises: the number of intervals N included in each period is determined by the following equation:
where R is the farthest distance that the receiving node can correctly receive the frame sent by the sending node, ρ is the average density of the nodes, ceil (x) represents the smallest integer greater than or equal to x.
3. The method of claim 1, wherein the method comprises: the interval selection rule of the nodes at the beginning of each period is as follows:
1) when N ═ 2: if the node last sent a beacon frame at or before the previous 2 nd cycle, it selects P1Has a probability of q1Wherein 0.5<q1<1, selecting P2Has a probability of 1-q1(ii) a If the node last sent the first 1 st cycle of the beacon frame, it selects P1Has a probability of 1-q1Selecting P2Has a probability of q1；
2) When N is present>And 2, time: if the node last sent a beacon frame at or before the first Nth period, it selects P1Has a probability of q1Selecting P2Has a probability of 1-q1The probability of selecting other intervals is 0; if the node last transmitted the beacon frame the first nth period, where 1<n<N, then it selects PN-n+1Has a probability of q2，0.5<q2<1, selecting PN-nHas a probability of
Selection of PN-n+2Has a probability of
The probability of selecting other intervals is 0; if the node last sent the first 1 st cycle of the beacon frame, it selects PN-1Has a probability of 1-q1Selecting PNHas a probability of q1The probability of selecting the other interval is 0.
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