Adaptive backoff optimization method for CSMA / CA mechanism based on IEEE802.15.4

Abstract

The invention belongs to the technical field of wireless sensor networks, and particularly relates to an adaptive backoff optimization method based on the CSMA / CA mechanism of IEEE802.15.4, comprising dividing data services into two priorities, and each node maintaining two queues corresponding to the two priorities; the node monitors and counts the number of ACK frames received in a beacon period during an active period, and sets the maximum number of ACK frames that can be received in a superframe period as a threshold; the node counts the number of times the node performs channel detection and the number of times the channel is detected to be in an idle state when a data frame is successfully transmitted, and performs a node state evaluation NSE; when data packets of different priorities arrive at the node, the number of backoff times and the length of a contention window are initialized, and a backoff index is adjusted according to the priority of the data service, the threshold and the node state evaluation; and a backoff cycle process is executed according to the backoff index. The invention improves the throughput of the network and reduces the system delay.

Description

Technical Field

[0001] The invention belongs to the technical field of wireless sensor networks, and in particular relates to an adaptive backoff optimization method based on a CSMA / CA mechanism of IEEE802.15.4. Background Art

[0002] At present, the Internet of Things technology has been widely used in all walks of life. As the core of the Internet of Things, wireless sensor networks use perception technology to realize the perception detection of the Internet of Things system. For example, existing perception technologies such as laser, sound, smell, temperature, infrared and other sensors can realize various perception and recognition needs; wireless network transmission technology, such as WIFI, Bluetooth, LoRa, ZigBee and other wireless technologies, can conveniently transmit data. Compared with other wireless technologies, ZigBee based on the IEEE802.15.4 standard has been widely used in smart cities, smart agriculture, smart construction sites and other scenarios due to its advantages such as self-organizing network, low power consumption, low cost and low complexity.

[0003] In the IEEE802.15.4 standard, the MAC layer uses a superframe structure to organize the time allocation of channel occupancy, such as Figure 1 As shown in the figure, the superframe period is the time between two beacon frames and consists of an active period and an inactive period. The active period is divided into a contention access period CAP and a non-contention access period CFP. During the contention access period, any node that wants to send data competes with other nodes that want to send data for the right to use the channel through the CSMA / CA algorithm. The non-contention access period is divided into some guaranteed time slots (GTS) for designated devices to send data without competition. After that, the node enters a sleep state during the inactive period.

[0004] Since only one node is allowed to transmit data at the same time, a binary backoff algorithm is used in the CSMA / CA mechanism to avoid conflicts. Figure 2 The algorithm contains three parameters: backoff number NB, contention window length CW, and backoff index BE. The node that needs to transmit data to the coordinator first selects the backoff index BE in the range (0, 2 BE -1) for a period of time, and perform idle channel assessment (CCA) after the backoff period ends. After two consecutive CCA detections that the channel is idle, the node starts to send data to the coordinator, otherwise BE increases by 1 and performs random backoff again. The above actions are repeated until the maximum backoff number NB is reached or the channel is idle twice.

[0005] As the network load continues to increase, the binary backoff algorithm of the CSMA / CA mechanism does not show good robustness. The simple change of the three parameters limits the efficiency of node channel access, increases the probability of network conflict and retransmission, and causes node packet failure, resulting in network congestion. In addition, different services have different real-time requirements. In addition to meeting the basic low-power data transmission, wireless sensor networks also need to respond to business needs with special delay requirements in a timely manner. However, in IEEE802.15.4, the protocol stipulates that emergency data (GTS request frames, command frames, data frames carrying alarm information, etc.) must compete with ordinary data frames for time slots for transmission, resulting in increased delays for emergency business data and failure to deliver them in a timely manner. Summary of the invention

[0006] In order to improve the channel access efficiency of priority services and reduce transmission delay, the present invention proposes an adaptive backoff optimization method based on the CSMA / CA mechanism of IEEE802.15.4, which specifically includes the following steps:

[0007] S1, divide the data service into two priorities P1 and P2, each node maintains two queues Q1 and Q2 corresponding to the two priorities P1 and P2, and the node gives priority to the queue with higher priority;

[0008] S2. The node monitors and counts the number of ACK frames received during the beacon period. i , set the maximum number of ACK frames that can be received within the superframe period to the threshold N inf ;

[0009] S3, the node counts the number of times the node performs channel detection when a data frame is successfully transmitted and the number of times the channel is detected to be idle, and performs node status evaluation NSE;

[0010] S4. When packets of different priorities arrive at the node, the node initializes the number of backoffs and the length of the contention window, based on the priority of the data service and the threshold N. inf and node status evaluation to adjust the backoff index;

[0011] S5. Execute a backoff cycle process according to the backoff index;

[0012] S6. After the backoff period ends, perform channel detection. If the current channel detection is idle, the length of the contention window is reduced by 1, and it is determined whether the value of the length of the contention window is zero. If it is zero, the channel competition is successful and the node obtains the right to use the channel. Otherwise, perform channel detection again and re-execute step S6; if the channel detection is busy, execute step S7;

[0013] S7, the backoff number is incremented by 1, the contention window length is set to 2, the backoff index is incremented by 1, and the backoff range is set according to the backoff index value range;

[0014] S8. Determine whether the backoff times are greater than the maximum backoff times. If so, the channel contention fails. Otherwise, randomly select a backoff index value according to the backoff range set in step S7, and execute step S5.

[0015] Furthermore, a greedy rate is set to allow packets with lower priority to enter queues with higher priority randomly, which specifically includes the following process:

[0016] After the data packet arrives, the priority of the data packet is determined, and the high-priority data packet is assigned to the corresponding high-priority queue according to the priority;

[0017] If the data packet has a lower priority, a random number μ is generated between (0, 1);

[0018] The random number μ is compared with the greedy rate ε. If μ is less than ε, the low-priority data enters the low-priority queue, otherwise it enters the high-priority queue.

[0019] Furthermore, step S4 specifically includes the following steps:

[0020] S41, when initializing the values ​​of the backoff number and the length of the contention window, if the data packet is of high priority, a backoff index is randomly selected in [0, 2], and if the data frame is of low priority, a backoff index is randomly selected in [2, 4]. Generally, the backoff number is initialized to 0 and the length of the contention window is initialized to 2;

[0021] S42, judging the number of ACK frames Ni and the threshold N inf If the number of ACK frames Ni is greater than the threshold N inf , then the backoff index is incremented by 1, and the process goes to step S44; if the number of ACK frames is N i Less than the threshold N inf Then jump to step S43;

[0022] S43, judging the value of the node status evaluation, if it is greater than the value of the node status evaluation 0.5, then the backoff index is reduced by 1; otherwise, the backoff index is increased by 1;

[0023] S44, in [0, 2 BE -1] randomly selects the backoff period within the range;

[0024] Among them, BE is the backoff exponent.

[0025] Further, in step S8, setting the backoff range according to the range of the backoff index value includes:

[0026]

[0027] Among them, RangeBackoff is a backoff range set according to the range of the backoff index value.

[0028] Furthermore, a superframe period is the time between two beacon frames. Each superframe period consists of an active period and an inactive period. The active period includes a contention access period and a non-contention access period. Nodes compete for channel use rights during the contention access period.

[0029] Furthermore, the priority of time-sensitive tasks is high, and the priority of other tasks is low.

[0030] Furthermore, the value of the current node status evaluation NSE is the ratio of the number of times the node detects the channel idle state during the last channel access to the total number of channel detections.

[0031] The adaptive backoff parameter optimization method based on time slot CMSA / CA proposed in the present invention takes into account the node service priority allocation and queuing strategy, designs a service priority improvement strategy to differentiate services, and on this basis, triggers from the perspective of adaptive optimization of different backoff parameters, so that the network selects different backoff periods according to different backoff parameters, thereby improving the channel access efficiency of different priority services of nodes. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a schematic diagram of the IEEE 802.15.4 superframe structure;

[0033] Figure 2 It is a schematic diagram of the CSMA / CA algorithm based on the IEEE 802.15.4 standard;

[0034] Figure 3 A schematic diagram of a priority service queue of the present invention;

[0035] Figure 4 is a flowchart for implementing the priority service strategy of the present invention;

[0036] Figure 5 It is a flow chart of the implementation of the adaptive backoff algorithm based on CSMA / CA of the present invention. DETAILED DESCRIPTION

[0037] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0038] The present invention proposes an adaptive backoff optimization method for the CSMA / CA mechanism based on IEEE802.15.4, which specifically includes the following steps:

[0039] S1, divide the data service into two priorities P1 and P2, each node maintains two queues Q1 and Q2 corresponding to the two priorities P1 and P2, and the node gives priority to the queue with higher priority;

[0040] S2. The node monitors and counts the number of ACK frames received during the beacon period. i , set the maximum number of ACK frames that can be received within the superframe period to the threshold N inf ;

[0041] S3, the node counts the number of times the node performs channel detection when a data frame is successfully transmitted and the number of times the channel is detected to be idle, and performs node status evaluation NSE;

[0042] S4. When packets of different priorities arrive at the node, the node initializes the number of backoffs and the length of the contention window, based on the priority of the data service and the threshold N. inf and node status evaluation to adjust the backoff index;

[0043] S5. Execute a backoff cycle process according to the backoff index;

[0044] S6. After the backoff period ends, perform channel detection. If the current channel detection is idle, the length of the contention window is reduced by 1, and it is determined whether the value of the length of the contention window is zero. If it is zero, the channel competition is successful and the node obtains the right to use the channel. Otherwise, perform channel detection again and re-execute step S6; if the channel detection is busy, execute step S7;

[0045] S7, the backoff number is incremented by 1, the contention window length is set to 2, the backoff index is incremented by 1, and the backoff range is set according to the backoff index value range;

[0046] S8. Determine whether the backoff times are greater than the maximum backoff times. If so, the channel contention fails. Otherwise, randomly select a backoff index value according to the backoff range set in step S7, and execute step S5.

[0047] Different service requirements in wireless sensor networks have different requirements for latency and bandwidth. Periodically acquired monitoring data does not require high bandwidth and latency, while time-sensitive events need to be transmitted in a timely manner. Since time-sensitive services still need to compete with ordinary data frames for access to channels in the CAP stage, their transmission latency is high and channel utilization is low. The purpose of the present invention is to improve the CSMA / CA algorithm in the IEEE 802.15.4 protocol, and propose a channel access algorithm with adaptive backoff parameters based on CMSA / CA to improve the channel access efficiency of priority services and reduce transmission latency.

[0048] To achieve the above purpose, this embodiment considers improving the channel access efficiency of node priority services from the perspectives of service priority allocation, queuing strategy, and adaptive optimization of backoff parameters. First, in view of the problem that time-sensitive services (such as GTS request commands) need to compete with data frames for time slots for transmission, a service priority improvement strategy is designed to distinguish services. On this basis, in order to further improve the channel access efficiency, the existing CSMA / CA improved algorithm is studied. Then, by introducing channel state information, a channel access algorithm based on CMSA / CA is proposed to adaptively improve backoff parameters, which specifically includes the following steps:

[0049] (1) In order to differentiate services at the node during the access contention phase, different services arriving at the node are first differentiated and data services are divided into two priorities, P1 and P2. Each node maintains two priority queues, Q1 and Q2. Figure 3 ,Take two nodes and a receiving node as an example. Each node has two queues, one is a high priority queue Q1, and the other is a low priority queue Q2. The node preferentially sends the services in the high priority queue to the receiving node, and the high priority data packets directly enter the high priority queue and wait.

[0050] (2) In order to avoid the "starvation" of low-priority services, the greed rate ε is set, and the low-priority services of the node are selected according to the ε value to enter the high-priority queue for channel access. As an optional implementation plan, the priority queuing strategy specifically includes:

[0051] A. Determine the priority of the data packet after it arrives;

[0052] B. If it is low priority data, a random number μ is generated between (0, 1);

[0053] C. Compare the random number μ with the greedy rate ε. If μ is less than ε, the low-priority data enters the low-priority queue, otherwise it enters the high-priority queue;

[0054] D. The node transmits the high priority queue data first, and then transmits the low priority queue data.

[0055] (3) During the active period, the node listens and counts the number of ACK frames N received during the beacon period. i , and design the threshold N based on the maximum number of ACK frames that can be received within the superframe period inf .

[0056] (4) The node counts the number of times the node performs channel detection and the number of times the channel is detected to be idle when a data frame is successfully transmitted, and performs node status evaluation NSE.

[0057] (5) When packets of different priorities arrive at the node, the values ​​of NB (backoff times) and CW (contention window length) are initialized, and the backoff index BE and backoff time BE are adaptively adjusted according to the priority allocation in (1) and (3) ACK information and (4) status evaluation information. The adjustment mechanism is as follows:

[0058] A. Determine the data service priority. If the data frame is of high priority, adjust the backoff time BE to be randomly selected within [0, 2]. If the data frame is of low priority, adjust the backoff time BE to be randomly selected within [2, 4].

[0059] B. Then determine the number of ACK frames N i With threshold N inf If the number of ACK frames is N i Greater than the threshold N inf , then the backoff time BE is automatically increased, and jump to step D; if the number of ACK frames N i Less than the threshold N inf , indicating that there are fewer nodes transmitting data frames within the superframe time, or there are too many nodes to send data frames, the network is congested, and a large number of collisions occur when sending data frames. Therefore, it is necessary to continue to judge and proceed to step C.

[0060] C. Determine the value of node status evaluation NSE. When NSE is greater than 0.5, it means that there are fewer nodes transmitting data frames, and the backoff time BE decreases automatically. When NSE is less than 0.5, it means that there are too many nodes to send data frames, and the backoff time BE increases automatically.

[0061] D. In [0, 2 BE -1] range and randomly select the backoff period.

[0062] (6) Execute the backoff cycle.

[0063] (7) After the backoff period ends, channel detection is performed. If the current channel detection is idle, CW is automatically decremented and a check is made to see if the value of CW is zero. If it is zero, the channel competition is successful and the node obtains the right to use the channel. Otherwise, channel detection is performed again and step (7) is re-executed. If the channel detection is busy, execute (8).

[0064] (8) NB (backoff times) is incremented, CW value is reset to 2, BE value is incremented, and adaptive backoff range RangeBackoff is set according to the range of BE value. Because a large number of collisions may be caused by the same random number allocation during the backoff period, and a large number of backoff cycles increase the energy consumption of a network with a high collision probability, this embodiment uses different backoff cycles to reduce the probability of data collision in the network. The backoff range RangeBackoff is expressed as:

[0065]

[0066] (9) Determine whether the NB is greater than its maximum backoff times. If it is greater, the channel competition fails. Otherwise, randomly select according to the backoff range set in (8) and jump to (6).

[0067] The adaptive backoff parameter optimization method based on time slot CMSA / CA proposed in the present invention takes into account the node service priority allocation and queuing strategy, designs a service priority improvement strategy to differentiate services, and on this basis, triggers from the perspective of adaptive optimization of different backoff parameters, so that the network selects different backoff periods according to different backoff parameters, thereby improving the channel access efficiency of different priority services of nodes.

[0068] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. An adaptive backoff optimization method based on the CSMA / CA mechanism of IEEE802.15.4, characterized in that: The following steps are involved: S1. Divide data services into two priorities P1 and P2. Each node maintains two queues Q1 and Q2 corresponding to the two priorities P1 and P2. The node preferentially transmits the queue with higher priority. When the node receives the data service, it stores the high-priority service in the queue corresponding to the high-priority service, and generates a random number μ between (0, 1) for the low-priority data service. Compare the random number μ with the greed rate ε of the device. If μ is less than ε, the low-priority data enters the low-priority queue, otherwise it enters the high-priority queue. S2. The node monitors and counts the number of ACK frames received during the beacon period. i , set the maximum number of ACK frames that can be received within the superframe period to the threshold N inf ; S3. The node counts the number of times the node performs channel detection and the number of times the channel is detected to be idle when a data frame is successfully transmitted, and performs a node status evaluation NSE. The value of the current node status evaluation NSE is the ratio of the number of times the node detects the channel idle state during the last channel access to the total number of channel detections. S4. When packets of different priorities arrive at the node, the node initializes the number of backoffs and the length of the contention window, based on the priority of the data service and the threshold N. inf And the node status evaluation adjusts the backoff index. The process of adjusting the backoff index includes: Determine the priority of the data service. If it is a high-priority service, the backoff index BE is randomly selected within [0, 2]. If the data frame is a low-priority service, the backoff index BE is randomly selected within [2, 4]. Compare the number of ACK frames N i With threshold N inf , if N i Greater than N inf , then the backoff index BE increases by 1; If N i Less than N inf , and the value of the node status evaluation NSE is greater than 0.5, the backoff index BE is reduced by 1; If N i Less than N inf , and the value of the node status evaluation NSE is less than 0.5, the backoff index BE is incremented by 1; S5. Calculate the backoff period according to the backoff index, and backoff according to the backoff period. The backoff period calculated according to the backoff index is expressed as: Among them, RangeBackoff is the backoff range set according to the range of the backoff index value; BE is the backoff index; S6. After the backoff period ends, perform channel detection. If the current channel detection is idle, the length of the contention window is reduced by 1, and it is determined whether the value of the length of the contention window is zero. If it is zero, the channel competition is successful and the node obtains the right to use the channel. Otherwise, perform channel detection again and re-execute step S6; if the channel detection is busy, execute step S7; S7, the backoff number is incremented by 1, the length of the contention window is set to 2, the backoff index is incremented by 1, and the backoff range is set according to the range of the backoff index value; S8. Determine whether the backoff times are greater than the maximum backoff times. If so, the channel contention fails. Otherwise, randomly select a backoff index value according to the backoff range set in step S7, and execute step S5.

2. The method for adaptive backoff optimization based on CSMA / CA mechanism of IEEE802.15.4 according to claim 1, characterized in that: The superframe period is the time between two beacon frames. Each superframe period consists of an active period and an inactive period. The active period includes a contention access period and a non-contention access period. Nodes compete for channel usage rights during the contention access period.

3. The method for adaptive backoff optimization based on CSMA / CA mechanism of IEEE802.15.4 according to claim 1, characterized in that: Time-sensitive tasks are given high priority, and other tasks are given low priority.

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