GAF-based MAC protocol for Wireless Sensor Networks

A wireless sensor network consists of a large number of low-cost micro-sensor nodes deployed in the monitoring area. A multi-hop self-organizing network system is formed through wireless communication, it is mainly used to collect, disseminate, and process sensing information.

Unlike traditional wireless self-organizing networks, wireless sensor networks have a large number of nodes and are densely distributed. Due to environmental impact and energy depletion, nodes are prone to faults; environmental Interference and node failure are prone to changes in the network topology. In addition, the energy, processing, storage, and communication capabilities of nodes are limited. Therefore, the primary purpose of wireless sensor networks is to efficiently use energy. The Media Access Control (MAC) Protocol of wireless sensor networks must focus on energy conservation and adopt a compromise mechanism, users can choose to extend the network life cycle, increase network throughput, and reduce communication latency.

Currently, for different sensor networks, researchers have proposed multiple MAC protocols from different aspects, and there is no unified classification method. Based on the fixed distribution channel or random access channel method, sensor network MAC protocols are divided into time division multiplexing (TDMA), random competition, and other MAC protocols. TDMA with fixed distribution channels can naturally perform low duty cycle operations on nodes, because they only need to enable the wireless module in their own time slot to complete sending and receiving, but its scalability is poor, time Synchronization is a huge overhead for the system. Due to the low data rate and low latency requirements of wireless sensor networks, the current practical energy-saving MAC protocol is basically based on competitive protocols. A large number of experiments and theoretical analysis show that the energy waste of wireless sensor nodes mainly comes from idle listening, conflicts, crosstalk and control. Therefore, combined with the existing wireless sensor network MAC protocol, the hierarchical topology control concept is introduced to establish an energy-efficient wireless sensor network protocol for analysis, simulation and verification, it has research significance.

1. Competitive MAC protocol analysis

1.1 S-MAC Protocol

Based on 802.11MAC protocol, S-MAC protocol is a sensor network MAC protocol based on energy-saving requirements of sensor networks. The S-MAC Protocol assumes that the sensor network data transmission is usually less, nodes collaborate to complete the common task, the network can process and integrate data to reduce the amount of data communication, the network can tolerate a certain degree of delay. Research shows that sensor energy is mainly consumed among nodes, and idle listening accounts for about 1/3 of the node communication energy. for the purpose of saving energy, the S-MAC protocol mainly adopts the low duty cycle mechanism of periodic listening/sleep, and controls the node to be sleep as much as possible to reduce the energy consumption of the node. However, S-MAC Protocol has the following problems: All nodes in the same virtual cluster in S-MAC protocol should switch from sleep state to active state at the same time, and began to compete with the channel, while a large number of nodes have no data transmission task, these nodes waste a lot of energy for Channel competition and idle listening.

1.2 T-MAC Protocol

The T-MAC (Timeout MAC) protocol is proposed based on the S-MAC protocol. The duration of the S-MAC Protocol is limited by latency requirements and cache size, while the listening time depends primarily on the message rate. Therefore, to ensure reliable message transmission, the node's periodic activity time must adapt to the highest communication load, resulting in a relatively low network load and a relatively high idle listening time on the node, in view of this deficiency, a T-MAC protocol is proposed, which dynamically adjusts the node activity time according to the communication traffic while keeping the length of the periodical listener unchanged, and sends messages in burst mode, reduces idle listening time. However, because a large number of nodes without data transmission to the channel competition and idle listening still cause a lot of energy waste, in addition to the implementation of the T-MAC protocol, there will be a problem of early sleep, resulting in reduced network throughput. Therefore, it uses two methods to improve the data throughput reduction caused by early sleep: (1) future request sending mechanism. (2) Full buffer priority mechanism, but the effect is not very satisfactory.

2. MAC protocol design based on topology control structure

All nodes in S-MAC and T-MAC virtual clusters are periodically transferred from sleep to working state, participating in channel competition and data transmission, while most nodes do not have data transmission tasks, as a result, they are idle for most of their activities, wasting a lot of energy. Aiming at S-MAC and T-MAC, a new MAC protocol GS-MAC (Geo graphical SeNSor MAC) is proposed ). After the GAF topology control algorithm is introduced in the GS-MAC protocol, the number of active nodes is reduced, the convergence speed is accelerated, and the idle listening and data collision of a large number of nodes on the channel are reduced.

2.1 GAF Improved Algorithm

The GAF (Geographical Adaptive Fidelity) algorithm is a clustering algorithm based on the Geographical location of nodes. This algorithm divides the monitoring area into virtual cells and splits nodes into corresponding cells based on their location information; each cell regularly generates a cluster head node. Only the cluster head node remains active, and other nodes enter the sleep state. The GAF algorithm is executed in two stages: the first stage is the division of virtual cells; the second stage is the election of cluster heads.

Virtual Cell Division: Based on the node location information and communication radius, the network area is divided into several virtual cells to ensure that any two nodes in the adjacent cells can communicate directly. To ensure that any two nodes in two adjacent cells can communicate directly, the following relationship must be met:

In formula (1), R indicates the communication radius of all nodes, and r indicates the edge length of a square virtual cell.

Cluster header election: In the GAF algorithm, the cluster header undertakes more data processing and communication, consuming a relatively large amount of energy. In the improved GAF algorithm, the node surplus energy is taken into account in cluster head election, and nodes with more surplus energy are elected as the cluster heads. Random cluster head Election Algorithm: nodes only know their own energy information and location information. Assume that a cluster header election starts at the Tr time and sends a Test message to any node N in the cell with probability P. The probability P is proportional to the residual energy. If the test message succeeds, the message M (Ep, N) occurs, and the Ep is the node N residual energy. If the message fails to be sent, node N enters the listening status. If no message is sent in a time slot, it indicates that no node competition in the time slot is successful, and a new round of election is started. Otherwise, if a node competition succeeds, send M (Ep, n) The message acts as the cluster header, and other nodes in the cell listen to the message M and add it to the cluster.

2.2 GS-MAC protocol description

In the GS-MAC Protocol, only the cluster head node enters the active state 1 and figure 2.

In the new protocol, due to the introduction of the topological structure mechanism, the idle listening time of some nodes can be reduced, and only the cluster head nodes are kept active. In the cluster head election, the remaining energy of the nodes is considered, it balances the remaining energy of nodes in a local range and prolongs the network life cycle.

Cluster head node maintenance and S-MAC protocol similar to the working/sleep mechanism, each cluster head node periodically and directly adjacent to the cluster head node by receiving and broadcasting SYNC data frame to exchange scheduling information; CSMA/CA mechanism and random backoff time are used; DATA transmission is completed through the RTS/CTS/DATA/ACK communication process, and its sleep schedule is not followed before DATA transmission is completed; the adaptive traffic listening mechanism is used to reduce the transmission latency of messages.

2.3 Performance Analysis

The S-MAC protocol and the new protocol (GS-MAC) were compared in terms of energy consumption and transmission delay. the energy consumption is the total energy consumption from a certain number of packets sent from the source node to the target node, the latency is an end-to-end latency. Set the simulation scenario to 300 Mbit/s * 300 Mbit/s and set it to 10 Mbit/s ~ 50 nodes, R = 300 m, r = 100 m, and 9 virtual cells. The simulation parameter selection is shown in table 1.

Figure 3 shows that when the number of nodes is equal to 10, almost every virtual cell has only one node, the GAF topology control algorithm is less efficient, basically close to the S-MAC protocol, as the number of nodes increases, the efficiency of GAF topology control algorithms is significantly improved, and the energy consumption is significantly reduced.

To compare the performance of the two protocols in terms of network latency performance. 50 fixed nodes are evenly arranged in the scenario of 300 Mbit/s × 100 Mbit/s. The multi-hop network topology is used to test the end-to-end data latency. The source node generates 20 messages, each of which is bytes, all messages are not sharded, Repeat 10 experiments under the condition of light traffic load, the average message latency of each test is 4, because the source node in the GS-MAC competition cluster head node formation delay, because of the large difference in the first forwarding hop latency between the two protocols, with the forward transmission of the message, the latency of the backend queue is less, but because of the existence of the status in the GS-MAC, the delay of GS-MAC protocol is slightly higher than that of S-MAC protocol.

To sum up, the GS-MAC protocol with topology control can further reduce the energy consumption of nodes and prolong the network lifetime under the condition of normal delay.

3 conclusion

By analyzing the performance of wireless sensor network S-MAC protocol, in the case of less data traffic, most sensor nodes are in idle listening state, a large amount of node energy is wasted, the GAF topology control algorithm is introduced, this greatly reduces the number of nodes in the idle listening state with normal network latency. The simulation shows that the GS-MAC protocol has high energy efficiency and prolongs the service life of the network.