One of the important aspects in wireless sensor networks is time synchronization. Many applications such as military activity monitoring, environmental monitoring and forest fire monitoring require highly accurate time synchronization. Time synchronization assures that all the sensor nodes in wireless sensor network have the same clock time. It is not only essential for aforementioned applications but it is mandatory for TDMA scheduling and proper duty cycle coordination. Time synchronization is a challenging problem due to energy constraints. Most of the existing synchronization protocols use fixed nodes for synchronization, but in the proposed synchronization, algorithm mobile nodes are used to synchronize the stationary nodes in the sensing field. In this paper, we propose a new time synchronization algorithm, named controlled mobility time synchronization (CMTS) with the objective to achieve the higher accuracy while synchronizing the nodes. The proposed approach is used in this paper to synchronize the nodes externally by using the mobile nodes. Simulation results exhibit that proposed controlled mobility time synchronization increases the synchronization precision and reduces the energy consumption as well as synchronization error by reducing the collisions and retransmissions.
Revolutionary technological changes in the field of microelectronics and wireless communications have resulted in the emergence of wireless sensor networks (WSN). The network is composed of numerous motes equipped with miniature sensors. The advantageous features of these motes are low cost, small size, multi-functionality and mobility. In modern era, wireless sensor networks have grown in popularity and are being used in various applications such as target tracking surveillance, seismic sensing, environmental monitoring and biomedical health monitoring [
In the recent past, many valuable researches have been conducted for time synchronization in WSNs [
This paper proposes Controlled Mobility Time Synchronization (CMTS) technique for wireless sensor networks. Major objective of CMTS is to reduce the synchronization error besides energy consumption of the nodes by reducing the communication distance among nodes. CMTS also conserves the energy of the nodes by reducing the collisions. The proposed CMTS is analyzed by simulation and the results show that proposed algorithm dissipates less energy than TTS and TSPN [
The rest of the paper is organized as follows: Section 2 presents related work; Section 3 describes the proposed mobile node model and mobile node based time synchronization; Section 4 contains the simulations and analysis; and finally we conclude the paper in Section 5.
In WSNs, there exist many time synchronization protocols. Some of the protocols synchronize the nodes internally by placing the nodes on common notion of time and some synchronize externally by adjusting the clocks of the sensor nodes with global clock. But none of the existing protocols exploits the mobility of the node for time synchronization.
Wang et al. proposed two-hop time synchronization protocol (TTS) with the objective to decrease the synchronization overhead and to offer more precise network-wide synchronization [
Djenouri et al. [
Wu et al. proposed average time synchronization (ATS) by pairwise message exchange with packet delay [
Thomas Schmid et al. proposed the temperature compensated time synchronization (TCTS) which exploits the temperature of the sensor to automatically calibrate the oscillation of the sensor nodes to eliminate the effect of the environmental temperature changes which leads to increase in the resynchronization period [
Long term and large scale time synchronization protocol (2LTSP) for WSNs has been proposed by Huang et al. with the objective to keep the large scale WSN synchronized for long time [
Ganeriwal et al. proposed timing-sync protocol for sensor networks (TPSN) which uses the sender receiver synchronization approach to synchronize the clock of the sensor nodes. Here, the logical hierarchical structure of sensor nodes is established to communicate between sender and receiver. The nodes of the network are synchronized hierarchically to the root node’s clock. This approach is not suitable for mobile nodes but suitable for the network with highly constrained computational energy and bandwidth.
There are various applications which require accurate time synchronization of the nodes in the WSNs. The existing protocols use the clock of the base station to synchronize the sensor nodes. Large size WSNs uses multi-hop communication for synchronization which reduces the time synchronization accuracy level and also consumes more energy. Therefore, to enhance the precision level of clock synchronization and to decrease the energy consumption, a new time synchronization scheme is proposed by exploiting the mobility of the nodes. By introducing the mobility to some of the nodes in WSNs, we can improve the clock synchronization accuracy level as well as reduce the energy consumption by reducing the communication range.
The proposed model consists of a set of stationary sensor nodes and a set of mobile nodes. These mobile nodes have a locomotion capability which allows them to move throughout a sensing region [
・ Long Network Life Time;
・ Improved Performance;
・ Accurate Clock Synchronization;
・ Additional Channel Capacity.
The mobility pattern of the mobile nodes defines the movement of mobile nodes and their velocity, location and acceleration changes over a period of time. In Controlled Mobility Time Synchronization model (CMTS), the movement of a mobile node is not arbitrary but is controlled and the mobile nodes move on a specific path and broadcast the time messages to the sensor nodes which fall in the range of the mobile nodes. The objective of the Controlled Mobility Time Synchronization (CMTS) is to reduce communication range to ensure reliable communication between mobile nodes and sensor nodes.
In CMTS model, each mobile node (MN) moves on a specific path as shown in
Following assumptions are made:
・ There is a unique ID for each node in the WSN;
・ During the initial synchronization phase, there is no fault in the mobile nodes and sensor nodes;
・ If any CH fails due to energy, then clustering algorithm immediately selects a new CH;
・ Each mobile node is equipped with GPS system and adequate energy;
・ The network is consists of N sensor nodes deployed in a square field with flat topology;
・ Sensor nodes can directly communicate (one hop communication) with mobile node;
・ Communication within the square field is not subjected to multipath fading. The communication channel is symmetric;
・ Nodes are left unattended after deployment and battery re-charge is not possible.
Calculation of the controlled path for the mobile nodes is based on the communication range of the nodes. The optimum number of controlled paths for the sensing field is defined by following equation:
The path is defined in such a way that mobile nodes directly communicate with the sensor nodes. The number of paths in a row is calculated as
where
Similarly x-axis and y-axis values of the square in second row are defined by increasing the value of i.
Similarly x-axis and y-axis value of the square for next row can be obtained.
1. While all SNs are not synchronized do
2. MNj selects destination point d (where d = 3R/2)
3. MNj starts moving on controlled path towards destination where 0 ≤ V ≤ λ
4. if (MNj location = Xend or Yend) then (j = 1 to k where k is number of MNs)
MNj changes its direction by 90 degree left or right;
5. While (MNj is moving) do
6. MNj broadcasts (Syn_message, ti,
7. MNj waits for reply;
8. if SN receives (Syn_message, ti,
send reply (Syn_reply, ti, ti+1, ti+2) to MNJ
9. if (MNj reaches its destination) then
Halt for
Repeat step 7 to 9
10. if (MNj receives (Syn_reply, ti, ti+1, ti+2) from SNs) then
MNj broadcasts (Syn_pkt, ti, ti+1, ti+2, ti+3,
11. if (SNs receives (Syn_pktti, ti+1, ti+2, ti+3,
calculate α1, α2, θ;
synchronize the node;
12. endif
13. endif
14. endif
15. endif
16. end while
17. endif
18. end while
19. Repeat step from 2 to 19 in next synchronization phase
As shown in
In the proposed algorithm, mobility of nodes is considered. Due to mobility, pair wise synchronization could not find out the precise time difference between two nodes due to varying distance between two nodes. In the proposed algorithm, mobile node (MN) moves with some known constant velocity V and sensor node exchanges the time stamp messages as shown in
Here
where
where
where
Finally, the sensor node can set the time with mobility factor between MN and static node by using the following equation.
When the mobile node is synchronizing the sensor nodes using the synchronization time ts, then sensor nodes set their time using the following equation.
Number of halts required by a mobile node, while moving on controlled path depends on the value of d which can be derived as:
After obtaining the value of d, it is easy to compute the number of times mobile node halts during one loop of the path by using the following equation:
In proposed Controlled Mobility Time Synchronization Model, mobile nodes move along the fixed path and broadcast time synchronization message, which will be received by all the nodes within the communication range. This message contains the time, velocity, direction of the mobile node and acceleration.
Initially the mobile node will be in a state of rest, therefore, initial velocity will be zero and when the mobile node starts moving then it reaches to a perfect velocity
where
Since it is known that how much time a mobile node is taking to obtain the perfect velocity and also know the time to reach in halt stage. Now the time spent by the mobile node at constant velocity depends on the distance travelled by the mobile node from perfect velocity to zero velocity and can be calculated as follows.
where
Now, the total time for a mobile node to complete one loop along the controlled path can be calculated by the following equation:
whereas
Halting of a mobile node increases the round trip time of the mobile node along the controlled path. If the size of the controlled path is long then there may be some sensor nodes keep on waiting for synchronization message from mobile nodes in initial phase of synchronization. Therefore, keeping in view the problem halting time after moving distance d may be removed and the mobile node moves without halt with a perfect velocity (i.e. constant velocity). Total time taken by a mobile node to take one round of the controlled path is given by following equation:
While implementing the proposed Controlled Mobility Time Synchronization Algorithm, it is important to find out the optimum number of mobile nodes required to synchronize the entire sensing field. Range of the mobile node is used to find out the optimum number of nodes required to synchronize the total area of the wireless sensor network. Following equation provides the optimum number of mobile nodes.
where X and Y are the dimensions of the sensing region and R is the communication range of mobile node. This can be verified by using
The purpose of this section is to analyze the performance of the proposed Controlled Mobility Time Synchronization modes using simulation. CMTS is using 4 mobile nodes during the simulation. The sensors are deployed over a square sized area of 300 m × 300 m with adjustable communication range and fixed sensing range. Simulation parameters are shown in
Initially the performance of proposed algorithms has been compared with each other, and then compared with Tow-hop time synchronization (TTS) [
Simulation results in
Parameter | Value |
---|---|
Number of nodes | Fixed/Variables |
Simulation area | 300 m ´ 300 m |
Initial energy per node | 2 J |
Transmission range | 75 m |
Coefficient of free space model | 10 pJ/bit/m2 |
Coefficient of free space model | 0.0013 pJ/bit/m4 |
50 nJ/bit | |
Data transmission rate | 250 kbps |
Maximum speed of MN (Vmax) | 2.5 m/s |
Initial clock skew | 5 ppm |
Initial offset | 8 ppm |
The objective of another experiment is to compare the energy consumption of the proposed protocols between CMTS and other existing protocols. It can be seen from
The objective of this experiment is to compare synchronization accuracy of CMTS with other protocols in terms of number of hops. Simulation results in
Time synchronization in wireless sensor networks is challenging problem due to high delay variability and resource constraints of nodes. In critical applications, time synchronization is very important to know the exact time of the event. This paper proposes a mobile node based time synchronization algorithms named CMTS for wireless sensor networks. In CMTS, mobile nodes move on a fixed path with certain velocity. Synchronization process is initiated by mobile node and it synchronizes sensor nodes falling in its communication range. The objective of mobile nodes is to reduce the communication overhead for sensor nodes and to do synchronization with higher precision. Simulation results show that the proposed algorithm is able to save the energy, reduce synchronization time and improve synchronization accuracy.
Gautam, G.C. and Kaushal, N.C. (2017) Controlled Mobility Time Synchronization for WSNs. Wireless Sensor Network, 9, 1-15. http://dx.doi.org/10.4236/wsn.2017.91001