Finding Optimized Transmission Power for Clustering of S-MAC Protocol in WSNs Anusorn Chungtragarn and Phongsak Keeratiwintakorn Department of Electrical Engineering King Mongkut’s Institute of Technology North Bangkok, Thailand Email:
[email protected],
[email protected]. Abstract - By using a sleep schedule technique, S-MAC can reduce the energy consumption in a wireless sensor network; thus, breaking the network into a virtual cluster. Using a different sleep schedule in each cluster causes problems in the S-MAC protocol. This paper aims to investigate the appropriate cluster size for the wireless sensors networks (WSNs) in terms of the average node number per virtual cluster. This is to show the effect of cluster size on the network performance parameter such as latency and energy consumption. The experiment results are obtained using the NS-2 simulator.
I.
INTRODUCTION
Wireless Sensor Networks (WSNs) are known as very limited capacity ad hoc networks, where a node is composed of a low-computation CPU, a low power transmitter, and limited battery energy which is our concern in this paper. Energy of each node is significantly spent for running a circuit for computation and for transmitting data. Therefore, WSNs are normally working together as a large group that consists of hundreds or thousands of nodes in order to minimize the energy used for computation and transmission. To further reduce the energy usage, nodes are divided into clusters. Only nodes in a cluster “wake up” at the same time to convey information while other nodes “sleep” to save energy. Normally, while waking up, a node spends more energy than while sleeping. However, the number of clusters becomes an issue of energy saving. In a large cluster, a great number of nodes would wake up, and this would cause many nodes spend energy while waking up. However, with a large number of nodes, transmission range can be short or more effective, and this would reduce the energy for transmission. Therefore, the problem statement is what should be the optimal size of a cluster to save most of the energy. In this paper, we investigate the energy usage and delay in transmission as our trade-off factor to create an optimal size by adjusting the node transmission power. In our work, we use S-MAC as a protocol to create a cluster and a schedule for wake-up and sleep period for nodes. S-MAC or Sensor-MAC is a well-known medium access control protocol for WSNs [1]. Basically, it is designed to reduce the sensor node energy consumption and to extend the network lifetime by reducing the number of events that could waste energy by pressing nodes to sleep without any loss of data from transmission. The common events in wireless networking that could waste the energy are (i) the collision of data packets, (ii) the overhearing – sensor nodes receive the packets belonging to other nodes, (iii) the control packet overhead – the overhead is used to control sensor node communication, and (iv) the idle listening – sensor
nodes stay in idle, and no data packet is transmitted at this state. S-MAC assumes that WSNs are low traffic load networks. Sensor nodes stay idle for a long time, and start transmitting a data packet when they have detected particular events from sensing. Therefore, the sensor nodes do not need to listen to a communication channel all the time. S-MAC introduces a sleep scheduling algorithm that sensor nodes sleep for most of the time and wake up only to send data and to synchronize with networks. Thus, one S-MAC cycle time is divided into a sleeping period and a wakeup period. The wakeup period consists of SYNC period, RTS/CTS period, and data transmission period. In every SYNC period, sensor nodes broadcast a SYNC packet to neighbor nodes. Nodes also use the receiving SYNC packets to synchronize with neighbor nodes in the network. The SYNC packet contains sender’s next sleeping time which tells receiving nodes when the next transmission would take place for the next cycle. The RTS/CTS period is used to request for transmission and to response with a permission to transmit. Then, sensor nodes can send or receive data. The sleep schedule starts when sensor nodes are deployed to the workspace. Then every node keeps listening to the channel for a random time period from their neighboring nodes for a SYNC packet. If a sensor node does not receive any SYNC packet at the end of the period, it will generate a sleep schedule, and then broadcast the schedule within a SYNC packet. Sensor nodes receiving a SYNC packet during the listening period will use the sleep schedule attached in the SYNC packet. The node which generates a sleep schedule is known as a synchronizer node, while the node which uses the sleep schedule is called a follower node. The sleep schedule is randomly generated, depending on the first random listening period. Thus, there would be many sleep schedules, and they are later arranged into several virtual clusters, each of which has a different sleep schedule. In the case where a node receives more than one SYNC packet, the node will have more than one schedule and it is called a border node. This node has to be active in every schedule because it works as a joint between each virtual cluster. Being active, this node consumes more energy than the other two nodes—a synchronizer node and a follower node. The sleep schedule in S-MAC can reduce the network energy consumption by introducing a low duty cycle for each node. By using the sleep schedule, S-MAC can trade off the latency with the energy saving. However, S-MAC performance would be decreased when using it in a network
that does not match with the S-MAC network assumption, for example, a quick response network with a high duty cycle such as an emergency response network or the first responder network. Thus, S-MAC still has such disadvantages such as large latency or uncontrollable sensing data delivery time. II.
RELATED WORK.
S-MAC has been developed to minimize energy consumption as much as possible without any control of delivery time in a network. Using the sleep schedule induces the delay time to the network while the sleep delay proportionally increases to the number of data hops which is also proportionally increase to the number of clusters. The energy consumption model for S-MAC has been proposed to help the protocol designer [2]. From this model, we can estimate S-MAC network energy consumption in different parameters such as a duty cycle and a packet inter-arrival period. However, the large latency is still the biggest disadvantage of S-MAC. S-MAC algorithm has been improved to solve the latency problem. The adaptive sleeping algorithm is added to original S-MAC [3]. Its main idea is to wake up the node to receive the control overhead packet (RTS, CTS) to pass on to a next hop node. This algorithm can reduce the sleep delay time by increasing a number of nodes to hear the control overhead packet of approximately a half of nodes in a network. AC-MAC has been developed from the S-MAC concept with an algorithm to reduce the sleep delay time [4]. The difference is that AC-MAC can adapt the duty cycle according to the traffic load. AC-MAC uses the packet queue at the MAC layer to make a decision whether congestion occurs, and then adapts the duty cycle according to the decision. This method can save energy more efficiently than the S-MAC. Some studies extend the simulation to the real topology of sensor networks and proposed an algorithm to save even more energy [5][6]. The virtual cluster in the S-MAC network is a cause of the border node problem. The border node is active for every schedule it knows; thus, consuming more energy, and later becoming a dead border node. In a large WSN, there would be more border nodes; hence, the overall energy-efficient performance could then be degraded. Additionally, a black hole could occur when border nodes die. Global Schedule Algorithm (GSA) has been introduced to solve the border node problem [7]. GSA forces every node in the network to use the same sleep schedule. GSA uses the schedule age to specify which sleep schedule should exist in a network. The most age sleep schedule has been selected as the only one to be used in the network. Schedule Unifying Algorithm (SUA) also forces nodes to use a unique schedule, that is, SUA uses the schedule which is generated from the highest priority address synchronizer as a unique schedule in the network [8]. III. S-MAC SIMULATION EXTENSION. We extend S-MAC simulation to study the effect of transmission power to the S-MAC network performance by using NS-2 network simulator [9]. The transmission power is
the critical issue of S-MAC network energy consumption. The more the transmission power, the more the energy consumption. In solving the latency problem, we have to sacrifice the energy since it is inversely proportional to the latency. The energy*latency product is used to indicate the optimum trade-off [10][11]. Increasing transmission power results in the longer transmission hop based on our assumption that the latency performance would be better because it can reduce the number of data hops to the destination. These cause not only the hop range but also the cluster size to be changed. The coverage area of transmission is calculated from Two-Ray Ground propagation model [12]. Pr = PtGtGr(ht)2(hr)2 / d4L
(1)
The Pt and Pr are the transmitting and the receiving power, and the Gt and Gr are the transmitting and the receiving antenna gains, respectively. The ht and hr are the transmitting and the receiving antenna height, and d is a propagation distance and L is a loss factor. From (1), more transmission power results in a longer propagation distance, and a larger cluster size. Additional performance issue to be examined in our paper is the effect of the cluster size on the S-MAC network performance. The more the number of virtual clusters, the more the border nodes are inside the network. Our simulation scenario employs the 2,000 m x 2,000 m workspace with the 7 x 7 grid topology of sensors, each of which is separated with 100 m in x-axis and y-axis as shown in Figure 1.
Figure 1. Simulation topology
Using the adaptive sleeping S-MAC and varying the sensor node transmission power, we observed how the cluster size changed. We set up the traffic load with CBR from node 0 to node 48 to measure the end-to-end delay time, the successful rate, and the average energy consumption. The other simulation parameters are shown in Table I. Table I Simulation Parameters. Parameter Value(s) NS-2 Version 2.29 Simulation Time. 3000 sec. Number of Nodes. 49 Topologies. 2000m x 2000m flat grid Node Initial Energy. 100 J. Antenna Height. 1.5 Transmission Power. 24 – 541 mW. Propagation Model Two- Ray Ground Model Routing Protocol. DSR. Application CBR with 7.0s packet arrival period
IV. SIMULATION RESULTS. In our simulation results, the S-MAC virtual clusters are randomly generated; depending on which sensor nodes win the contention of the channel, and broadcast the SYNC packet first. We estimate the transmission distance from the propagation model, and know the number of nodes receiving the packet for each transmission power level. Consider Node 24 in Figure 1, with 24mW transmission power, Node 24 can reach Nodes 17, 23, 25 and 27, which are the expected number of nodes per cluster. If we expect the larger cluster size, we have to increase the transmission power. There are seven transmission power levels each of which is calculated according to more expected numbers of nodes, which are 8, 12, 20, 24 and 28 nodes per cluster. Figure 2 shows the average number of nodes per cluster according to different transmission power, showing with 95% confidence interval. By plotting the result versus the transmission power, we can see the average node number increase when transmission power increases. Depending on S-MAC algorithm, some virtual clusters may have node number more or less than the expected number of nodes in a transmission radius which is calculated from the propagation model. The result clearly shows that if we want a bigger cluster we should increase the transmission power. The effects of increasing transmission power to network performance are shown in Figure 3 and Figure 4.
Figure 2. Cluster Size
Figure 4. Successful Rate
In Figure 3, by increasing transmission power, the network tends to have lower delay. The reduction of the delay could be because the higher transmission power probably reduces the number of hops along paths. Figure 4 shows the effect of transmission power to the successful rate. It seems that higher transmission power could make more successful rate because of the shorter path. However, we cannot conclude the network performance from this figure because the successful rate also depends of the S-MAC sleep schedule. From analyzing the raw simulation result, we see that most of dropping packets occur at the send buffer queue. Because of using a long sleep period, a node may establish a route too slow; therefore, the packets are dropped due to the timeout or the buffer overflow. If routes can be establishes faster, we can send data packets with less delay and more successful rate. The average delay time, the average successful rate, and the average energy consumption are metrics used for S-MAC performance evaluation. To study the impact of the transmission power or the virtual cluster size on the three important performance metrics as mentioned before, we propose a trade-off factor. The trade-off factor (K) is calculated by means of average end-to-end delay time (l), average energy consumption (E) and a successful rate (S). If the network can completely pass the packet along the route, the route node should consume more energy since the packet is dropped at the queue buffer or when a collision occurs. Our assumption is the average energy consumption proportional to the successful rate. E∝S E = K1S
(2) (3)
Not only the successful rate but also the average energy consumption are inversely proportional to the average endto-end delay is calculated. If the S-MAC network can send packets from a source to a destination with low latency, the S-MAC network has to reduce the sleep time, which means the S-MAC network consumes more energy. E ∝ 1/l E = K2/l Figure 3. Average end-to-end delay
(4) (5)
Combining the effect of the average end-to-end delay and the successful rate by multiplying (3) and (5), then we have the trade-off factor as follows. E2 = KS/l K = E/(S/l)1/2
(6) (7)
The trade-off factor indicates the network performance in terms of the average end-to-end delay time, the successful rate and the average energy consumption. If the trade-off factor is lower, it means we can obtain a better network performance since it consumes less energy, has a lower average end-to-end delay and achieves more successful rate. Data collected from the simulation are plotted on the tradeoff constant versus transmission power as shown in Figure 5.
Figure 5. Trade-Off Factor Performance
For the trade-off factor, we expect to obtain the lowest trade-off factor from transmission power levels. With the longer transmission distance, we can transmit packets with fewer hops, which mean the lower average end-to-end delay. The trade-off factor indicates the balance between the energy consumption and the average end-to-end delay. From the results in Figure 5, by increasing transmission power we could make the average end-to-end delay time lower, but consuming more energy. In terms of energy-efficient, it is not optimization. With a very large cluster size, node cannot transmit packets much faster as expected because of more collisions occurred. The SYNC packet collision rate increases when the cluster size is larger. Sometimes, it makes sensor nodes lose synchronization with their neighboring nodes. V. CONCLUSION AND FURTHER WORK In conclusion, increasing transmission power could change the average number of nodes in a cluster, or making a cluster size larger. It is also shown that by increasing the transmission power the trade-off factor is increased. It means less delay and more energy consumption. However, this advantage is exponentially reduced as the transmission power is increased due to more packet collision in a larger cluster of nodes. Using the trade-off factor we can find the optimize transmission power for our topology that is the
lowest point of trade-off factor, we obtain 29.114mW transmission power. For our future work, modification of the S-MAC algorithm to support multiple sleep schedules will be studied because each schedule corresponds to different transmission power. Swapping the multi-sleep schedules in S-MAC to match with the traffic load is also suggested. Lastly, the collision problem in a large virtual cluster is another worthwhile issue as it affects the performance of energy efficiency and latency. REFERENCES [1]
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