An Enhanced Aloha based Medium Access Control Protocol for Underwater Sensor Networks

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1 An Enhanced Aloha based Medium Access Control Protocol for Underwater Sensor Networks Abdul Gaffar. H 1, a and P.Venkata Krishna 2,b 1 School of Computer Science and Engineering, VIT University, Vellore, India. 2 Department of Computer Science, Sri Padmavathi Mahila Visvavidyalayam, Tirupathi, India. Abstract. In Underwater Acoustic Sensor Networks (UASN) designing Medium Access Control (MAC) protocol is a challenging issue. This is due to low bandwidth and high propagation delay in UASN. This paper proposes a MAC protocol for UASN called Aloha with Virtual Back off Algorithm (Aloha- V). The VBA approach is the collision avoidance mechanism for wireless networks. In this proposed work we use VBA with non-counter sharing. The Aloha-V MAC protocol reduces collision probability thereby improves the system throughput by restricting the number of transmissions carried out by the node. The Aloha-V protocol is compared with the existing underwater Aloha protocol. The simulation results show that the proposed method reduces the number of collisions and significantly increases the throughput compared to classical underwater Aloha protocol. Keywords: Underwater Acoustic Sensor Networks, MAC, Aloha, VBA, collision. INTRODUCTION In recent times, Underwater Acoustic Sensor Networks (UASN) has been playing an important role in underwater communications. UASN supports many applications like undersea exploration, oceanography data collection, water pollution monitoring, disaster prevention and so on. Underwater sensor networks are a collection of sensors. Using this, vehicles are deployed to perform cooperative observation tasks over a given region [1]. Compared to terrestrial sensor networks, UASN is different in terms of low bandwidth, high propagation delay and high bit error rate. So designing a MAC protocol for UASN is a challenging task. The main task of MAC protocol is to avoid data packet collision and reduces the number of retransmissions to improve throughput. In general, MAC protocols are classified into contention free and contention based protocols. The contention free protocols like Time division multiple access (TDMA) [2], Frequency division multiple access (FDMA) [3] are not suitable for UASN due to large and random deploying setting with distributed control and burst data [4]. On the other hand, the contention based protocol- Carrier sense multiple access (CSMA) too does not perform well in UWASN [5]. Compared to above mentioned MAC protocols, Aloha [6] is simple and less complex. Aloha is a contention based MAC protocol. Due to long propagation delay and low bandwidth in acoustic signal, the Aloha MAC is more suitable. The problem with Aloha is that the packet collision rate is high. VBA [7] proposed the solution for packet collision in a wireless network. In VBA, each node is limited to access the channel. The number of contenting nodes is reduced and the collision chances also reduced in VBA. So it improves the throughput of the network. Hence, in this paper, we propose Aloha-V, which reduces the data packet collisions and improve throughput. The rest of the paper is organized as follows. Section 2 briefly explains the related works on MAC protocols for UASN. Section 3 gives a system model for the proposed method and section 4 discusses the proposed MAC protocol. Section 5 shows the results and finally section 6 concludes this paper. RELATED WORK The existing MAC protocols for terrestrial sensor networks are not suitable for UASN due to the peculiar characteristic of acoustic signals. In this section, we discuss the Aloha and its variant protocols. In pure Aloha, when the nodes have a packet to transmit, it is sent immediately. If the receiver successfully receives the packet it sends an acknowledgement to sender. And when the sender receives the acknowledgement, the transmission is successful. If more than two nodes transmit a packet at the same time a collision occurs, which will lead to retransmission. The other each node can send the packet only to the beginning of the slot. In [8], simple mathematical analysis and simulations for pure Aloha and slotted Aloha in underwater situations are presented. The results show that the performance of Aloha and S-Aloha is same under heavy traffic. In [9], the paper proposes two variants of Aloha protocols, namely, Aloha with Collision Avoidance (Aloha CA) and Aloha with Avoidance Notification (Aloha AN). Both the protocols utilize overhearing concept. The packet has two segments- one for header and other data. The header segment contains essential information. Each node maintains a local database table to store overhearing information from neighboring nodes. This table is utilized by the node, before transmitting to ensure that no collision with other nodes occurs. With Aloha-CA it needs less amount of memory and control messages. In Aloha AN, the node first transmits small advance notification packet (NTF) which has a normal header segment. The lag time is the waiting time between the NTF and data packet and is used by the node to extract information from multiple NTF packets. This is useful for the node to avoid collisions and the Aloha AN needs more resources. The combination of Aloha and handshaking protocol for underwater sensor networks is presented in [10]. The proposed MAC protocol is utilized to explore the advantages of both the Aloha and handshaking 7251

2 protocols. The authors present a mathematical model to reduce end-to-end delay and increase the throughput. The authors in [11], consider the advantages of Aloha and Multiple Access with Collision Avoidance (MACA) protocols. If the load is low, it uses aloha method. If the load is high, it uses MACA protocol. Hence this protocol performs better than Aloha protocol. In [13], the authors investigate the impact of spatial temporal uncertainty in underwater networks. From the analysis given, it states that the S-Aloha is significantly affected by the spatial dimension of uncertainty. So they propose a guard band to add to transmission slots in S-Aloha to improve the performance. In [14], the authors propose a Backoff tuning for Aloha protocol. Backoff is the waiting time also known as Contention Window (CW). The CW plays an importance role in collision avoidance. Here, the Backoff tuning mechanism is utilized to maximize the throughput of Aloha protocol. Each node in the network dynamically adjusts their CW to reduce collision and improve throughput of the network. Authors of [15] explained the Aloha protocol performance in underwater sensor networks. It also presents the theoretical analysis of frame collision probability and throughput efficiency. This paper proposes two variants of enhanced S-Aloha protocols to reduce the impact of propagation delay in underwater sensor networks. In [16] the authors have proposed an analytical model for Aloha protocol in multi hop underwater acoustic networks. The proposed model is identified and validated here. The paper [17] discusses the busy terminal problem in underwater sensor networks. The authors developed a theoretical model for Aloha to analyze the successful transmission probability. And also develop a nodal throughput model which can increase the nodal throughput. In [12] the authors present a variation of Aloha called slotted Aloha. In this paper, the authors presented a model for analyzing slotted Aloha variants. SYSTEM MODEL The goal of the defined method is to reduce collisions by controlling the number of transmission attempts by the nodes. The number of transmission attempts is controlled by the sequence number to access the channel. The system is set up in such a way that all the nodes approach a sink node in one hop communication. Assume there are n nodes and these nodes are communicating with the sink node as shown in Figure 1. Figure 1. System model Let us evaluate the collision probability of Aloha-V protocol. Let be the probability of successful access to the channel. Then the probability of unsuccessful access to the channel is (1) Let be the sequence number and limits the number of successful and unsuccessful attempts. The relationship between probability and sequence number is given in [7] as, And From equation (2), increasing the sequence number, increasing the successful channel access and equation (3), increasing the sequence number, reduces the unsuccessful channel access. ALOHA-V: ALOHA WITH VIRTUAL BACKOFF ALGORITHM In this section, we present an overview of Aloha for UASN and VBA. Then we propose our MAC protocol for UASN. Overview of ALOHA Aloha [18] is a contention based MAC protocol for UASN. In Aloha, the sending process is as follows: the node generates a packet; it sends immediately and waits for an ACK packet. If it receives the ACK packet, then the transmission is successful. In case the node does not receive an ACK packet, there is a chance for collisions or link errors. In this case, the node retransmits the packet. If the retransmit reaches the maximum limit, the node drops the packet. In the receiving process, the node listens to the channel for data packet and sends the ACK packet as a reply. (2) (3) Overview of VBA The VBA methodology in [7] is used to improve the throughput as well as reducing the number of collisions in wireless networks. In VBA, the nodes have limits to access the channel, so the number of collisions is reduced. Each node in VBA maintains a counter, when the nodes want to transmit 7252

3 the data it senses the channel, and if the channel is free, the node checks the counter value and if it is less than or equal to the sequence number, then the channel is accessed and increment the counter. Otherwise the channel access is deferred, and the counter is incremented. Two variances of VBA are presented: one is a VBA-CS (virtual backoff with counter sharing) and VBA-NCS (VBA with non-counter sharing). [7] Proposed work The Aloha is a simple MAC protocol. The main drawback of the Aloha is that there are more data packet collisions. The lack of collision avoidance method in Aloha leads to increasing in collisions. The advantage of VBA method is to reduce the number of collisions. Hence we propose a new MAC protocol for UASN which is the combination of Aloha with VBA method. In Aloha-V (see algorithm 1), each sensor node in the underwater environment maintains a counter for counting the number of times it transmits the data. Initially, all the node s counter values are zero. The sequence number is used to limit the number of transmissions to the nodes. The value of the sequence number is an integer value. If a node is ready to transmit a packet, first it checks its counter value with the sequence number. If the counter value is less than or equal to the sequence number, then the node sends the data packet and waits for acknowledgement (ACK) packet from the receiver. Algorithm 1: Aloha V Input: Sequence Number(k), Number of nodes(n); Output: Channel access for i in 1 to n do initialize counter[i] to 0 end for for i in 1 to n do while channel access[i] do if counter[i] is not equal to k then access channel; else defer access; end if increment counter[i] end while end for Simulation settings We study the performance of Aloha V protocol using Aqua Sim [18], a NS-2 based simulator. We compare the performance of Aloha-V protocol with Aloha protocol. The nodes employ half duplex mode with omnidirectional antenna for data transmission. We deployed all nodes in a random manner and with one hop distance. Each node can hear the transmission of other nodes. In our simulation, one node acts as a receiver while the remaining nodes are senders. The receiver node is placed in the center of the network environment. All the sender nodes have data to transmit to the receiver. The data packet generation of nodes follows the passion process with data rate λ. The simulation parameters are given in table I. Simulation parameter Channel Propagation Number of nodes Topology area Propagation speed Transmission range Sequence number Transmission power Reception power Table 1: Simulation Parameters Value Underwater channel Underwater propagation m * 500m 1500 m/s 100m 5 3 W 0.75 W Performance Metrics In our study, we consider the following metrics: throughput, and number of packet collision. The throughput is the ratio of total number of bytes received successfully to the simulation time. The number of collisions is the total number of packet collision during the simulation. Simulation results Effect of varying the Traffic In this set of simulations, the traffic rate is varied from 0.02 to 0.2 packets per second (pkt/s). The simulation time is 1000 s and the modem bit rate is 10kbps. The receiver successfully receives the data packet, it replies with an ACK packet. The transmissions did successfully, the node increment its counter value. Otherwise, the node waits for some random amount of time, and then resends the packet up to the maximum limit. Once the limit is reached, the node drops the packet. The number of transmissions per node will be limited by the sequence number K. PERFORMANCE ANALYSIS In this section, we analyze the performance of Aloha V protocol and compare Aloha-V protocol with existing underwater Aloha protocol. Figure 2: Throughput versus Traffic rate 7253

4 The throughput performance is given in Figure 2. This graph is plotted different traffic rate with throughput. The results depict that the throughput of Aloha-V is higher than Aloha. The traffic rate is 0.18 (pkt/s) both the Aloha-V and Aloha getting highest throughput. Figure 5: Number of collision versus Bit rate In Figure 5, the graph plotted is the bit rate versus the number of collisions. In Aloha V, the collision is controlled by sequence number. From the result, it is known that Aloha V reduces the number of collisions compared to Aloha. Figure 3: Number of collisions versus Traffic rate The Figure 3 shows the performance of traffic rate with number of collisions. The absence of a collision avoidance mechanism in Aloha, the number of collisions increased. From the graph, the number of collisions in Aloha-V is very less compared to Aloha. At traffic rate 0.2 kbps the number of collisions in Aloha is 4099 and Aloha V is CONCLUSION This paper describes the collision avoidance technique in underwater sensor networks. We proposed Aloha V protocol which reduces collisions during data transmission. This protocol executes Aloha protocol with existing Virtual Backoff Algorithm with non-counter sharing. The proposed method reduces the number of collisions that occur in UASN and to increase the throughput of the network. From the simulation results, the Aloha V performed better in terms of high throughput and low packet collisions compared to the existing Aloha protocol. REFERENCES Figure 4: Throughput versus with Bit rate Effect of varying the Bit rate The graph plotted with Bit rate versus throughput is given in figure 4. In this simulation, the bit rate is varied from 20 to 65 kbps to evaluate the performance of the proposed protocol. It is depicted that the throughput of Aloha V is outperform with Aloha protocol. The sequence number is limited to access the channel so the throughput of the Aloha V is increased compared to Aloha. [1] Akyildiz, I. F., Pompili, D., & Melodia, T. (2005). Underwater acoustic sensor networks: research challenges. Ad hoc networks, 3(3), [2] Chen, Y. D., Lien, C. Y., Chuang, S. W., & Shih, K. P. (2011, June). DSSS: a TDMA-based MAC protocol with dynamic slot scheduling strategy for underwater acoustic sensor networks. In OCEANS, 2011 IEEE-Spain (pp. 1-6). IEEE. [3] Cheng, X., Qu, F., & Yang, L. (2011, August). Single carrier FDMA over underwater acoustic channels. In Communications and Networking in China (CHINACOM), th International ICST Conference on (pp ). IEEE. [4] De, S., Mandal, P., & Chakraborty, S. S. (2011). On the characterization of Aloha in underwater wireless networks. Mathematical and Computer Modelling, 53(11), [5] Xiao, Y., Zhang, Y., Gibson, J. H., Xie, G. G., & Chen, H. (2011). Performance analysis of ALOHA and p-persistent ALOHA for multi-hop underwater acoustic sensor networks. Cluster Computing, 14(1),

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