(MAC) protocol for sensor networks that provides a high ... One of the chief requirements in pro- ... protocol involves a clustered-hierarchical network organi-.
An Adaptive Low Power Reservation Based MAC Protocol for Wireless Sensor Networks Saurabh Mishra and Asis Nasipuri Department of Electrical & Computer Engineering University of North Carolina at Charlotte Charlotte, NC 28223-0001 Abstract A sensor network is a collection of wireless sensor nodes, each equipped with embedded processor, memory, and a wireless transceiver, that may be designed to perform distributed sensing and tracking operations. The sensor nodes use ad hoc networking principles to communicate amongst themselves. However, limitations of battery power in the sensor nodes and the bursty and correlated nature of traffic in the network introduce additional constraints for designing the communication protocols for sensor networks over that of traditional ad hoc networks. This paper proposes a reservation based medium access control (MAC) protocol for sensor networks that provides a high probability of success in packet transmissions, low average energy consumption, and adaptability to the traffic requirements to maximize the data throughput.
1 Introduction Advances in micro fabrication and wireless technologies have resulted in the development of small low-cost wireless sensors that have embedded processor, memory, and low-powered wireless transceiver on the same chip. Each of these “smart” sensor nodes can perform limited on-board processing and wireless networking functions in addition to detecting various kinds of signals such as temperature, light, movements, magnetic and acoustic signals. The potential of forming a wireless ad hoc network of a large number of wireless sensors has generated significant interest in using such networks for unmanned distributed surveillance operations. Applications include environmental monitoring, industrial process monitoring and control, robotics, intrusion detection, and target detection and tracking in tactical environments [9, 2, 4, 1]. The locations of sensor nodes are usually not engineered or predetermined. Nodes in the network have to coordinate among themselves to set-up an ad hoc network for efficient and reliable communication. However, sensor networks have unique characteristics and communication requirements which make them different from traditional ad hoc networks. One of the chief requirements in pro-
tocols for sensor networks is energy conservation. Sensor nodes are battery powered. Because of inaccessibility and the large number of sensors, it is difficult to replace or recharge their batteries. In order for sensor networks to be viable, battery power must be conserved for maximizing their lifetime. An additional difference is that the traffic in sensor networks is highly sporadic and correlated in nature. While for most of the time sensors may remain dormant, a sudden activity may trigger a large number of nodes within close proximity to become alive and start transmitting their observations. It is desired that optimum network-efficiency is maintained under this bursty traffic condition. To address the issue of energy conservation, many MAC protocols have been proposed in recent times that are based on TDMA. TDMA is an ideal choice in sensor networks because a node can sleep when it is not involved in transmission or reception. Appropriate scheduling of transmission times of different nodes can resolve collisions and provide a high probability of success. However, design of optimum TDMA frame structure is an issue which needs to be addressed. If the TDMA frame size is too large in comparison to the number of packets to be transmitted, the scheme will result in low channel utilization. On the other hand, if the frame size is too small, then it will not be possible to accommodate all the transmissions. In this paper we propose a MAC protocol which addresses both of the main requirements of sensor networks i.e., low energy use, and adaptation to traffic. The proposed protocol involves a clustered-hierarchical network organization and uses a TDMA-like frame structure with contention based slot reservation, a centralized schedule establishment process, and slotted data packet transmission. Our protocol includes a practical scheme for adapting the frame size in order to achieve high channel utilization as well as high probability of success for data packet transmission. Extensive simulation experiments are performed to evaluate the efficiencies of clusterization and adapting the frame size in terms of probability of success, throughput and energy consumption in the network.
2 Related Work In recent times many reservation-based energy efficient MAC protocols have been proposed for wireless networks [6, 7, 5, 10, 3]. These reservation based protocols mainly employ TDMA as the primary channel access method. In [5] a selforganization protocol for wireless sensor networks is proposed. Each node maintains a TDMA-like frame, called the superframe, in which nodes schedule different time slots to communicate with their neighbors. To avoid interference between adjacent links, the protocol assigns different channels to potentially interfering links. A drawback of this scheme is that it takes lot of time in initially configuring the network and rescheduling is necessary whenever a node goes in or out of the network. Also, the scheme has low bandwidth utilization. In [10], each node determines the transmission time of its next packet and piggybacks this information over its current DATA packet. The destination includes this information on its ACK packet, allowing all listening nodes to take note of this schedule. An in-depth analysis on the choice of network model for sensor networks to meet energy-efficiency is provided in [3], in a protocol called LEACH. It compares the following three models: (a) a direct-communication model, where each sensor node sends its data directly to the gateway node, which is assumed to be present within the coverage area of the network,(b) a multi-hop model, where nodes route their data to the sink through intermediate nodes, and (c) a clustered-hierarchy model, where the entire network is assumed to be covered by a number of small clusters, each having a cluster-head. Inside a cluster, the nodes can communicate with their neighbors in a multi-hop fashion and with the cluster-head using a direct (one-hop) link. The authors conclude that the clustered-hierarchy model is more suited for sensor networks as the use of clusters leverages the advantage of a small transmit distance for most nodes which result in low energy consumption per node. LEACH employs a TDMA scheme for communication between the cluster-head and other nodes in the cluster and has been shown to achieve good energy-efficiency when nodes are static and constant. However, if nodes enter or leave the cluster area or change their data rate, it cannot adapt. Performance evaluations of LEACH to determine the effect of clusterization on network throughput were not reported in [3].
3 Protocol Description Our proposed protocol is based on a clusteredhierarchical network organization that is similar to LEACH. Inside each cluster, communication is based on a TDMA-like frame structure. The protocol adapts the TDMA frame size to maintain high channel utilization as
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Figure 1: Superframe structure.
well as desired probability of success for packet transmission.
3.1 Network setup and synchronization After the deployment of the sensors in the field, the network organizes itself into clusters. All the nodes in the network contend to take the role of a cluster-head. Each node waits for a random period of time before transmitting a packet to broadcast a claim to become a cluster-head. The first node to capture the medium by transmitting the packet in a given neighborhood wins and assumes the role of the cluster-head in that neighborhood. A node’s neighborhood is defined by the area covered by the transmission range of a node. All other nodes which hear the transmission before transmitting their own packet accept this node as the cluster head. In case they receive multiple packets for clusterhead claim, they choose the one with largest signal power. This is justified, as the one with larger signal power will come from a nearer node. Inside each cluster it is the job of the cluster head to maintain synchronization for TDMA schedule amongst all the nodes in that cluster. It does so by periodically transmitting a beacon signal that can be heard by all the nodes in the cluster.
3.2 Channel structure We consider a single shared channel that is divided into synchronized superframes, as shown in Figure 1. The boundaries between superframes are marked by epochs, which are maintained by beacon signals transmitted by the cluster-head. Each superframe is further divided in four parts: (a) a short Control slot, (b) an unslotted contentionbased window, called Reservation Request (RR) window, (c) a short Reservation Confirmation (RC) slot and, (d) a slotted Data window, which consists of a number of Data slots. The Control slot is used by the cluster head to broadcast control information such as the length of the next superframe and requests for cluster-head rotation. The cluster-head consumes more power than ordinary nodes and it is desirable to rotate its role amongst the nodes in the
cluster. After taking the responsibility of the cluster-head for a predetermined number of superframes, the clusterhead decides to relinquish its role. It sends this information in the Control slot of the superframe. At the end of this superframe all other nodes in the cluster start contending for their claim to become the new cluster-head. In the RR window, all nodes that wish to transmit Data packets compete to access the medium. This is done by sending a Reservation Request (RR) packet to the cluster-head. An RR packet includes the identities of the source and the intended destination. On successful reception of this RR packet the cluster-head reserves a Data slot for the source to transmit the Data packet in the Data window. A cluster-head can employ different policies for ordering the reservation. It can reserve the Data slots for different nodes in the order it receives the RR packets from them, or it can reserve the Data slots based on priorities, as conveyed by the node in the RR packet. The cluster-head will be awake throughout the RR window for receiving the RR packets. All other nodes wake up only when they need to transmit the RR packet. In the RC slot, the cluster-head broadcasts the Reservation Confirmation (RC) packet, which contains the Data packet transmission schedule of all the nodes whose RR packets were successfully received during the RR window. All nodes wake-up in the RC slot in order to receive the RC packet. Thereafter, each source node goes into a sleep mode and wakes up only in its designated Data slot in the Data window to transmit the Data packet. The corresponding destination node will also do the same and wakeup to receive this Data packet in the corresponding slot. All nodes will sleep during the Data slots not assigned to them for either transmission or reception.
3.3
Collision Avoidance in the Contention Window
A key factor that determines the efficiency of channel usage in this scheme is the probability of success of the RR packets in the contention window. In order to reduce collisions between RR packets, we use a random access scheme in the RR window that is based on an adaptation of nonpersistant CSMA [8], under the constraint of a finite window size for transmission. Consider that the superframe is fixed at size-N, which implies that the superframe can accommodate exactly N number of RR and Data packets, in the RR and Data windows respectively. Any node (source node), which has data to send to another node (destination node) starts by picking a random time inside the RR window to transmit the RR packet. If the medium is found to be free at the chosen time for transmitting the RR packet, it transmits. Otherwise it reschedules the transmission time for this RR packet after some random backoff time and repeats the same procedure. In case a source reaches the end of the RR window without being able to transmit the RR packet,
it makes a fresh attempt for transmission in the following superframe. In case more than one node chooses the same backoff time, there will be a collision and the corresponding RR packets are lost. The choice of the backoff time before making another transmission attempt is an important issue. We propose that this is chosen randomly from the window [0, Tb ] where Tb is a parameter. If Tb is small, more number of nodes can choose the same backoff time, which can result in a collision. If Tb is large, transmission from more number of nodes can be deferred to the end of the RR window and some nodes may never get a chance to transmit due to the expiration of the RR window. We found that smaller values of Tb provide better results when the network is fully connected i.e. the transmit power of nodes is high enough for any node in the network to hear any other node. However, for a cluster-based scenario, where nodes transmit with lower power, better results are achieved by choosing the maximum value of Tb i.e. Tb = RRWndEndTime.
3.4 Adapting the Superframe Size Inside a cluster, the number of contending RR packets may vary due to two reasons. Firstly, the number of active nodes in the cluster may vary due to context-dependent events in the network, such as an appearance of a target signal that triggers transmissions from many sensors at the same time. Secondly, the transmission requirements from individual nodes may also vary. Since both of the above situations are expected to be common in most applications of sensor networks, it is desired that the length of the TDMA frame is made adaptive to maintain efficient channel reservation under all situations. We explore a technique for adapting the superframe length that is based on an estimate of the probability of transmission failures of the RR packets in a superframe. According to this scheme, the superframe is extended when the number of failures of RR packet transmissions exceed a predetermined value (indicating that the RR window is too short) , and it is shortened when there is a very small number of RR transmission failures (indicating, contention window is being under-utilized). To determine the number of RR transmission failures, we use a variable called C ON TENTION I NDEX in each node, which keeps the count of the number of superframes in which a particular RR packet could not be sent. Initially C ONTENTION I NDEX for each node is set to zero. If a node is unable to transmit the RR packet during the entire duration of the RR window in a superframe, the value of its C ONTENTION I NDEX is incremented. This variable is transmitted in the RR packets so that the cluster-head can obtain an estimate of the average contention index (ACI) by taking the average of the C ONTENTION I NDEX values from all RR packets that are successfully transmitted in a given RR window. The pro-
cess of changing the window sizes is implemented by the cluster-head. At the end of each epoch (superframe), the cluster-head takes a decision whether to increase, decrease, or leave unchanged the current value of the superframe size by using the following rule: τ1 < ACI τ2 < ACI ≤ τ1 τ3 < ACI ≤ τ2 ACI < τ3 for K superframes
−→ −→ −→ −→
increase by ∆1 increase by ∆2 no change decrease by ∆3
The cluster-head broadcasts the new value of the superframe length in the Control slot and starts broadcasting the beacon signals according to the new frequency. All nodes set their superframe size and all window lengths accordingly.
4 Performance Evaluation The efficiency of our protocol depends upon the following: (a) effective medium access in the contention period for achieving slot reservation, (b) the effect of dynamicadaptation of the superframe size for minimizing congestion while maintaining a high bandwidth utilization, and (c) the effect of clusterization for increasing the overall network efficiency in terms of throughput and power consumption. In this section we present performance results to demonstrate the effectiveness of these characteristics. The metrics for the performance evaluation here include probability of success of RR packets, average data throughput, and energy consumption. We use the RFMACSIM simulator, which is a discrete event network simulator and has detailed implementations of wireless transmission, channel models, and receiver characteristics. We consider a network of a fixed number of wireless sensor nodes that are located in a uniform grid with inter-node spacing of 5m. The length of the RR packets is 50 Bytes, that of RC packets is 800 Bytes and a Data packet is 1000 Bytes long. For all the cases, we assume that Data packets are generated randomly and independently in each of the sensor nodes. All simulations are run for 500 secs. We assume a single common channel that has a capacity of 1 Mbps.
4.1
Evaluation of adaptation of window size
To evaluate the effectiveness of the proposed frame size adaptation scheme, we evaluate the performance of the proposed MAC protocol, with and without frame size adaptation, while varying the number of active nodes in the cluster with time. All active nodes are assumed to have the same average packet generation rate of λ. Initially, the superframe size is set to size-30 and we start with 30 active nodes in the cluster at T = 0. We then change the number of active nodes in the cluster with time, as follows: 0 ≤ T
