HARDWARE-AWARE COMMUNICATION PROTOCOLS IN LOW ENERGY WIRELESS SENSOR NETWORKS Fanchun Jin , Hyeong-Ah Choi , and Suresh Subramaniam Department of Computer Science Department of Electrical & Computer Engineering The George Washington University Washington, DC 20052 email: jinfc,hchoi,suresh @gwu.edu ABSTRACT As the energy consumption in sensor nodes is domi- nated by the radio transmission/reception circuitry, com- munication protocols must be designed for economy in radio communications. Sensor nodes are generally equipped with short-range radios that have various char- acteristics including data rate, power consumption in transmit, receive, and sleep modes, and time to switch from one mode to another. These parameters can have significant effects on the performance of communication protocols in low energy sensor networks. In this paper, we consider a protocol called STEM that was proposed in [6] and optimize the protocol’s parameters by consid- ering the radio characteristics. I NTRODUCTION Wireless sensor networks are ad hoc networks com- posed of a large number of small sensor nodes and one or more “base stations” which process the data received from the sensor nodes and take necessary control actions. Such networks are typically used for monitoring and surveil- lance functions. A sensor node collects sensory data from its environment, and such data are processed by the node circuitry to determine if an “event” has occurred. Once an event occurs, the node needs to transmit some data col- lected from the event to a base station. The data transmis- sion phase involves sending the data through other sensor nodes that act as intermediate nodes in a multi-hop net- work. The sensor nodes in wireless sensor networks have a limited energy supply that usually cannot be renewed. Thus, the lifetime of a network is constrained by the amount of energy that is spent by the sensor nodes in performing their operation of sensing, processing, and transmitting data to the control centers. Furthermore, the power consumption in sensor nodes is dominated by the radio transmission/reception circuitry. Hence, the com- munication protocols must be designed such that the en- ergy consumed by the radio circuitry is small as possible. There has been much research on energy-efficient proto- cols recently [2], [3], [6]. In this paper, we focus on a protocol proposed in [6] called STEM (Sparse Topology Energy Management). The protocol in [6] works as follows. Each node is as- sumed to be equipped with two radio transceivers – one for data transfer and one for control. Let us suppose that a node A has sensed an event and wants to transfer some data to node B (which is an intermediate node on the path to a base station). Prior to the data transfer, the link be- tween A and B must be established, as shown in Figure 1. In order to conserve energy during the period when a node is only in the monitoring state and does not have any data to transmit or receive, the STEM protocol does the fol- lowing. The data radio is in sleep mode until a link to a neighboring node is activated through a control message exchange between the node and one of its neighbors. The control radio of a node sleeps most of the time and wakes up periodically in order to listen to possible requests from neighboring nodes which want to transfer some data to the node. STEM has two versions called STEM-B and STEM-T, where B stands for beacon and T for tone. In this paper, we will restrict ourselves to STEM-B, and re- fer to it simply as STEM. Node A transmits a series of beacons (control packets containing A’s and B’s MAC ad- dresses) to node B until one of the beacons is received by node B, which then acknowledges the receipt of this packet and turns on its data radio. Upon receiving the ac- knowledgement from B, A turns on its data radio and the 1