A Simulation Framework for Sensor Networks in J-Sim

December 23, 2003

1. Introduction

This tutorial is intended to describe our work in incorporating wireless sensor network simulation in J-Sim. Our work builds up on the autonomous component architecture (ACA) of J-Sim. The tutorial is organized as follows: Section 2 presents the simulation architecture overview; this includes the description of the different types of nodes that are found in a typical wireless sensor network. Section 3 lists the tasks that we completed. Section 4 describes, using block diagrams, the layers of the protocol stack of each type of node mentioned in Section 2. Each layer in the protocol stack is a component. Section 5 gives a detailed description of the ports and contracts that enable different layers to interact in the initiator-reactor mechanism that is a fundamental concept of J-Sim. Finally, Section 6 provides a sample script of a wireless sensor network simulation; the script shows how layer (component) binding is deferred until a complete system is actually being built.

2. Overview of Sensor Networks Architecture

In a typical wireless sensor network, sensors monitor a specific environment (e.g., an airport, a battlefield, etc.) and upon detecting a relevant stimulus (signal), sensors send reports to a sink (user) node. There are different types of stimuli; for example, acoustic and seismic. Stimuli are generated by target nodes. Hence, a wireless sensor network simulation environment consists of three main types of nodes: sensor nodes, target nodes, and sink (user) nodes. Target nodes generate signals (stimuli). Sensor nodes detect the signals (stimuli) that are generated by a target and forward the necessary data to the sink nodes. Finally, a sink node receives data collected by sensors.

Since the nature of signal propagation between target nodes and sensor nodes is inherently different from that between sensor nodes and sink nodes, two different models for channels and protocol stacks are needed. Namely, a sensor node is equipped with (1) a sensor protocol stack, which enables it to capture signals generated by target nodes through a sensor channel, and (2) a wireless protocol stack, which enables it to send reports to the sink nodes through a wireless channel. This operation is illustrated in Figure 1. The combination of the sensor protocol stack and the wireless protocol stack is usually called the sensor function model [1]. A sensor node also has a power model that embodies the energy-producing components (e.g., battery) and the energy-consuming components (e.g., radio and CPU) that enables a sensor to fulfill the functions of the sensor function model. Figure 2 shows an internal view of the three types of nodes [2].

Figure 1: A typical wireless sensor network environment

Figure 2.a: Internal view of a sensor node (dashed line)

Figure 2.b: Internal view of a target node (dashed line)

Figure 2.c: Internal view of a user (sink) node (dashed line)

3. Completed tasks

In order to incorporate wireless sensor network simulation in J-Sim, we made a new package: drcl.inet.sensorsim. All of the implemented classes described below are defined inside this new package:

Target Nodes:

This includes the implementation of the following classes:
TargetAgent: implements a target node. This involves generating signals (stimuli) and passing them to the lower layers in order to be transmitted over the sensor channel.

TargetPacket: implements the packet that represents the signal generated by a target node.
SensorMobilityModel: a derived class of drcl.inet.mac.MobilityModel. Since the target nodes may move (e.g., moving tanks), their mobility pattern, speed and location are handled by SensorMobilityModel.
SensorNodePositionTracker: keeps track of the locations of all the nodes. This location information is being reported by the SensorMobilityModel component of each node to the SensorNodePositionTracker. A major function done by this class is that it determines the sensor nodes that are within the transmission radius of a target node, and hence, should receive a copy of the signal generated by that target node.
SensorPhy: This class plays two roles depending on whether an instance of it exists in the protocol stack of a target node or a sensor node. If it is defined in the protocol stack of a target node, its function is to receive a signal generated by the TargetAgent, query the SensorMobilityModel component to get the most up-to-date location of the target node, and forward the generated signal (together with the location information) to the sensor channel component.
SensorPositionReportContract: defines the contract needed to define the interface between SensorPhy and SensorMobilityModel and also between SensorMobilityModel and SensorNodePositionTracker.