Performance Characterization and Architecture Exploration of PicoRadio Data Link Layer using VCC Mei Xu and Rahul C. Shah EE249 Class Project Fall 2001 Abstract - PicoRadio is an ongoing project at the Berkeley Wireless Research Center that is trying to develop tiny meso-scale nodes capable of forming ad-hoc networks. The key challenges in the project are the small form factor and energy constraints at every node. For this, we need smart protocols that are energy efficient while meeting all requirements of the application. To test the behavior of the protocols that are developed, we defined the protocol stack in VCC. We then used VCC Architectural Services to provide a performance characterization of the data link protocol layer. Using this characterization, we explored different target architectures and obtained an architecture that is efficient as well as met all the constraints of the protocol operation. This is crucial for the design of the next generation PicoNode. At the same time, the project has identified new services VCC needs to support for simulating such large-scale embedded systems. I. INTRODUCTION Recently there has been a lot of interest in building and deploying sensor networks – dense wireless networks of heterogeneous nodes collecting and disseminating environmental data. There is a multiplicity of scenarios in which such networks might find uses, such as environmental control in office buildings, robot control and guidance in automatic manufacturing environments, interactive toys, the smart home providing security, identification, and personalization, and interactive museums. Crucial to the success of these ubiquitous networks is the availability of small, lightweight, low-cost network elements, which we call PicoNodes [1]. These nodes must be smaller than one cubic centimeter, weigh less than 100 grams, and cost substantially less than one dollar. Even more important, the nodes must use ultra-low power to eliminate frequent battery replacement. We envision a power-dissipation level below 100 microwatts, as this would enable self-powered nodes using energy extracted from the environment, an approach called energy scavenging or harvesting. Trying to network a large number of such low-power mobile nodes is a challenging problem that has recently been the focus of many researchers. We recently defined the entire protocol stack of PicoRadio in the Virtual Component Co-Design (VCC) environment [2][3]. This includes the application, network and data link layers. We then performed behavioral simulations to verify the behavior of all the layers. For this project, we completed the full definition of the data link and MAC layer of the PicoNodes and created a small network of 5-6 nodes. We then used VCC Architectural Services to provide a performance characterization of the data link protocol layer. Since the data link layer does not need an entire network to test, a few nodes provided a fairly accurate characterization. Also, this is the most complex layer in the protocol stack, thus we clearly needed to understand whether the sub-blocks could meet the required constraints. Using this characterization, we explored different target architectures and obtained an architecture that is efficient as well as met all the constraints of the protocol operation. This is useful as an input for the design of the next generation PicoNode. The paper is organized as follows. Section II introduces sensor networks and the PicoRadio project. Section III describes the protocol stack and its functionality with particular emphasis on the data link layer. Section IV discusses the VCC environment and methodology while the architectures we explored are described in Section V. Results and observations on the performance simulations are in Section VI, concluding with Section VII. II. SENSOR NETWORKS AND THE PICORADIO PROJECT Sensor networks typically consist of hundreds of nodes, deployed for the purpose of environment monitoring and control. Let us consider a Smart Building scenario, one of the key applications forthe PicoRadio project at the Berkeley Wireless Research Center. This is aimed at controlling the environment of a typical office environment using a distributed building monitor and control approach. Thus the three main functions in a sensor network are sensing, controlling and actuating. These functions could be on separate nodes or co-located on the same physical node. In addition, each physical node also has a logical repeater function which helps in multi-hop routing. We thus define three types of nodes – sensors, controllers and actuators. Also the bit rates in sensor networks are fairly low, about a few hundred bits per sec per node. At most, the peak bit rate supported will be about 10 kb/s, which can enable simple voice messaging (not real time). Sensor data is also highly redundant, which means that end-to-end reliability is not a requirement for most data packets. Based on these requirements, we have got a first-order description of the entire protocol stack and are implementing it on our testbed. We are also analyzing the protocol description to design the architecture of our final chip, PicoNode III. III. DESIGN OF THE PICONODE The three main layers we concentrate on for designing the PicoNode are the physical, data link layer and network layer [4]. A. Physical Layer Communication between two nodes requires creating a physical link between two radios. The physical layer handles the communication across this physical link, which involves modulating and coding the data so that the intended receiver can optimally decode it in the presence of channel non-idealities and interference. B. Data Link Layer (DLL) The data link layer’s [5] primary functions are to provide access control, channel assignment, neighbor list management and power control. It also has a location sub-system that computes the x, y and z-coordinates based on the received signal strength of neighboring nodes and the presence of certain anchors in the network that know their exact positions. The DLL coordinates channel assignment such that each node gets a locally unique channel for transmission, while the channels are globally reused. There is also a global broadcast channel that is used for common control messages and for waking up nodes. Each node has two radio receivers, one of which runs at 100% duty cycle, but is very low bit rate and consumes very little power. The second
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