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Sunday, 21 February 2016

System Architecture Directions for Networked Sensors



ABSTRACT

Technological progress in integrated, low-power, CMOS communication devices and sensors makes a rich design space of networked sensors viable. They can be deeply embedded in the physical world and spread throughout our environment like smart dust. The missing elements are an overall system architecture and a methodology for systematic advance. To this end, we identify key requirements, develop a small device that is representative of the class, design a tiny event-driven operating system, and show that it provides support for efficient modularity and concurrency-intensive operation. Our operating system fits in 178 bytes of memory, propagates events in the time it takes to copy 1.25 bytes of memory, context switches in the time it takes to copy 6 bytes of memory and supports two level scheduling. The analysis lays a groundwork for future architectural advances.


1. INTRODUCTION

As the post-PC era emerges, several new niches of computer system design are taking shape with characteristics that are quite different from traditional desktop and server regimes. Many new regimes have been enabled, in part, by “Moore’s Law” pushing a given level of functionality into a smaller, cheaper, lower-power unit. In addition, three other trends are equally important: complete systems on a chip, integrated low-power communication, and integrated low-power transducers. All four of these trends are working together to enable the networked sensor. The basic microcontroller building block now includes not just memory and processing, but non-volatile memory and interface resources, such as DACs, ADCs, UARTs, interrupt controllers, and counters. Communication can now take the form of wired, short-range RF, infrared, optical, and various other techniques [18]. Sensors now interact with various fields and forces to detect light, heat, position, movement, chemical presence, and so on. In each of these areas, the technology is crossing a critical threshold that makes networked sensors an exciting regime to apply systematic design methods. Today, networked sensors can be constructed using commercial components on the scale of a square inch in size and a fraction of a watt in power. They use one or more microcontrollers connected to various sensor devices and to small transceiver chips. One such sensor is described in this study. Many researchers envision driving the networked sensor down to microscopic scale by taking advantage of advances in semiconductor processes. This includes having communication integrated on-chip with a rich set of microelectromechanical (MEMS) sensors and CMOS logic at extremely low cost [37, 5]. They envision that this smart dust will be integrated into the physical environment, perhaps even powered by ambient energy [31], and used in many smart space scenarios. Alternatively, others envision ramping up the functionality associated with one-inch devices dramatically. In either scenario, it is essential that the network sensor design regime be subjected to the same rigorous, workload-driven, quantitative analysis that allowed microprocessor performance to advance so significantly over the past 15 years. It should not be surprising that the unique characteristics of this regime give rise to very different design trade-offs than current general-purpose systems. This paper provides an initial exploration of system architectures for networked sensors. The investigation is grounded in a prototype “current generation” device constructed from off-the-shelf components. Other research projects [37, 5] are trying to compress this class of devices onto a single chip. The key missing technology is the system software support to manage and operate the device. To address this problem, we have developed a tiny microthreaded OS, called TinyOS. It draws on previous architectural work on lightweight thread support and efficient network interfaces. While working in this design regime two issues emerge strongly: these devices are concurrency intensive - several different flows of data must be kept moving simultaneously; and the system must provide efficient modularity - hardware specific and application specific components must snap together with little processing and storage overhead. We address these two problems with our tiny microthreaded OS. Analysis of this solution provides valuable initial directions for future architectural innovation. 

Section 2 outlines the design requirements that characterize the networked sensor regime and guide our microthreading approach. Section 3 describes our baseline, currenttechnology hardware design. Section 4 develops our TinyOS for devices of this general class. Section 5 evaluates the effectiveness of the design against a collection of preliminary benchmarks. Section 6 contrasts our approach with that of prevailing embedded operating systems. Finally, Section 7 draws together the study and considers its implications for architectural directions.


2. NETWORKED SENSOR CHARACTERISTICS

This section outlines the requirements that shape the design of network sensor systems; these observations are made more concrete by later sections....


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