INTRODUCTION In recent years, the growing interest in the wire- less sensor network (WSN) has resulted in thou- sands of peer-reviewed publications. Significant results in this area have enabled many civilian and military applications, and several start-up companies and large corporations are investing considerable amounts of resources in this tech- nology. Most deployed wireless sensor networks measure scalar physical phenomena, such as temperature, pressure, humidity, or location of objects. In general, sensor networks are designed for data-only delay-tolerant applications with low bandwidth demands. The integration of low-power wireless net- working technologies with inexpensive hardware such as complementary metal–oxide semicon- ductor (CMOS) cameras and microphones is now enabling the development of distributed, networked systems that we refer to as wireless multimedia sensor networks (WMSNs), that is, networks of wireless, interconnected smart devices that enable retrieving video and audio streams, still images, and scalar sensor data. As an example, the Cyclops image-capturing and inference module [1], designed for extremely lightweight imaging, can be interfaced with a host mote such as Crossbow’s MICA2 or MICAz, thus realizing an imaging device with processing and transmission capabilities. WMSNs will enable the retrieval of multimedia streams and will store, process in real-time, cor- relate, and fuse multimedia content captured by heterogeneous sources. We envision that users will be able to gather information about the physical environment by issuing simple textual queries, thus accessing multiple remote WMSNs connected to the Internet through application level gateways. The characteristics of a WMSN diverge con- sistently from traditional network paradigms, such as the Internet and even from scalar sensor networks. Most potential applications of a WMSN require the sensor network paradigm to be rethought to provide mechanisms to deliver multimedia content with a predetermined level of quality of service (QoS). Whereas minimizing energy consumption has been the main objective in sensor network research, mechanisms to effi- ciently deliver application-level QoS and to map these requirements to network-layer metrics, such as latency and jitter, have not been primary concerns. Delivery of multimedia content in sen- sor networks presents new, specific system design challenges, which are the object of this article. We discuss the state of the art and the main research challenges for the development of WMSNs. We begin the discussion by describing the main applications enabled by WMSNs and by introducing a reference architecture. Then, we point out the major factors influencing the design of WMSNs. Next, we discuss existing solutions and open research issues at the appli- cation, transport, network, link, and physical lay- ers of the communication stack, respectively. Finally, we conclude the article. IEEE Wireless Communications • December 2007 32 1536-1284/07/$20.00 © 2007 IEEE Internet W IRELESS S ENSOR N ETWORKING IAN F. AKYILDIZ, GEORGIA INSTITUTE OF TECHNOLOGY TOMMASO MELODIA, STATE UNIVERSITY OF NEW YORK AT BUFFALO KAUSHIK R. CHOWDURY, GEORGIA INSTITUTE OF TECHNOLOGY ABSTRACT In recent years, the growing interest in the wireless sensor network (WSN) has resulted in thousands of peer-reviewed publications. Most of this research is concerned with scalar sensor networks that measure physical phenomena, such as temperature, pressure, humidity, or loca- tion of objects that can be conveyed through low-bandwidth and delay-tolerant data streams. Recently, the focus is shifting toward research aimed at revisiting the sensor network paradigm to enable delivery of multimedia content, such as audio and video streams and still images, as well as scalar data. This effort will result in distribut- ed, networked systems, referred to in this paper as wireless multimedia sensor networks (WMSNs). This article discusses the state of the art and the major research challenges in archi- tectures, algorithms, and protocols for wireless multimedia sensor networks. Existing solutions at the physical, link, network, transport, and application layers of the communication protocol stack are investigated. Finally, fundamental open research issues are discussed, and future research trends in this area are outlined. W IRELESS M ULTIMEDIA S ENSOR N ETWORKS : A S URVEY The uthors discusse the state of the art and the major research challenges in architectures, algorithms, and protocols for wireless multimedia sensor networks. Authorized licensed use limited to: IEEE Xplore. Downloaded on October 18, 2008 at 18:10 from IEEE Xplore. Restrictions apply.