## 1 Introduction  
Wireless multimedia sensor networks (WMSNs) have become a promising research area in recent years. Unlike traditional wireless sensor networks (WSNs), which are typically deployed to collect scalar data (e.g., temperature and humidity), WMSNs are also able to collect multimedia data including audio, still image, video, and even live media streams with the addition of a microphone and a camera to the sensor nodes. With significant advances in embedded systems, image processing techniques, and communication technologies, WMSNs are anticipated to be used for a wide range of applications, such as security surveillance, real-time traffic and environmental monitoring, health care, and so on [1; 2; 3]. As a descendant of traditional WSNs, WMSNs inherit most of their advantages including deployment simplicity, scalability, resilience, and self-organizing capabilities. However, WMSNs are also confronted with many new challenges, one of which is the demand for highly energy-efficient and QoS-aware computation algorithms and communication protocols. Such requirements are crucial because WMSNs are by nature quite resource constrained in terms of energy, processing capacity, and communication bandwidths, while at the same time they also generate large volumes of multimedia data that need to be delivered over the shared wireless medium urgently and reliably. Consequently, most techniques originally developed for traditional WSNs cannot be directly used in WMSNs. Instead, new techniques ranging from the application layer down to the physical layer need to be designed and existing ones from traditional WSNs should get modified before they are applied to WMSNs. Fortunately over the years, the research community has done a lot of work, and there have been various surveys that sum up such previous efforts. Leading the way are the top-down surveys that cover from sensor hardware to every individual layer in the network model, as well as to cross-layer designs [4; 5; 6; 7]. Detailed studies about existing testbeds regarding to hardware architectures and software components are presented in [8; 9]. References [10; 11; 12] survey image processing algorithms in sensor networks.Reliable transport protocols are investigated in [13]. Energy efficient and/or QoS aware routing protocols are compared in [14; 15; 16]. Respectively, QoS-aware MAC protocols and multi-channel MAC protocols are reviewed in [17; 18]. While [PERSON]. AISkaif etc. provide detailed energy models and numerical studies for MAC protocols in low data rate WMSNs in [19]. References [20; 21] analyze cross-layer solutions for multimedia delivery over sensor networks. References [22] provides an insight into the energy problem that concerns all sensor networks, and [23] in specific discusses energy efficiency with regard to target tracking applications. Finally, security issues and privacy problems are summarized in [24; 25]. Our work differs from previous surveys mainly in the following three grounds. Firstly, we outlined the four nuclear characteristics of WMSNs: Battery-powered, real-time video data, large volume of sensory data, and directional sensor coverage. For each characteristic, we couple it with the respective requirements and corresponding solutions in the hope that it will aid researchers from different areas to better grasp the challenges in their own field and help to clarify what needs to be done in their particular research directions. Moreover, we are also the first to give formal definitions to local processing, multimedia in-network processing, and camera calibrations. Secondly, to provide network planners and policy makers with a clearer overall understanding of WMSNs, we present the characteristics of WMSNs together with the imposed requirements and the existing solutions in a novel way such that all of these aspects are illustrated as an inter-connected unibody instead of scattered fragments.