Nevertheless, the rapid development and progress of sensors, MEMS, embedded computing, in addition to the availability of inexpensive CMOS (Complementary Metal Oxide Semiconductor) cameras and microphones coupled with the significant progress in distributed signal processing and multimedia source coding techniques, allowed for the emergence of so called wireless multimedia sensor networks. As a result, Wireless Multimedia Sensor Network (WMSN) [6] is a network of wirelessly interconnected sensor nodes equipped with multimedia devices, such as cameras and microphones, and capable to retrieve video and audio streams, still images, as well as scalar sensor data. WMSNs promise a wide range of potential applications in both civilian and military areas which require visual and audio information such as surveillance sensor networks, law-enforcement reports, traffic control systems, advanced health care delivery, automated assistance to elderly telemedicine, and industrial process control. In these applications multimedia support has the potential of enhancing the level of information collected, enlarging the range of coverage, and enabling multi-resolution views [7] (_i.e._, in comparison to the measurements of scalar data). WMSNs have also additional characteristics and challenges, in addition to those of WSNs, because of the nature of the real time multimedia data such as high bandwidth demand, real-time delivery, tolerable end-to-end delay, and proper jitter and frame loss rate. Moreover, there are many different resource constraints in WMSNs involving energy, bandwidth, data rate, memory, buffer size and processing capability because of the physically small size of the sensors and the nature of the multimedia application that is typically producing a huge amount of data. Therefore, to meet the quality of service (QoS) requirements and to use the network scarce resources in a fair and efficient manner, these characteristics of WMSNs along with other research issues such as coverage and security --as shown in Figure 1--become a concern, and should be considered probably at the different layers of the communication protocol stack. We outline and discuss these issues in detail in the following sections. Moreover, given the relatively high redundancy in the visual sensor data, WMSNs have additional requirements such as in-node multimedia processing techniques (e.g., distributed multimedia source coding and data compression), application-specific QoS requirements, and multimedia in-network processing techniques (e.g., storage management, data fusion and aggregation). These mentioned characteristics, challenges, and requirements of designing WMSNs open many research issues and future research directions to develop protocols, algorithms, architectures, devices, and testbeds to maximize the network lifetime while satisfying the quality of service requirements of the various applications. In this paper, we survey the state of the art in the proposed algorithms, protocols, and hardware for the development of WMSN and discuss their open research issues. Also, we outline in detail the research challenges at different layers of the communication protocol stack. Although our survey paper and the work presented in [6] are similar in the coverage, they have many major differences. First, our paper is more up-to-date and covers all research aspects of WMSNs, (e.g., network architecture, communication layer stack, cross layer design, challenge issues like security and coverage, and hardware and testbeds). Some of those aspects were not covered in the previous work. Second, new classifications,  
Figure 1: Research Challenges in WMSNs. (e.g., source coding techniques) are covered in more details. Third, a complete classification of the hardware platforms used in WMSN based on their functionalities and capabilities pertaining to wireless motes, camera motes, and testbeds is included in our paper. Moreover, we survey all the hardware devices and prototypes used in WMSN and compare among their specifications and features.