## 7 Application Layer in WMSN
### Multimedia Source Coding Techniques  
In order to have the ability to handle multimedia content over wireless sensor networks and to support real time multimedia applications, multimedia processing and source coding techniques have been widely used in the application layer of WMSN. Multimedia processing techniques aim to reduce the amount of multimedia traffic transferred over the network by extracting the useful information from the captured images and videos while in the same time maintaining the application-specific QoS requirements. However, in WMSN, these techniques should be designed in such a way that they meet current hardware capabilities, more power efficient to match the battery constrains in WMSN, and have high compression efficiency to reduce the size of the multimedia content and to meet the available supported data rate and bandwidth in the network. The existing video coding techniques that have been used in the literature for WMSN vary in their high compression efficiency, error resiliency, and low encoding-decoding complexity and can be classified into four groups:  
* Layered Coding (LC) [65] is type of video source coding technique by which the original data is encoded to one important base layer (coarse version) and one or more less important successive enhancement layers (to get the fine version). At the destination side, the base layer can be combined again with all or subset of the higher-quality layers to achieve the desired level of video resolution. However, the loss of the base layer makes the information received from the the enhancement layers useless. The same principle of Layered Coding technique was used in [58] and [61]. A similar concept of LC was used in the work presented in [66] that proposes an image-pixel-position information based resource allocation scheme to transmit wavelet-based compressed images with best effort quality in WMSNs. Using wavelet for image compression will transform the image to coefficients **that** describe its information with different significant-level values that can be further compressed more easily to have at the end some large magnitude coefficients and many small magnitude coefficients with **\"0\"** bits. These small magnitude coefficients stand for the image-pixel-position information that is more important than the large magnitude coefficients that contains the image-pixel-value information (_i.e._, brightness). It is shown that the communication loss or bit errors in position information will have significantly higher effect on the overall quality of the received image than the loss or errors in value information. This is because the correct decoding of position data segment depends on the correct decoding of previous position data segments only, but the correct decoding of value data segment depends on previous bit-planes of both position data and value data segments. So, by allocating the scarce resources in WMSN on the position information more than the magnitude value information, position data segments are effectively protected to enhance image quality while value data segments are less protected to improve energy efficiency. * Predictive Video Coding (PVC) [67], used in MPEG-x and H.26x standards, is based on the idea of reducing the bit rate generated by the source encoder by exploiting data statistics. PVCcoding employs two modes for encoding the video: 1) Intra-frame coding mode (I-frame) that is used to reduce the redundancy within one frame by exploiting the spatial correlation in the frame, and 2) Inter-frame coding mode (P-frame) or motion compensated predictive that is used to reduce data redundancy in subsequent frames by exploiting both spatial and Temporal correlation. Performance evaluation of PVC over Stargate and TelosB is conducted in [68] showing the energy consumption in both video compression and transmission. * Multiple Description Coding (MDC) [69] is used to enhance the error resiliency of video delivery by splitting the multimedia content to two or more independent and equal important streams (multiple descriptions).