## 1 Introduction  
The recent widespread use of small, inexpensive multimedia sensors such as CMOS imaging sensors and hardware improvements to wireless sensor nodes have led to the creation of wireless multimedia sensor networks (WMSNs), which are capable of acquiring multimedia content such as sound and images. WMSNs can be used in many applications such as security programs and the monitoring of dangerous areas and can improve the quality of application services provided by networks, as compared with traditional wireless sensor networks, which transmit text-based information [1-5]. However, multimedia content has several unique characteristics as compared with text-based information, and thus requires anew transfer protocol for WMSNs. First, the size of multimedia content, especially image data, is much greater than that of text-based data. Multimedia data consist of large-sized groups of numerical values, whereas text-based data such as temperature or brightness are usually expressed as a single numerical value. For example, the temperature of a place can be expressed as 25 or 30, which requires just one or two bytes, for which a small packet is sufficient. However, one small (64 \(\times\) 64) image of a place would require 4096 bytes, which in turn requires many packets to transmit because the packet size in a wireless sensor network is usually quite small, _i.e.,_ from 10s\(\sim\)100s bytes. Second, multimedia data is quite sensitive to data loss, whereas text-based data is relatively tolerant of data loss. The loss of a small fraction of image data leads to the discarding of the entire image or to a drastic reduction in image quality. Because multimedia content is quite large, it requires many packets that cannot be lost or dropped, if high quality is to be realized. As a consequence, in order to deal with multimedia content, a network protocol should provide end-to-end reliability of packet transmission, in consideration of the characteristics of multimedia content [6-15]. However, existing protocols in WMSN are not designed to accommodate multimedia content. These protocols are usually based on the protocols of traditional networks such as the internet, where there are frequent packet losses whenever the network becomes congested. Thus, packets with multimedia content in WMSN can be dropped in the case of congestion. Some research efforts on WMSNs have focused on congestion control, but this research does not guarantee end-to-end packet delivery. Moreover, in WMSNs, packet transmission takes place through the air, a medium in which many errors or losses can occur, so that packet transmission in WMSNs is generally regarded as quite unreliable. Some research that has claimed to provide reliable packet transmission usually seeks to reduce the packet loss or error that occurs in node-to-node transmission, not end-to-end. For this reason, a new transport protocol for WMSN should be developed to facilitate the reliable transfer of large, loss-sensitive multimedia content [16-17]. In this paper, we propose a Reliable Asynchronous Image Transfer protocol, RAIT. Our focus is upon the transfer of images, which constitute an important form of multimedia content. The proposed RAIT seeks to provide the reliable transfer of whole packets of an image from a sensor node to the sink node or gateway in a WMSN. RAIT prevents packet drop and packet loss by introducing a double sliding window. One sliding window is designed to prevent packet drop from taking place inside the node due to congestion, and the other is designed to prevent packet loss in the node-to-node transfer. For the sliding windows to work properly, the lower sensor node, which sends the image packet, must know the queue state of the upper sensor node, which receives the image packet. The sender node should stop sending packets whenever the queue of the receiver node becomes full. The network protocol should therefore be coordinated with the queuing layer.