Costs in Digital Library Databases

Digital libraries involve various types of data like text, audio , images and video. The data objects are typically very large and of the order of hundreds and thousands of kilobytes. In a digital library, these data objects are distributed in a wide area network. Retrieving large data objects in a wide area network has a high response time. We have conducted experiments to measure the communication overhead in the response time. We have studied the correlation between communication and size of image, between communication and time of day and the communication delay to various sites in a local and wide area network. Images are amenable to losing data without losing semantics of the image. Lossy compression techniques reduce the quality of the image and reduce the size leading to a lower communication delay. We compared the communication delay between compressed and uncompressed images and studied the overhead due to compression and decompression. This enabled us to study the tradeoo between communication time and quality of the image.