health care organization's needs over the course of a single day. The information objects that support health care vary substantially in size and complexity. While simple text effectively represents the content of a care provider's notes, consultation reports, and the name-value pairs of common laboratory test results, many health problems require the acquisition and communication of clinical images such as X rays, computed tomography (CT), and magnetic resonance imaging (MRI). The electronic forms of these images, which often must be compared with one another in multiple image sets, comprise tens to hundreds of megabytes of information that may need to be communicated to the end user within several seconds or less. Medical information demands on digital networks are thus notable for their irregularity and the tremendous variation in the size of transmitted files. When such files need to be transmitted in short times, very high bandwidths may be required and the traffic load may be extremely bursty.
No capabilities have yet been deployed across the Internet to ensure QOS. Virtually all Internet service providers (ISPs) offer only best-effort service, in which they make every effort to deliver packets to their correct destination in a timely way but with no guarantees on latency or rates of packet loss. Round-trip times (or latencies) for sending messages across the Internet between the East and West Coasts of the United States are generally about 100 milliseconds, but latencies of about 1 second do occurand variations in latency between 100 milliseconds and 1 second can be observed even during a single connection.1 Such variability is not detrimental to asynchronous applications such as e-mail, but it can render interactive applications such as videoconferencing unusable. Similarly, the rates of packet loss across the Internet range from less than 1 percent to more than 10 percent; high loss rates degrade transmission quality and increase latencies as lost packets are retransmitted. Furthermore, because many applications attempt to reduce congestion by slowing their transmission rates, packet loss directly affects the time taken to complete a transaction, such as an image transfer, over the network.
Several approaches can be taken to improve QOS across the Internet, with varied levels of effectiveness. For example, Internet users can upgrade their access lines to overcome bottlenecks in their links to ISPs, but such efforts affect bandwidth and latency into and out of their own site only. They provide no means for assuring a given level of QOS over any distance. Similarly, ISPs can attempt to improve service by expanding the capacity of their backbone links. However, as described below, such efforts provide no guarantees that bandwidth will be available when needed and contain no mechanisms for prioritizing message traffic in the face of congestion. To overcome these limitations, efforts are under way to develop specific protocols for providing QOS guarantees across thecontinue