The seven papers in this special issue focus on developments in the area of vehicular networks.

In May 1996, a group of major hardware and software manufacturers jointly announced a set of guidelines forming an industry standard for a new class of devices called network computers (NCs). The new industry standard-called the Network Computer Reference Profile 1-is designed to accelerate the development of low-cost computing. The group of firms, consisting of Apple, IBM, Oracle, Sun Microsystems and Netscape Communications, unveiled plans to provide network computer operating software and a variety of NC hardware models by the end of 1996. The NC Reference Profile 1 considerably builds upon existing Internet and World Wide Web technology. This paper gives a brief overview of the goals pursued by the introduction of NC Reference Profile 1. It surveys the technologies employed for NCs and discusses their application and market potential in different environments.

Continuous media such as audio and video require a certain quality-of-service (QoS) when transferred through computer networks. The selection of a network route for a particular media stream should hence take into account which route is best-suited for providing this QoS. QoSFinder is a method for QoS-based routing of multimedia streams. It is based on a path vector protocol that takes into account throughput, delay, and loss rate of individual route segments. A simulation of QoSFinder shows that its heuristic is superior to metrics that are only based on one of these parameters. QoSFinder increases the probability of finding suitable paths through networks for distributed multimedia applications.

HeiTS, the Heidelberg Transport System, is a multimedia communication system for real-time delivery of digital audio and video.HeiTS operates on top of guaranteed-performance networks that apply resource reservation techniques.To make HeiTS also work with networks for which no reservation scheme can be realized (for example, Ethernet or existing internetworks), we implement an extension to HeiTS which performs media scaling at the transport level: The media encoding is modified according to the bandwidth available in the underlying networks.Both transparent and non-transparent scaling methods are examined.HeiTS lends itself to implement transparent temporal and spatial scaling of media streams.At the HeiTS interface, functions are provided which report information on the available resource bandwidth to the application so that non-transparent scaling methods may be used, too.Both a continuous and discrete scaling solution for HeiTS are presented.The continuous solution uses feedback messages to adjust the data flow.The discrete solution also exploits the multipoint network connection mechanism of HeiTS.Whereas the first method is more flexible, the second technique is better suited for multicast scenarios.The combination of resource reservation and media scaling seems to be particularly well-suited to meet the varying demands of distributed multimedia applications.

The author presents the time capsule abstraction to describe how timed data shall be stored, exchanged, and accessed in a real-time system. When data are written into a time capsule, a time stamp and a duration are associated with the data item. When it is read, a time stamp is used to select the data item. The time capsule abstraction includes the notion of clocks that ensure periodic data access that is typical for continuous-media applications. By modifying the parameters of a clock, effects such as time lapses or slow motion can be achieved.>