Support for multimedia applications by general purpose computing platforms has been the subject of considerable research. Much of this work is based on an evolutionary strategy in which small changes to existing systems are made. The approach adopted here is to start ab initio with no backward compatibility constraints. This leads to a novel structure for an operating system. The structure aims to decouple applications from one another and to provide multiplexing of all resources, not just the CPU, at a low level. The motivation for this structure, a design based on the structure, and its implementation on a number of hardware platforms is described. 2 processor sees a performance which is in uenced by the load on the other virtual processors, and mechanisms to control this interference are generally not available. Multimedia applications require such mechanisms.One way of controlling this interference is by providing multiple real processors. For example, many multimedia applications (or parts thereof), run on processors on peripheral cards so that the main processor is not involved. Moreover, the code running on the peripheral is likely to be embedded and there is no danger of competing applications using the peripheral at the same time. The same approach is also used in mainframes where the use of channel processors reduces the I/O demands on the central processors, in particular ensuring that the central processors do not get overloaded by I/O interrupts.Our aim in Nemesis is to allow a general purpose processor to be used to provide the functions one would nd in a specialised DSP peripheral while providing the same control of interference across virtual processors as can be achieved with distinct hardware. We wish to retain the exibility of the virtual processor system so that resources can be used more e ciently than in a dedicated-peripheral approach.In approaching the design of an operating system with these goals, the immediate question of revolution versus evolution arises. Should one attempt to migrate a current operating system (or indeed use a current operating system) in order to meet these goals, or should one start afresh? The reasons why current general purpose operating systems are not appropriate are well established. Similarly, hard real time solutions which require static analysis are not appropriate in a situation where the application mix is dynamic.General purpose operating systems with \real time threads" in which the real time behaviour is provided by static priority are also inappropriate, unless one is running a single multimedia application or can a ord to perform an analysis of the complete system in order to assign priorities. A better solution might be to take an existing operating system and modify its scheduling system to support multimedia applications { perhaps one reason for the di culty in performing such a scheduler transplant is that knowledge of the characteristics of the scheduler often migrates to other components making the e ect of replacement unpredictable.This, toget...

When the problem of routing multicast connections in networks has been previously considered, the emphasis has been on the source transmitting to a fixed set of destinations (the multicast group). There are some applications where destinations will join and leave the multicast group. Under these conditions, computing an "optimal" spanning tree after each modification may not be the best way t o proceed. An alternative is t o make modest alterations to an existing spanning tree to derive a new one. A n extreme, though non-optimal, variation of this i s to use minimal cost source t o destination routing for each destination, effectively ignoring the existing multicast tree. We examine just how non-optimal these trees are in random general topology networks and conclude that they are worse by only a small factor. The factor is reduced still further if a hierarchy is imposed upon the random network t o give a more realistic model.

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Most research in network programmability has stressed the exibility engendered by increasing the ability of users to con gure network elements for their own purposes, without addressing the larger issues of how such advanced control systems can coexist both with each other and with more conventional ones. The Tempest framework presented here extends beyond the provision of simple network programmability to address these larger issues. In particular we show how network programmability can be achieved without jeopardizing the integrity of the network as a whole, how network programmability ts in with existing networks and how programmability can be o ered at di erent levels of granularity. Our approach is based on the Tempest's ability to dynamically create Virtual Private Networks (VPNs) over a switched transport architecture, e.g. an ATM network. Each VPN is assigned a set of network resources which can be controlled using either a well known control system or a control system tailored to the speci c needs of a distributed application. The rst level of programmability in the Tempest is fairly course grained: an entire virtual network can be programmed by a third party. At a ner level of granularity the Tempest allows user supplied code to be injected into parts of an operational virtual network, thus allowing application speci c customisation of network control. The paper shows how the Tempest framework allows these new approaches to coexist with more conventional solutions.

Air quality monitoring plays an increasingly important role in providing accurate air pollution data for assessing the impacts of air pollution on public health. Development of proper sensor networks, by deploying the right air pollution sensors at the right place, in order to meet the needs of different groups in the city and provide the much needed public services, deserves careful attention, especially when smart city development is being considered. However, air quality monitoring can be a costly measure. To tackle such a challenge, air pollution sensor placement can be carefully designed to achieve certain optimal citizen-centric objectives in the absence of field information, which can be formulated as an optimal sensor placement problem. In this paper, we propose three citizen-centric objectives for the optimal sensor placement problem, which does not require the prior deployment of pollution sensors for obtaining any field information. By citizen-centric, we mean that sensor placement puts the citizens' welfare at the center of attention and be able to fulfill the following objectives: 1) better assessing the vulnerable people's exposure to air pollution; 2) maximizing overall satisfaction of obtaining public information on existing air quality; and 3) better monitoring traffic emissions. We formulate the optimization problem for each scenario and propose an effective method to solve the problem accordingly. Last but not least, we conduct a case study in the city of Cambridge to evaluate the feasibility and effectiveness of our proposed methods. Our case study has shown that in order to optimize our citizen-centric objectives, there is a need to re-orient the current sensor placement strategies in the city of Cambridge, U.K.