real-time services, on the other hand, are in a constant T h i s paper introduces a m e c h a n i s m f o r optimally graceful QoS degradation in distributed real-time m u b timedia services, Declarative Scheduling. W e assume that f o r each m e d i u m of a multimedia service there are a set of implementations, both with differing resource consumption and provided &OS, thus allowing t o explozt both, resource-resource an.d resource-QoS tmdeo.tfs. If the a m o u n t of available resources changes, the servzce m a y locally adapt without changiiig the provided QoS by scheduling a different set of iiiiplenieiitatioii,s with different resource requirements. Should this kind of rescheduling not be possible, the service m a y adapt b y scheduling implementations with a lower QoS providable with the resources available. In the laiter case Declarative Scheduling degrades the QoS optimally, i.e. the d-fference between the provided QoS niad the requested QoS is m i n i m a l with respect t o th,e user's prefe re n, ces . Th. e p ro b 1 e m , which imp 1 e i n e lit at i o 11 s to se 1 e ct t o m i n i m i z e th.e QoS degradation i s sfi.owiz i o be a so f a r unconsidered instance of the NP-coinplefe knapsack problem. A heuristic f o r this knapsack problem is proposed and shown t o be suficiently e f i c i e n t and accurate t o implement Declarative Scheduling. I Introduction T h e recent, strong interest in multimedia coiiiiiiunication led to the development of two different approaches t o real-time communication: Hard real-time communication reserves the required resources beforehand to guarantee the qua.lity of service (QoS), while the QoS soft real-time communication depends at any time on the amount of resources available[S]. Hard real-time services must adjust their QoS during their initialization, if the optimum amount of resources ca.nnot be reserved. Further, if the operating system should reclaim some of the resources while the service is running[7], the &OS must be adjusted, too. Soft 0-8186-7436-9/96 $5.00 0 1996 IEEE Proceedings of MULTIMEDIA '96 need to adapt t o the amount of resources available[8]. Recent research on soft real-time services has been concentrating on optimizing the QoS of a video transmission with respect to the amount of network throughput a.vailable, see e.g. [l], disrega.rding ot,lier resources like CPU time or buffer memory, while there seems t o be little work on high-level QoS adaptation in the area of hard real-time services.In this paper we a.re interested in optimizing the local QoS experienced by one participant of a distributed multiinedia service with respect t o the user QoS requirements, and const,ra.ined by the amount of resources available at the local computer system. In other words, if resource shortage a.t the local computer system forbids t o present incoming d a t a and generate outgoing d a t a as requested by the user, the QoS should only degrade as little as necessary, and in a.ccordance wit,li the user's preferences.T h e mechanism proposed in t,liis paper is re...
