Abstract
Multimedia data are characterized by large, complex values with
temporal dependencies. Such data are used by distributed multimedia
applications offering long-lived multimedia presentations in
distributed environments. An end-client may choose to specify quality
requirements for a multimedia presentation, using quality of service
(QoS) parameters known from traditional communication technology. The
client's demands and requirements should be satisfied throughout the
entire presentation, thus QoS management is required. These
management tasks may be performed by software components residing at
each functional component in a distributed multimedia system, for
example the network or database system. We refer to such a software
component as a QoS manager, who, among other things, is responsible
for adjusting the level of QoS prior to and during a multimedia
presentation. Whenever the user's requirements regarding QoS are not
satisfied, the adaptation mechanism is responsible for bringing the
QoS level within the accepted boundaries.
We focus on adaptation mechanisms for QoS in a multimedia database
management system (MMDBMS) offering services to applications for
distance learning called Learning-on-Demand (LoD). LoD offers
interactive services and a high degree of user interactions.
In the OMODIS (Object-Oriented Modeling and Database Support for
Distributed Multimedia Systems) project at the Department of
Informatics at the University of Oslo, a framework for QoS management
has been developed. The framework defines messages for interaction
between QoS managers in a dynamic, reconfigurable QoS management
hierarchy. Each manager consists of 7 internal agents, one being a
QoS adaptation agent. In our work we propose messages for interaction
inside a QoS manager connected to a MMDBMS and a QoS management
hierarchy. To decide the tasks of a manager for a MMDBMS we identify a
set of events that may occur in a MMDBMS supporting LoD applications.
These events are used to design protocols for internal communication
between the agents of a QoS manager for a MMDBMS.
The internal messages are realized through formal specifications in
Promela (protocol meta language). The formal specifications are used
to simulate and validate the behavior of the internal message passing.
Simulations and conformance testing are used to show that the
protocols as specified, are correct with respect to a set of
correctness criteria, and that the protocols support the agents in
performing their QoS management tasks.
Temporal claims and correctness requirements are used during
validation. The validation results show that the protocols for
internal communication meet all correctness criteria. In particular,
the protocols are free of deadlocks and livelocks. All protocol
executions end in legal end states, and they all follow our definition
of legal sessions in respect to the session states defined in the
OMODIS QoS framework.
In addition, we propose an adaptation model for the QoS adaptation
agent of the QoS manager. The model defines behavior for local QoS
adaptation prior to and during a presentation. Our adaptation model is
based on QoS monitoring results of the local component. Thus, it is
capable of discovering QoS violations caused by the MMDBMS at an
earlier stage than adaptation models depending exclusively upon
monitoring results from the client side. Problems at the client side
will not arise until an end-to-end QoS violation has occurred.