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systems. At this stage, no real-time constraints or QoS guarantees are taken into
account. At a second stage, we use a temporal logic based language to describe the
ordering constraints over the functional behaviors and also the time-critical QoS
requirements. LOTOS has been proven to be an efficient and abstract language for
the specification of concurrent and non-determinism behaviors. QTL features real-
time capabilities, which are required to express real-time properties.
3 Case Studies
In case studies, we illustrate aspect-oriented specification by three examples. First
example is the specification of multimedia stream, Second example is the specifica-
tion of the alternating bit protocol and third example is the specification of an elevator
control system that illustrates the development process including static structure,
dynamic behaviors and the weaving of time aspect by integration of Informal Specifi-
cation and formal Specification with aspect-oriented Approach.
3.1 Aspect-Oriented Formal Specification of Multimedia Stream
In this example, the case of a multimedia stream is presented, and a structure linking a
multimedia data source and a data sink. The data source and the data sink are assumed
to be communicating asynchronously over an unreliable channel. We use LOTOS to
specify the abstract behaviors of multimedia stream and QTL to specify real-time
aspect of multimedia stream.
LOTOS is used for specifying the abstract behaviors. We believe there are a num-
ber of advantages in using process algebra such as LOTOS for the representation of
abstract behaviors. Process algebraic techniques generally feature an elegant set of
operators for developing concurrent systems. Thus, succinct expressions of communi-
cating concurrent processes can be made. Similarly, the emphasis on non-determinism
encourages elegant specification and abstracts away from implementation details.
Furthermore, rich and tractable mathematical models of the semantics of process
algebra have been developed. In LOTOS, this model is based upon concepts of
equivalence through observation of the external behaviors of a specification and is
derived from the seminal work of Robin Milner . Finally, due to the standardization of
LOTOS and the application oriented nature of the language a large number of support
tools have been developed. It should be stressed that we use standard LOTOS and do
not require any semantic alterations to model time. The LOTOS specification there-
fore describes the possible event orders in the system, but does not relate events to
real-time. Some advantages of using standard LOTOS are that changes are not re-
quired to the standard and existing toolkits can still be used.
It feels that a temporal logic offers the most natural means for expressing re-
quirements of distributed systems. It has been demonstrated that such logics enable
the abstract expression of requirements and facilitate rigorous reasoning about these
requirements. More specifically, for our application domain, a real-time temporal
logic is required in order to specify the real-time requirements prevalent in distrib-
uted multimedia systems. Note that the added expressiveness of real-time temporal
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