"Detecting Feature Interactions in Telecommunications Systems Designs"

Mohammed Faci's PhD Thesis

University of Ottawa
1995

Abstract

A basic telephone service is used to establish a communication session
between two users. A telephone feature, such as call waiting, call
forwarding, and three way calling, is defined as an added functionality
of the basic telephone service. A feature interaction is the
interference of the functionality of one telephone feature with the
functionality of another telephone feature, meaning that the invocation
of the first feature modifies the functionality of the other active
feature, or even prevents its functionality altogether. This problem
has become a major obstacle for the extension of telephone systems with
new services. Our contributions in this thesis are the following:

(1) define a model for specifying telephone systems and their services;
(2) formalize the notion of feature interaction; and
(3) develop a design methodology for detecting feature interactions at
    the specification level.

In the first part of the thesis, we define a model for structuring the
components of telephone specifications. In this model, a specification
is expressed as a set of communicating processes representing three
types of constraints: local constraints, end-to-end constraints, and
global constraints.  Each of these constraints is defined and used in
our specifications.

In the second part of the thesis, we develop a methodology for
detecting feature interactions. Central to our methodology is the
formalization of the notion of feature interaction. Intuitively, we say
that an interaction exists between n features if one of the features
cannot exhibit its behaviour when integrated into POTS in combination
with other features. We formalize this concept by defining the
composition and integration of features. Composition expresses the
synchronization of features on their common actions with POTS and their
interleaving on their independent actions. Integration expresses the
extension of POTS with the n features, such that each feature is able
to execute all of its actions which are allowed in the context of POTS,
when the other features are disabled.

Then, we reason about interactions in terms of the conformance relation
studied in testing theory, in the following way. An interaction exists
between n features if their integration does not conform to their
composition. A set of concrete telephony examples is used to show the
applicability of the methodology.  The specification language used is
LOTOS, which turns out to be ideally suited to express our methodology.
However, our method is general enough to be adapted to other
specification languages with similar constructs.