Modular Design of Call Control Layer in Telephony Software

An important property of a telephony system is the call control model on which it is based. It is noted that many call control models in the past, especially those in PSTN/ISDN networks follow centralized model. For such a model, typical is significant coupling of modules belonging to different services with the basic call control module which is aware of all active telephony features in the system. Although sometimes based on distributed model, VoIP call control models still manifest some of the listed problems of their predecessors. In this paper we present a fully distributed model which exhibits minimal coupling of modules belonging to different services and a simple basic call control module. The model is based on taxonomies of call control services which are presented in the paper. Also, the implementation of several typical services is described.