Voice service interworking for PSTN and IP networks

This article presents an overview of the main technical problems to be addr essed for the provision of interoperable services between IP telephony and the PSTN. The pivotal element of the solution resides in an interworking fu nction. This function is typically implemented in a gateway whose requireme nts and behavior are here analyzed in terms of signaling and control protoc ols (control plane) as well as user data transfer (user plane). The present ation is structured around these two planes. The control plane defines the set of signaling protocols to be used in each networking context and the tr anslation between them. Detailed scenarios illustrate the signal translatio n in the gateway allowing for the establishment of a hybrid phone call. The user plane is responsible for adapting the user data to the properties of each network channel and determines the quality of service of the voice cal l in terms of delay and speech quality.