Video Telephony is the real time exchange of voice and video between end-users. It is the basis of a wide range of applications (e.g. Multiparty games, distance learning). Quality of service (QoS) enables network performance control for meeting specific applications and/or end-user requirements. It is a differentiating factor for service providers. Evolved Packet Core (EPC) is the new core network for 3GPP 4G networks. Home Subscriber Server (HSS) is the standardized master database of 3GPP next generation networks including video telephony networks and EPC. It contains the subscription related information that is needed to support the network entities when they handle sessions. The constant increase in the number of subscribers is one of the challenges for future mobile networks including video telephony networks and EPC. Virtualization is a technique used to emulate the physical characteristics of resources. It enables efficiency in resource usage and is a key technology for scalability and elasticity.  
This thesis proposes an architecture for QoS Enabled video telephony with a Virtualized HSS (VHSS) in a 3GPP 4G environment. It makes two main contributions. Firstly, it proposes a differentiated QoS service delivery platform that relies on EPC. This platform enables the provisioning of a refined differentiated QoS scheme which allows prioritization between different sessions of a same video telephony application running on a same network. This new scheme is a differentiating factor for service providers. Second it proposes a preliminary mechanism for a scalable and elastic HSS in order to cope with the increasing number of subscribers. This is done by decomposing the HSS into three main layers (diameter layer, database computation layer and storage layer). Each of these layers are virtualized and can grow/shrink independently. We have built a proof of concept prototype to demonstrate the feasibility of the proposed architecture. Performance measurements have also been made to evaluate viability.