I. Introduction
Since the publication of the seminal paper by Kelly [1], the network utility maximization (NUM) framework has been used for various resource allocation problems in communication networks. Especially, it is extensively used to study rate control in the Internet (See [2] and references therein.). In the NUM framework, each user has a utility function which represents its satisfaction to the service according to the amount of resource allocation. In this framework, it is important to assign an appropriate utility function to each user that can model its satisfaction to the service accurately.