Motion information scalability is an important requirement for a fully scalable video codec, especially in low bit rate or small resolution decoding scenarios. So far, several layered coding and motion vector precision scalability approaches have been proposed. However, it is still vague on the required functionalities of a fully scalable motion model and how it interacts with other scalabilities, such as spatial, temporal and quality, in a scalable video codec. In this paper, we first define the functionalities required by a fully scalable motion model. Based on these requirements, a fully scalable motion model will be proposed and, moreover, the associated rate distortion optimized estimation techniques will also be provided as a companion. Several simulation results will be presented to summarize the advantages of proposed motion model.