Real-time distributed collaboration applications are characterized by close co-operation among processes. Concurrency control and timeliness of the response are essential requirements for such applications. As mobile systems are characterized by frequent disconnections and physical mobility from one cell to another, providing real-time distributed collaboration for mobile systems is a real challenge. In this paper, we propose a framework for concurrency control in real-time distributed collaboration applications for mobile systems. We view mobile systems as a three-tier architecture and propose a data-centric framework for optimistic concurrency control. The framework is built on top of a totally ordered exactly-once multicast protocol. Our model ensures that number of messages on the wired network is independent of the number of mobile hosts in the system. We have implemented and tested our framework by developing a collaborative text editor application over exactly-once multicast protocol.