Projects with fairly well defined requirements, sufficient specifications and very little ambiguity can be executed as fixed time and fixed price projects. Based on this project details estimation for the engagement is done in terms of deliverables, timelines, cost and the execution / delivery contract is prepared. Enhancement or any changes are provisioned and managed through change management process. Payments are made either based on milestones, that is specific delivery or completion of specific task or periodic at fixed interval of time.
This model limits the risks associated with the project as well as ensures timely and within budget delivery. However this model limits change flexibility especially the changes which impact delivery and / or budgets.