A fixed payment over the duration of the contract. This is usually preferred when you are looking to exploit a range of services, share the risks and rewards, seeking a long term support.
The project is signed off for a fixed price, for a pre-determined time frame. You can choose this model, if you are looking at a temporary engagement and usually preferred by established enterprises or corporations.
Some companies prefer to take support on specific functions, for specific time period. Here, we will work with you to customize the service that you will require, with cost and time frame. This model works well when you are clear about your requirements and deliverables.