As per D&C, except that the contractor must also maintain the facility for a specified period - usually between 10 and 30 years - in return for a largely fixed monthly fee. The fixed monthly fee covers planned maintenance activities. Unplanned maintenance, which arises for reasons unrelated to fault by the contractor, is typically paid for on a cost reimbursement or schedule of rates basis. Maintenance payments can also be linked to performance (i.e. abated if the facility is not available due to need to carry out maintenance work, or fails to perform at specified levels). This model motivates the contractor to design and build the facility in a manner that will minimise the combined design, construction and maintenance costs, which results in a lower whole-of-life cost for government. This model is also commonly referred to as Design, Build & Maintain (DBM). Another variant is the Design, Build, Operate & Maintain (DBOM) model, where the contractor designs and builds a facility and then operates and maintains it for a specified period.