A time fence is a period during which certain types of change to the master production schedule are possible. IFS/Master Scheduling recognizes two such periods: the demand time fence and the planning time fence. For Level 0 (zero) parts, the system generates both values automatically. For Level 1 parts, you must specify these values.
The demand time fence is the shorter of the two periods. It normally extends from the current day until the end of the lead time for the part. The demand time fence represents the length of time it would take you to produce the part if all subassemblies were in stock. Most, if not all, of the demand falling within this period is based on actual customer orders. This helps you differentiate between actual and forecast demand and thus avoid duplication. During the demand time fence, the master schedule is frozen, meaning that changes are extremely difficult to make.
A demand time fence of 0 means that no demand time fence applies. It is important to understand that the fence is defined in terms of work days. So for example, if the demand time fence is entered as 0 and the last master schedule run date is Monday, with a conventional five day work calendar, then the demand time fence ends on the previous Friday. On the other hand, if the demand time fence is entered as 4 and the last master schedule run date is Monday, then the demand time fence extends through Thursday. As a more extreme example, if the demand time fence is entered as 1, last master schedule run date is Monday, and the work calendar only defines Wednesdays as work days, then the demand time fence extends through Wednesday.
The planning time fence is a worst-case scenario. It represents the cumulative lead time of the part, i.e., the length of time it would take to produce the part if none of the subassemblies were in stock. To calculate the planning time fence, you use the critical path method. This method assumes that the total time required to build the part is the sum of the durations of the subassemblies. If more than one subassembly exists at the same level within the part structure, the assumed duration is the longest time required to produce a subassembly at that level. The demand falling within the planning time fence is a mixture of actual customer orders and forecasts.
During the time period beyond the demand time fence but within the planning time fence, changes to the master production schedule require consideration of management trade-offs. Beyond the planning time fence, the master production scheduler is free to change the schedule. The planning time fence is calculated in a manner similar to the demand time fence.
Before a part reaches the planning time fence, it is not associated with any shop or purchase orders. The system forecasts the supply and generates master schedule proposals. As a part moves within the planning time fence, you must analyze the system-generated proposed receipts and convert them into manual proposals, firm planned shop orders, or released shop orders. You must perform this step manually because, once the part is within the planning time fence, any changes you make may adversely affect lower levels of the structure.