In brief: It is a common situation in practice that a "small" change in the master plan turns into dozens of messages to the purchasing department within a few days. The master production schedule (MPS) is the input of material requirements planning (MRP); every figure changed in the MPS propagates to all lower levels along the bill of materials. This article covers the time fences that keep the MPS stable, the ATP calculation used as the basis for order promising, ways to reduce MRP "nervousness", and how MRP and JIT/kanban can be operated together in the same factory. For basic concepts, the What Is Production Planning? and What Is Materials Planning? pages can be consulted. In this article:Why Should the MPS Be Stable?How Are Time Fences Set Up?How Are Orders Promised with ATP?How Is MRP Nervousness Reduced?How Do MRP and JIT/Kanban Work Together in the Same Factory?Technology and Digitalization: Closed-Loop Planning with ERP, APS, and MESImplementation: Checklist Why Should the MPS Be Stable? A single quantity or date change touching the near term gives rise to dozens of rescheduling messages in MRP for subassemblies, semi-finished goods, and purchased materials. Following this, the purchasing department puts pressure on the supplier, preparations in production are disrupted, and urgent orders take precedence over regular orders. On the other hand, never changing the MPS is not a solution either; a plan that never changes cannot respond to customer demand. Time fences tie the balance between these two needs to a defined rule instead of leaving it to the bargaining power of individuals on a given day. How Are Time Fences Set Up? ZoneTypical HorizonChange RuleDemand Source Frozen zoneUp to the demand time fence (usually the final assembly / final operation time)Only with the approval of a defined authority; with the cost calculatedFirm customer orders only Slushy (semi-flexible) zoneBetween the demand time fence and the planning time fence (up to the cumulative production and procurement lead time)With material and capacity check and master planner approvalOrders + remaining forecast Liquid (free) zoneBeyond the planning time fenceFreely, according to the planning system and S&OP decisionsForecast (consumed by orders) Instead of determining fence lengths intuitively, it is recommended to derive them from the product's cumulative procurement and production lead time. When the MPS quantity that uses a long-lead-time material is changed after that material has been ordered, the cost shows up as excess stock or urgent procurement expense. Fence lengths should be reviewed at least once a year, and updated without waiting for the annual review when lead times change. How Are Orders Promised with ATP? Delivery date requests from the sales department must be answered on the basis of available to promise (ATP) quantity rather than stock on hand. ATP is the quantity that can be allocated to new orders after previously promised customer orders are deducted from current stock and planned MPS production. In the example below, the starting inventory is 50 units (figures are for illustration purposes): Week12345 MPS production100-100-100 Customer orders received8030602010 Discrete ATP40-20-90 For week 1: 50 (stock) + 100 (production) − 80 − 30 (orders up to the next MPS) = 40. For week 3: 100 − 60 − 20 = 20. For week 5: 100 − 10 = 90. Suppose the sales team submits a new request of 30 units for week 3. 20 units of this request can be met from week 3, and the remaining 10 units from the ATP of week 1; the week 5 ATP is preserved for later-dated requests. The main benefit of ATP is that it prevents a quantity that is in stock but already allocated to another customer from being sold a second time. How Is MRP Nervousness Reduced? MRP nervousness is the situation in which a small change in input leads to large and frequent plan changes at lower levels. The most common causes encountered in practice and the corresponding solution recommendations are given below. MPS changes inside the fence: Limited with firm planned orders and the frozen zone rule.Lot sizing rules: Rules such as periodic order quantity can shift order dates back and forth with small demand changes; for critical items, a fixed order quantity or minimum lot can be preferred.Safety stock at every level: Protection against uncertainty is preferably provided at the finished goods or MPS level, and at lower levels with safety time.Small rescheduling messages: A threshold is defined for shift suggestions of a few days, and messages below the threshold are not acted on.Data accuracy: When inventory records and bills of materials are wrong, MRP continuously produces corrections; cycle counting and BOM verification are basic requirements. How Do MRP and JIT/Kanban Work Together in the Same Factory? At this point in trainings, the question "MRP or kanban?" frequently comes up. In many factories, using the two methods together rather than seeing them as alternatives to each other gives better results. MRP anticipates material and capacity requirements over a long horizon; kanban runs daily replenishment with a pull system in repetitive and stable flows. The prerequisite for this is that the MPS is leveled enough to carry the kanban loops. A fluctuating MPS continually leaves kanban quantities insufficient. The widely used formula for the number of cards is N = D × L × (1 + α) / C. For example, with daily demand D = 480 units, replenishment time L = 0.5 days, safety factor α = 10%, and container capacity C = 40 units, N = 480 × 0.5 × 1.1 / 40 = 6.6 → calculated as 7 kanbans. Item TypeRecommended MethodRationale Purchased items with long lead times and high valueMRPRequires demand visibility and advance ordering Internal semi-finished goods produced repetitivelyKanbanStable consumption; low stock through pull Low-value fasteners, consumablesTwo-bin system / vendor-managed inventoryPlanning effort is kept low relative to value Order-specific (project-related) componentsMRP (project/order-linked)Different requirement for every order MPS stability determines the reliability of MRP, and leveling of the MPS determines the workability of kanban. Technology and Digitalization: Closed-Loop Planning with ERP, APS, and MES Defining time fences and ATP rules on paper is the relatively easy part of the process. Whether these rules work in practice depends on how accurately planning and production systems pass data to each other. When the loop between plan, actuals, and correction is closed digitally, who changed what in the frozen zone and how "nervous" MRP has become cease to be a matter of guesswork and become visible and manageable. Enterprise Resource Planning (ERP) is the enterprise transaction system that records orders, inventory, purchasing, production, and finance transactions in a common database; it is the master data and transaction source for planning. MRP calculations are mostly run inside the ERP. The accuracy of bill of materials, routing, lead time, and open order data directly determines the reliability of the MRP output. Advanced Planning and Scheduling (APS) is a planning system based on optimization and heuristic methods that produces feasible plans and schedules by considering capacity, material, and priority constraints at the same time. It checks the MPS draft against capacity and material constraints simultaneously and detects infeasible plans before they are passed to MRP. Available-to-Promise and Capable-to-Promise (ATP/CTP) is an approach that, when promising orders, checks not only stock and planned production but also capacity and material availability together. When used together with APS, it ensures that where ATP is insufficient, order promises are given according to capacity and material availability. Manufacturing Execution System (MES) is the system that links the plan and actual production by monitoring shop floor work orders, machine status, production quantities, and scrap in near real time. It feeds back actual production, stoppages, and scrap immediately, making the plan-versus-actual gap in the frozen zone measurable. Internet of Things (IoT) is the infrastructure in which sensors on machines, vehicles, warehouses, and products transmit data automatically over a network; location, temperature, status, and consumption data are collected without human input. Operating and stoppage data from machine sensors make capacity assumptions realistic; with containers fitted with weight or fill sensors, the kanban signal can be transmitted automatically (e‑kanban). Artificial Intelligence (AI) and Machine Learning (ML) are methods that learn patterns from data to produce forecasts, classifications, and recommendations; machine learning is the sub-field of artificial intelligence that learns from data. By forecasting the probability of delay by supplier and line, they generate risk warnings on order promise dates and prioritize which MRP recommendations should be acted on. Artificial intelligence should not be expected to replace time fence discipline. It is still the planner who applies the rule; artificial intelligence provides the planner with earlier and more selective information. Instead of long lists of rescheduling messages, highlighting only those messages that endanger delivery is a practical way of reducing nervousness. However, for this to work, the master data in the ERP and the actuals coming from the MES must be reliable. When the fence rule is not followed, an algorithm only speeds up the noise; the priority is a stable MPS, and smart message prioritization follows it. Implementation: Checklist Calculate cumulative procurement and production lead times for finished goods families.Define demand and planning time fences according to these lead times.Write an authority and cost approval rule for changes in the frozen zone.Link the order promising process to the ATP calculation.Set a threshold for MRP rescheduling messages; use firm planned orders.Classify items one by one among MRP, kanban, and the two-bin system.Audit bill of materials, routing, and lead time data in the ERP regularly; link actuals from the MES to the weekly MPS review. Related Articles Lean Production and Kanban Systems: Pull Logic, Kanban Count Calculation, and ImplementationCampaign Planning and Changeover Times in the Process IndustryWhich Decision in Which Meeting in the S&OP Cycle? Calendar, Decision Logic, and Scenarios Frequently Asked Questions How is the length of time fences determined? For the demand time fence, the final assembly or final operation time is generally used; for the planning time fence, the product's cumulative procurement and production lead time is used. When the lead times change, the fences also need to be updated. Are ATP and stock on hand the same thing? No. Stock on hand is the physical quantity sitting on the shelf. ATP is the quantity that can be promised to new orders after planned production is added to this stock and orders already given are deducted. Should MRP or kanban be used? In most factories, it is appropriate to use both methods together. Long-lead-time and project-related items are managed with MRP, and repetitive and stable flows with kanban. This decision should not be made once for the whole factory; it should be handled item by item. Why should the MPS be leveled for kanban? Kanban quantities are calculated according to average consumption. When the MPS fluctuates sharply from day to day, the loops fall short and stockouts become more frequent. A leveled MPS reduces this fluctuation. What is the difference between APS and MRP in the ERP? MRP assumes capacity is unlimited while calculating material requirements. APS, on the other hand, creates a feasible plan and schedule by considering capacity and material constraints together. In many setups, the ERP records transactions and APS supports planning and scheduling decisions. References Jacobs, F. R., Berry, W. L., Whybark, D. C., & Vollmann, T. E. (2011). Manufacturing Planning and Control for Supply Chain Management (6th ed.). McGraw-Hill.Monden, Y. (2011). Toyota Production System: An Integrated Approach to Just-In-Time (4th ed.). CRC Press.ASCM. ASCM Supply Chain Dictionary (APICS Dictionary): entries for "time fence", "available-to-promise", and "system nervousness". Related Optimum Planlama Trainings factory Production Planning and Detailed Scheduling Better manage orders, capacity-material requirements, and production flow. Detailed Information trolley Purchasing, Warehouse and Inventory Management Build reliable systems for supplying materials to production and customers. Detailed Information handshake Sales and Operations Planning, S&OP/IBP Build the balance of demand and supply with competitive tactics and consensus. 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