The supermarkets may consist of multiple units to perform same checkout operation of sales, which are usually set all together besides the entrance of the supermarket. Each unit contains one employee. This kind of a system is called a multiple-server system with single service facility, in other words multiple checkouts counters (service units) with sales checkout as a service available in a system. There are two possible models for multiple-server system: Single-Queue Multiple-Server model, and Multiple-Queue Multiple-Server model.