Bakery A A Bakery Simulation

Bakery A We start of with the simplest possible simulation - the bakery mentioned in Lecture 2. We need: a) A way to generate customer arrivals at appropriate intervals; b) A way to serve those customers at an appropriate rate; c) A way to send those customers on their way. Arena provides us (among others), with three distinct modules: an Arrival module, a Server module and a Depart module. Start Arena from the submenu of the Start menu of Windows (whichever version is on your computer - 95 or more recent). You may find a couple of dialogs telling you about the academic version of Arena and introductory materials. You may want to spend some time reading and following through. Once you are done, you will have a blank center for your screen - the area where you will build your models.

Bakery A The first thing to do is to select File/New. Then click on the Attach… label and double-click on Common.tpo - a rather extensive tool-bar will appear on the left along with a number of (pre-existing) buttons and color choices on the right. We will use FOUR buttons from the Common tool-bar: Arrive, Depart, Server and Simulate. Click on Arrive, move the cursor to the blank area in the middle (it will become a cross-hair) and click where you would like the Arrive icon to appear (stay on the left of center). Click on Server, move to the right of the Arrive icon you just placed and click to place the Server icon. Repeat for the Depart module. Repeat once more for the Simulate module, but place its icon below the other three, near the center of the region. Placement is NOT crucial, it is just convenient to start with some design principles (movement of customers from left to right, etc…) and stay with it unless the configuration does not permit it. You would have the picture on the next slide:

Bakery A

The black-filled box indicates the module is selected. We now have all the pieces, but we have to decide how to use them and how to connect them. Double-click on the box of the Arrive module - notice that it has a name below it (Arrive 1): each module placed has a default name that we can use or change at will. A dialog box will appear, and we will discuss its contents.

Bakery A It has, as you can see, three regions: and Enter Data region, an Arrival Data region and a Leave Data region. Enter Data:The DEFAULT name of the station is Arrive 1. If you were to drop the list, you would find Depart 1 and Server 1 (as you might expect). You could now type BakeryIn, to give it a (possibly) more appropriate name. There are two other buttons in that area: you may click on them to find out what their dialogs look like - make no changes there. Arrival Data: we will leave Batch Size at 1 (one customer at a time); First Creation could be set to 0.0; Max Batches should be left empty.

Bakery A Time Between has a drop-down list. You can either fill-in your own recipe, or you can pick one of the ones given. Drop the list and pick EXPO(Mean). When the expression appears in the window, replace Mean with some number (say 0.833), indicating that you have chosen an exponential arrival discipline with mean inter-arrival time of time units. There is a further Mark Time Attribute. You could either leave it blank or choose Queue Time (as the only possibility) from the drop-down list. You can check the Assign… and Animate… buttons. Make sure you make no changes to the dialog boxes (exit them with Cancel…). At this point things should look like:

Bakery A Leave Data: Check out the two further dialog buttons - leave them with Cancel - and now we are left with the question “where to”? Leave the Route and StNm buttons as they are. The drop-won list for Station contains the names BakeryIn, Server 1, Depart 1. Since we are going from the door to the counter, type in BakeryCounter (you could have chosen Server 1, but one should always use names that are meaningful in the context). For Route Time we could leave 0, or we could insert a small value (say 0.05) to indicate that it takes a short while to walk from the door to the counter.

Bakery A When the dialog looks like the one on the right, you are done, and you can click on the OK button, ready for the next step. Now you are ready to double-click on the Server box…

Bakery A The initial dialog looks as on the right, and we need to fill enough of it so our simulation will work. Enter Data: we will leave the Label as is, and choose BakeryCounter from the Station drop- down list. Check the dialog for Tran In. Server Data : notice that, as you typed BakeryCounter, the Resource window changed from Server 1_R (the default) to BakeryCounter_R (your name). Leave Capacity at 1 (one employee behind the counter), Capacity Type at Capacity (check what choices the drop- down list gives you). Resource Statistics remains checked. For Process Time, choose EXPO(Mean) from the drop-down list.

Bakery A Replace Mean by 0.7. Check out the dialogs for the four buttons - leave each with Cancel. Leave Area: examine the extra dialogs, leave Route and StNm checked, the drop-down list for Station has BakeryIn, BakeryCounter and Depart 1 as choices. Type BakeryOut, and give a Route Time of 0.05 (same time to get out as to get in… change it if you don’t like it). When the dialog looks like the one on the next slide, click OK - done with the Server module.

Bakery A OK. Now double-click on the Depart box, to bring up its own dialog.

Bakery A Enter Data: leave the Label blank; go to the drop-down list and choose BakeryOut. Examine the options given by the dialogs corresponding to the three buttons. Count: check Individual Counter. Tally: leave as is. When the dialog looks like the one on the next Slide, click OK.

Bakery A Notice that, as you return to the Simulation area, a blue box has appeared on top of the Depart box: it will show the exit count. We are almost done. The simulation is set up, except for some last decisions. They refer to the contents of the Simulate box. Double-click on it, to bring up its dialog.

Bakery A Project: just type something appropriate into the boxes - dates are according to the American system (MM/DD/YY). Replicate: each replication will give you one sample point. Start with 1, but be prepared to run many, and then run statistics on your results. Leave Beginning Time at 0.0; put 1000 for Length of replication; leave all the rest blank. Click OK. Click on the red check mark on the strip at the bottom of the simulation area. You should have a small dialog that tells you everything is OK. Accept it and go on. There is one small problem left: you would like to have a bit more animation. It would be nice to see something move from the BakeryIn through the BakeryCounter to the BakeryOut boxes.

Bakery A At the right of the design area you will find a collection of buttons. Choose the one with the R (the Route button). Click on it a the dialog box at the right will appear. Clicking OK will let you draw a route, from the center of one of the red boxes to the next one on the route - with corners if needed. The route from entrance to counter would be:

Bakery A Repeat, using the red box to the right of the Server and that above the Exit. You should end up with something like this: Hit the Run button - at the bottom left of the Arena window, just like a tape player button - and watch. The numbers in the blue window on top of the Depart module will keep changing…

Bakery A At the end of the run, you will be asked if you want to see the results. Answer YES. A NotePad (or WordPad - if large enough) document will come up, with the collected statistic. You fill find Tally Variables: which contain cycle-time and queue-time statistics (average, max and min time units in queue, for example). Discrete-Change Variables: contain resource utilization (busy), resource availability (1.000 if the resource never fails or goes for a break), and number-in-queue statistics. Notice that number-in-queue, and average-time-in-queue are fairly large, and might indicate that there are too many customers for the number of counter-persons available… More runs first, though…

Bakery A Exercises. a) Change the mean Inter-Arrival time and the mean Service Time of the Server. Try distributions other than the Exponential. Check through the Arena Help System to find out more. What happens if the mean Service Time is longer than the mean Inter-Arrival Time? Run at least one example. b) What happens if you add a counter worker? HOW can you add a counter worker? The Server module will allow you to do this - you need to find out. It isn’t hard. Set up a system with 3 counter workers to take care of the customers. c) Many Italian Bars require the customer to pay BEFORE asking the counter people for the the cup of coffee or the sandwich, or whatever. Insert a cashier between the arriving customer and the counter. Obviously, you will have to choose an appropriate Service Distribution for the cashier. The cashier should be able to serve enough people to keep your counter workers occupied most of the time…

Bakery A d) Under “usual” conditions - you pay after being served your cup of coffee, the queue time at the counter is enough for you to reach a decision, so no EXTRA time is devoted to making up your mind. In the Italian Bar case, you will waste another 0.3 time units between the moment you enter the door and the moment you join the cashier’s queue. Model this. What would you need to do with the REST of the system, to offset this extra time? Show the validity of your “intuition” through a simulation.